HomeMy WebLinkAboutFRCC HEALTH CARE CAREERS CENTER - SPA180002 - SUBMITTAL DOCUMENTS - ROUND 1 - GEOTECHNICAL (SOILS) REPORTGeotechnical Subsurface Exploration Program
Front Range Community College:
Health Care Career Center
Fort Collins, Colorado
Prepared For:
Front Range Community College
4616 S. Shields Street
Fort Collins, CO 80526
Attn: Dennis DeRemer
Job Number: 18-0040 November 26, 2018
Project Site
TABLE OF CONTENTS
Page
Purpose and Scope of Study ..................................................................................... 1
Proposed Construction .............................................................................................. 1
Site Conditions .......................................................................................................... 2
Subsurface Exploration ............................................................................................. 2
Laboratory Testing .................................................................................................... 3
Subsurface Conditions .............................................................................................. 3
Seismic Classification ................................................................................................ 5
Foundation/Floor System Overview ........................................................................... 6
Foundation System ................................................................................................... 8
Floor System ............................................................................................................ 10
Observatory Foundation and Floor System ............................................................... 13
Lateral Earth Pressures ........................................................................................... 23
Water Soluble Sulfates ............................................................................................ 24
Soil Corrosivity ........................................................................................................ 24
Exterior Flatwork ...................................................................................................... 27
Project Earthwork ..................................................................................................... 29
Excavation Considerations ....................................................................................... 33
Utility Lateral Installation and Backfilling ................................................................... 34
Surface Drainage .................................................................................................... 37
Subsurface Drainage ................................................................................................ 40
Pavement Sections ................................................................................................. 42
Closure and Limitations ........................................................................................... 47
Locations of Test Holes ................................................................................... Figure 1
Logs of Test Holes .................................................................................. Figure 2 to 4
Legend and Notes ........................................................................................... Figure 5
Axial Capacity Reductions of Closely Spaced Pier / Pile Elements .................. Figure 6
Lateral Capacity Reductions of Closely Spaced Pier / Pile Elements .............. Figure 7
Summary of Laboratory Test Results .............................................................. Table 1
Summary of Soil Corrosion Test Results ......................................................... Table 2
Pavement Thickness Calculations ............................................................. Appendix A
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PURPOSE AND SCOPE OF STUDY
This report presents the results of a geotechnical evaluation performed by GROUND
Engineering Consultants, Inc. (GROUND) for Front Range Community College in
support of design of the proposed Health Care Career Center in Lafayette, Colorado.
Our study was conducted in general accordance with GROUND’s proposal No. 1809-
1662 with Front Range Community College, dated September 18th, 2018.
A field exploration program was conducted to obtain information on the subsurface
conditions. Material samples obtained during the subsurface exploration were tested in
the laboratory to provide data on the engineering characteristics of the on-site soils. The
results of the field exploration and laboratory testing are presented herein.
This report has been prepared to summarize the data obtained and to present our
findings and conclusions based on the proposed development/improvements and the
subsurface conditions encountered. Design parameters and a discussion of engineering
considerations related to the proposed improvements are included herein. This report
should be understood and utilized in its entirety; specific sections of the text, drawings,
graphs, tables, and other information contained within this report are intended to be
understood in the context of the entire report. This includes the Closure section of the
report which outlines important limitations on the information contained herein.
This report was prepared for design purposes of Front Range Community
College(FRCC) based on our understanding of the proposed project at the time of
preparation of this report. The data, conclusions, opinions, and geotechnical parameters
provided herein should not be construed to be sufficient for other purposes, including the
use by contractors, or any other parties for any reason not specifically related to the
design of the project. Furthermore, the information provided in this report was based on
the exploration and testing methods described below. Deviations between what was
reported herein and the actual surface and/or subsurface conditions may exist, and in
some cases those deviations may be significant.
PROPOSED CONSTRUCTION
Current plans provided show an approximately 30,000 square foot footprint for the
proposed Health Care Career Center along with a supporting paved drive lane and
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parking spaced on the south side of the proposed facility. The existing observatory is
also planned to be relocated to the south of its current position. Additionally,
construction will likely include installation of underground utilities. Provided preliminary
building loads were 275 kips for maximum column loads and 2 kips per linear foot for
wall loads. Provided preliminary grading plans indicate that the anticipated finished
floor elevation is approximately 5080.5 feet in elevation. Cuts of up to approximately 9
feet and fills of up to approximately 4 feet will be required to establish anticipated
grades. If proposed construction, including the assumed loading conditions, differ from
those described above, or changes subsequently, GROUND should be notified to re-
evaluate the parameters in this report.
SITE CONDITIONS
At the time of our exploration, the project site was the northwest corner of the Larimer
Campus of Front Range Community College near the intersection of E. Harmony Road
and S. Shields Street at Fort Collins. The site supported grass and trees. The site is
bordered by the Harmony Library to the south, existing FRCC buildings to the east, West
Harmony Road to the north, and South Shields Street to the west. The topography
across the site was variable with notable slopes falling from the northwest to the
southeast at slopes up to approximately 15 percent. There is an existing drainage ditch
and detention on the eastern portion of the site.
Fill was observed in several of the test holes likely associated with previous site grading
at the project site. The exact extents, limits, and composition of any man-made fill were
not determined as part of the scope of work addressed by this study, and should be
expected to exist at varying depths and locations across the site.
SUBSURFACE EXPLORATION
The subsurface exploration for the project was conducted on October 31st, and
November 1st, 2018. A total of eleven (11) test holes were drilled at the project site.
Nine (9) test holes were drilled within the approximate footprint of the proposed building.
One (1) test hole was drilled near the proposed future location of the observatory.
One(1) test hole were drilled within the proposed parking lot expansion. The test holes
were drilled with a truck-mounted, continuous flight power auger rig to evaluate the
subsurface conditions as well as to retrieve soil samples for laboratory testing and
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analysis. The foundation test holes were drilled to depths of approximately 24 to 39 feet
below existing grades and the pavement test hole was drilled to approximately 5 feet
below existing grades. A representative of GROUND directed the subsurface
exploration, logged the test holes in the field, and prepared the soil samples for transport
to our laboratory.
Samples of the subsurface materials were retrieved with a 2-inch I.D. California liner
sampler. The sampler was driven into the substrata with blows from a 140-pound
hammer falling 30 inches. This procedure is similar to the Standard Penetration Test
described by ASTM Method D1586. Penetration resistance values, when properly
evaluated, indicate the relative density or consistency of soils. Depths at which the
samples were obtained and associated penetration resistance values are shown on the
test hole logs.
The approximate locations of the test holes are shown in Figure 1. Logs of the
exploratory test holes are presented in Figures 2 to 4. Explanatory notes and a legend
are provided in Figure 5.
LABORATORY TESTING
Samples retrieved from our test holes were examined and visually classified in the
laboratory by the project engineer. Laboratory testing of soil samples obtained from the
subject site included standard property tests, such as natural moisture contents, grain
size analyses, liquid and plastic limits, and swell-consolidation testing. Water-soluble
sulfate and corrosivity testing was completed on selected samples of the soils as well.
Laboratory tests were performed in general accordance with applicable ASTM protocols.
Results of the laboratory testing program are summarized on Tables 1 and 2.
SUBSURFACE CONDITIONS
Geologic Setting - Published geologic maps, e.g., Colton (1978),1 depict the project site
as underlain by Slocum Alluvium (Qs) consisting of cobble and gravel with reddish
brown clay. The surficial deposits at the project site are mapped as being underlain by
1
Colton, R.B., 1978, Geologic map of the Boulder-Fort Collins-Greeley area, Front Range Urban Corridor,
Colorado: U.S. Geological Survey, Miscellaneous Investigations Series Map I-855-G, scale 1:100,000
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the Pierre Shale (Kp) consisting of silty shale, sandstone, and shale. A portion of that
geologic map is reproduced below.
The claystones and siltstones can be moderately to highly expanse and the formation
includes well cemented beds that can be very hard and difficult to excavate.
Local Conditions The subsurface conditions encountered in the test holes generally
consisted of a layer of topsoil2 materials approximately 1 foot in thickness. The topsoil
was underlain by fill materials that consisted of sandy clay with trace gravels that
generally extended to depths ranging from 3 to 6 feet below existing grades. Sand and
clay materials were encountered locally below the fill in test holes 2, 3 and 10. Sand and
clay with gravel was encountered below the sand and clay and fill materials. Claystone
bedrock was encountered below the sand and clay with gravel materials at depths
ranging from 8 to 35 feet below existing grades and continued to the test hole
termination depths of approximately 24.5 to 39 feet below existing grades.
2
‘Topsoil’ as used herein is defined geotechnically. The materials so described may or may not be
suitable for landscaping or as a growth medium for such plantings as may be proposed for the project.
Approximate Project Site
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It should be noted that coarse gravel, cobbles, boulders, and similarly sized fragments of
debris are not well represented in small diameter liner samples collected from 4-inch
diameter test holes. Therefore, such materials may be present at varying depths at the
project site.
Fill materials consisting of sandy clay with trace gravels were fine to coarse grained
locally, low to medium plastic, relatively compact, slightly moist to moist, and light brown
to dark brown in color.
Sand and Clay materials were fine to coarse grained with local gravel, low to medium
plastic, stiff/medium dense to hard/dense, moist, and brown to red-brown in color with
local caliche deposits.
Sand and Gravel with Clay materials were fine to coarse grained with gravel, non to
medium plastic, stiff/medium dense to hard/dense, moist to wet, and brown to red-brown
to yellow-brown in color with local caliche deposits.
Claystone Bedrock was fine grained with siltstone layers, medium to highly plastic,
hard to very hard, dry to slightly moist, and brown-gray in color.
Swell-Consolidation Testing yielded results ranging from 0.9 percent swell to 0.3
percent consolidation and one swell of 0.2 percent at various surcharge pressures based
on estimated overburden pressure.
Groundwater was encountered in the test holes at the time of drilling at approximate
elevations of 5069 to 5076 feet. When measured 5 days later in test holes 1, 3, 7, and 9
groundwater was at elevations ranging from approximately 5072.5 to 5076 feet.
Groundwater levels can be expected to fluctuate, however, in response to annual and
longer-term cycles of precipitation, irrigation, surface drainage, nearby rivers and creeks,
land use, and the development of transient, perched water conditions.
SEISMIC CLASSIFICATION
According to the 2015 International Building Code® (Section 1613 Earthquake Loads),
“Every structure, and portion thereof, including nonstructural components that are
permanently attached to structures and their supports and attachments, shall be
designed and constructed to resist the effects of earthquake motions in accordance with
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ASCE 7, excluding Chapter 14 and Appendix 11A. The seismic design category for a
structure is permitted to be determined in accordance with Section 1613 (2015 IBC) or
ASCE 7.” Exceptions to this are further noted in Section 1613.
Based on extrapolation of available data to depth and our experience in the project area,
we consider the site likely to meet the criteria for a Seismic Site Classification of C
according to the 2015 IBC classification (Section 1613.3.2). If, however, a quantitative
assessment of the site seismic properties is desired, then sampling or shear wave
velocity testing to a depth of 100 feet or more should be performed.
Utilizing the United States Geological Survey’s Seismic Design Maps Tool
(http://geohazards.usgs.gov/designmaps/us/application.php), assuming a Site Class C
the project area is indicated to possess an SDS value of 0.149g and an SD1 value of
0.066g for the site latitude and longitude. If however, local codes require quantitative
assessment of the site then assuming a Site Class D the tool provides an SDS value of
0.198g and an SD1 value of 0.094g for the site latitude and longitude.
FOUNDATION/FLOOR SYSTEM OVERVIEW
Geotechnical Considerations for Design: As stated previously cuts of up to 9 feet and
fills of up to 4 feet will be required to establish grades for the anticipated finished floor
elevation of 5080.5.
If simply filled and cut from current elevations the differential fills will likely settle
differentially based on depth. Additionally un-documented fills exist locally below the
anticipated finished floor elevation. Un-documented fills are generally not geotechnically
acceptable as they present an unquantifiable potential for heave and settlement.
