HomeMy WebLinkAboutTHE QUARRY BY WATERMARK - FDP210016 - SUBMITTAL DOCUMENTS - ROUND 1 - GEOTECHNICAL (SOILS) REPORT
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TABLE OF CONTENTS
SUMMARY .................................................................................................................................... 1
PURPOSE AND SCOPE OF STUDY ........................................................................................... 2
PROPOSED CONSTRUCTION .................................................................................................... 2
SITE CONDITIONS ...................................................................................................................... 3
SUBSURFACE CONDITIONS ...................................................................................................... 3
LABORATORY TESTING ............................................................................................................. 4
GEOTECHNICAL ENGINEERING CONSIDERATIONS .............................................................. 6
FOUNDATION RECOMMENDATIONS ........................................................................................ 7
FLOOR SLABS ........................................................................................................................... 11
CONCRETE FLATWORK ........................................................................................................... 12
SITE GRADING .......................................................................................................................... 13
SURFACE DRAINAGE ............................................................................................................... 15
SEISMIC DESIGN CRITERIA ..................................................................................................... 16
CLUBHOUSE SWIMMING POOL .............................................................................................. 17
LATERAL EARTH PRESSURES ................................................................................................ 18
PAVEMENT DESIGN ................................................................................................................. 19
DESIGN AND CONSTRUCTION SUPPORT SERVICES .......................................................... 21
LIMITATIONS ............................................................................................................................. 22
FIG. 1 – LOCATIONS OF EXPLORATORY BORINGS
FIGS. 2 through 4 – LOGS OF EXPLORATORY BORINGS
FIG. 5 – LEGEND AND NOTES
FIGS. 6 through 11 – SWELL-CONSOLIDATION TEST RESULTS
FIGS. 12 and 13 – REMOLDED SWELL-CONSOLIDATION TEST RESULTS
FIG. 14 – GRADATION TEST RESULTS
FIG. 15 – MOISTURE-DENSITY RELATIONSHIP (STANDARD PROCTOR)
TABLE I – SUMMARY OF LABORATORY TEST RESULTS
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SUMMARY
1. The subsurface conditions at the site were explored by drilling 26 exploratory borings at
the approximate locations shown on Fig. 1.
Pre-existing fill was encountered in some of the borings on the eastern and southern
portions of the site which consisted of sandy lean clays. The fill appeared to be re-worked
natural material from a source on or near the project site. The thickness of the fill
encountered in the borings ranged from approximately 2 to 5 feet below the existing
ground surface.
The natural clay soils at the surface or underlying the fill consisted of similar lean clays
with varying sand. At greater depths, natural granular soils were encountered and
consisted of silty clayey sands, and occasional poorly-graded sands and silts with gravel.
The natural clay soils were slightly moist to moist and generally very stiff to hard, although
some zones of medium stiff to stiff clay soils were encountered near groundwater. The
natural granular soils were slightly moist to wet when encountered below groundwater and
were generally medium dense to very dense based on sampler penetration resistance
values (blow counts).
Claystone and sandstone bedrock were encountered in all but two of the building borings
at depths ranging from approximately 14.5 to 23 feet below the existing ground surface.
The claystone bedrock was moist and hard to very hard. The sandstone bedrock was
moist and firm to very hard in consistency with partial cementation in some of the very
hard zones.
Groundwater was encountered during drilling at approximately 9 to 15 feet. A follow-up
groundwater measurement indicated stabilized groundwater levels at about 7.5 to 11.5
feet below the ground surface.
2. With proper subgrade preparation, shallow foundations and slab-on-grade construction is
feasible for the buildings. Post-Tensioned slabs and shallow spread footings placed as
described herein may be designed for net allowable bearing pressures of 2,000 and 3,000
psf, respectively. Further discussion on subgrade preparation is presented in this report.
3. The following table presents the recommended pavement thicknesses:
Paved Area
Full Depth
Asphalt
(inches)
Composite Section
(asphalt/ABC) inches
Parking Stalls 6.0 4.0 / 7
Fire Access and Drive Lanes
within the Parking Lots 7.0 4.5 / 9
ABC – Aggregate Base Course
Dumpster pads and other areas where truck turning movements are concentrated
should be paved with a minimum of 6.0 inches of Portland cement concrete.
