HomeMy WebLinkAboutFOOTHILLS MALL REDEVELOPMENT, PHASE 3 - FDP - FDP140007 - SUBMITTAL DOCUMENTS - ROUND 1 - RECOMMENDATION/REPORT41 Inverness Drive East, Englewood, CO 80112-5412 Phone (303) 289-1989 Fax (303) 289-1686 www.groundeng.com
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Subsurface Exploration Program
Geotechnical and Pavement Recommendations
Foothills Mall Redevelopment – Residential Structures
Fort Collins, Colorado
FINAL Submittal
Prepared for:
Walton Foothills Holdings, VI, LLC
5750 DTC Parkway, Suite 210
Greenwood Village, Colorado 80111
Attention: Mr. Adam Radcliffe
Job Number: 12-3649B November 26, 2012
EXECUTIVE SUMMARY
The content in the report provides geotechnical and pavement design recommendations for the
Foothills Mall Redevelopment. Below is a summary of the information contained in the report
for Phase 4.
The subsurface conditions encountered in the test holes generally consisted of a thin veneer of
asphalt, approximately 2 to 6 inches thick, underlain by sand and/or clay and gravel. These
materials were underlain by sandstone and claystone bedrock at depths ranging from
approximately 16 to 24 feet below existing grade. The test holes extended to depths of
approximately 20 to 40 feet below existing grades. Groundwater was encountered in some of
the test holes at depths ranging from approximately 12 to 23 feet below existing grades at the
time of drilling. GROUND constructed Test Holes 45 and 59 as observation/monitoring holes in
order to temporarily observe groundwater levels. Groundwater was encountered in Test Holes
45 and 59 at depths of approximately 14 and 28 feet, respectively, when measured 7 and 14
days later. The remainder of the test holes were backfilled immediately following drilling
operations.
Some of the proposed structures are anticipated to be located within and beyond the extents of
the existing structures. Provided that a uniform fill prism or at least 12 inches of properly
moisture-treated and compacted fill materials is placed beneath the shallow foundation and floor
system, the proposed structures could be founded on spread footings with slab-on-grade floors.
An allowable bearing pressure of 2,000 psf may be used for the spread footings assuming
properly drained soils and footing widths of 4 feet or less. In the event column loads are high
resulting in large footing sizes, a deep foundation system consisting of straight-shaft drilled piers
advanced into the underlying bedrock may be more cost-effective and practical for some or all
of these structures. Please refer to the report for a detailed explanation for this system.
GROUND recommends that a flexible pavement section consisting of a minimum of 6.5 inches
of full depth asphalt, a minimum of 5 inches of asphalt over a minimum of 6 inches of aggregate
base course equivalent composite section, or a rigid section consisting of at least 6 inches of
concrete be placed on areas intended for the driveways servicing the structures. Parking areas
may be paved with 6 or more inches of full depth asphalt, a minimum of 4.5 inches of asphalt
over a minimum of 6 inches of aggregate base course, or a rigid section consisting of at least 5
inches of concrete. GROUND recommends that trash collection zones as well as other
pavement areas subjected to high turning stresses or heavy truck traffic be provided with a rigid
pavement consisting of 6 or more inches of concrete. For enhanced performance, concrete
sections should be underlain by 6 inches of properly compacted aggregate base course.
Additional recommendations with respect to foundations, floor slabs, water-soluble sulfates,
corrosivity, exterior flatwork, project earthworks, excavation conditions, utility installation,
surface drainage, perimeter underdrains, and pavement sections are contained herein. This
executive summary should not be solely relied upon as a complete summary of the information
contained in this report; rather the entire contents of this report should be reviewed by the
Client/Owner/Project Team prior to design/construction.
TABLE OF CONTENTS
Page
Purpose and Scope of Study ...................................................................................... 1
Proposed Construction ................................................................................................ 1
Site Conditions ............................................................................................................ 2
Subsurface Exploration ............................................................................................... 3
Laboratory Testing ...................................................................................................... 3
Subsurface Conditions ................................................................................................ 4
Engineering Seismicity ................................................................................................ 6
Foundation/Floor System Overview .............................................................................. 7
Foundation System ..................................................................................................... 9
Floor System ............................................................................................................. 15
Mechanical Rooms/Mechanical Pads ........................................................................... 18
Exterior Flatwork ....................................................................................................... 18
Water Soluble Sulfates ................................................................................................ 21
Soil Corrosivity ............................................................................................................ 22
Lateral Earth Pressures ............................................................................................ 25
Project Earthwork ...................................................................................................... 27
Excavation Considerations ........................................................................................ 31
Utility Pipe Installation and Backfilling ......................................................................... 32
Surface Drainage ...................................................................................................... 35
Underdrain/Subsurface Moisture Infiltration ............................................................... 38
Pavement Conclusions/Recommendations ................................................................ 39
Closure and Limitations ............................................................................................. 45
Locations of Test Holes ..................................................................................... Figure 1
Logs of Test Holes ...................................................................................... Figures 2-3
Legend and Notes ............................................................................................. Figure 4
Compaction Test Results ..................................................................................... Figure 5
Summary of Laboratory Test Results ................................................................ Table 1
Summary of Soil Corrosion Test Results .............................................................. Table 2
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 1 of 48
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsurface exploration program performed by
GROUND Engineering Consultants, Inc. (GROUND) to provide geotechnical and
pavement recommendations for the proposed Foothills Mall Redevelopment - Phase 4
located near the intersection of South College Avenue and East Foothills Parkway in
Fort Collins, Colorado. Our study was conducted in general accordance with GROUND
Proposal No. 1207-1089 Revised, dated September 13, 2012.
Field and office studies provided information regarding surface and subsurface
conditions, including existing site vicinity improvements and groundwater. Material
samples retrieved during the subsurface exploration were tested in our laboratory to
assess the engineering characteristics of the site earth materials, and assist in the
development of our geotechnical recommendations. Results of the field, office, and
laboratory studies for the proposed facility are presented below.
This report has been prepared to summarize the data obtained and to present our
conclusions and recommendations based on the proposed construction and the
subsurface conditions encountered. Design parameters and a discussion of engineering
considerations related to construction of the proposed structures are included herein.
PROPOSED CONSTRUCTION
We understand that the proposed project is comprised of four phases. Phases 1
through 3 were addressed in a separate study, previously prepared and submitted to the
Client. The following presents a brief summary of the proposed construction associated
with Phase 4.
Phase 4 will consist of the demolition of two existing retail/commercial facilities and the
construction of five (5) residential buildings, ranging from three to five stories in height.
According to provided grading plans, finish floor elevations (FFEs) for the residential
buildings will range from approximately 5,000 feet to 5,012.5 feet. Therefore, it appears
that material cuts and fills up to approximately 4 feet will be necessary to facilitate
proposed construction.
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 2 of 48
Column loads were unavailable at the time of this report preparation. Additionally, we
understand that below level parking will be associated with Lot 3 (north side of Phase 3).
The approximate proposed building(s) layouts are shown in Figure 1. Development will
also include installation of underground utilities to service the proposed development.
SITE CONDITIONS
The Phase 4 project area consisted
of existing retail facilities along the
north side of the development and
asphalt-paved parking and drive
lanes along the east side of the
development. An existing
retail/commercial building was
located on the southeastern corner of
the project site. Landscaped
medians were also associated with
the project site. As-built drawings for
the existing structures were unavailable at the time of this report preparation. During our
exterior site reconnaissance, the existing retail structures appeared to be performing
satisfactorily. However, the pavement areas consisted of medium severity pavement
distress (see Pavement Conclusions/Recommendations section). Some of the
distressed areas have been previously crack sealed. The general topography across the
project site was relatively flat with slopes up to approximately 2 percent descending
toward the northeast.
Although not obviously encountered in the test holes, man-made fill may exist on-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 potentially
exist at varying depths and locations across the site.
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 3 of 48
SUBSURFACE EXPLORATION
The subsurface exploration for the project was conducted on September 24 through 26,
2012. A total of nineteen (19) test holes were drilled with a truck-mounted, continuous
flight power auger rig and limited access drill rig to evaluate the subsurface conditions as
well as to retrieve soil and bedrock samples for laboratory testing and analysis. The test
holes were advanced to depths ranging from approximately 20 to 40 feet below existing
grade. A representative of GROUND directed the subsurface exploration, logged the
test holes in the field, and prepared the soil and bedrock samples for transport to our
laboratory.