According to our field and laboratory analysis, it is GROUND’s opinion the materials
encountered in our exploration are generally suitable to support the proposed structure
on a shallow foundation system consisting of spread footings with a slab-on-grade floor
system. However, a uniform fill prism consisting of over-excavated and replaced site
materials should be established to mitigate the potentials for differential settlement and
the risks associated with undocumented fills.
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Foundation and Floor System: The building footprint should be over-excavated to a
common bottom elevation of approximately 5076 feet. This depth of over-excavation will
establish a uniform fill prism for floor slabs and spread footing foundations. The fill prism
should extend out past the building perimeter by at least 5 feet. The existing site
materials should be re-placed in moisture treated and compacted state in accordance
with the Project Earthwork section of this report.
Greater depths of over-excavation may be required if soft areas are exposed at the time
of constructions. A representative of the geotechnical engineer should be retained to
verify existing conditions during construction excavation. Due to the proximity of
groundwater near the bottom of the over-excavation, localized areas may require
stabilization methods such as stabilization rock, aggregate base course materials, and
geogrid in some combination to establish a firm platform for filling.
Additionally, dewatering operations and the construction of a mud mat may be required
to facilitate the construction of any proposed deeper foundations that will bear at or near
the groundwater.
The Contractor should take care to construct a fill layer of uniform composition to reduce
differential post-construction building movements. If materials are imported for, fill with
the exception of stabilization rock, they should not be mixed with site materials in the
same horizontal fill layer within the building footprint.
To use these parameters, the Owner must accept the risk of post-construction
foundation movement associated with shallow foundation systems placed on the on-site
soils. Utilizing the above parameters as well as other parameters in this report, we
estimate likely post-construction foundation and floor movements to be on the order of 1
inch, with 1/2 inch differential movements over spans of about 40 feet. Movement
estimates are difficult to predict and actual movements may be more or less
The conclusions and parameters provided in this report were based on the data
presented herein, our experience in the general project area with similar structures, and
our engineering judgment with regard to the applicability of the data and methods of
forecasting future performance. A variety of engineering parameters were considered as
indicators of potential future soil movements. Our parameters were based on our
judgment of “likely movement potentials,” (i.e., the amount of movement likely to be
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realized if site drainage is generally effective, estimated to a reasonable degree of
engineering certainty) as well as our assumptions about the owner’s willingness to
accept geotechnical risk. “Maximum possible” movement estimates necessarily will be
larger than those presented herein. They also have a significantly lower likelihood of
being realized in our opinion, and generally require more expensive measures to
address. We encourage the Client, upon receipt of this report, to discuss these risks
and the geotechnical alternatives with us.
FOUNDATION SYSTEM
The design and construction criteria presented below should be observed for a spread
footing foundation system. The construction details should be considered when
preparing project documents. The precautions and parameters provided below will not
prevent movement of the footings if the underlying materials are subjected to alternate
wetting and drying cycles. However, the recommended measures will tend to make the
movement more uniform, and reduce resultant damage if such movement occurs.
Geotechnical Parameters for Shallow Foundation Design:
1) Footings bearing on materials as described in the Foundation / Floor System
Overview section above may be designed for an allowable soil bearing
pressure (Q) of 2,500 psf.
These values may be increased by ⅓ for transient loads such as wind or seismic
loading.
Compression of the bearing soils for the provided allowable bearing pressure is
estimated to be 1 inch, based on an assumption of drained foundation conditions.
If foundation soils are subjected to an increase/fluctuation in moisture content,
the effective bearing capacity will be reduced and greater post-construction
movements than those estimated above may result.
2) To be able to use the allowable bearing capacity values presented above, strip
footings should be limited to 6 feet or less in width and pad footing should have a
maximum dimension of 10.5 feet. For other estimated settlements associated
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with allowable bearing pressure values or footing widths exceeding the
dimensions above please contact this office.
3) In order to reduce differential settlements between footings or along continuous
footings, footing loads should be as uniform as possible. Differentially loaded
footings will settle differentially.
Similarly, differential fill thickness beneath footings will result in increased
differential settlements.
4) Spread footings should have a minimum lateral dimension of 18 or more inches
for linear strip footings and 24 or more inches for isolated pad footings. Actual
footing dimensions, however, should be determined by the structural engineer.
5) All footings should bear at an elevation 3 or more feet below the lowest adjacent
exterior finish grades.
6) Continuous foundation walls should be reinforced top and bottom to span an
unsupported length of at least 10 feet.
7) Geotechnical parameters for lateral resistance to foundation loads are provided
in the Lateral Earth Pressures section of this report.
8) Connections to the building of all types must be flexible and/or adjustable to
accommodate the anticipated, post-construction movements.
Shallow Foundation Construction: The following should be considered during the
construction of spread footing foundations.
1) Care should be taken when excavating the foundations to avoid disturbing the
supporting materials. Hand excavation or careful backhoe soil removal may be
required in excavating the last few inches.
2) Footing excavation bottoms may expose loose, organic or otherwise deleterious
materials, including debris. Firm materials may become disturbed by the
excavation process. All such unsuitable materials should be excavated and
replaced with properly compacted fill or the footing deepened.
3) Foundation soils may be disturbed or deform excessively under the wheel loads
of heavy construction vehicles as the excavations approach footing bearing
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levels. Construction equipment should be as light as possible to limit
development of this condition. Track-mounted vehicles generally should be used
because they exert lower contact pressures. The movement of vehicles over
proposed foundation areas should be restricted.
4) All footing areas should be compacted with a vibratory plate compactor prior to
placement of reinforcing steel placement for concrete to consolidate loose
materials at the surface.
5) Compacted fill placed against the sides of the footings should be compacted in
accordance with the criteria in the Project Earthwork section of this report.
FLOOR SYSTEM
The following measures are recommended to reduce damage, which may result from
movement of the slab subgrade material. These measures will not eliminate potential
movements. If slab-on-grade construction is used in accordance with the following
criteria, as well as other applicable parameters contained in this report, we estimate that
potential slab movements may be on the order of 1 inch. The actual magnitude of
movement is difficult to estimate and may be more or less.
Geotechnical Parameters for Design of Slab-on-Grade Floors
1) Slab subgrade materials shall be over-excavated and replaced in general
accordance with the Foundation / Floor System Overview section above.
2) An allowable subgrade vertical modulus (K) of 100 pci may be utilized for lightly
loaded slabs supported by over-excavated on-site materials. This value is for a 1-
foot x 1-foot plate; they should be adjusted for slab dimension.
3) Based on our experience with reinforced slabs and conversations with the owner,
there is a concern with using 4 inch slab thicknesses and reinforcing steel such
as #4 bar mats. Slab thicknesses of 5 inches or greater should be considered.
Reinforcing clearance and cover can be difficult to achieve during construction of
thinner slabs. Lack of proper cover can be detrimental to the finish and
appearance of these slabs.
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4) Floor slabs should be separated from all bearing walls and columns with slip
joints, which allow unrestrained vertical movement.
Slip joints should be observed periodically, particularly during the first several
years after construction. Slab movement can cause previously free-slipping
joints to bind. Measures should be taken to assure that slab isolation is
maintained in order to reduce the likelihood of damage to walls and other interior
improvements.
5) Concrete slabs-on-grade should be provided with properly designed control
joints.
ACI, AASHTO and other industry groups provide guidelines for proper design
and construction concrete slabs-on-grade and associated jointing. The design
and construction of such joints should account for cracking as a result of
shrinkage, curling, tension, loading, and curing, as well as proposed slab use.
Joint layout based on the slab design may require more frequent, additional, or
deeper joints, and should reflect the configuration and proposed use of the slab.
Particular attention in slab joint layout should be paid to areas where slabs
consist of interior corners or curves (e.g., at column blockouts or reentrant
corners) or where slabs have high length to width ratios, significant slopes,
thickness transitions, high traffic loads, or other unique features. The improper
placement or construction of control joints will increase the potential for slab
cracking.
6) Interior partitions resting on floor slabs should be provided with slip joints so that
if the slabs move, the movement cannot be transmitted to the upper structure.
This detail is also important for wallboards and doorframes. Slip joints which will
allow 2 inches or more of differential vertical movement should be considered.
It may not be practical to construct slip joints capable of accommodating
movements of that magnitude. In such case, replacement of the slip joints or re-
establishment of slip capacity should be anticipated and incorporated into
building design. Accommodation for differential movement also should be made
where partitions meet bearing walls.
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7) Post-construction soil movements may not displace slab-on-grade floors and
utility lines in the soils beneath them to the same extent. Design of floor
penetrations, connections and fixtures should accommodate at least 2 inches of
differential movement.
8) Moisture can be introduced into a slab subgrade during construction and
additional moisture will be released from the slab concrete as it cures. A properly
compacted layer of free-draining gravel, 4 or more inches in thickness, should be
placed beneath the slabs. This layer will help distribute floor slab loadings, ease
construction, reduce capillary moisture rise, and aid in drainage.
The free-draining gravel should contain less than 5 percent material passing the
No. 200 Sieve, more than 50 percent retained on the No. 4 Sieve, and a
maximum particle size of 2 inches.
The capillary break and the drainage space provided by the gravel layer also
may reduce the potential for excessive water vapor fluxes from the slab after
construction as mix water is released from the concrete.
We understand, however, that professional experience and opinion differ with
regard to inclusion of a free-draining gravel layer beneath slab-on-grade floors. If
these issues are understood by the owner and appropriate measures are
implemented to address potential concerns including slab curling and moisture
fluxes, then the gravel layer may be deleted.
9) A vapor barrier beneath a building floor slab can be beneficial with regard to
reducing exterior moisture moving into the building, through the slab, but can
retard downward drainage of construction moisture. Uneven moisture release
can result in slab curling. Elevated vapor fluxes can be detrimental to the
adhesion and performance of many floor coverings and may exceed various
flooring manufacturers’ usage criteria.
Per the 2006 ACI Location Guideline, a vapor barrier is required under concrete
floors when that floor is to receive moisture-sensitive floor covering and/or
adhesives, or the room above that floor has humidity control.
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Therefore, in light of the several, potentially conflicting effects of the use vapor-
barriers, the owner and the architect and/or contractor should weigh the
performance of the slab and appropriate flooring products in light of the intended
building use, etc., during the floor system design process and the selection of
flooring materials. Use of a plastic vapor-barrier membrane may be appropriate
for some building areas and not for others.
In the event a vapor barrier is utilized, it generally should consist of a minimum
15 mil thickness, extruded polyolefin plastic (no recycled content or woven
materials), maintain a permeance less than 0.01 perms per ASTM E-96 or ASTM
F-1249, and comply with ASTM E-1745 (Class “A”). Vapor barriers should be
installed in accordance with ASTM E-1643.
Polyethylene (“poly”) sheeting (even if 15 mils in thickness which polyethylene
sheeting commonly is not) does not meet the ASTM E-1745 criteria and should
not, in general, be used as vapor barrier material. It can be torn and/or
punctured easily, does not possess necessary tensile strength, becomes brittle,
tends to decompose over time, and has a relatively high permeance.
Construction Considerations for Slab-on-Grade Floors
10) Loose, soft or otherwise unsuitable materials exposed on the prepared surface
on which the floor slab will be cast should be excavated and replaced with
properly compacted fill.
11) Concrete floor slabs should be constructed and cured in accordance with
applicable industry standards and slab design specifications.
12) All plumbing lines should be carefully tested before operation. Where plumbing
lines enter through the floor, a positive bond break should be provided.
OBSERVATORY FOUNDATION AND FLOOR SYSTEM (Drilled Pier Foundations)
Geotechnical Risk The proposed location of the observatory is underlain by
approximately 6 feet of undocumented fill materials and claystone bedrock materials. As
stated previously undocumented fills are generally not geotechnically acceptable as they
present an unquantifiable potential for heave and settlement. Additionally the claystone
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materials underlying the proposed observatory location exhibited potentials for post-
construction heave that can cause damaging, post-construction, structural movements.