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PURPOSE AND SCOPE OF STUDY
This report presents the results of a geotechnical engineering study and pavement thickness
design for “The Quarry” multifamily residential project to be constructed southeast of Hobbit Street
and South Shields Street in Ft. Collins, Colorado. The project site is shown on Fig. 1. The study
was conducted for the purpose of obtaining subsurface data and developing geotechnical
engineering recommendations for design and construction of the proposed development. This
study has been performed in general accordance with our Proposal No. P-20-847 to Watermark
Residential dated November 3, 2020.
A field exploration program consisting of exploratory borings was conducted to obtain information
on the subsurface conditions. Samples obtained during the field exploration were tested in the
laboratory to determine their classification and engineering characteristics. The results of the field
exploration and laboratory testing were analyzed to develop recommendations for foundation
type, building floor slabs, and site pavements. The results of the field exploration and laboratory
testing are presented herein.
This report has been prepared to summarize the data obtained during this study and to present
our conclusions and recommendations based on the proposed construction and the subsurface
conditions encountered. Design parameters and a discussion of geotechnical engineering
considerations related to construction of the proposed apartment buildings and pavement areas
are included in the report.
PROPOSED CONSTRUCTION
Based on the information provided to us, the proposed development will include the construction
of several multi-story apartment buildings including eight-3-story buildings and one-4-story
building. A total of ten, one- to two-story duplexes will be constructed at the northeast portion of
the property. A clubhouse with swimming pool will be constructed near the entrance off of South
Shields Street. Paved drive lanes and automobile parking will be constructed around the
buildings. Access to the new development will be located from the west off of South Shields
Street and from the north off of Hobbit Street. With the exception of the swimming pool, there is
no below-ground construction planned for this project.
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Finished floor elevations for the buildings were not available at the time of this report, however
we do not anticipate that a significant change of grade will be required to achieve finished floor
elevations.
If the proposed construction varies significantly from that generally described above or depicted
in this report, we should be notified to reevaluate the conclusions and recommendations provided
herein.
SITE CONDITIONS
The overall project site will be constructed on approximately 13.5 acres of vacant land. The site
is bounded by Hobbit Street on the north, South Shields Street on the west, the Spring Creek Trail
on the south and by single-family residential construction on the east. The Spring Creek itself is
located immediately south of the aforementioned trail.
Site topography is nearly level with existing grades trending down towards the southeast portion
of the site. Based on a cursory review of site grades, overall elevation differences across the
entire site are approximately 5 to 8 feet, with the more dramatic elevation changes occurring in
the far southeastern corner of the property.
Vegetation at the site was limited to sparse weeds and grasses. Large Cottonwood trees were
present along Spring Creek, just south of the southern property boundary.
SUBSURFACE CONDITIONS
Field Exploration: The subsurface conditions were explored by drilling 26 exploratory borings (17
building, 9 pavement) at the approximate locations shown on Fig. 1. The exploratory borings
were advanced through the overburden soils and bedrock using 4-inch diameter continuous flight
augers. Samples of the soils encountered in the borings were obtained with a 2-inch diameter
California-type drive sampler. The sampler was driven with blows from a 140-pound hammer
falling 30 inches. This sampling procedure is similar to the standard penetration test described
by ASTM D1586. Penetration resistance values indicate the relative density or consistency of the
subsurface soils.
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Graphic logs of the exploratory borings are presented on Figs. 2 through 4. A legend and notes
describing the subsurface soils encountered is presented on Fig. 5.
Subsurface Conditions: Pre-existing fill was encountered in some of the borings on the eastern
and southern portions of the site which consisted of sandy lean clays. The fill appeared to be re-
worked natural material from a source on or near the project site. The exact lateral and vertical
extents of the soil and the degree of compaction of the fill was not evaluated as part of this study.
The thickness of the fill encountered in the borings ranged from approximately 2 to 5 feet below
the existing ground surface.
The natural clay soils encountered at the surface/underlying the fill consisted of similar lean clays
with varying sand. At greater depths, natural granular soils were encountered and consisted of
silty clayey sands, and occasional poorly-graded sands and silts with gravel. The natural clay
soils were slightly moist to moist and generally very stiff to hard, although some zones of medium
stiff to stiff clay soils were encountered near groundwater. The natural granular soils were slightly
moist to wet when encountered below groundwater and were generally medium dense to very
dense based on sampler penetration resistance values (blow counts).