Monitoring/observation holes were installed in Test Holes 45 and 59 at the time of our
field exploration in order to temporarily observe groundwater levels. 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 and 3. Explanatory notes and a legend
are provided in Figure 4. GROUND utilized the Client-provided site plan indicating
existing features, etc., to approximately locate the test holes.
LABORATORY TESTING
Samples retrieved from our test holes were examined and visually classified in the
laboratory by the project engineer. Laboratory testing of soil and bedrock samples
obtained from the subject site included standard property tests, such as natural moisture
contents, dry unit weights, grain size analyses, swell-consolidation potential, unconfined
compressive strength, and liquid and plastic limits. Water-soluble sulfate and corrosivity
tests were completed on selected samples of the soils as well. A Proctor test was
completed on the representative composite bulk sample. Laboratory tests were
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 4 of 48
performed in general accordance with applicable ASTM and AASHTO protocols.
Results of the laboratory testing program are summarized on Tables 1 and 2.
SUBSURFACE CONDITIONS
Geologic Setting
The subject parcel consists largely of a sequence of sedimentary rock formations
deposited and preserved in a structural depression in north-central Colorado. In the
general project area, these sedimentary rocks dip eastward at low angles (less than 10
degrees, typically) and are overlain by a variety of surficial deposits including alluvial
(stream-laid) sediments, eolian (wind-blown) materials and colluvial (slope-wash)
deposits.
The bedrock deposits underlying the project area are mapped as Upper Cretaceous
Pierre Shale (upper unit and sandstone members) (Colton, 19781). In the project
vicinity, this formation consists predominately of shale, interbedded locally with siltstones
or sandstones. The sands/clays encountered above the Pierre Shale at the site are
interpreted to be alluvial deposits of the Pleistocene Slocum Alluvium.
Based on the published information reviewed for the site and our experience within
Denver, there are no mapped geologic hazards within or directly adjacent to the project
site.
Subsurface Conditions
The subsurface conditions encountered in the test holes generally consisted of a thin
veneer of asphalt, approximately 2 to 6 inches thick, underlain by sand and/or clay and
gravel. These materials were underlain by sandstone and claystone bedrock at depths
ranging from approximately 16 to 24 feet below existing grade. The test holes extended
to depths of approximately 20 to 40 feet below existing grades.
Groundwater was encountered in some of the test holes at depths ranging from
approximately 12 to 23 feet below existing grades at the time of drilling. GROUND
constructed Test Holes 45 and 59 as observation/monitoring holes in order to
1 Colton, Roger, 1978, Geologic Map of the Boulder, Fort Collins, and Greeley Area, Colorado, USGS Map I-
855G
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 5 of 48
temporarily observe groundwater levels. Groundwater was encountered in Test Holes
45 and 59 at depths of approximately 14 and 28 feet, respectively, when measured 7
and 14 days later. The remainder of the test holes were backfilled immediately following
drilling operations.
Subsurface Materials
Sand and Clay were interbedded, fine to meduim grained, low to highly plastic, medium
to very stiff/loose to medium dense, slightly moist to moist, light brown to reddish brown
in color, and occasionally calcareous.
Sand was silty to clayey, medium to coarse grained with occasional gravel, non-plastic
to low plastic, medium dense to dense, moist to wet, and reddish brown to light brown in
color.
Sand and Gravel were interbedded, coarse to gravel grained, non-plastic to low plastic,
medium dense to very dense, moist to wet, and reddish brown in color.
Sandstone and Claystone Bedrock (Comparably Unweathered Bedrock) were
interbedded, fine to medium grained, low to highly plastic, hard to very hard, dry to
moist, light brown in color, and occasionally iron-stained.
Sandstone Bedrock (Comparably Unweathered Bedrock) was silty to clayey,
medium to coarse grained, low plastic, hard to resistant, dry to moist, light brown in
color, and occasionally iron-stained. Please note that the sandstone may be cemented
and relatively resistant, which may complicate excavation such as deep foundations.
Swell-Consolidation Testing of samples of the on-site materials encountered in the
project test holes indicate a potential for heave/consolidation (See Table 1). A swell of
approximately 0.1 percent and consolidations of approximately 0.1 to 0.2 percent were
also measured at various surcharge loads.
The asphalt thicknesses are approximate and should be expected to vary throughout the
project site. Prospective contractors should not rely solely on this data for any purpose.
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 6 of 48
Groundwater levels can fluctuate, however, in response to annual and longer-term
cycles of precipitation, irrigation, surface drainage and land use, and the development
and drainage of transient, perched water conditions.
ENGINEERING SEISMICITY
According to the 2009 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
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 (2006/2009
IBC) or ASCE 7.” Exceptions to this are further noted in Section 1613.
Utilizing the USGS’s Earthquake Ground Motion Tool v.5.0.9a and site latitude/longitude
coordinates of 40.543527 and –105.074088 (obtained from Google Earth) respectively,
the project area is indicated to possess an SDS value of 0.238 and an SD1 value of
0.090.
Per 2009 IBC, Section 1613.5.2 Site class definitions, “Based on the site soil properties,
the site shall be classified as Site Class A, B, C, D, E or F in accordance with Table
1613.5.2. When the soil properties are not known in sufficient detail to determine the
site class, Site Class D shall be used unless the building official or geotechnical data
determines that Site Class E or F soil is likely to be present at the site”.
As permitted in Table 1613.5.2, in the event the soil shear wave velocity, vs, is not
known, site class shall be determined from standard penetration resistance, N, or from
soil undrained shear strength, su, calculated in accordance with Section 1613.5.5, for the
top 100 feet of subsurface soils.
Based on the soil conditions encountered in the test hole drilled on the site, our review of
applicable geologic maps, as well as our experience within the Project site vicinity,
GROUND estimates that a Site Class D according to the 2009 IBC classification (Table
1613.5.2) could be anticipated for seismic foundation design. This parameter was
estimated utilizing the above-referenced table as well as extrapolation of data beyond
the deepest depth explored. Actual shear wave velocity testing/analysis and/or
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 7 of 48
exploration to 100 feet was not performed. In the event the Client desires to potentially
utilize Site Class C for design, according to the 2006/2009 IBC, actual downhole seismic
shear wave velocity testing and/or exploration to subsurface depths of at least 100 feet,
should be performed. In the absence of additional subsurface exploration/analysis,
GROUND recommends a Site Class D be utilized for design.
The largest recorded earthquake (estimated magnitude 6.2 to 6.6) in Colorado occurred
in November 1882. While the specific location of this earthquake is very uncertain, it is
postulated to have occurred in the Front Range near Rocky Mountain National Park.
The most recent significant seismic movements associated with the historically active
Rocky Mountain Arsenal Fault (Commerce City, Colorado) occurred in the 1960s,
generating earthquakes up to magnitude 5.5. Since the early 1960s, numerous
earthquakes with magnitudes up to approximately 5, with the majority possessing
magnitudes of 2 to 4, have been experienced within the State. Earthquakes ranging in
magnitude from 3.7 (Craig, Colorado) to 3.9 (Eads, Colorado and Trinidad, Colorado)
occurred during the time period between July 2009 and August 2009. On August 23,
2011, a 5.3 magnitude earthquake occurred 9 miles west-southwest of Trinidad,
Colorado. Earthquakes with similar magnitudes, and potentially greater, are anticipated
to continue by the USGS, throughout the State. Therefore, the risk of damaging,
earthquake-induced ground motions at the site is considered to be relatively low given
the low, previously recorded, seismic magnitudes. Furthermore, based on the
subsurface conditions at the site and the risks associated with this nearest fault, the risk
of liquefaction of the site soils is considered low.
FOUNDATION AND FLOOR SYSTEM OVERVIEW
As stated, column loads for the buildings were unknown at the time of this report
preparation. In the event column loads are high resulting in large footing sizes, a deep
foundation system consisting of straight-shaft drilled piers advanced into the underlying
bedrock may be more cost-effective and practical for some or all of these structures.
Utilizing this option as well as other applicable recommendations provided in this report,
GROUND anticipates potential post-construction foundation movements of
approximately ½-inch.