This condition, if not mitigated, will affect improvements near the observatory site.
Mitigating the expansive materials and/or their effects, the undocumented fills, as well as
controlling the surface waters and shallow subsurface moisture changes, are the
principal geotechnical design considerations for the site. Specific geotechnical
parameters in these regards are provided in subsequent sections of this report.
Additional discussion and information regarding these recommendations and the
geotechnical risks that they address are provided below.
Additionally, it is our understanding that this structure is sensitive to vibration and
settlement and a deep foundation system is desirable to mitigate further mitigate these
when compared to a shallow foundation.
Depth of Wetting The “depth of wetting” (the depth to which foundation soils will gain
moisture and experience volume change over the design-life of a structure) estimated for
a given site strongly affects the anticipated performance of structures at that site. Based
on the data obtained at this site and our experience with similar geotechnical settings, a
‘depth of wetting’ of 20 feet was used to develop geotechnical parameters for foundation
system design. ‘Depths of wetting’ of 30, 40 or 70 feet or more have been considered
(e.g., Chao and others, 2006)3 and have been encountered locally in the field. Depths of
wetting of such magnitudes, however, generally are in unusual geologic conditions such
as the Dipping Bedrock Overlay District to the west, or identified forensically in unusual
circumstances such as a pipe leak that has remained un-repaired for an extended
period. In our experience, such deep ‘depths of wetting’ are considered only rarely in
engineering consulting practice in more typical geologic settings in the Colorado Front
Range area. GROUND considers a 20-foot depth of wetting to be appropriately
conservative for the proposed project. We can consider a more (or less) conservative
depth of wetting, however, upon request.
Observatory Foundation/Floor System Due to the movement potential in the existing
fill materials and the claystone materials encountered at the project site as well as the
nature of the observatory structure, it is GROUND’s opinion that the proposed
3 Chao, K-C, D.D. Overton, and J.D. Miller, 2006, The Effects of Site Conditions on the Predicted Time Rate
of Heave, Unsaturated Soils 2006, American Society of Civil Engineers, Special Publication No. 147, pp.
2086 – 2097.
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observatory facility be supported on straight-shaft drilled piers advanced into the
underlying bedrock and provided with a structural floor system. Additionally, building
entryways should be founded similarly so as to reduce the potential for differential
movement.
Although a drilled pier foundation system (of any length) will not eliminate the risk of
post-construction building movement, if the measures outlined in this report are
implemented effectively, the likelihood of acceptable building performance to a
reasonable degree of engineering certainty will be within local industry standards for
construction of a drilled pier foundation system on soils and bedrock of this nature.
Based on the conditions encountered in GROUND’s test holes, the assumptions outlined
herein, including effective maintenance of site drainage, we estimate likely post-
construction movements from heave and/or settlement of drilled pier foundations to be
on the order of ½ inch.
Observatory Drilled Pier Foundation System
Geotechnical Parameters for Drilled Pier Design Based on the results of the field
exploration, laboratory testing, and experience, the design criteria presented below
should be observed for a straight-shaft, drilled pier foundation system. In our experience
it can be beneficial to facilitate construction to use as few pier diameters / types as
possible.
Note that the minimum dead load and the minimum pier length indicated below were
developed to resist the uplift force that the expansive bedrock will exert on the surface of
the pier in the zone above the depth of wetting. The minimum length on that basis may
or may not be sufficient to provide the necessary axial capacity. The uplift loading also
should be used to develop the (minimum) reinforcing steel, as discussed below.
1) Drilled piers should bear in ‘comparatively unweathered’ bedrock underlying the
site. For design purposes, ‘comparatively unweathered’ bedrock may be taken to
be at and below a depth of approximately 9 feet below existing grades. For
bidding purposes, these elevations may vary.
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2) Drilled piers should be at least 18 inches in diameter and should be designed
with a maximum length to diameter ratio of 30 to 1. The actual length to diameter
ratios should be determined by the structural engineer.
3) Drilled piers should have a minimum length of 30 feet. The actual drilled pier
lengths should be determined by the structural engineer based on loading, etc.,
with further increases in length possibly required by the conditions encountered
during installation at each drilled pier location.
4) Drilled piers also should penetrate at least 10 feet into relatively un-weathered
bedrock or 3 drilled pier diameters, whichever is greater.
Based on the minimum length and bedrock penetration, and taking the top of
relatively un-weathered bedrock to be about 9 feet below existing grade, drilled
pier lengths on the order of 30 to 32 feet are anticipated to meet the geotechnical
criteria. Actual drilled pier lengths commonly will be greater due to structural
considerations, conditions in the drilled pier holes, actual depths to relatively un-
weathered bedrock, clean out, etc.
5) Drilled piers bearing in relatively un-weathered bedrock at and below depths of
30 feet may be designed for an allowable end bearing pressure of 30,000 psf.
The portion of the drilled pier penetrating comparatively un-weathered bedrock
may be designed for a skin friction value of 2,250 psf. 100 percent of the skin
friction may be used to resist both compressional loads and uplift. However, skin
friction above the depth of wetting indicated in the Observatory Foundation and
Floor System section of this report, i.e., in the upper 20 feet of the pier should be
ignored for axial load resistance.
6) Estimated settlement of properly constructed drilled piers will be low – on the
order of ½ inch – to mobilize skin friction.
7) Drilled piers should be designed for a minimum dead load pressure of 6,000 psf
based on drilled pier cross-sectional area.
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Where minimum dead load cannot be applied, it will be necessary to increase the
drilled pier length beyond the recommended minimum, even where the minimum
bedrock penetration has been achieved or exceeded. This can be accomplished
by assuming that skin friction on the extended zone acts in the direction to resist
uplift.
8) Drilled piers should be reinforced as determined by the structural engineer. At a
minimum, drilled piers should be reinforced for their full length to resist the tensile
loading created by the swelling soils and bedrock. Tension may be estimated as
an uplift skin friction of 1,050 psf applied to the upper 20 feet of each drilled pier.
Reinforcement design also should include any deficit between the dead load
applied in design and the minimum dead load provided above.
9) A 6-inch or thicker continuous void should be provided beneath grade beams,
drilled pier caps, foundation walls, and floor slabs. The void space should be
protected from backfill intrusion.
10) Based on the data obtained for this study and our experience with similar sites
and conditions, lateral load analysis using the Terzaghi method may take the
values tabulated below for the modulus of horizontal subgrade reaction (Kh) to be
characteristic of the soils and bedrock underlying the site, based on a simplified
soil / bedrock profile. Resistance to lateral loads by deep foundations should be
neglected in the upper 3 feet of soils, weather fill or native granular fill soils.
Horizontal Modulus Subgrade Reaction (Kh) – Terzaghi Method
Material
Kh based on
Foundation
Kh based on
Foundation
Approximate
Depth Range
Element Width /
Diameter
Element Width /
Diameter
1.5-Foot Diameter 2-Foot Diameter
Overburden
Fill and Sand
and Clay
3-9 80 tcf 60 tcf
Claystone
Bedrock
9+ feet 240 tcf 180 tcf
Note that the Kh values tabulated above are dependent on deep foundation
element width or diameter. If values for other widths / diameters are required,
please contact this office.
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11) Penetration of comparatively unweathered bedrock in drilled pier shafts should
be roughened artificially to assist the development of peripheral shear between
the drilled pier and bedrock. Artificially roughening of drilled pier holes should
consist of installing shear rings 3 inches high and 2 inches deep in the portion of
each drilled pier penetrating comparatively unweathered bedrock and below a
depth of 20 feet. The shear rings should be installed 18 inches on centers.
However, the specifications should allow a geotechnical engineer to waive the
requirement for shear rings depending on the conditions actually encountered in
individual drilled pier holes.
12) Groups of closely spaced drilled piers placed to support concentrated loads will
require an appropriate reduction of the estimated capacities. Reduction of axial
capacity generally can be avoided by spacing drilled piers at least 3 diameters
center to center. At this spacing or greater, no reduction in axial capacities or
horizontal soil modulus values is required. Drilled pier groups spaced less than 3
diameters center to center should be studied on an individual basis to determine
the appropriate axial capacity reduction(s). The settlement of closely spaced
groups of drilled piers should also be studied on an individual basis.
Linear arrays of drilled piers, however, must be spaced at least 8 diameters
center to center to avoid reductions in lateral capacity when loaded in line with
the array (parallel to the line connecting the drilled pier centers). Linear arrays of
drilled piers spaced more closely than 8 diameters center to center should be
studied to determine the appropriate lateral capacity reductions in that direction.
Refer to Figures 6 and 7 for additional information regarding reductions in lateral
and axial capacity for closely spaced caissons
Drilled Pier Construction The following should be considered during the construction
of drilled pier foundations.
13) The depth of comparatively unweathered bedrock should be determined in the
field at each drilled pier location and may differ from other information provided
herein.
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14) Lenses or beds of relatively soft bedrock, including coal or lignite seams, (not
suitable for foundation support) may be encountered in the bedrock materials.
Such circumstances may result in lengthening the drilled piers.
15) The bedrock beneath the site was hard to very hard and resistant, particularly
with depth. The pier-drilling contractor should mobilize equipment of sufficient
size and operating capability to achieve the design lengths and bedrock
penetration.
If refusal is encountered in these materials, a geotechnical engineer should be
retained to evaluate the conditions to establish whether true refusal has been
met with adequate drilling equipment.
16) Groundwater was encountered in test hole 10 at a depth of approximately 22 feet
below existing grade the time of drilling. Additionally, groundwater was observed
as shallow as 4 feet below the existing grade at the proposed observatory
location in the test holes advanced for the Health Care Career Center to the
northwest. Groundwater, combined with granular soils, often results in caving
during pier installation. Seating of the casing in the upper layers of the bedrock
may not create positive cutoff of water infiltration. The contractor should be
prepared to address this condition.
17) In no case should concrete be placed in more than 3-inches of water, unless
placed through an approved tremie method. The proposed concrete placement
method should be discussed during the pre-construction meeting by the Project
Team.
18) Where groundwater and unconsolidated soils and/or caving bedrock materials
are encountered, the installation procedure of drilled piers can be a concern.
Commonly in these conditions, the drilling contractor utilizes casing and slurry
during excavation of the drilled pier holes, which may adversely affect the axial
and/or lateral capacities of the completed drilled piers. During casing withdrawal,
the concrete should have sufficient slump and must be maintained with sufficient
head above groundwater levels to displace the water or slurry fully to prevent the
creation of voids in the drilled pier.
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Because of these considerations, the drilling contractor should submit a written
procedure addressing the use of casing, slurry, and concrete placement prior to
commencement of drilled pier installation.
19) Drilled pier holes should be properly cleaned prior to placement of concrete.
20) Concrete utilized in the drilled piers should be a fluid mix with sufficient slump so
that it will fill the void between reinforcing steel and the drilled pier hole wall, and
inhibit soil, water, and slurry from contaminating the concrete. The concrete
should be designed with a minimum slump of no less than 5 inches.
21) Concrete should be placed by an approved method to minimize mix segregation.
22) Concrete should be placed in a drilled pier on the same day that it is drilled.
Failure to place concrete the day of drilling may result in a requirement for
lengthening the drilled pier. The presence of groundwater or caving soils may
require that concrete be placed immediately after the pier hole drilling is
completed.
23) The contractor should take care to prevent enlargement of the excavation at the
tops of drilled piers, which could result in “mushrooming” of the drilled pier top.
24) Sonic integrity testing (sonic echo or cross-hole sonic) can be performed at the
discretion of the structural engineer to assess the effectiveness of the drilled pier
construction methods and to check any suspect piers. Additional information on
sonic integrity testing can be provided upon request.
Observatory Structural Floor System
Structural floors should be supported on grade beams and straight-shaft drilled piers in
the same manner as the building structure. Requirements for the number and position
of additional piers to support the floors will depend upon the span, design load, and
structural design, and should be developed by the Structural Engineer. Geotechnical
recommendations for design and installation of drilled piers are provided in the
Observatory Drilled Pier Foundation System section of this report.