Claystone and sandstone bedrock were encountered in all but two of the building borings at
depths ranging from approximately 14.5 to 23 feet below the existing ground surface. The
claystone bedrock was moist and hard to very hard. The sandstone bedrock was moist and firm
to very hard in consistency with partial cementation in some of the very hard zones.
Groundwater was encountered during drilling at approximately 9 to 15 feet. A follow-up
groundwater measurement indicated stabilized groundwater levels at about 7.5 to 11.5 feet below
the ground surface. Water levels may fluctuate with time, and fluctuate upward in response to
heavy precipitation and landscape irrigation.
LABORATORY TESTING
Selected samples obtained from the exploratory borings were visually classified by the project
engineer. Laboratory testing was performed on selected samples to determine in-situ soil
moisture content and dry unit weight, liquid and plastic limits, swell-consolidation behavior,
moisture-density relationship (standard Proctor) and concentration of water-soluble sulfates. The
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results of the laboratory testing program are shown adjacent to the graphical boring logs on Figs.
2 through 4, plotted graphically on Figs. 6 through 15, and summarized the attached Table I. The
testing was conducted in general accordance with recognized ASTM International and CDOT test
procedures.
Swell-Consolidation Testing: Swell-consolidation testing was conducted on samples of the on-
site overburden clays to determine their swell or compressibility characteristics under loading and
when wetted. Additional swell-consolidation testing was conducted on clayey samples which
were remolded to near 95% of the maximum dry density at, and above optimum moisture content
to help determine the suitability of the on-site material for use as structural fill below foundations
and floor slabs.
Each sample was prepared and placed in a confining ring between porous discs. A selected
surcharge pressure of either 200 or 1,000 psf was applied to the samples, and each sample was
allowed to compress to a stabilized height before being submerged in water. The sample height
was monitored until deformation practically ceased under each load increment.
The results of the laboratory swell-consolidation testing indicate that the tested in-situ samples of
the on-site clays exhibited generally low swell potential in the majority of the samples and very
high swell characteristics in two tested samples. The high to very high swell potential is due in
part to the relatively low moisture content and relatively high in-situ dry density of the samples.
One in-situ sample of clay exhibited additional compression under additional loading, which we
believe to be the result of sample disturbance during sampling and handling.
The remolded samples of the on-site clay exhibited low swell potential. Results of the swell-
consolidation tests are presented on Figs. 6 through 13.
Water-Soluble Sulfates: The concentration of water-soluble sulfates measured in samples of the
on-site clay soils obtained from the exploratory borings ranged from 0.07% to 0.19%. These
concentrations of water-soluble sulfates represent a Class S0 to borderline Class S1/S2 severity
exposure of sulfate attack on concrete exposed to these materials. These degrees of attack are
based on a range of Class S0, Class S1, Class S2, and Class S3 severity exposure as presented
in ACI 201.2R.
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Based on the laboratory data and our experience, we recommend all concrete exposed to the on-
site materials meet the cement requirements for Class S2 exposure as presented in ACI 201.2R.
Alternatively, the concrete could meet the Colorado Department of Transportation’s (CDOT)
cement requirements for Class 2 exposure as presented in Section 601.04 of the latest version
of the CDOT Standard Specifications for Road and Bridge Construction.
GEOTECHNICAL ENGINEERING CONSIDERATIONS
Pre-Existing Fill: As indicated, fill was encountered in a handful of borings at the site, which
extended to as deep as 5 feet below the ground surface. Deeper concentrations of fill may be
present. Pre-existing fill beneath foundations and floor slabs present a problem, particularly if the
existing fill was placed in an uncontrolled fashion. As such, uncontrolled fill can be subject to
variable and unpredictable settlement and/or heave-related movement under structural loading.
Accordingly, the existing fill at the site should be considered non-engineered in its present
condition. Therefore, all existing fill beneath foundation and floor slab areas should be over-
excavated, and replaced with structural fill according to the material and placement
recommendations provided in the “Site Grading” section of this report.