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 8 of 48
Additionally, but not equal in anticipated building performance (post-construction
movements), the structures could be founded on a shallow foundation system consisting
of spread footings with a slab-on-grade floor system. Some of these structures are
anticipated to be located within and beyond the extents of the existing structures. As
previously stated, Lot 3 (structures on the north side of the Phase 4) will consist of a
below grade level. Therefore, it is GROUND’s opinion that following demolition and
proper backfill of the existing structures, the proposed residential structures may be
founded on a shallow foundation system consisting of spread footings with a slab-on-
grade floor system provided that a uniform layer of properly moisture-density treated
materials is placed beneath the entire building footprint(s) of the proposed structures to
reduce differential settlements between footings or along continuous footings as well as
across floor slabs. Following the construction of the fill prism, a shallow foundation
system consisting of spread footings with a slab-on-grade floor system may be
constructed for the proposed building(s). Utilizing this option as well as other applicable
recommendations provided in this report, GROUND anticipates potential movements on
the order of 1 inch.
In the event an existing structure is not located within a new building footprint or the
proposed building contains a below grade level, it is our opinion that the upper 12 inches
beneath footings and slabs be scarified, moisture-conditioned, and re-compacted in
accordance the Project Earthwork section of our report.
The fill prism should extend laterally at least 5 feet beyond the building beneath any
building appurtenances including entryways, patios, courtyards, etc. Fill materials may
consist of moisture-density treated on-site materials or approved import materials.
These materials should be placed in accordance with the recommendations provided in
the Project Earthwork section of our report.
Additionally, groundwater was encountered at depths of approximately 12 to 23 feet
below existing grades at the time of drilling and at depth of approximately 14 and 28 feet
in Test Holes 45 and 59, respectively, when measured 7 and 14 days later. Therefore,
an underdrain system/dewatering system may be necessary during and following
construction of structures with below grade levels. The Project Team should consider
possibly designing the below grade levels watertight, similar to a tank, or utilizing a drain
and pump/back-up pump with alarm system(s) capable of removing groundwater and
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 9 of 48
associated recharge rates. It is very difficult to accurately predict both potential
groundwater quantities as well as recharge rates. The structure should be designed to
account for minor seepage through the walls. The Project Civil Engineer or
Hydrologist/Hydrogeologist should be consulted to provide additional guidance regarding
this condition.
Existing foundation elements should be entirely removed and the resultant excavation
properly backfilled in accordance with the Project Earthwork section of this report.
Additionally, if portions of the existing foundations are below grade, i.e. mechanical
rooms, grease traps, etc., the excavation and backfill should consist of the entire
building footprint to the depth of the lowest foundation element.
Often times post-tensioned slab foundations are desired for similar structure types. We
can provide parameters for these systems upon request.
FOUNDATION SYSTEM
Spread Footings
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 recommendations 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. Based on the assumption of effective surface and subsurface drainage away
from the building as well as the recommendations presented herein, we anticipate the
following system would result in movement potentials on the order of 1 inch. Movement
estimates are difficult to predict and actual movements may be more or less.
1) Footings bearing on properly prepared materials may be designed for an
allowable soil bearing pressure (Q) of 2,000 psf. As stated, the buildings should
include construction of a uniform fill prism or be placed on a minimum of 12
inches of properly moisture-density treated site generated materials (see
previous). Based on this allowable bearing capacity, we anticipate post-
construction settlements to be on the order of 1 inch. Fills should be constructed
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 10 of 48
in accordance with the recommendations provided in the Project Earthwork
section of this report.
Increased bearing capacities can potentially be provided with additional
overexcavation and compaction efforts. Our office should be contacted in the
event these are desired.
2) The recommended allowable bearing pressure was based on an assumption of
drained conditions and footing widths of 4 feet or less. If foundation materials are
subjected to increase fluctuations in moisture content, the effective bearing
capacity will be reduced and greater post-construction movements than those
estimated above may result. We should be contacted if planned footing
widths exceed 4 feet.
3) Footing excavation bottoms may expose loose, organic 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.
5) 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 thicknesses beneath
footings will result in increased differential settlements.
6) Spread footings should have a minimum footing dimension of 14 or more inches.
Actual footing dimensions, however, should be determined by the Structural
Engineer, based on the design loads.
7) Footings should be provided with adequate soil cover above their bearing
elevation for frost protection. Footings should be placed at a bearing elevation 3
or more feet below the lowest adjacent exterior finish grades.
8) Continuous foundation walls should be reinforced top and bottom to span an
unsupported length of at least 10 feet.
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 11 of 48
9) The lateral resistance of spread footings will be developed as sliding resistance
of the footing bottoms on the foundation materials and by passive soil pressure
against the sides of the footings. Sliding friction at the bottom of footings may be
taken as 0.33 times the vertical dead load.
10) Compacted fill placed against the sides of the footings should be compacted to at
least 95 percent relative compaction in accordance with the recommendations in
the Project Earthwork section of this report.
11) 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.
12) Foundation soils may be disturbed or deform excessively under the wheel loads
of heavy construction vehicles as the excavations approach footing levels.
Construction equipment should be as light as possible to limit development of
this condition. The use of track-mounted vehicles is recommended since they
exert lower contact pressures. The movement of vehicles over proposed
foundation areas should be restricted.
13) All footing areas should be compacted with a vibratory plate compactor prior to
placement of concrete.
14) The Civil Design Engineer(s) and contractor should evaluate the possible
sources of water in the project area over the life of the structure, and provide a
design/construction agenda that ensures not to allow moisture to infiltrate the
foundation/structure supporting materials before, during, or after construction.
Drilled Piers
The design criteria presented below should be observed for a straight-shaft pier
foundation system. The construction details should be considered when preparing
project documents.
1) Piers may be designed for an allowable end bearing pressure of 30,000 psf and
skin friction values of 3,000 psf for the portion of the pier penetrating comparably
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 12 of 48
unweathered bedrock. This skin friction value assumes the installation of shear
rings. The upper 1 foot of bedrock penetration should be ignored in all load
calculations.
2) Piers also should be designed for a minimum dead load pressure of 5,000 psf
based on pier end area only. If the minimum dead load requirement cannot be
achieved and the piers are spaced as far apart as is practical, the pier length
should be extended beyond the minimum length to make up the dead load
deficit. This can be accomplished by assuming the skin friction located in the
extended zone acts in the direction to resist uplift. This value may be increased
by 1/3 for transient loads such as wind or seismic loading.
3) Piers should penetrate at least 10 feet into comparably unweathered bedrock
and have a minimum length of 30 feet. Based on the depth to bedrock
encountered in the test holes, piers approximately 30 to 34 feet in length should
meet these minimum criteria. Both criteria for minimum pier length and minimum
bedrock penetration should be met. However, the actual pier lengths should be
based on the specific design loads, the requirement for minimum dead load
pressure, etc., as determined by the Structural Engineer, as well as the actual
conditions encountered in the field at each pier location during installation.
4) A minimum pier diameter of 18 inches is recommended to facilitate proper
cleaning and observation of the pier hole. Larger pier diameters than the
minimum may be needed to accommodate the anticipated significant loads, as
well to comply with diameter to length ratios recommend by the structural
engineer.
5) Piers may be designed to resist lateral loads assuming a soil horizontal modulus
of 90 tcf in overburden sands, gravels, and clays and 400 tcf in competent
sandstone and claystone bedrock.
6) Bedrock penetration in pier holes should be roughened artificially to assist the
development of peripheral shear between the pier and bedrock. Artificially
roughening of pier holes should consist of installing shear rings 3 inches high and
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 13 of 48
2 inches deep in the lower 10 feet of each hole. The shear rings should be
installed 18 inches on centers.
The specifications should allow the Geotechnical Engineer to waive the
requirement for shear rings depending on the conditions actually encountered in
individual pier holes, however.
7) Groups of piers required to support concentrated loads will require an
appropriate reduction of the estimated bearing capacity based on the effective
envelope area of the pier group.
Reduction of axial capacity can be avoided by spacing piers at least 3 diameters
center to center. Pier groups spaced less than 3 diameters center to center
should be studied on an individual basis to determine the appropriate axial
capacity reductions(s).
8) Piers should be reinforced for their full length to resist the ultimate tensile load
created by the on-site materials. Adequate reinforcement should be designed to
resist the deficit between the design dead load on the pier and the uplift
pressures acting on the pier perimeter in the upper 15 feet of material penetrated
by the pier. Tension may be estimated on the basis of an uplift pressure of 750
psf in the upper 15 feet of material penetrated by the pier, and on the surface
area of the pier.
9) Based on the results of our field exploration, laboratory testing, and our
experience with similar, properly constructed, drilled pier foundations, we
estimate pier settlement will be low, on the order of ½-inch to mobilize skin
friction. The settlement of closely spaced piers will be larger and should be
studied under an individual basis.