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Crawl Space Structural floors should be constructed to span above a well-ventilated
crawl space permitting utility lines to be installed above the swelling materials. The
crawl space should be adequately sized to allow access to and maintenance of utility
piping and have a minimum clearance of 24 inches. Piping connections through floors,
grade beams, or foundation walls should allow for differential movement between the
piping and the structural element through which the piping is penetrating. Additionally,
prior to or during the construction of the planned addition may be an opportune time to
assess the ventilation and general condition of crawl space beneath the existing gym.
This space should also be provided with adequate ventilation similar to that of the new
crawl space.
A vapor barrier meeting ASTM E-1745 (Class “A”) should be considered for installation
below all structurally supported floors and if utilized, should be properly attached/sealed
to foundation walls/drilled piers above the void material. The sheet material should not
be attached to horizontal surfaces such that condensate might drain to wood or
corrodible metal surfaces.
Use of polyethylene (“poly”) sheeting as a vapor barrier is not recommended.
Polyethylene (“poly”) sheeting (even if 15 mils in thickness which polyethylene sheeting
commonly is not) does not meet the ASTM E-1745 criteria and is not recommended for
use as vapor barrier material. It can be easily torn and/or punctured, does not possess
the necessary tensile strength, gets brittle, tends to decompose over time, and has a
relatively high permeance.
New buildings generally lack ventilation due primarily to systematic efforts to construct
air-tight, energy-efficient structures. Therefore, areas such as crawl spaces beneath
structural floors are typically areas of elevated humidity which never completely dry.
This condition can be aggravated in some locations by shallow groundwater or a
perched groundwater condition, which can result in saturated soils within several feet of
the finished building pad grade. Persistently warm, humid conditions in the presence of
cellulose, which is found in many typical construction materials, creates an ideal
environment for the growth of molds, fungi and mildew. Published data suggest links
between molds and illnesses. Therefore, GROUND recommends that crawl spaces
beneath structural floors be provided with adequate, active ventilation systems or other
active mechanisms such as specially designed HVAC systems to reduce the potential
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for mold, fungus and mildew growth. Crawl spaces should be inspected periodically so
that remedial measures can be taken in a timely manner, should mold, fungus or mildew
be present and require removal.
The Owner must be willing to accept the risks of potential mold, fungus and mildew
growth when electing to utilize a structural floor system. Additionally, the contractor is
solely responsible for the means and methods during construction including adequate
ventilation, and any observation or testing performed during construction does not
relieve the contractor of that responsibility.
All plumbing lines should be carefully tested before operation. Where utility lines enter
through the floor, positive bond breaks should be provided. Utility lines can be displaced
by soils and bedrock movements, which are not reflected in the building. Design and
installation of associated fixtures should accommodate this potential differential
movement, which could be on the order of 8 inches or more and should allow for repair /
maintenance.
Slab-on-void Alternative Based on GROUND’s experience with similar projects, as an
alternate floor system, the proposed floor could be constructed as a structural slab on
void form in lieu of a structural floor system over a ventilated crawl space. In the event a
structural slab on void form is utilized, a minimum void form thickness of 6 inches
should be utilized. Greater void thickness may accommodate increased heave
movement. The Client/Owner should be aware that to our knowledge, there is no floor
system that will provide the same tolerance for floor movements as would be provided
by a structural floor system placed over a well-ventilated crawl space.
Please note that by utilizing a structural floor spanning above a crawl space, as indicated
previously, this would permit utility lines to be installed (hung) above the swelling soils
and bedrock. GROUND would anticipate that utilities placed in soil trenches beneath a
structural slab on void may be subjected to these expansive materials. Piping
connections through the floor should allow for differential movement between the piping
and floor system (flexible). All plumbing lines should be carefully tested before
operation. Where utility lines enter through the floor, positive bond breaks should
be provided. Utility lines can be displaced by soil and bedrock movements, which are
not reflected in the building. Design and installation of associated fixtures should
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accommodate this potential differential movement. Previously, we stated that entryway
floor slabs should also be constructed as structural floors. In the event this is not to be
the case, again, the Client/Owner should be aware that these elements may also be
subjected to expansive materials and may experience significant structural movements.
LATERAL EARTH PRESSURES
The at-rest, active, and passive conditions for the on-site backfill is summarized on the
table below. Base friction may be combined with passive earth pressure if the
foundation is in a drained condition. The values for the on-site material in the upper 10
feet provided in the table below were approximated utilizing a unit weight of 127 pcf and
a phi angle of 27 degrees.
Lateral Earth Pressures (Equivalent Fluid Unit Weights)
Material Type
Water
Condition
At-Rest
(pcf)
Active
(pcf)
Passive(pcf)
Friction
Coefficient
CDOT Class 1
Structure Fill
Drained 55 37 400 (max. 4,000 psf) 0.45
On-Site Backfill Drained 70 48 300(max. 3,000 psf) 0.34
The upper 1 foot of embedment should be neglected for passive resistance, however.
Where this passive soil pressure is used to resist lateral loads, it should be understood
that significant lateral strains will be required to mobilize the full value indicated above,
likely 1 inch or more. A reduced passive pressure can be used for reduced anticipated
strains, however.
The lateral earth pressures indicated above are for a horizontal upper backfill slope. The
additional loading of an upward sloping backfill as well as loads from traffic, stockpiled
materials, etc., should be included in the wall/shoring design. GROUND can provide the
adjusted lateral earth pressures when the additional loading conditions and site grading
are clearly defined.
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WATER-SOLUBLE SULFATES
The concentration of water-soluble sulfates measured in selected samples retrieved
from the test holes ranged up to 0.02 percent by weight (see Table 2). Such a
concentration of water-soluble sulfates represents a negligible degree of sulfate attack
on concrete exposed to these materials. Degrees of attack are based on the scale of
'negligible,' 'moderate,' 'severe' and 'very severe' as described in the “Design and
Control of Concrete Mixtures,” published by the Portland Cement Association (PCA).
The Colorado Department of Transportation (CDOT) utilizes a corresponding scale with
4 classes of severity of sulfate exposure (Class 0 to Class 3) as described in the
published table below.
REQUIREMENTS TO PROTECT AGAINST DAMAGE TO
CONCRETE BY SULFATE ATTACK FROM EXTERNAL SOURCES OF SULFATE
Severity of
Sulfate
Exposure
Water-Soluble
Sulfate (SO4)
In Dry Soil
(%)
Sulfate (SO4)
In Water
(ppm)
Water
Cementitious
Ratio
(maximum)
Cementitious
Material
Requirements
Class 0 0.00 to 0.10 0 to 150 0.45 Class 0
Class 1 0.11 to 0.20 151 to 1500 0.45 Class 1
Class 2 0.21 to 2.00 1501 to 10,000 0.45 Class 2
Class 3 2.01 or greater 10,001 or greater 0.40 Class 3
Based on these data no special sulfate-resistant cement appears necessary in project
concrete.
SOIL CORROSIVITY
The degree of risk for corrosion of metals in soils commonly is considered to be in two
categories: corrosion in undisturbed soils and corrosion in disturbed soils. The potential
for corrosion in undisturbed soil is generally low, regardless of soil types and conditions,
because it is limited by the amount of oxygen that is available to create an electrolytic
cell. In disturbed soils, the potential for corrosion typically is higher, but is strongly
affected by soil chemistry and other factors.
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A preliminary corrosivity analysis was performed to provide a general assessment of the
potential for corrosion of ferrous metals installed in contact with earth materials at the
site, based on the conditions existing at the time of GROUND’s evaluation. Soil
chemistry and physical property data including pH, and sulfides content were obtained.
Test results are summarized on Table 2.
pH Where pH is less than 4.0, soil serves as an electrolyte; the pH range of about 6.5 to
7.5 indicates soil conditions that are optimum for sulfate reduction. In the pH range
above 8.5, soils are generally high in dissolved salts, yielding a low soil resistivity
(AWWA, 2010). Testing indicated pH values of approximately 8.5 and 8.4.
Reduction-Oxidation testing indicated negative potentials: approximately -116 and -111
millivolts. Such low potentials typically create a more corrosive environment.
Sulfide Reactivity testing for the presence of sulfides indicated ‘positive’ results. The
presence of sulfides in the site soils also suggests a more corrosive environment.
Soil Resistivity In order to assess the “worst case” for mitigation planning, samples of
materials retrieved from the test holes were tested for resistivity in the in the laboratory,
after being saturated with water, rather than in the field. Resistivity also varies inversely
with temperature. Therefore, the laboratory measurements were made at a controlled
temperature.
A measurement of electrical resistivity indicated values of approximately 2,287 and
5,044 ohm-centimeters in samples of the site earth materials.
Corrosivity Assessment The American Water Works Association (AWWA, 20104) has
developed a point system scale used to predict corrosivity. The scale is intended for
protection of ductile iron pipe but is valuable for project steel selection. When the scale
equals 10 points or higher, protective measures for ductile iron pipe are suggested. The
AWWA scale (Table A.1 Soil-test Evaluation) is presented below. The soil characteristics
refer to the conditions at and above pipe installation depth.
4 American Water Works Association ANSI/AWWA C105/A21.5-05 Standard.
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Table A.1 Soil-test Evaluation
Soil Characteristic / Value Points
Resistivity
<1,500 ohm-cm ..........................................................................................… 10
1,500 to 1,800 ohm-cm ................................................................……......…. 8
1,800 to 2,100 ohm-cm .............................................................................…. 5
2,100 to 2,500 ohm-cm ...............................................................................… 2
2,500 to 3,000 ohm-cm .................................................................................. 1
>3,000 ohm-cm ................................................................................… 0
pH
0 to 2.0 ............................................................................................................ 5
2.0 to 4.0 ......................................................................................................... 3
4.0 to 6.5 ......................................................................................................... 0
6.5 to 7.5 ......................................................................................................... 0 *
7.5 to 8.5 ......................................................................................................... 0
>8.5 ...................................................................................................... 3
Redox Potential
< 0 (negative values) ........................................................................................ 5
0 to +50 mV ................................................................................................…. 4
+50 to +100 mV ............................................................................................… 3½
> +100 mV ............................................................................................... 0
Sulfide Reactivity
Positive ........................................................................................................…. 3½
Trace .............................................................................................................… 2
Negative .......................................................................................................…. 0
Moisture
Poor drainage, continuously wet ..................................................................…. 2
Fair drainage, generally moist ....................................................................… 1
Good drainage, generally dry ..................................................................... 0
* If sulfides are present and low or negative redox-potential results (< 50 mV) are
obtained, add three points for this range.
We anticipate that drainage at the site after construction will be effective. However,
based on the values obtained for the soil parameters, the overburden soils appear to
comprise a highly corrosive environment for metals (10.5 points).
If additional information are needed regarding soil corrosivity, the American Water Works
Association or a Corrosion Engineer should be contacted. It should be noted, however,
that changes to the site conditions during construction, such as the import of other soils,
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or the intended or unintended introduction of off-site water, may significantly alter
corrosion potential.
EXTERIOR FLATWORK
Proper design, drainage, construction, and maintenance of the areas between individual
buildings and parking/driveway areas are critical to the satisfactory performance of the
project. Sidewalks, entranceway slabs and roofs, fountains, raised planters, and other
highly visible improvements commonly are installed within these zones, and distress in
or near these improvements is common. Commonly, soil preparation in these areas
receives little attention because they fall between the building and pavement (which are
typically built with heavy equipment). Subsequent landscaping and hardscape
installation often is performed by multiple sub-contractors with light or hand equipment,
and over-excavation / soil processing is not performed. Therefore, particular care should
be taken by the design team, contractor, and pertinent subcontractors take particular
care with regard to proper subgrade preparation around the structure exteriors.
Similar to slab-on-grade floors, exterior flatwork and other hardscaping placed on the
soils encountered on-site may experience post-construction movements due to volume
change of the subsurface soils and the relatively light loads that they impose. Both
vertical and lateral soil movements can be anticipated as the soils experience volume
change as the moisture content varies. Distress to rigid hardscaping likely will result.
The following measures will help to reduce damages to these improvements.