Foundations and Floor Slabs: At the time of this report, we had not been provided with a grading
plan. Based on the subsurface conditions encountered in the borings and the topography of this
site, we assume soil conditions at foundation and floor slab subgrade elevation may consist of
moderate to highly expansive clays. Foundations placed on expansive material such as the on-
site clays soils are likely to experience movement in excess of normally accepted tolerances
should the soils become subject to moisture changes. The safest approach to limit potentially
excessive foundation movement due to potential moisture-related expansion is to support the
building on a deep foundation system using straight-shaft piers drilled into bedrock or helical piers
bearing in bedrock or dense granular soils. Using a deep foundation system has the advantage
of bottoming the piers in a zone of relatively stable moisture content and concentrating the loads
to help offset uplift forces from expansive soil and bedrock.
Floor slabs also present a problem where potentially expansive materials are present near floor
slab elevation because sufficient dead load cannot be imposed on them to resist the uplift
generated when the materials are wetted and expand. The most positive method to avoid slab
damage as a result of ground heave is to construct a structural floor above a well-vented crawl
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space. The structural floor would be supported on grade beams and piers the same as the main
structure. Alternatively, a “slab-on-void” construction approach may be used by constructing a
structurally supported reinforced slab on void form material.
We recognize deep foundation systems in combination with structural floors may be cost
prohibitive to the project. Based on our experience, we believe slab-on-grade floors supported
on a zone of compacted fill should be a practical and cost-effective alternative to structural floors
for the proposed building. Additionally, the over-excavation required for slab-on-grade
construction would also allow the use of a shallow foundation system bearing on the prepared fill.
Acceptable performance will also rely on minimizing water infiltration into the soils by providing
good surface drainage and implementing sensible landscaping and irrigation practices. The
Owner should understand and accept the risk of distress resulting from some foundation and slab
movement even though mitigation measures are used to reduce the potential for building and slab
distress resulting from ground heave.
If the potential for foundation settlement or uplift is not acceptable, a deep foundation system in
combination with a structural floor should be used. Kumar & Associates can provide
recommendations for deep foundations and structural floors, if requested.
Given the above discussion and based on the proposed construction and the subsurface
conditions encountered in the borings, we believe shallow foundations and slab on grade
construction is feasible for the site structures. Shallow foundations may consist of Post-
Tensioned (PT) slabs or spread footings placed on a minimum zone of re-conditioned structural
fill placed and compacted to the recommendations presented in the “Site Grading” section of this
report.
FOUNDATION RECOMMENDATIONS
Post-Tensioned Slabs: We assume that PT-slab foundation design will be conducted in
accordance with the Post-Tensioning Institute’s (PTI) design approach. PTI’s current design
approach is outlined in their publication "Design of Post-Tensioned Slabs-On-Ground (Third
Edition, 2004)" and subsequent addenda, which revised the approach that was outlined in their
publication "Design and Construction of Post-Tensioned Slabs-On-Ground (Second Edition,
1996)". It is the opinion of K+A and many other geotechnical engineers practicing in this area
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that the guidelines provided in both the Second and Third Editions are empirical methods
developed for application in other parts of the country, and may not be strictly applicable for local
conditions due to the method not taking into account direct measurements of a soil’s swell-
consolidation characteristics, which are routinely used for foundation design in the Denver area.
The International Building Code (IBC) permits designing PT-slabs in accordance with the methods
outlined in either the Second or Third Editions. The values presented for design are based on
guidelines in the PTI’s Third Edition.
The design and construction criteria presented below should be observed for a PT-slab
foundation. The construction details should be considered when preparing project documents.
1. We recommend that PT-slab foundations be supported on a prepared subgrade consisting
of at least 3 feet of properly moisture-conditioned and compacted structural fill extending
to undisturbed natural soil. The minimum depth of fill should be measured from the bottom
level of the turned down edge of the slab. The over-excavation for the compacted fill zone
should extend beyond the limits of the PT-slab foundation to a minimum distance equal to
the depth of over-excavation. Loose or soft material encountered within the foundation
excavation should be removed and replaced with compacted structural fill according to the
recommendations presented in the “Site Grading” section of this report.
2. PT-slab foundations bearing on compacted fill material placed as recommended herein
should be designed for a maximum allowable bearing pressure of 2,000 psf.
3. Based on the methodology in PTI’s Third Edition, the slabs should be designed using the
following criteria:
Criteria Center Lift Edge Lift
Moisture variation (em)
(ft.) 9.0 4.6
Differential swell (ym) (in) 0.4 0.3
The parameters used to calculate these values include a controlling soil suction (pF) of
3.6, and a depth to constant soil suction of 7 feet in accordance with the PTI Manual 3rd
Edition. These parameters were selected from the PTI Design Manual based on soil index
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parameters and our opinion regarding the site's swell and compressibility potential; they
are not actual measurements or estimates of soil suction and soil moisture distributions
across the site.