10) A minimum void form of 6 inches should be provided beneath grade beams to
reduce the potential of the swelling soil and bedrock from exerting uplift forces on
the grade beams, as well as to concentrate pier loadings. The same void should
also be provided beneath necessary pier caps.
Foothills Mall Redevelopment-Phase 4
Residential Structures
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Final Submittal
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11) Groundwater was encountered at depths of approximately 12 to 23 feet below
existing grades at the time of drilling and at depth of approximately 14 and 28
feet in Test Holes 45 and 59, respectively, when measured 7 and 14 days later.
Therefore, the use of casing may be required for pier installation. The
requirements for casing can sometimes be reduced by placing concrete
immediately upon cleaning and observing the pier hole. In no case should
concrete be placed in more than 3 inches of water, unless placed through an
approved “tremie” method.
12) Pier holes should be properly cleaned prior to placement of concrete.
13) Concrete utilized in the piers should be a fluid mix with sufficient slump so that it
will fill the void between reinforcing steel and the pier hole wall. We recommend
the concrete have a minimum slump in the range of 5 to 7 inches. Concrete
should be placed by an approved “tremie”-type method or other methods such as
the utilization of a long steel pipe or “elephant trunk” to reduce mix segregation.
The “tremie” should be extended down into the center of the drilled pier shaft in
order to provide a clear pathway through the reinforcement cage. A centering
chute that extends to shallow depths may not be sufficient.
14) Concrete should be placed in piers the same day they are drilled. Failure to
place concrete the day of drilling will normally result in a requirement for
additional bedrock penetration. The presence of groundwater or caving soils at
the time of pier installation may require that concrete be placed immediately after
the pier hole drilling is completed.
15) The Contractor should take care to prevent enlargement of the excavation at the
tops of piers, which could result in mushrooming of the pier top. Mushrooming of
pier tops can increase uplift pressures on the piers.
16) The bedrock beneath the site is hard to very hard and relatively resistant. These
conditions should be anticipated during construction. The pier drilling Contractor
should mobilize equipment of sufficient size and operating capability to achieve
the required penetration into the bedrock. GROUND recommends a high-torque,
commercial rig be used. If refusal is encountered in these materials, the
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Job No. 12-3649B Ground Engineering Consultants, Inc. Page 15 of 48
Geotechnical Engineer should be retained to evaluate the conditions to establish
that true refusal has been met with adequate drilling equipment.
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 recommendations 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.
1) Floor slabs should be placed on properly prepared materials. As stated,
construction of a uniform fill prism (determined by the greatest depth of
excavation necessary during building demolition) or scarified, moisture-
conditioned, and re-compacted to a depth of at least 12 inches below the slabs (if
existing foundation elements are not present within proposed footprint) will be
required. These materials should be placed in accordance with the
recommendations in the Project Earthwork section of our report.
2) The prepared surface on which the floor slabs will be cast should be observed by
the Geotechnical Engineer prior to placement of reinforcement. Exposed loose,
soft, or otherwise unsuitable materials should be excavated and replaced with
properly compacted fill, placed in accordance with the recommendations in the
Project Earthwork section of this report.
3) Floor slabs should be separated from all bearing walls and columns with slip
joints, which allow unrestrained vertical movement.
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.
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4) 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 door frames. A slip joint which
will allow sufficient vertical movement is recommended. If slip joints are placed
at the tops of walls, in the event that the floor slabs move, it is likely that the wall
will show signs of distress, especially where the floors meet the exterior wall.
5) Concrete slabs-on-grade should be placed on properly prepared subgrade. They
should also be constructed and cured according to applicable
standards and be provided with properly designed and constructed control joints.
The design and construction of such joints should account for cracking as a
result of shrinkage, tension, and loading; curling; as well as proposed slab use.
Joint layout based on the slab design may require more frequent, additional, or
deeper joints, and should also be based on the ultimate use and configuration of
the slabs. Areas where slabs consist of interior corners or curves (at column
blockouts or around corners) or where slabs have high length to width ratios,
high degree of slopes, thickness transitions, high traffic loads, or other unique
features should be carefully considered. The improper placement or construction
of control joints will increase the potential for slab cracking. ACI, AASHTO, and
other industry groups provide many guidelines for proper design and construction
of concrete slabs on grade and the associated jointing.
6) Floor slabs should be adequately reinforced. Recommendations based on
structural considerations for slab thickness, jointing, and steel reinforcement in
floor slabs should be developed by the Structural Engineer. Placement of slab
reinforcement continuously through the control joint alignments will tend to
increase the effective size of concrete panels and reduce the effectiveness of
control joints.
7) All plumbing lines should be carefully tested before operation. Where plumbing
lines enter through the floor, a positive bond break should be provided. Flexible
connections allowing sufficient vertical movement should be provided for slab-
bearing mechanical equipment.
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8) Moisture can be introduced into a slab subgrade during construction and
additional moisture will be released from the slab concrete as it cures. GROUND
recommends placement of a properly compacted layer of free-draining gravel, 4
or more inches in thickness, 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. 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.
Therefore, in light of the several, potentially conflicting effects of the use of vapor
barriers, the Owner and the Architect and/or Flooring 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 vapor barrier may be appropriate for
some buildings and not for others.
In the event a vapor barrier is utilized, it 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.
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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 necessary tensile strength, gets brittle, tends to
decompose over time, and has a relatively high permeance.
Slab movements are directly related to the increases in moisture contents to the
underlying soils after construction is completed. The precautions and recommendations
itemized above will not prevent the movement of floor slabs if the underlying materials
are subjected to excessive moisture. However, these steps will reduce the damage if
such movement occurs.
MECHANICAL ROOMS/MECHANICAL PADS
Often, slab-bearing mechanical rooms/mechanical equipment are incorporated into
projects. Our experience indicates these are located as partially below-grade or
adjacent to the exterior of a structure. GROUND recommends these elements be
founded on the same type of foundation systems as the main structure. Furthermore,
mechanical connections must allow for potential differential movements.
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, GROUND
recommends that 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
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Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 19 of 48
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.
Ideally, subgrade soils beneath project sidewalks, paved entryways and patios, masonry
planters and short, decorative walls, and other hardscaping should be placed on the
same amount of processed soil as those recommended for the floor slabs, or greater.
Provided the owner understands the risks identified above, we believe that subgrade
under exterior flatwork or other (non-building) site improvements could be scarified to a
minimum depth of 12 inches. This should occur prior to placing any additional fill
required to achieve finished design grades. This processing depth will not eliminate
potential movements. The excavated soil should be replaced as properly moisture-
conditioned and compacted fill as outlined in the Project Earthwork section of this report.
As stated above, greater depths of moisture-density conditioning of the subgrade soils
beyond the above minimum will improve hardscape performance. Movement will occur,
some of which could be significant, especially if sufficient surface drainage is not
maintained.
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.
In no case should exterior flatwork extend to under any portion of the building where
there is less than several inches of 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.
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
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case should water be allowed to pond on or near any of the site improvements or a
reduction in performance should be anticipated.
Water Features
Locations of water features planned with the project site should be provided to GROUND
in order to evaluate the proximity to structures and the necessity of underdrains and/or
liners.
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.
1) Maintaining a maximum water/cement ratio of 0.45 by weight for exterior
concrete mixes.
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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) Including ‘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.
WATER-SOLUBLE SULFATES
The concentrations of water-soluble sulfates measured in selected samples retrieved
from the test holes ranged from less than the detectable limit of 0.01 percent to 0.03
percent by weight (See Table 2). Such concentrations of water-soluble sulfates
represent 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.
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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, percent,
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 GROUND, makes no recommendation for use of a special, sulfate-
resistant cement 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 conditions for a variety of reasons but primarily soil chemistry.
A 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, oxidation-reduction (redox) potential, sulfides, and moisture
content were obtained. Test results are summarized on Table 2.
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.
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Measurements of electrical resistivity indicated values ranging from approximately 2,844
to 13,925 ohm-centimeters in samples of retrieved soil. The following table presents the
relationship between resistivity and a qualitative corrosivity rating2:
Corrosivity Ratings Based on Soil Resistivity
Soil Resistivity (ohm-cm) Corrosivity Rating
>20,000 Essentially non-corrosive
10,000 – 20,000 Mildly corrosive
5,000 – 10,000 Moderately corrosive
3,000 – 5,000 Corrosive
1,000 – 3,000 Highly corrosive
<1,000 Extremely corrosive
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 resistivity3.