Provided the owner understands the risks identified above, the subgrade under exterior
flatwork or other (non-building) site improvements should be scarified to a depth of 12 or
more inches. The scarified soil should be replaced as properly moisture-conditioned and
compacted fill as outlined in the Project Earthwork section of this report.
Prior to placement of flatwork, a proof roll should be performed to identify areas that
exhibit instability and deflection. The soils in these areas should be removed and
replaced with properly compacted fill or stabilized.
Flatwork should be provided with effective control joints. Increasing the frequency of
joints may improve performance. ACI recommendations should be followed regarding
construction and/or control joints.
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In no case should exterior flatwork extend to under any portion of the building where
there is less than 2 inches of vertical clearance between the flatwork and any element of
the building. Exterior flatwork in contact with brick, rock facades, or any other element of
the building can cause damage to the structure if the flatwork experiences movements.
GROUND does not recommend tying of exterior flatwork and/or hardscapes to the
building including floor slabs, bearing walls, or columns. The exterior flatwork should be
independent and be allowed to move independently from the structure and its
components.
As discussed in the Surface Drainage section of this report, proper drainage also should
be maintained after completion of the project, and re-established as necessary. In no
case should water be allowed to pond on or near any of the site improvements or a
reduction in performance should be anticipated.
Concrete Scaling: Climatic conditions in the project area including relatively low
humidity, large temperature changes and repeated freeze – thaw cycles, make it likely
that project sidewalks and other exterior concrete will experience surficial scaling or
spalling. The likelihood of concrete scaling can be increased by poor workmanship
during construction, such as ‘over-finishing’ the surfaces. In addition, the use of de-icing
salts on exterior concrete flatwork, particularly during the first winter after construction,
will increase the likelihood of scaling. Even use of de-icing salts on nearby roadways,
from where vehicle traffic can transfer them to newly placed concrete, can be sufficient
to induce scaling. Typical quality control / quality assurance tests that are performed
during construction for concrete strength, air content, etc., do not provide information
with regard to the properties and conditions that give rise to scaling.
We understand that some municipalities require removal and replacement of concrete
that exhibits scaling, even if the material was within specification and placed correctly.
The contractor should be aware of the local requirements and be prepared to take
measures to reduce the potential for scaling and/or replace concrete that scales.
In GROUND’s experience, the measures below can be beneficial for reducing the
likelihood of concrete scaling. It must be understood, however, that because of the other
factors involved, including weather conditions and workmanship, surface damage to
concrete can develop, even where all of these measures were followed.
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1) Maintaining a maximum water/cement ratio of 0.45 by weight for exterior
concrete mixes.
2) Include Type F fly ash in exterior concrete mixes as 20 percent of the
cementitious material.
3) Specify a minimum, 28-day, compressive strength of 4,500 psi for all exterior
concrete.
4) Include ‘fibermesh’ in the concrete mix. also may be beneficial for reducing
surficial scaling.
5) Cure the concrete effectively at uniform temperature and humidity. This
commonly will require fogging, blanketing and/or tenting, depending on the
weather conditions. As long as 3 to 4 weeks of curing may be required, and
possibly more.
6) Avoid placement of concrete during cold weather so that it is not exposed to
freeze-thaw cycling before it is fully cured.
7) Avoid the use of de-icing salts on given reaches of flatwork through the first
winter after construction.
We understand that commonly it may not be practical to implement some of these
measures for reducing scaling due to safety considerations, project scheduling, etc. In
such cases, additional costs for flatwork maintenance or reconstruction should be
incorporated into project budgets.
PROJECT EARTHWORK
The earthwork criteria below are based on our interpretation of the geotechnical
conditions encountered in the test holes. Where these criteria differ from applicable
municipal specifications, e.g., for trench backfill compaction along a public utility line, the
latter should be considered to take precedence.
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General Considerations: Site grading should be performed as early as possible in the
construction sequence to allow settlement of fills and surcharged ground to be realized
to the greatest extent prior to subsequent construction.
Prior to earthwork construction, existing concrete, asphalt, vegetation, and other
deleterious materials should be removed and disposed of off-site. Relic underground
utilities should be abandoned in accordance with applicable regulations, removed as
necessary, and properly capped.
Topsoil present on-site should not be incorporated into ordinary fills. Instead, topsoil
should be stockpiled during initial grading operations for placement in areas to be
landscaped or for other approved uses.
Existing Fill Soils: Man-made fill materials were encountered in select test holes during
subsurface exploration. Complete contents and composition of the man-made fill
materials are not known; therefore, some of the excavated man-made fill materials may
not be suitable for replacement as backfill. We anticipate that the majority of the fill
materials will be suitable for re-use as fill; however, a geotechnical engineer should be
retained during site excavations to observe the excavated fill materials and provide
recommendations for its suitability for reuse.
Drainage During Construction The contractor should take pro-active measures to
control surface waters during construction, to direct them away from excavations and
into appropriate drainage structures. Wetting of foundation soils during construction can
have adverse effects on the performance of the proposed facility.
Filled areas should be graded to drain effectively at the end of each work day.
Use of Existing Native Soils: Overburden soils that are free of trash, organic material
(including all firewood, wood chips, etc.), construction debris, and other deleterious
materials are suitable, in general, for placement as compacted fill. Organic materials
should not be incorporated into project fills.
Fragments of rock, cobbles, and inert construction debris (e.g., concrete or asphalt)
larger than 3 inches in maximum dimension will require special handling and/or
placement to be incorporated into project fills. In general, such materials should be
placed as deeply as possible in the project fills. A Geotechnical Engineer should be
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consulted regarding appropriate direction for usage of such materials on a case-by-case
basis when such materials have been identified during earthwork. Standard parameters
that likely will be generally applicable can be found in Section 203 of the current CDOT
Standard Specifications for Road and Bridge Construction.
Imported Fill Materials: If it is necessary to import material to the site, the imported
soils should be free of organic material, and other deleterious materials. Imported
material should consist of soils that have less than 50 percent passing the No. 200
Sieve and should have a plasticity index of less than 15. Representative samples
of the materials proposed for import should be tested and approved prior to transport to
the site.
Fill Platform Preparation: Prior to filling, the top 8 to 12 inches of in-place materials on
which fill soils will be placed should be scarified, moisture conditioned and properly
compacted in accordance with the parameters below to provide a uniform base for fill
placement. If over-excavation is to be performed, then these parameters for subgrade
preparation are for the subgrade below the bottom of the specified over-excavation
depth.
If surfaces to receive fill expose loose, wet, soft or otherwise deleterious material,
additional material should be excavated, or other measures taken to establish a firm
platform for filling. The surfaces to receive fill must be effectively stable prior to
placement of fill.
General Considerations for Fill Placement: Fill soils should be thoroughly mixed to
achieve a uniform moisture content, placed in uniform lifts not exceeding 8 inches in
loose thickness, and properly compacted. No fill materials should be placed, worked,
rolled while they are frozen, thawing, or during poor/inclement weather conditions.
Where soils supporting foundations or on which foundation will be placed are exposed to
freezing temperatures or repeated freeze – thaw cycling during construction – commonly
due to water ponding in foundation excavations – bearing capacity typically is reduced
and/or settlements increased due to the loss of density in the supporting soils. After
periods of freezing conditions, the contractor should re-work areas affected by the
formation of ice to re-establish adequate bearing support.
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Care should be taken with regard to achieving and maintaining proper moisture contents
during placement and compaction. Materials that are not properly moisture conditioned
may exhibit pumping, rutting, and deflection at high moisture contents.
Compaction areas should be kept separate, and no lift should be covered by another
until relative compaction and moisture content within the specified ranges are obtained.
Compaction Specifications: Soils that classify as GP, GW, GM, GC, SP, SW, SM, or
SC in accordance with the USCS classification system (granular materials) should be
compacted to 95 or more percent of the maximum modified Proctor dry density at
moisture contents within 2 percent of optimum moisture content as determined by ASTM
D1557.
Soils that classify as ML or CL should be compacted to 95 or more percent of the
maximum standard Proctor density at moisture contents within 2 percent of the optimum
moisture content as determined by ASTM D698.
Use of Squeegee: Relatively uniformly graded fine gravel or coarse sand, i.e.,
“squeegee,” or similar materials commonly are proposed for backfilling foundation
excavations, utility trenches (excluding approved pipe bedding), and other areas where
employing compaction equipment is difficult. In general, GROUND does not recommend
this procedure for the following reasons:
Although commonly considered “self compacting,” uniformly graded granular materials
require densification after placement, typically by vibration. The equipment to densify
these materials is not available on many job-sites.
Even when properly densified, uniformly graded granular materials are permeable and
allow water to reach and collect in the lower portions of the excavations backfilled with
those materials. This leads to wetting of the underlying soils and resultant potential loss
of bearing support as well as increased local heave or settlement.
It is GROUND’s opinion that wherever possible, excavations be backfilled with approved,
on-site soils placed as properly compacted fill. Where this is not feasible, use of
“Controlled Low Strength Material” (CLSM), i.e., a lean, sand-cement slurry (“flowable
fill”) or a similar material for backfilling should be considered.
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Where “squeegee” or similar materials are proposed for use by the contractor, the
design team should be notified by means of a Request for Information (RFI), so that the
proposed use can be considered on a case-by-case basis. Where “squeegee” meets
the project requirements for pipe bedding material, however, it is acceptable for that use.
Settlements: Settlements will occur in filled ground, typically on the order of 1 to 2
percent of the fill depth. If fill placement is performed properly and is tightly controlled, in
GROUND’s experience the majority (on the order of 60 to 80 percent) of that settlement
will typically take place during earthwork construction, provided the contractor achieves
the compaction levels recommended herein. The remaining potential settlements likely
will take several months or longer to be realized, and may be exacerbated if these fills
are subjected to changes in moisture content. GROUND anticipates some degree of
post-construction movement/distress as a result of settlement.
Cut and Filled Slopes: Permanent site slopes supported by on-site soils up to 5 feet in
height may be constructed no steeper than 3 : 1 (horizontal : vertical) in site soils. Minor
raveling or surficial sloughing should be anticipated on slopes cut at this angle until
vegetation is well re-established. Surface drainage should be designed to direct water
away from slope faces.
Steeper slope angles and heights may be possible but will require detailed slope stability
analysis based on final proposed grading plans. GROUND can be retained to evaluate
this on a case by case basis, if needed.
EXCAVATION CONSIDERATIONS
Excavation Difficulty: Test holes for the subsurface exploration were advanced to the
depths indicated on the test hole logs by means of conventional, truck-mounted,
geotechnical drilling equipment. We anticipate no significant excavation difficulties in the
majority of the site with conventional heavy-duty excavation equipment in good working
condition.
Excavation Slopes: Temporary, un-shored excavation slopes up to 8 feet in height be
cut no steeper than 1.5 (H) to 1 (V) in the on-site soils in the absence of seepage. Some
surface sloughing may occur on the slope faces at these angles. Where seepage or
flowing groundwater is encountered in shallow project excavations, a Geotechnical
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Engineer should be retained to evaluate the conditions and provided additional direction,
as appropriate. The risk of slope instability will be significantly increased in areas of
seepage along excavation slopes.
Should site constraints prohibit the use of the recommended slope angles, temporary
shoring should be used. The shoring should be designed to resist the lateral earth
pressure exerted by structure, traffic, equipment, and stockpiles. GROUND can be
retained to provide shoring design upon request.
Any excavations in which personnel will be working must comply with all OSHA
Standards and Regulations (CFR 29 Part 1926). The contractor’s “responsible person”
should evaluate the soil exposed in the excavations as part of the contractor’s safety
procedures. GROUND has provided the information above solely as a service to the
client, and is not assuming responsibility for construction site safety or the contractor’s
activities.
Surface Water and Groundwater: Good surface drainage should be provided around
temporary excavation slopes to direct surface runoff away from the slope faces. A
properly designed swale should be provided at the top of the excavations. In no case
should water be allowed to pond at the site. Slopes should be protected against erosion.