4. The exterior perimeter slab beams should have sufficient embedment for frost protection.
The down-turned edges should have a minimum of 36 inches of soil cover.
5. Once the building pad area has been prepared as described above, it should be protected
from excessive wetting or drying until after the foundation has been completed.
6. We recommend an experienced PT-slab contractor construct the slabs. Representatives
of the geotechnical and structural engineer should check the foundation excavations and
tendon positions prior to placement of concrete, respectively. Fill placement and subgrade
preparation should be observed and tested by a representative of the geotechnical
engineer.
Spread Footings: Shallow spread footings are feasible for foundation support of the site
structures. Similar to PT-Slab foundations, footings should be placed on a minimum of 3 feet of
properly moisture-conditioned and compacted structural fill extending to undisturbed natural soil.
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.
1. Footings placed as described above should be designed for a net allowable soil bearing
pressure of 3,000 psf.
2. Based on experience, we estimate total settlement for footings designed and constructed
as discussed in this section will be less than 1-inch. Differential settlements across the
building are estimated to be approximately ½ to ¾ of the total settlement.
3. Spread footings should have a minimum footing width of 16 inches for continuous footings
and of 24 inches for isolated pads.
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4. Exterior footings and footings beneath unheated areas should be provided with adequate
soil cover above their bearing elevation for frost protection. Placement of foundations at
least 36 inches below the exterior grade is typically used in this area.
5. The lateral resistance of a spread footing placed as described herein will be a combination
of the sliding resistance of the footing on the foundation materials and passive earth
pressure against the side of the footing. Resistance to sliding at the bottoms of the
footings can be calculated based on a coefficient of friction of 0.30. Passive pressure
against the sides of the footings can be calculated using an equivalent fluid unit weight of
190 pcf. The above values are working values with a factor of safety of 2.0 applied.
6. Structural fill placed beneath footings should meet the placement and compaction
recommendations presented in the “Site Grading” section of this report. Compacted fill
placed against the sides of the footings to resist lateral loads should be a non-expansive
material.
7. Excessive wetting or drying of the foundation excavations should be avoided during
construction. Care should be taken to provide adequate surface drainage during the
excavation of footings, and the contractor should have equipment available for removing
water from excavations following precipitation, if needed. Footing excavations that are
inundated as a result of uncontrolled surface runoff may soften, requiring possible
additional moisture conditioning and re-compaction of the exposed subgrade soils, or
removal of soft subgrade soils and replacement with new compacted structural fill.
8. Continuous foundation walls should be reinforced top and bottom to span an unsupported
length of at least 10 feet.
9. A representative of the geotechnical engineer should observe all footing excavations prior
to concrete placement.
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FLOOR SLABS
For slab on grade floors, the following measures should be taken to reduce damage which could
result from movement should the under-slab materials be subjected to changes in moisture
content.
1. Floor slabs should be supported on a minimum 5-foot-thick layer of properly compacted
structural fill meeting the material and placement criteria in the “Site Grading” section of
this report.
2. Floor slabs should be separated from all bearing walls and columns with expansion joints
which allow unrestrained vertical movement.
3. Non-bearing 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. Slip joints that will allow at least 2 inches
of vertical movement are recommended.
If wood or metal stud partition walls are used, the slip joints should preferably be placed
at the bottoms of the walls so differential slab movement will not damage the partition wall.
If slab-bearing masonry block partitions are constructed, the slip joints will have to be
placed at the tops of the walls. If slip joints are provided at the tops of walls and the floors
move, it is likely the partition walls will show signs of distress, such as cracking. An
alternative, if masonry block walls or other walls without slip joints at the bottoms are
required, is to found them on pad-supported grade beams and to construct the slabs
independently of the foundation. If slab-bearing partition walls are required, distress may
be reduced by connecting the partition walls to the exterior walls using slip channels.
4. Floor slab control joints should be used to reduce damage due to shrinkage cracking.
Joint spacing is dependent on slab thickness, concrete aggregate size, and slump, and
should be consistent with recognized guidelines such as those of the Portland Cement
Association (PCA) or American Concrete Institute (ACI). The joint spacing and slab
reinforcement should be established by the designer based on experience and the
intended slab use.