Testing indicated pH values ranging from approximately 7.5 to 8.5.
The American Water Works Association (AWWA) 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 recommended. The AWWA scale is
presented below. The soil characteristics refer to the conditions at and above pipe
installation depth.
2 ASM International, 2003, Corrosion: Fundamentals, Testing and Protection, ASM Handbook, Volume 13A.
3 American Water Works Association ANSI/AWWA C105/A21.5-05 Standard
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Table A.1 Soil-test Evaluation 4
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 Content
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 good. Nevertheless,
based on the values obtained for the soil parameters, the overburden soils/bedrock
appear(s) to comprise a corrosive environment for metals.
If additional information or recommendations are needed regarding soil corrosivity,
GROUND recommends contacting the American Water Works Association or a
4 American Water Works Association ANSI/AWWA C105/A21.5-05 Standard
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Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 25 of 48
Corrosion Engineer. It should be noted, however, that changes to the site conditions
during construction, such as the import of other soils, or the intended or unintended
introduction of off-site water, may alter corrosion potentials significantly.
LATERAL EARTH PRESSURES
Structures which are laterally supported and can be expected to undergo only a limited
amount of deflection should be designed for “at-rest” lateral earth pressures. The
cantilevered retaining structures will be designed to deflect sufficiently to mobilize the full
active earth pressure condition, and may be designed for “active” lateral earth pressures.
“Passive” earth pressures may be applied in front of the structural embedment to resist
driving forces.
The at-rest, active, and passive earth pressures in terms of equivalent fluid unit weight
for the on-site backfill and CDOT Class 1 structure backfill are summarized on the table
below. Base friction may be combined with passive earth pressure if the foundation is in
a drained condition. The use of passive pressure under a saturated condition is not
recommended. The values for the on-site material in the upper 10 feet provided in the
table below were approximated utilizing a unit weight of 121 pcf and a phi angle of 26
degrees.
Lateral Earth Pressures (Equivalent Fluid Unit Weights)
Material Type Water
Condition
At-Rest
(pcf)
Active
(pcf)
Passive
(pcf)
Friction
Coefficient
On-Site Sand
and Clay Backfill
Drained 68 47 300 0.33
Submerged 95 85 - 0.33
Structure Backfill
(CDOT Class 1)
Drained 55 35 400 0.45
Submerged 90 80 -- 0.45
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If the selected on-site soil meets the criteria for CDOT Class 1 structure backfill as
indicated in the Project Earthwork section of this report, the lateral earth pressures for
CDOT Class 1 structure backfill as shown on the above table may be used. To realize
the lower equivalent fluid unit weight, the selected structure backfill should be placed
behind the wall to a minimum distance equal to the retained wall height.
The lateral earth pressures recommended 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.
Wall Drainage
Retaining walls should be provided with drains at the heels of the walls, or with weep
holes, or both, to help reduce the development of hydrostatic loads.
The underdrain system should consist of perforated PVC drainpipe at least 4 inches in
diameter, free-draining gravel, and filter fabric. 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 drainpipe should be
surrounded on the sides and top with 6 or more inches of free-draining gravel. The
gravel surrounding the drainpipe and/or the pipe itself should be wrapped with filter
fabric to reduce the migration of fines into the drain system. The Civil Engineer should
design the actual layout, outlets, and locations.
In addition to surrounding the drain pipes with at least 6 inches of free-draining gravel,
the gravel should extend upward to within 12 inches of the backfill surface behind the
wall or the wall should be backed with a layer of geocomposite drainage medium, e.g.,
an appropriate MiraDrain® product or equivalent. The gravel or drainage product
backing the wall should be in hydraulic connection with the wall heel drain. If gravel is
selected, it should be separated from the enclosing soils by a layer of filter fabric to
reduce the migration of fines into the drainage system. Damp proofing should be
applied to the backside of rigid types of retaining walls.
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PROJECT EARTHWORK
The following information is for private improvements; public roadways or utilities
should be constructed in accordance with applicable municipal / agency
standards.
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 structures, asphalt/concrete, 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. Remnant foundation elements should be
entirely removed and the resultant excavation properly backfilled. The Geotechnical
Engineer should be contracted to test the excavation backfill during placement.
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.
It is not possible to accurately correlate subgrade stability with information derived from
site observations made during the geotechnical exploration or subsequent laboratory
testing. It is often our experience that when pavements are removed, the pavement
subgrade experiences instability when subjected to building construction and/or traffic
loading, even when laboratory testing suggests reasonable moisture contents and
density. Therefore, it may be necessary to stabilize the majority of the existing subgrade
prior to repaving. This may require reprocessing of existing soils or removal and
replacement with other site materials or imported soil. Our office should be retained to
observe the subgrade condition and stability during the removal process. If additional or
more specific information is required, then we suggest removal of several large sections
of these existing pavement areas for evaluation prior to design or bidding.
Existing Fill Soils: Although not obviously encountered in the test holes, man-made fill
may exist on site. Actual contents and composition of the man-made fill materials are
not known; therefore, some of the excavated man-made fill materials may not be
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suitable for replacement as backfill. The Geotechnical Engineer should be retained
during site excavations to observe the excavated fill materials and provide
recommendations for its suitability for reuse.
Use of Existing Native Soils: Overburden soils that are free of trash, organic material,
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
consulted regarding appropriate recommendations for usage of such materials on a
case-by-case basis when such materials have been identified during earthwork.
Standard recommendations that likely will be generally applicable can be found in
Section 203 of the current CDOT Standard Specifications for Road and Bridge
Construction.
Sandstone fragments should not exceed 3 inches in largest dimension and
claystone/siltstone fragments should be reduced to a soil-like mass.
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 relatively impervious 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 by the Geotechnical Engineer prior to transport to the site.
Imported Structural Fill: Select granular materials imported for use as structural fill
should meet the criteria for CDOT Class 1 Structure Backfill as tabulated below.
Representative samples of proposed imported soils should be tested and approved by
GROUND prior to transport to the site.
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CDOT Class 1 Structure Backfill
Sieve Size or
Parameter
Acceptable Range
2-inch Sieve 100% passing
No. 4 Sieve 30% to 100% passing
No. 50 Sieve 10% to 60% passing
No. 200 Sieve 5% to 20% passing
Liquid Limit < 35 %
Plasticity Index < 6 %
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 recommendations below to provide a uniform base for
fill placement. If over-excavation is to be performed, then these recommendations 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.
Fill Placement: Fill materials should be thoroughly mixed to achieve a uniform moisture
content, placed in uniform lifts not exceeding 8 inches in loose thickness, and properly
compacted.
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, MH, CL or CH should be compacted to 100 percent of the
maximum standard Proctor density beneath Building Structures and compacted to 95
percent of the maximum standard Proctor density in all other areas at moisture contents
from 1 percent below to 3 percent above the optimum moisture content as determined
by ASTM D698. In addition, these fill soils must exhibit as-placed swells of 0.5 percent
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or less, against a 1,000 psf surcharge. Materials represented by samples exhibiting
more than 0.5 percent swell upon wetting against a 1,000-psf surcharge should be re-
worked at increased moisture contents and re-compacted in accordance with the
recommendations herein.
No fill materials should be placed, worked, rolled while they are frozen, thawing, or
during poor/inclement weather conditions.
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 significant pumping, rutting, and deflection at moisture contents near
optimum and above. The contractor should be prepared to handle soils of this type,
including the use of chemical stabilization, if necessary.
Compaction areas should be kept separate, and no lift should be covered by another
until relative compaction and moisture content within the recommended ranges are
obtained.
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.
GROUND recommends that wherever possible, excavations be backfilled with approved,
on-site soils placed as properly compacted fill. Where this is not feasible, use of
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“Controlled Low Strength Material” (CLSM), i.e., a lean, sand-cement slurry (“flowable
fill”) or a similar material for backfilling should be considered.
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.
Cut and Filled Slopes: Permanent site slopes supported by on-site soils up to 10 feet
in height may be constructed no steeper than 3:1 (horizontal : vertical). 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.
EXCAVATION CONSIDERATIONS
The test holes for the subsurface exploration were excavated to the depths indicated by
means of truck-mounted, flight auger drilling equipment. We anticipate no significant
excavation difficulties in the majority of the site with conventional heavy-duty excavation
equipment in good working condition.