Erosion along the slopes will result in sloughing and could lead to a slope failure.
Groundwater was measured as shallow as 5076 feet in the test holes approximately 4.5
feet below anticipated finished floor elevations. Therefore, groundwater may be
encountered in shallow utility trenches A properly designed and installed de-watering
system may be required during the construction in these sections of trench. The risk of
slope instability will be significantly increased in areas of seepage along the excavation
slopes. If seepage is encountered, the slopes should be re-evaluated by a geotechnical
engineer.
UTILITY LATERAL INSTALLATION AND BACKFILLING
The measures and criteria below are based on GROUND’s evaluation of the local,
geotechnical conditions. Where the parameters herein differ from applicable municipal
requirements, the latter should be considered to govern.
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Pipe Support: The bearing capacity of the site soils appeared adequate, in general, for
support of buried utilities. The pipes + contents, typically, are less dense than the soils
which will be displaced for installation. Therefore, GROUND anticipates no significant
pipe settlements in these materials where properly bedded.
Excavation bottoms may expose soft, loose or otherwise deleterious materials, including
debris. Firm materials may be disturbed by the excavation process. All such unsuitable
materials should be excavated and replaced with properly compacted fill. Areas allowed
to pond water will require excavation and replacement with properly compacted fill. The
contractor should take particular care to ensure adequate support near pipe joints which
are less tolerant of extensional strains.
Where thrust blocks are needed, they may be designed utilizing the parameters set forth
in the Later Earth Pressures section of this report
Trench Backfilling: Some settlement of compacted soil trench backfill materials should
be anticipated, even where all the backfill is placed and compacted correctly. Typical
settlements are on the order of 1 to 2 percent of fill thickness. However, the need to
compact to the lowest portion of the backfill must be balanced against the need to
protect the pipe from damage from the compaction process. Some thickness of backfill
may need to be placed at compaction levels lower than recommended or specified (or
smaller compaction equipment used together with thinner lifts) to avoid damaging the
pipe. Protecting the pipe in this manner can result in somewhat greater surface
settlements. Therefore, although other alternatives may be available, the following
options are presented for consideration:
Controlled Low Strength Material: Because of these limitations, we recommend
backfilling the entire depth of the trench (both bedding and common backfill zones) with
“controlled low strength material” (CLSM), i.e., a lean, sand-cement slurry, “flowable fill,”
or similar material along all trench alignment reaches with low tolerances for surface
settlements.
We recommend that CLSM used as pipe bedding and trench backfill exhibit a 28-day
unconfined compressive strength between 50 to 150 psi so that re-excavation is not
unusually difficult.
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Placement of the CLSM in several lifts or other measures likely will be necessary to
avoid ‘floating’ the pipe. Measures also should be taken to maintain pipe alignment
during CLSM placement.
Compacted Soil Backfilling: Where compacted soil backfilling is employed, using the
site soils or similar materials as backfill, the risk of backfill settlements entailed in the
selection of this higher risk alternative must be anticipated and accepted by the
Client/Owner.
We anticipate that the on-site soils excavated from trenches will be suitable, in general,
for use as common trench backfill within the above-described limitations. Backfill soils
should be free of vegetation, organic debris and other deleterious materials. Fragments
of rock, cobbles, and inert construction debris (e.g., concrete or asphalt) coarser than 3
inches in maximum dimension should not be incorporated into trench backfills.
If it is necessary to import material for use as backfill, the imported soils should meet the
requirements set for in the Project Earthwork section of this report. Representative
samples of the materials proposed for import should be tested and approved prior to
transport to the site.
Soils placed for compaction as trench backfill should be conditioned to a relatively
uniform moisture content, placed and compacted in accordance with the parameters in
the Project Earthwork section of this report.
Pipe Bedding: Pipe bedding materials, placement and compaction should meet the
specifications of the pipe manufacturer and applicable municipal standards. Bedding
should be brought up uniformly on both sides of the pipe to reduce differential loadings.
As discussed above, we recommend the use of CLSM or similar material in lieu of
granular bedding and compacted soil backfill where the tolerance for surface settlement
is low. (Placement of CLSM as bedding to at least 12 inches above the pipe can protect
the pipe and assist construction of a well-compacted conventional backfill, although
possibly at an increased cost relative to the use of conventional bedding.)
If a granular bedding material is specified, GROUND recommends that with regard to
potential migration of fines into the pipe bedding, design and installation follow ASTM
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D2321. If the granular bedding does not meet filter criteria for the enclosing soils, then
non-woven filter fabric (e.g., Mirafi® 140N, or the equivalent) should be placed around
the bedding to reduce migration of fines into the bedding which can result in severe,
local surface settlements. Where this protection is not provided, settlements can
develop/continue several months or years after completion of the project. In addition,
clay or concrete cut-off walls should be installed to interrupt the granular bedding section
to reduce the rates and volumes of water transmitted along the utility alignment which
can contribute to migration of fines.
If granular bedding is specified, the contractor should not anticipate that significant
volumes of shallow on-site soils may be suitable for that use. Materials proposed for use
as pipe bedding should be tested for suitability prior to use. Imported materials should
be tested and approved prior to transport to the site.
SURFACE DRAINAGE
The site soils are relatively stable with regard to moisture content – volume relationships
at their existing moisture contents. Other than the anticipated, post-placement
settlement of fills, post-construction soil movements will result primarily from the
introduction of water into the soils underlying the proposed structure, hardscaping and
pavements. Based on the site surface and subsurface conditions encountered in this
study, we do not anticipate a rise in the local water table sufficient to approach grade
beam or floor elevations. Therefore, wetting of the soils likely will result from infiltrating
surface waters (precipitation, irrigation, etc.), and water flowing along constructed
pathways such as bedding in utility pipe trenches.
The following drainage measures should be followed both for during construction and as
part of project design. The facility should be observed periodically to evaluate the
surface drainage and identify areas where drainage is ineffective. Routine maintenance
of site drainage should undertaken throughout the design life of the proposed facility. If
these measures are not implemented and maintained effectively, the movement
estimates provided in this report could be exceeded.
1) Wetting or drying of the underslab or foundation bearing areas should be avoided
during and after construction. Permitting increases/variations in moisture to the
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adjacent or supporting soils may result in increased total and/or differential
movements.
2) Positive surface drainage measures should be provided and maintained to
reduce water infiltration into foundation soils.
The ground surface surrounding the exterior of each building should be sloped to
drain away from the foundation in all directions. A minimum slope of 12 inches in
the first 10 feet should be constructed in the areas not covered with pavement or
concrete slabs, or a minimum of 3 percent in the first 10 feet in the areas covered
with pavement or concrete slabs. Reducing the slopes to comply with ADA
requirements or other reasons may be necessary but may result in an increased
potential for moisture infiltration and subsequent volume change of the underling
soils.
In no case should water be allowed to pond near or adjacent to foundation
elements, hardscaping, etc.
3) Drainage also should be established to direct water away from sidewalks and
other hardscaping as well as utility trench alignments which are not tolerant of
moisture-volume changes in the underlying soils or flow of infiltrating water.
The ground surface near foundation elements should be able to convey water
away readily. Cobbles or other materials that tend to act as baffles and restrict
surface flow should not be used to cover the ground surface near the
foundations.
Where the ground surface does not convey water away readily, additional post-
construction movements and distress should be anticipated.
4) In GROUND’s experience, it is common during construction that in areas of
partially completed paving or hardscaping, bare soil behind curbs and gutters,
and utility trenches, water is allowed to pond after rain or snow-melt events.
Wetting of the subgrade can result in loss of subgrade support and increased
settlements / increase heave. By the time final grading has been completed,
significant volumes of water can already have entered the subgrade, leading to
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subsequent distress and failures. The contractor should maintain effective site
drainage throughout construction so that water is directed into appropriate
drainage structures.
In no case should water be permitted to pond adjacent to or on sidewalks,
hardscaping, or other improvements as well as utility trench alignments, which
are likely to be adversely affected by moisture-volume changes in the underlying
soils or flow of infiltrating water.
5) Roof downspouts and drains, if used, should discharge well beyond the
perimeter of the structure foundation, or be provided with positive conveyance
off-site for collected waters.
If roof downspouts and drains are not used, then surface drainage design should
anticipate concentrated volumes of water adjacent to the buildings.
6) Irrigation water – both that applied to landscaped areas and over-spray –
commonly is a significant cause of distress to improvements. Where (near-)
saturated soil conditions are sustained, distress to nearby improvements should
be anticipated.
To reduce to potential for such distress, vegetation requiring watering should be
located 10 or more feet from the building perimeter, flatwork, or other
improvements. Irrigation sprinkler heads should be deployed so that applied
water is not introduced near or into foundation/subgrade soils. Landscape
irrigation should be limited to the minimum quantities necessary to sustain
healthy plant growth.
Use of drip irrigation systems can be beneficial for reducing over-spray beyond
planters. Drip irrigation also can be beneficial for reducing the amounts of water
introduced to building foundation soils, but only if the total volumes of applied
water are controlled with regard to limiting that introduction. Controlling rates of
moisture increase beneath the foundations, floors and other improvements
should take higher priority than minimizing landscape plant losses.
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Where plantings are desired within 10 feet of the building, plants should be
placed in water-tight planters, constructed either in-ground or above-grade, to
reduce moisture infiltration in the surrounding subgrade soils. Planters should be
provided with positive drainage and landscape underdrains.
As an alternative involving only a limited increase in risk, the use of water-tight
planters may be replaced by local shallow underdrains beneath the planter beds.
7) Plastic membranes should not be used to cover the ground surface near the
building without careful consideration of other components of project drainage.
Plastic membranes can be beneficial to directing surface waters away from the
building and toward drainage structures. However, they effectively preclude
evaporation and transpiration of shallow soil moisture. Therefore, soil moisture
tends to increase beneath a continuous membrane. Where plastic membranes
are used, additional shallow, subsurface drains should be installed.
Perforated “weed barrier” membranes that allow ready evaporation from the
underlying soils may be used.
SUBSURFACE DRAINAGE
As a component of project civil design, properly functioning, subsurface drain systems
(underdrains) can be beneficial for collecting and discharging saturated subsurface
waters. Underdrains will not collect water infiltrating under unsaturated (vadose)
conditions, or moving via capillarity, however. In addition, if not properly constructed and
maintained, underdrains can transfer water into foundation soils, rather than remove it.
This will tend to induce heave or settlement of the subsurface soils, and may result in
distress. Underdrains can, however, provide an added level of protection against
relatively severe post-construction movements by draining saturated conditions near
individual structures should they arise, and limiting the volume of wetted soil.
Professional opinion varies regarding the potential benefits relative to the cost of an
underdrain system. Therefore, the owner and the design team and contractor should
assess the net benefit of an underdrain system as a component of overall project
drainage. (GROUND does not specifically recommend underdrains at this site with the
exception of below grade levels.)
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If, however, below-grade or partially below-grade level(s) are incorporated into project
design, then an underdrain system should be included to protect those portions of the
building. Damp-proofing should be applied to the exteriors of below-grade elements.
The provision of Tencate MiraFi® G-Series backing (or comparable wall drain provisions)
on the exteriors of (some) below-grade elements may be appropriate, depending on the
intended use. If a (partially) below-grade level is limited in extent, the underdrain
system, etc., may be local to that area.
Geotechnical Parameters for Underdrain Design: Where an underdrain system is
included in project drainage design, it should be designed in accordance with the
parameters below. The actual underdrain layout, outlets, and locations should be
developed by a civil engineer.
An underdrain system should be tested by the contractor after installation and after
placement and compaction of the overlying backfill to verify that the system functions
properly.
1) An underdrain system for a building should consist of perforated, rigid, PVC
collection pipe at least 4 inches in diameter, non-perforated, rigid, PVC discharge
pipe at least 4 inches in diameter, free-draining gravel, and filter fabric, as well as
a waterproof membrane.
2) The free-draining gravel should contain less than 5 percent passing the No. 200
Sieve and more than 50 percent retained on the No. 4 Sieve, and have a
maximum particle size of 2 inches. Each collection pipe should be surrounded
on the sides and top (only) with 6 or more inches of free-draining gravel.