We recommend that temporary, un-shored excavation slopes up to 10 feet in height be
cut no steeper than 1:1 (horizontal : vertical) in the site soils in the absence of seepage.
Sloughing on the slope faces should be anticipated at this angle. Local conditions
encountered during construction, such as groundwater seepage and loose sand, will
require flatter slopes. Stockpiling of materials should not be permitted closer to the tops
of temporary slopes than 5 feet or a distance equal to the depth of the excavation, which
ever is greater.
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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 building, traffic, equipment, and stockpiles. GROUND can provide
shoring design upon request.
Groundwater was encountered in the test holes at depths ranging from approximately 12
to 23 feet below existing grades at the time of drilling and in Test Holes 45 and 59 at
depths of approximately 14 and 28 feet, respectively, when measured 7 and 14 days
later. Therefore, groundwater may be encountered in some sections of a trench or
within the excavation of the structures. A properly designed and installed de-watering
system may be required during the construction in these sections of the trench or below
grade levels. 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 the Geotechnical Engineer.
Good surface drainage should be provided around temporary excavation slopes to direct
surface runoff away from the slope faces. A properly designed drainage swale should
be provided at the top of the excavations. In no case should water be allowed to pond at
the site. Slopes should also be protected against erosion. Erosion along the slopes will
result in sloughing and could lead to a slope failure.
Excavations in which personnel will be working must comply with all OSHA Standards
and Regulations. Project excavations and shoring should be observed regularly by the
Geotechnical Engineer throughout construction operations. 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.
UTILITY PIPE INSTALLATION AND BACKFILLING
Pipe Support: The bearing capacity of the site soils appeared adequate, in general, for
support of the proposed water line. The pipe + water 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.
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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 for an allowable passive soil
pressure of 250 psf per foot of embedment, to a maximum of 2,500 psf. Sliding friction
at the bottom of thrust blocks may be taken as 0.33 times the vertical dead load.
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 200 psi so that re-excavation is not
unusually difficult.
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.
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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 be free
of vegetation, organic debris, and other deleterious materials. Imported material should
consist of relatively impervious 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.
Soils placed for compaction as trench backfill should be conditioned to a relatively
uniform moisture content, placed and compacted in accordance with the
recommendations 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
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
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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 sewer alignment which
can contribute to migration of fines.
If granular bedding is specified, the contractor should not anticipate that significant
volumes of on-site soils will be suitable for that use. Materials proposed for use as pipe
bedding should be tested by a geotechnical engineer for suitability prior to use.
Imported materials should be tested and approved by a geotechnical engineer prior to
transport to the site.
SURFACE DRAINAGE
The following drainage measures are recommended for design, construction, and should
be maintained at all times after the project has been completed:
1) Wetting or drying of the foundation excavations and underslab areas should be
avoided during and after construction as well as throughout the improvements’
design life. Permitting increases/variations in moisture to the adjacent or
supporting soils may result in a decrease in bearing capacity and an increase in
volume change of the underlying soils and/or differential movement.
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. Ideally, we recommend a minimum slope of 12 inches in the first
10 feet in the areas not covered with pavement or concrete slabs, or a minimum
3 percent in the first 10 feet in the areas covered with pavement or concrete
slabs. However, we realize that these recommended slopes cannot always be
designed for this type of development. Therefore, lesser slopes can be used
provided that positive surface drainage is implemented and routinely maintained
throughout the life of the facility. In the event water is allowed to infiltrate the
foundation soils, an increase in potential movements of the structures will occur.
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For areas of reduced slopes, subsurface drainage systems should be
implemented in the design.
3) Reducing the slopes to comply with ADA requirements 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. However, if positive surface
drainage is implemented and maintained directing moisture away from the
building, lesser slopes can be utilized. In no case should water be allowed to
pond near or adjacent to foundation elements.
4) On some sites it is common to have slopes descending toward buildings. Such
slopes can be created during grading even on comparatively flat sites. In such
cases, even where the recommendation above regarding slopes adjacent to the
building is followed, water may flow to and beneath the building with resultant
additional post-construction movements. Where the final site configuration
includes graded or retained slopes descending toward the building or flatwork,
interceptor drains should be installed between the building and the slope. In
addition, where irrigation is applied on or above slopes, drainage structures
commonly are needed near the toe-of-slope to prevent on-going or recurrent wet
conditions.
5) 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.
6) Roof downspouts and drains should discharge well beyond the perimeters of the
structure foundations (minimum 10 feet), or be provided with positive conveyance
off-site for collected waters.
7) Based on our experience with similar facilities, the project site may consist of
landscaping/watering near the building. Provided that positive, effective surface
drainage is initially implemented and maintained throughout the life of the facility,
vegetation that requires little to no watering may be located within 10 feet of the
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building perimeter. Irrigation sprinkler heads should be deployed so that applied
water is not introduced near or into foundation/subgrade soils. The area
surrounding the perimeter of the building should be constructed so that the
surface drains away from the structure. Additionally, it is very important that
landscape maintenance is performed such that the amount of moisture is strictly
controlled so that the quantity of moisture applied is limited to that which is
necessary to sustain the vegetation; in no case should saturated or marshy
conditions be allowed to occur near any of the site improvements (including
throughout the landscaped islands in parking areas). Periodic inspections should
be made by facility representatives to make sure that the landscape irrigation is
functioning properly and that excess moisture is not applied.
8) Use of drip irrigation systems can be beneficial for reducing over-spray beyond
planters. Drip irrigation can also be beneficial for reducing the amounts of water
introduced to foundation/subgrade soils, but only if the total volumes of applied
water are controlled with regard to limiting that introduction. Controlling rates of
moisture increase in foundation/subgrade soils should take higher priority than
minimizing landscape plant losses.
9) Where plantings are desired within 10 feet of a building, GROUND recommends
that the plants 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.
Colorado Geological Survey – Special Publication 43 provides additional
guidelines for landscaping and reducing the amount of water that infiltrates into
the ground.
10) Detention ponds commonly are incorporated into drainage design. When a
detention ponds fills, the rate of release of the water is controlled and water is
retained in the pond for a period of time. Where in-ground storm sewers direct
surface water to the pond, the granular pipe bedding also can direct shallow
groundwater or infiltrating surface water toward the pond. Thus, detention ponds
can become locations of enhanced and concentrated infiltration into the
subsurface, leading to wetting of foundation soils in the vicinity with consequent
heave or settlement. Therefore, unless the pond is clearly down-gradient from
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the proposed buildings and other structures that would be adversely affected by
wetting of the subgrade soils, including off-site improvements, GROUND
recommends that the detention pond should be provided with an effective, low
permeability liner. In addition, cut-off walls and/or drainage provisions should be
provided for the bedding materials surrounding storm sewer lines flowing to the
pond.
11) Plastic membranes should not be used to cover the ground surface adjacent to
foundation walls. Perforated “weed barrier” membranes that allow ready
evaporation from the underlying soils may be used.
UNDERDRAIN/SUBSURFACE MOISTURE INFILTRATION
Installation of an underdrain system is common practice for projects of this type. All
below grade levels, partial below grade levels, crawl spaces, or other below grade void
spaces should be provided with an underdrain system. If properly constructed,
backfilled, and maintained, an effective underdrain system can collect free water that
may otherwise infiltrate foundation/subgrade soils. Underdrains will not collect water
infiltrating under unsaturated (vadose) conditions, or moving via capillarity.
Furthermore, an underdrain not properly functioning can allow more moisture to infiltrate
the foundation/subgrade soils and induce volume change of the soils, which may result
in distress. Wetting or drying of the foundation excavations and underslab areas should
be avoided during and after construction as well as throughout the life of the facility.
Permitting increases/variations in moisture to the supporting soils may result in a
decrease in bearing capacity and an increase in settlement, heave, and/or differential
movement.
Various elements of the project design, as well as site conditions before and after
construction impact the need for incorporating an underdrain system into the project
design. Design information regarding landscaping, flatwork, slopes, etc., was not
available at the time this report was prepared, so it is therefore difficult to evaluate the
need for an underdrain system. Upon request, our office is available to help evaluate
the incorporation of an underdrain system or systems.
Underdrain systems typically consist of rigid, perforated PVC drain pipe at least 4 inches
in diameter, free-draining gravel, a water-proof membrane, and filter fabric constructed
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at a minimum slope of 1 percent. Upon completion and receipt of the final grading
information and the selection of foundation type(s), GROUND can provide a detail of the
perimeter drain as it relates to the proposed foundation system and minimum and
maximum depth dimension from finish floor to the pipe invert. Additionally, GROUND
can review the underdrain layout plans as they comply with this geotechnical study.
As stated previously, the Project Team should consider possibly designing the below
grade levels watertight, similar to a tank, or utilizing a drain and pump/back-up pump
with alarm system(s) capable of removing groundwater and associated recharge rates.
It is very difficult to accurately predict both potential groundwater quantities as well as
recharge rates. The Project Civil Engineer or Hydrologist/Hydrogeologist should be
consulted to provide additional guidance regarding this condition as well as providing
groundwater flow direction and average/estimated rates.
PAVEMENT CONCLUSIONS/RECOMMENDATIONS
Pavement thicknesses, based on our exploration program, ranged from 2 inches to 6
inches. Pavement distresses throughout the pavement area consisted of medium
severity longitudinal cracking and alligator/fatigue cracking. Some areas, appear to
consist of recent localized preventive M & R (maintenance and rehabilitation) methods
including full-depth patching, and thin asphalt overlays. At the time of this report
preparation, it is unknown what areas will be reconstruction or rehabilitated.
Pavement Rehabilitation
If an overlay is desired, areas consisting of longitudinal/transverse cracking should be
sealed. In the event cracks greater than 2 inches in width are observed, removal and
replacement of the asphalt in these severe cracking areas should take place prior to the
overlay.
A mill and overlay program consisting of at least 2 inches of asphalt may be feasible for
pavements with at least 4.5 to 5 inches of existing asphalt if the resulting milled surface
is stable enough to avoid “breaking through” the stable milled asphalt surface with heavy
trucks and paving equipment. According to our core depths within the pavement areas,
a mill and overlay program appears feasibly in most areas. If the owner chooses to
conduct a minimum 2-inch mill and overlay program on site pavements, GROUND
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should be notified to evaluate the stability of a milled surface test section. Based on the
condition of the milled surface, total removal may be required. Contractor bid schedules
should contain costs for both scenarios.
Although a mill and overlay program is a more cost effective means of improving a
pavements structural capacity and correcting minor surface undulations, it should be
noted that the risk of reflective cracking exists anytime a distressed pavement (a
pavement containing various levels and types of cracking) is overlaid.
GROUND recommends that GPR analysis be performed prior to performing a mill and
overlay program.
Pavement Reconstruction
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 recommended 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 potential
pavement subgrade materials classify as A-1-a to A-6 soils in accordance with the
American Association of State Highway and Transportation Officials (AASHTO)
classification system.
For the site soils, based on our experience, a resilient modulus of 4,195 psi was
assumed for use in the pavement design. It is important to note that significant
decreases in soil support have been observed as the moisture content increases above
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the optimum. Pavements that are not properly drained may experience a loss of the soil
support and subsequent reduction in pavement life.
Design Traffic
GROUND attempted to retrieve traffic information, however, this information was
unavailable. Based on our experience with similar facilities, an equivalent 18-kip daily
load application (EDLA) value of 5 was assumed for the general parking lot areas. The
EDLA value of 5 was converted to an equivalent 18-kip single axle load (ESAL) value of
36,500 for a 20-year design life. In areas of heavy truck traffic and drive lanes, an
equivalent 18-kip daily load application (EDLA) value of 10 was assumed. The EDLA
value of 10 was converted to an equivalent 18-kip single axle load (ESAL) value of
73,000 for a 20-year design life. If design traffic loadings differ significantly from these
assumed values, GROUND should be notified to re-evaluate the pavement
recommendations below.
Pavement Design
The soil resilient modulus and the assumed ESAL value were used to determine the
required design structural number for the project pavements. The required structural
number was then used to develop recommended 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 and a terminal
serviceability of 2.0 were utilized for design of the pavement sections. A structural
coefficient of 0.40 was used for hot bituminous asphalt and 0.12 was used for aggregate
base course. The minimum pavement sections recommended by GROUND are
tabulated below.
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Recommended Minimum Pavement Sections
Location
Full Depth
Asphalt
(inches Asphalt)
Composite
Section
(inches Asphalt
/ inches
Aggregate
Base)
Rigid
Section
(inches
Concrete)
Private Parking
Lot 6 4.5 / 6 5
Private Drive
Lanes and
Heavy Truck
Traffic
6.5 5 / 6 6
It has been GROUND’s experience that if properly constructed and maintained, a
composite pavement section can provide better long-term performance. We recommend
that primary delivery truck routes such as the dock area, trash collection area, as well as
other pavement areas subjected to high turning stresses or heavy truck traffic be
provided with rigid pavements consisting of 6 or more inches of Portland cement
concrete. For enhanced performance, concrete sections should be underlain by 6 inches
of properly compacted aggregate base. Reinforcement bar should be considered in rigid
pavements to reduce differential movement when cracking occurs.
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.
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. Concrete should have a minimum modulus
of rupture of third point loading of 650 psi. Normally, concrete with a 28-day compressive
strength of 4,000 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.
In areas of repeated turning stresses we recommend that the concrete pavement joints
be fully tied or doweled. We suggest that civil design consider joint layout in accordance
with CDOT’s M Standards. Standard plans for placement of ties and dowels, etc.,
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(CDOT M Standards) for concrete pavements can be found at the CDOT website:
http://www.dot.state.co.us/DesignSupport/
If composite flexible sections are placed, the aggregate base material should meet the
criteria of CDOT Class 6 aggregate base course. Base course should be placed in
uniform lifts not exceeding 8 inches in loose thickness and compacted to at least 95
percent of the maximum dry density a uniform moisture contents within 3 percent of the
optimum as determined by ASTM D1557 / AASHTO T-180, the “modified Proctor.”
Subgrade Preparation
Shortly before placement of pavement, including aggregate base, the exposed subgrade
soils should be scarified and/or processed to a depth of at least 12 inches, mixed to
achieve a uniform moisture content and then re-compacted in accordance with the
recommendations provided in the Project Earthwork section of this report. Subgrade
preparation should extend the full width of the pavement from back-of-curb to back-of-
curb. As stated in the Exterior Flatwork section, greater depths of subgrade processing
will further reduce potential pavement movements. The shallow processing depth
indicated above will not eliminate potential movements.
It is not possible to accurately correlate subgrade stability with information derived from
site observations made during the geotechnical exploration or subsequent laboratory
testing. It is often our experience that when pavements are removed, the pavement
subgrade experiences instability when subjected to construction and/or traffic loading,
even when laboratory testing suggests reasonable moisture contents and
density. Therefore, it may be necessary to stabilize the majority of the existing subgrade
prior to repaving. This may require reprocessing or chemical stabilization of existing
soils or removal and replacement with other site materials or imported soil. Our office
should be retained to observe the subgrade condition and stability during the removal
process. If additional or more specific information is required, then we suggest removal
of several large sections of these pavement areas for evaluation prior to design or
bidding.
The Contractor should be prepared either to dry the subgrade materials or moisten
them, as needed, prior to compaction. It may be difficult for the contractor to achieve
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and maintain compaction in some on-site soils encountered without careful control of
water contents. Likewise, some site soils likely will “pump” or deflect during compaction
if moisture levels are not carefully controlled. The Contractor should be prepared to
process and compact such soils to establish a stable platform for paving, including use
of chemical stabilization, if necessary.
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. Passing a proof roll is an additional
requirement, beyond placement and compaction of the subgrade soils in accordance
with the recommendations in this report. Some soils that are compacted in accordance
with the recommendations herein may not be stable under a proof roll, particularly at
moisture contents in the upper portion of the acceptable range.
Additional Observations
The collection and diversion of surface drainage away from paved areas is extremely
important to the 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.
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 45 of 48
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. Distress of this type is likely to occur
even where the subgrade has been prepared properly and the asphalt has been
compacted properly. As stated, some of these types of distress should be anticipated.
The use of thick base course or reinforced concrete pavement can minimize this. Our
office should be contacted if these alternates are desired.
The design 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.
CLOSURE
Geotechnical Review
The author of this report should be retained to review project plans and specifications to
evaluate whether they comply with the intent of the recommendations in this report. The
review should be requested in writing.
The geotechnical recommendations presented in this report are contingent upon
observation and testing of project earthworks 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 recommendations in this report, or by providing alternative
recommendations.