3) The gravel surrounding the collection pipe(s) should be wrapped with filter fabric
(MiraFi 140N® or the equivalent) to reduce the migration of fines into the drain
system.
4) The waterproof membrane should underlie the gravel and pipe, and be attached
to the foundation grade beam or stem wall.
5) The underdrain system should be designed to discharge at least 5 gallons per
minute of collected water.
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6) The high point(s) for the collection pipe flow lines should be below the lowest
foundation bearing elevation. Multiple high points can be beneficial to reducing
the depths to which the system would be installed.
The collection and discharge pipe for the underdrain system should be laid on a
slope sufficient for effective drainage, but a minimum of 1 percent. (Flatter
gradients may be used but will convey water less efficiently and entail an
increased risk of local post-construction movements.)
Pipe gradients also should be designed to accommodate at least 1 inch of
differential movement after installation along a 50-foot run.
7) Underdrain ‘clean-outs’ should be provided at intervals of no more than 100 feet
to facilitate maintenance of the underdrains. Clean-outs also should be provided
at collection and discharge pipe elbows of 60 degrees or more.
8) The underdrain discharge pipes should be connected to one or more sumps from
which water can be removed by pumping, or to outlet(s) for gravity discharge.
We suggest that collected waters be discharged directly into the storm sewer
system, if possible.
PAVEMENT SECTIONS
A pavement section is a layered system designed to distribute concentrated traffic loads
to the subgrade. Performance of the pavement structure is directly related to the
physical properties of the subgrade soils and traffic loadings. The standard care of
practice in pavement design describes the flexible pavement section as a “20-year”
design pavement: however, most flexible pavements will not remain in satisfactory
condition without routine maintenance and rehabilitation procedures performed
throughout the life of the pavement. Pavement designs for the private pavements were
developed in general accordance with the design guidelines and procedures of the
American Association of State Highway and Transportation Officials (AASHTO).
Subgrade Materials Based on the results of our field exploration and laboratory testing,
the majority of potential pavement subgrade materials classify as sandy clay soils.
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Based on our experience at similar sites, an R-Value of 5 was estimated for the likely on-
site pavement subgrade materials. An R-Value of 5 converts to a resilient modulus of
3,025 psi based on CDOT correlation tables. It is important to note that significant
decreases in soil support have been observed as the moisture content increases above
the optimum. Pavements that are not properly drained may experience a loss of the soil
support and subsequent reduction in pavement life.
Anticipated Traffic Based on our experience with similar projects an equivalent 18-kip
daily load application (EDLA) value of 5 was assumed for the general parking areas, an
EDLA value of 10 was assumed for light vehicle drive lanes. The EDLA values of 5 and
10 were converted to equivalent 18-kip single axle load (ESAL) values of 36,500 and
73,000 respectively for a 20-year design life. If anticipated traffic loadings differ
significantly from these assumed values, GROUND should be notified to re-evaluate the
pavement recommendations below.
Pavement Sections The soil resilient modulus and the ESAL values were used to
determine the required design structural number for the project pavements. The
required structural number was then used to develop the pavement sections. Pavement
designs were based on the DARWin™ computer program that solves the 1993 AASHTO
pavement design equations. A Reliability Level of 80 percent was utilized to develop the
pavement sections, together with a Serviceability index loss of 2.5. An overall standard
of deviation of 0.44 also was used. Structural coefficients of 0.44 and 0.11 were used
for hot bituminous asphalt and aggregate base course, respectively. The resultant
minimum pavement sections that should be used at the facility are tabulated below.
Minimum Pavement Sections
Location
Full Depth
Asphalt
Composite Section
(inches Asphalt)
(inches Asphalt / inches
ABC)
Parking Areas 6.0 4.0 / 8
Light Vehicle Drive
Lanes
6.5 4.5 / 8
ABC = Aggregate base course, PCCP = Portland cement concrete pavement
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Heavy traffic areas/routes serving the facility that impose high stress on the pavement
such as trash collection areas or where trucks or tractor-trailers start, stop, or turn
sharply such as loading dock areas should be provided with rigid pavements consisting
of 6.5 or more inches of portland cement concrete underlain by 6 or more inches
of properly compacted CDOT Class 5 or 6 Aggregate Base Course. (An equivalent
composite flexible section for these areas would not perform as well as the concrete
section where heavy vehicles are parked, stop suddenly, turn repeatedly, etc.)
Pavement Materials Asphalt pavement should consist of a bituminous plant mix
composed of a mixture of aggregate and bituminous material. Asphalt mixture(s) should
meet the requirements of a job-mix formula established by a qualified engineer and
applicable local municipality design requirements.
Aggregate base material should meet the criteria of CDOT Class 5 or 6 Aggregate Base
Course. Base course should be placed in and compacted in accordance with the
standards in the Project Earthwork section of this report.
Concrete pavements should consist of a plant mix composed of a mixture of aggregate,
Portland cement and appropriate admixtures meeting the requirements of a job-mix
formula established by a qualified engineer and applicable local municipality design
requirements. Concrete should have a minimum modulus of rupture of third point
loading of 650 psi. Normally, concrete with a 28-day compressive strength of 4,500 psi
should develop this modulus of rupture value. The concrete should be air-entrained with
approximately 6 percent air and should have a minimum cement content of 6 sacks per
cubic yard. Maximum allowable slump should be 4 inches for hand-placed concrete.
Machine-placed concrete may require a lower slump.
These concrete mix design criteria should be coordinated with other project
requirements including any criteria for sulfate resistance presented in the Water-Soluble
Sulfates section of this report. To reduce surficial spalling resulting from freeze-thaw
cycling, we suggest that pavement concrete meet the requirements of CDOT Class P
concrete. In addition, the use of de-icing salts on concrete pavements during the first
winter after construction will increase the likelihood of the development of scaling.
Placement of flatwork concrete during cold weather so that it is exposed to freeze-thaw
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cycling before it is fully cured also increases its vulnerability to scaling. Concrete placing
during cold weather conditions should be blanketed or tented to aid in curing.
Concrete pavements should contain sawed or formed joints. CDOT and various industry
groups provide guidelines for proper design and concrete construction and associated
jointing. In areas of repeated turning stresses the concrete pavement joints should be
fully tied and doweled. Example layouts for joints, as well as ties and dowels, that may
be applicable can be found in CDOT’s M standards, found at the CDOT website:
http://www.dot.state.co.us/DesignSupport/. PCA, ACI and ACPA publications also
provide useful guidance in these regards.
Subgrade Preparation Shortly before paving, the pavement subgrade should be
excavated and/or scarified to a minimum depth of 12 inches, moisture-conditioned and
properly re-compacted.
Subgrade preparation should extend the full width of the pavement from back-of-curb to
back-of-curb. The subgrade for sidewalks and other project hardscaping also should be
prepared in the same manner.
Criteria and standards for fill placement and compaction are provided in the Project
Earthwork section of this report. The contractor should be prepared either to dry the
subgrade materials or moisten them, as needed, prior to compaction. Localized
stabilization efforts such as chemical stabilization or removal and replacement with
aggregate base may be used in areas that do not stabilize with conventional moisture-
density treatment.
Where adequate drainage cannot be achieved or maintained, excavation and
replacement should be undertaken to a greater depth, in addition to the edge drains
discussed below.
Proof Rolling Immediately prior to paving, the subgrade should be proof rolled with a
heavily loaded, pneumatic tired vehicle. Areas that show excessive deflection during
proof rolling should be excavated and replaced and/or stabilized. Areas allowed to pond
prior to paving will require significant re-working prior to proof-rolling. Establishment of a
firm paving platform (as indicated by proof rolling) is an additional requirement beyond
proper fill placement and compaction. It is possible for soils to be compacted within the
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limits recommended in the Project Earthwork section of this report and fail proof rolling,
particularly in the upper range of indicated moisture contents.
Additional Considerations The collection and diversion of surface drainage away from
paved areas is extremely important to satisfactory performance of the pavements. The
subsurface and surface drainage systems should be carefully designed to ensure
removal of the water from paved areas and subgrade soils. Allowing surface waters to
pond on pavements will cause premature pavement deterioration. Where topography,
site constraints or other factors limit or preclude adequate surface drainage, pavements
should be provided with edge drains to reduce loss of subgrade support. The long-term
performance of the pavement also can be improved greatly by proper backfilling and
compaction behind curbs, gutters, and sidewalks so that ponding is not permitted and
water infiltration is reduced.
Landscape irrigation in planters adjacent to pavements and in “island” planters within
paved areas should be carefully controlled or differential heave and/or rutting of the
nearby pavements will result. Drip irrigation systems are recommended for such
planters to reduce over-spray and water infiltration beyond the planters. Enclosing the
soil in the planters with plastic liners and providing them with positive drainage also will
reduce differential moisture increases in the surrounding subgrade soils.
In our experience, infiltration from planters adjacent to pavements is a principal source of
moisture increase beneath those pavements. This wetting of the subgrade soils from
infiltrating irrigation commonly leads to loss of subgrade support for the pavement with
resultant accelerating distress, loss of pavement life and increased maintenance costs.
This is particularly the case in the later stages of project construction after landscaping
has been emplaced but heavy construction traffic has not ended. Heavy vehicle traffic
over wetted subgrade commonly results in rutting and pushing of flexible pavements,
and cracking of rigid pavements. Where the subgrade soils are expansive, wetting also
typically results in increased pavement heave. In relatively flat areas where design
drainage gradients necessarily are small, subgrade settlement or heave can obstruct
proper drainage and yield increased infiltration, exaggerated distress, etc. (These
considerations apply to project flatwork, as well.)
Front Range Community College
Health Care Career Center
Fort Collins, Colorado
Job No. 18-0040 Ground Engineering Consultants, Inc. Page 47 of 51
Also, GROUND’s experience indicates that longitudinal cracking is common in asphalt-
pavements generally parallel to the interface between the asphalt and concrete
structures such as curbs, gutters or drain pans. This of this type is likely to occur even
where the subgrade has been prepared properly and the asphalt has been compacted
properly.
The anticipated traffic loading does not include excess loading conditions imposed by
heavy construction vehicles. Consequently, heavily loaded concrete, lumber, and
building material trucks can have a detrimental effect on the pavement. GROUND
recommends that an effective program of regular maintenance be developed and
implemented to seal cracks, repair distressed areas, and perform thin overlays
throughout the life of the pavements.
Most pavements will not remain in satisfactory condition and achieve their “design lives”
without regular maintenance and rehabilitation procedures performed throughout the life
of the pavement. Maintenance and rehabilitation measures preserve, rather than
improve, the structural capacity of the pavement structure. Therefore, GROUND
recommends that an effective program of regular maintenance be developed and
implemented to seal cracks, repair distressed areas, and perform thin overlays
throughout the lives of the pavements. The greatest benefit of pavement overlaying will
be achieved by overlaying sound pavements that exhibit little or no distress.
Crack sealing should be performed at least annually and a fog seal/chip seal program
should be performed on the pavements every 3 to 4 years. After approximately 8 to 10
years after construction, patching, additional crack sealing, and asphalt overlay may be
required. Prior to overlays, it is important that all cracks be sealed with a flexible,
rubberized crack sealant in order to reduce the potential for propagation of the crack
through the overlay. If actual traffic loadings exceed the values used for development of
the pavement sections, however, pavement maintenance measures will be needed on
an accelerated schedule.
CLOSURE AND LIMITATIONS
Geotechnical Review The author of this report or a GROUND principal should be
retained to review project plans and specifications to evaluate whether they comply with
Front Range Community College
Health Care Career Center
Fort Collins, Colorado
Job No. 18-0040 Ground Engineering Consultants, Inc. Page 48 of 51
the intent of the measures discussed in this report. The review should be requested in
writing.
The geotechnical conclusions and parameters presented in this report are contingent
upon observation and testing of project earthwork by representatives of GROUND. If
another geotechnical consultant is selected to provide materials testing, then that
consultant must assume all responsibility for the geotechnical aspects of the project by
concurring in writing with the parameters in this report, or by providing alternative
parameters.