Materials Testing
The client should consider retaining a Geotechnical Engineer to perform materials
testing during construction. The performance of such testing or lack thereof, 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
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 46 of 48
or pertinent subcontractor is ultimately responsible for managing the quality of their work;
furthermore, testing by the geotechnical engineer does not preclude the contractor from
obtaining or providing whatever services they deem necessary to complete the project in
accordance with applicable documents.
Limitations
This report has been prepared for the Walton Foothills Holdings VI, LLC as it pertains to
the proposed Foothills Mall Redevelopment, Phase 4, as described herein. It may not
contain sufficient information for other parties or other purposes. The owner or any
prospective buyer relying upon this report must be made aware of and must agree to the
terms, conditions, and liability limitations outlined in the proposal.
In addition, GROUND has assumed that project construction will commence by
Fall/Winter 2013. Any changes in project plans or schedule should be brought to the
attention of the Geotechnical Engineer, in order that the geotechnical recommendations
may be re-evaluated and, as necessary, modified.
The geotechnical conclusions and recommendations in this report relied upon
subsurface exploration at a limited number of exploration points, as shown in Figure 1,
as well as the means and 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.
If during construction, surface, soil, bedrock, or groundwater conditions appear to be at
variance with those described herein, the Geotechnical Engineer should be advised at
once, so that re-evaluation of the recommendations may be made in a timely manner. In
addition, a contractor who relies upon this report for development of his scope of work or
cost estimates 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 is responsible for obtaining
the additional geotechnical information that is necessary to develop his workscope and
cost estimates with sufficient precision. This includes current depths to groundwater,
etc.
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 47 of 48
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.
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 project improvements are understood by the Client, Project
Owner (if different), or properly conveyed to any future owner(s). Utilizing these
recommendations for planning, design, and/or construction constitutes understanding
and acceptance of recommendations or information provided herein, potential risks,
associated improvement performance, as well as the limitations inherent within such
estimations. If any information referred to herein is not well understood, it is imperative
for the Client, Owner (if different), or anyone using this report to contact the author or a
company principal immediately.
Performance of the proposed structures and pavement will depend on implementation of
the recommendations 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 the owner.
The scope of services that GROUND provided for the subject project was based on
GROUND’s understanding that the Client is the project owner, and the project consisted
of apartment units intended for rent or lease, rather than condominiums or other
residential units for sale to individuals. As such, the recommendations provided and the
services performed were based on GROUND’s understanding of the Client’s tolerance
for risk of foundation movement, potential distress in the building finishes and other
resulting damage. As a result, GROUND’s recommendations and services may not be
applicable to, or sufficient for, individual homeowners. Therefore, should the Client or its
successors in interest wish to convert these apartment units to condominiums or other
“fee ownership” residential units, then GROUND should be contacted in writing prior to
such conversion to permit GROUND to evaluate the suitability and application of
GROUND’s recommendations and/or services to the proposed use at the time of the
proposed conversion and to advise potential buyers or risks related to geotechnical
conditions.
Foothills Mall Redevelopment-Phase 4
Residential Structures
Fort Collins, Colorado
Final Submittal
Job No. 12-3649B Ground Engineering Consultants, Inc. Page 48 of 48
This report was prepared in accordance with generally accepted soil and foundation
engineering practice in the project area at the date of preparation. GROUND makes no
warranties, either expressed or implied, as to the professional data, opinions or
recommendations 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.
GROUND appreciates the opportunity to complete this portion of the project and
welcomes the opportunity to provide the Owner with a cost proposal for construction
observation and materials testing prior to construction commencement.
Sincerely,
GROUND Engineering Consultants, Inc.
Amy Crandall, E.I.
Reviewed by Andrew J. Suedkamp, P.E.
Maximum dry density = 130.7 pcf 127.5 pcf
Optimum moisture = 8.1 % 9.0 %
Elev/ Classification Nat.
Sp.G. LL PI
% > % <
Depth USCS AASHTO Moist. #4 No.200
ROCK CORRECTED TEST RESULTS UNCORRECTED MATERIAL DESCRIPTION
Project No. Client: Remarks:
Project:
Date:
Location: TH 40 to 48 Sample Number: 176
GROUND ENGINEERING CONSULTANTS, INC.
ENGLEWOOD, CO. Figure
ASTM D 698-07 Method A Standard
SC-SM A-2-4(0) 20 7 12.0 30.0
12-3649 Walton Foothills Holdings, VI, LLC, c/o
09/21/
5
Dry density, pcf
100
110
120
130
140
150
Water content, %
- Rock Corrected - Uncorrected
2 4 6 8 10 12 14
8.1%, 130.7 pcf 9.0%, 127.5 pcf
Test specification:
ASTM D 4718-87 Oversize Corr. Applied to Each Test Point
COMPACTION TEST REPORT For Curve No. 176
Foothills Mall Redevelopment
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Sample Location Natural Natural Percent Atterberg Limits Percent Unconfined Water USCS AASHTO
Test Moisture Dry Passing Liquid Plasticity Swell Compressive Soluble pH Resistivity Classifi- Classifi- Soil or
Hole Depth Content Density Gravel Sand No. 200 Limit Index (Surcharge Strength Sulfates cation cation Bedrock Type
No. (feet) (%) (pcf) (%) (%) Sieve (%) (%) Pressure PSF) (psf) (%) (ohm-cm) (GI)
40 4 17.5 108.2 - - 61 28 9 - - - - - CL A-4(3) sandy lean CLAY
41 10 16.2 110.2 12 36 52 36 17 - - 0.01 7.5 2,844 CL A-6(8) SAND and CLAY
42 27 19.4 111.3 - - 79 42 17 - - - - - CL A-7-9(14) CLAYSTONE BEDROCK
43 8 4.5 - 10 79 11 NV NP - - - - - SP-SM A-1-b(0) poorly-graded SAND with silt
44 5 20.7 105.3 - - 22 37 16 0.1 (650) - - - - SC A-2-6(0) clayey SAND
45 4 12.1 - 42 15 43 32 9 - - - - - GC A-4(1) clayey GRAVEL with sand
46 9 26.4 92.3 - - 67 30 14 - - - - - CL A-6(7) sandy lean CLAY
47 13 23.3 100.9 - - 47 23 10 - - - - - SC A-4(0) SAND and CLAY
48 4 16.1 105.2 - - 39 29 8 -0.2 (500) - - - - SC A-4(0) clayey SAND
49 10 10.7 119.0 - - 37 25 10 - - - - - SC A-4(0) clayey SAND
50 24 6.0 - - - 22 19 4 - - - - - SC-SM A-2-4(0) silty, clayey SANDSTONE
51 3 18.4 86.6 - - 22 38 5 - - 0.03 8.1 8,656 SM A-2-4(0) silty SAND
52 3 19.0 100.8 - - 25 31 5 - - <0.01 8.5 13,925 SM A-2-4(0) silty SAND
53 24 23.5 96.8 - - 90 51 23 - - - - - CH A-7-6(24) CLAYSTONE BEDROCK
54 15 3.3 121.8 - - 25 20 4 - - - - - SC-SM A-2-4(0) silty, clayey SAND
55 10 3.7 - 22 66 12 NV NP - - - - - SM A-1-b(0) silty SAND with gravel
56 4 17.0 105.9 - - 58 35 13 -0.1 (500) - - - - CL A-6(5) sandy lean CLAY
57 3 12.5 96.1 - - 89 35 13 0.1 (375) - - - - CL A-6(12) sandy lean CLAY
58 19 18.5 110.5 - - 85 48 21 - - - - - CL A-7-6(20) CLAYSTONE BEDROCK
59 5 16.3 106.1 - - 47 33 11 - - - - - SC A-6(2) SAND and CLAY
40-48 1-5 8.1* 130.7* 12.0 58.0 30 20 7 - - - - - SC-SM A-2-4(0) silty, clayey SAND
* Indicates optimum moisture content and maximum standard Proctor density (ASTM D-698) Job No. 12-3649-B
Gradation
TABLE 2
SUMMARY OF LABORATORY TEST RESULTS
Sample Location Water Redox Sulfides USCS
Test Soluble pH Potential Content Resistivity Classifi- Soil or
Hole Depth Sulfates cation Bedrock Type
No. (feet) (%) (mV) (ohm-cm)
41 10 0.01 7.5 -25 Positive 2,844 CL sandy lean CLAY
51 3 0.03 8.1 -63 Positive 8,656 SM silty SAND
52 3 < 0.01 8.5 -89 Trace 13,925 SM silty SAND
Job No. 12-3649-B