Materials Testing Front Range Community College should consider retaining a
geotechnical engineer to perform materials testing during construction. The
performance of such testing or lack thereof, however, in no way alleviates the burden of
the contractor or subcontractor from constructing in a manner that conforms to
applicable project documents and industry standards. The contractor or pertinent
subcontractor is ultimately responsible for managing the quality of his work; furthermore,
testing by the geotechnical engineer does not preclude the contractor from obtaining or
providing whatever services that he deems necessary to complete the project in
accordance with applicable documents.
Limitations This report has been prepared for Front Range Community College as it
pertains to design of the proposed Health Care Career Center as described herein. It
should not be assumed to contain sufficient information for other parties or other
purposes. The Client has agreed to the terms, conditions, and liability limitations
outlined in our agreement between Front Range Community College and GROUND.
Reliance upon our report is not granted to any other potential owner, contractor, or
lender. Requests for third-party reliance should be directed to GROUND in writing;
granting reliance by GROUND is not guaranteed.
In addition, GROUND has assumed that project construction will commence by Spring /
Summer 2019. Any changes in project plans or schedule should be brought to the
attention of a geotechnical engineer, in order that the geotechnical conclusions in this
report may be re-evaluated and, as necessary, modified.
The geotechnical conclusions in this report were based on subsurface information from a
limited number of exploration points, as shown in Figure 1, as well as the means and
Front Range Community College
Health Care Career Center
Fort Collins, Colorado
Job No. 18-0040 Ground Engineering Consultants, Inc. Page 49 of 51
methods described herein. Subsurface conditions were interpolated between and
extrapolated beyond these locations. It is not possible to guarantee the subsurface
conditions are as indicated in this report. Actual conditions exposed during construction
may differ from those encountered during site exploration. In addition, a contractor who
obtains information from this report for development of his scope of work or cost
estimates does so solely at his own risk and may find the geotechnical information in this
report to be inadequate for his purposes or find the geotechnical conditions described
herein to be at variance with his experience in the greater project area. The contractor
should obtain the additional geotechnical information that is necessary to develop his
workscope and cost estimates with sufficient precision. This includes, but is not limited
to, information regarding excavation conditions, earth material usage, current depths to
groundwater, etc. Because of the necessarily limited nature of the subsurface
exploration performed for this study, the contractor should be allowed to evaluate the
site using test pits or other means to obtain additional subsurface information to prepare
his bid.
If during construction, surface, soil, bedrock, or groundwater conditions appear to be at
variance with those described herein, a geotechnical engineer should be retained at
once, so that our conclusions for this site may be re-evaluated in a timely manner and
dependent aspects of project design can be modified, as necessary.
The materials present on-site are stable at their natural moisture content, but may
change volume or lose bearing capacity or stability with changes in moisture content.
Performance of the proposed structure and pavement will depend on implementation of
the conclusions and information in this report and on proper maintenance after
construction is completed. Because water is a significant cause of volume change in
soils and rock, allowing moisture infiltration may result in movements, some of which will
exceed estimates provided herein and should therefore be expected by Front Range
Community College.
ALL DEVELOPMENT CONTAINS INHERENT RISKS. It is important that ALL aspects
of this report, as well as the estimated performance (and limitations with any such
estimations) of proposed improvements are understood by Front Range Community
College. Utilizing the geotechnical parameters and measures herein for planning,
design, and/or construction constitutes understanding and acceptance of the
Front Range Community College
Health Care Career Center
Fort Collins, Colorado
Job No. 18-0040 Ground Engineering Consultants, Inc. Page 50 of 51
conclusions with regard to risk and other information provided herein, associated
improvement performance, as well as the limitations inherent within such estimates.
Ensuring correct interpretation of the contents of this report by others is not the
responsibility of GROUND. If any information referred to herein is not well understood, it
is imperative that Front Range Community College contact the author or a GROUND
principal immediately. We will be available to meet to discuss the risks and remedial
approaches presented in this report, as well as other potential approaches, upon
request.
This report was prepared in accordance with generally accepted soil and foundation
engineering practice in the project area at the date of preparation. Current applicable
codes may contain criteria regarding performance of structures and/or site
improvements which may differ from those provided herein. Our office should be
contacted regarding any apparent disparity.
GROUND makes no warranties, either expressed or implied, as to the professional data,
opinions or conclusions contained herein. Because of numerous considerations that are
beyond GROUND’s control, the economic or technical performance of the project cannot
be guaranteed in any respect.
This document, together with the concepts and conclusions presented herein, as an
instrument of service, is intended only for the specific purpose and client for which it was
prepared. Re-use of, or improper reliance on this document without written authorization
and adaption by GROUND Engineering Consultants, Inc., shall be without liability to
GROUND Engineering Consultants, Inc.
Front Range Community College
Health Care Career Center
Fort Collins, Colorado
Job No. 18-0040 Ground Engineering Consultants, Inc. Page 51 of 51
GROUND appreciates the opportunity to complete this portion of the project and
welcomes the opportunity to provide Front Range Community College with a proposal
for construction observation and materials testing.
Sincerely,
GROUND Engineering Consultants, Inc.
Kelsey Van Bemmel, P.E. Reviewed by Joseph Zorack, P.E.
Cient: Front Range Community College
Project No.: 18-0040
(psi) (ksf)
1 9 14.6 116.2 - - 47.7 26 9 - - - - SC A-4 (1) Clayey SAND
2 8 18 106.4 - - 61.6 48 21 -0.9 1000 - - s(CL) A-7-6 (12) Sandy CLAY
3 30 12.8 121 - - 58.1 38 14 - - 98.3 14.16 s(CL) A-6 (6) Sandy CLAYSTONE
4 8 6.7 103.3 2 87 11 NV NP - - - - SP-SM A-2-4 (0) SAND with silt; trace gravel
5 7 11.5 121.8 7 55 38 31 13 - - - - (SC) A-6(1) Clayey SAND
7 5 13.3 118 - - 41.7 35 15 - - - - SC A-6 (3) Clayey SAND
8 9 23.6 100.6 - - 79.7 40 18 - - - - (CL)s A-6 (14) CLAY with Sand
8 14 11.8 123.8 - - 51.6 33 13 - - 83.5 12.02 s(CL) A-6 (4) Sandy CLAYSTONE
10 4 20 101.3 - - 52.6 42 18 - - - - s(CL) A-7-6 (7) Sandy CLAY
10 9 28.2 94 - - 86 65 31 2.0 1000 - - MH A-7-5 (31) SILTSTONE
*Negative indicates collapse, SD = Sample disturbed, NV = No value, NP = Non-plastic
Front Range Community College
Health Care Career Center
TABLE 1: SUMMARY OF LABORATORY TEST RESULTS
Gradation
Gravel
(%)
USCS
Equivalent
Classification
Sample Location
Sand
(%)
Surcharge
(psf)
Volume
Change
(%)*
Plasticity
Index
Liquid
Limit
Fines
(%)
Depth
(feet)
Test
Hole
No.
Natural
Dry
Density
(pcf)
Natural
Moisture
Content
(%)
Sample Description
AASHTO
Equivalent
Classification
(Group Index)
Unconfined
Compressive
Strength
Atterberg Limits Swell/Collapse
Cient: Front Range Community College
Project No.: 18-0040
2 8 0.02 8.5 -116 Positive 2287 s(CL) A-7-6 (12)
7 5 < 0.01 8.4 -111 Positive 5044 SC A-6 (3)
*Performed by eAnalytics Laboratory.
Front Range Community College
Health Care Career Center
TABLE 2: SUMMARY OF SOIL CORROSION TEST RESULTS
AASHTO
Equivalent
Classification
(Group Index)
Water
Soluble
Sulfates
(%)
Sulfide
Reactivity*
pH
Redox
Potential
(mV)
Resistivity
(ohm-cm)
USCS
Equivalent
Classification
Sample Description
Sandy CLAY
Clayey SAND
Test
Hole
No.
Depth
(feet)
Sample Location
Appendix A
Pavement Thickness Calculations
Page 1
1993 AASHTO Pavement Design
DARWin Pavement Design and Analysis System
A Proprietary AASHTOWare
Computer Software Product
Network Administrator
Flexible Structural Design Module
Front Range Commuinty College
Health Care Career Center
Fort Collins,CO
Vehicle only Parking areas
Full Depth Asphalt
Flexible Structural Design
18-kip ESALs Over Initial Performance Period 36,500
Initial Serviceability 4.5
Terminal Serviceability 2
Reliability Level 80 %
Overall Standard Deviation 0.44
Roadbed Soil Resilient Modulus 3,025 psi
Stage Construction 1
Calculated Design Structural Number 2.55 in
Specified Layer Design
Layer
Material Description
Struct
Coef.
(Ai)
Drain
Coef.
(Mi)
Thickness
(Di)(in)
Width
(ft)
Calculated
SN (in)
1 Hot Mix Asphalt 0.44 1 6 - 2.64
Total - - - 6.00 - 2.64
Page 1
1993 AASHTO Pavement Design
DARWin Pavement Design and Analysis System
A Proprietary AASHTOWare
Computer Software Product
Network Administrator
Flexible Structural Design Module
Front Range Commuinty College
Health Care Career Center
Fort Collins,CO
Vehicle only Parking areas
Composite Pavement Section
Flexible Structural Design
18-kip ESALs Over Initial Performance Period 36,500
Initial Serviceability 4.5
Terminal Serviceability 2
Reliability Level 80 %
Overall Standard Deviation 0.44
Roadbed Soil Resilient Modulus 3,025 psi
Stage Construction 1
Calculated Design Structural Number 2.55 in
Specified Layer Design
Layer
Material Description
Struct
Coef.
(Ai)
Drain
Coef.
(Mi)
Thickness
(Di)(in)
Width
(ft)
Calculated
SN (in)
1 Hot Mix Asphalt 0.44 1 4 - 1.76
2 Aggregate Base Course 0.11 1 8 - 0.88
Total - - - 12.00 - 2.64
Page 1
1993 AASHTO Pavement Design
DARWin Pavement Design and Analysis System
A Proprietary AASHTOWare
Computer Software Product
Network Administrator
Flexible Structural Design Module
Front Range Commuinty College
Health Care Career Center
Fort Collins,CO
Light Vehicle Drive Lanes
Full Depht Asphalt Pavement
Flexible Structural Design
18-kip ESALs Over Initial Performance Period 73,000
Initial Serviceability 4.5
Terminal Serviceability 2
Reliability Level 80 %
Overall Standard Deviation 0.44
Roadbed Soil Resilient Modulus 3,025 psi
Stage Construction 1
Calculated Design Structural Number 2.82 in
Specified Layer Design
Layer
Material Description
Struct
Coef.
(Ai)
Drain
Coef.
(Mi)
Thickness
(Di)(in)
Width
(ft)
Calculated
SN (in)
1 Hot Mix Asphalt 0.44 1 6.5 - 2.86
Total - - - 6.50 - 2.86
Page 1
1993 AASHTO Pavement Design
DARWin Pavement Design and Analysis System
A Proprietary AASHTOWare
Computer Software Product
Network Administrator
Flexible Structural Design Module
Front Range Commuinty College
Health Care Career Center
Fort Collins,CO
Light Vehicle Drive Lanes
Composite Pavement
Flexible Structural Design
18-kip ESALs Over Initial Performance Period 73,000
Initial Serviceability 4.5
Terminal Serviceability 2
Reliability Level 80 %
Overall Standard Deviation 0.44
Roadbed Soil Resilient Modulus 3,025 psi
Stage Construction 1
Calculated Design Structural Number 2.82 in
Specified Layer Design
Layer
Material Description
Struct
Coef.
(Ai)
Drain
Coef.
(Mi)
Thickness
(Di)(in)
Width
(ft)
Calculated
SN (in)
1 Hot Mix Asphalt 0.44 1 4.5 - 1.98
2 Aggregate Base Course 0.11 1 8 - 0.88
Total - - - 12.50 - 2.86