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5. Floor slabs should not extend beneath exterior doors or over foundation grade beams,
unless saw cut at the beam after construction.
6. If moisture-sensitive floor coverings will be used, mitigation of moisture penetration into
the slabs, such as by use of a vapor barrier may be required. If an impervious vapor
barrier membrane is used, special precautions will be required to prevent differential
curing problems which could cause the slabs to warp. American Concrete Institute (ACI)
302.1R addresses this topic.
7. All plumbing lines should be tested before operation. Where plumbing lines enter through
the floor, a positive bond break should be provided. Flexible connections should be
provided for slab-bearing mechanical equipment.
8. The geotechnical engineer should evaluate the suitability of proposed under-slab fill
material.
The precautions and recommendations itemized above will not prevent the movement of floor
slabs if the underlying materials are subjected to alternate wetting and drying cycles. However,
the precautions should reduce the damage if such movement occurs.
CONCRETE FLATWORK
The depth of over-excavation and replacement beneath exterior flatwork immediately adjacent to
the buildings including sidewalks and areas where reduction of heave potential is considered
important should be done in accordance with the recommendations provided in the “Floor Slabs”
section of this report. Where reduction of movement potential is less of a concern such as for
patios and sidewalks located more than 10 feet from the building, subgrade preparation may be
done in accordance with the subgrade preparation recommendations provided in the “Pavement
Design” section of this report. Proper surface drainage measures as recommended in following
sections of this report are also critical to limiting moisture- or frost-related movement.
Upward heave-related movement of exterior flatwork adjacent to the building may result in
adverse drainage conditions with runoff directed toward the building. In addition, upward
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movement of exterior flatwork may restrict movement of outward swinging doors. Site grading
and drainage design should consider those possibilities, particularly at entryways.
SITE GRADING
Site Preparation: Prior to placing new fills, the exposed subgrade should be scarified to a depth
of 12 inches, adjusted to a moisture content within 2 percentage points of optimum, and
recompacted to at least 95% of the standard Proctor (ASTM D698) maximum dry density. The
compacted subgrade should be proof rolled with a heavily-loaded pneumatic-tired vehicle or a
heavy, smooth-drum roller compactor. Areas that deform excessively during proof rolling should
be removed and replaced to achieve a reasonably stable subgrade prior to placement of structural
fill.
Temporary Excavations: We assume that temporary excavations will be constructed by over-
excavating the slopes to a stable configuration where enough space is available. Excavations
generally will extend through existing fill and/or natural clay soils and are not anticipated to
encounter groundwater. All excavations should be constructed in accordance with OSHA
requirements, as well as state, local and other applicable requirements. The on-site natural clay
soils will generally classify as Type B soils. Excavations encountering perched groundwater could
require much flatter side slopes than those allowed by OSHA.
Material Specifications: The following material specifications are presented for fills on the project
site.
1. Fills Placed at the Site: The on-site soils are suitable for re-use as structural fill beneath
buildings and pavements. If imported fill is required, the following requirements should be
met:
Percent Passing No. 200 Sieve Maximum 50
Liquid Limit Maximum 30
Plasticity Index Maximum 15
Fill source materials not meeting the above liquid limit and plasticity index criteria may be
acceptable if the swell potential when remolded to 95% of the ASTM D698 (standard
Proctor) maximum dry density at optimum moisture content under a 200 psf surcharge
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pressure does not exceed 0.5%. Evaluation of potential import sources would require
determination of laboratory moisture-density relationships and swell consolidation tests on
remolded samples.
2. Utility Trench Backfill: Material excavated from the utility trenches may be used for backfill
provided it does not contain unsuitable material or particles larger than 4 inches.
3. Material Suitability: The on-site soils including the existing fills, are generally suitable for
use as structural fill material. The on-site overburden soils are generally expected to
possess low swell potential when moisture conditioned and compacted according to the
recommended criteria.
All fill material should be free of vegetation, brush, sod and other deleterious substances
and should not contain rocks, debris or lumps having a diameter of more than 4 inches.
The geotechnical engineer should evaluate the suitability of proposed import fill materials
prior to placement.
Compaction Specifications: We recommend the following compaction criteria be used on the
project:
Moisture Content: Fill should be placed at a moisture content between 0 and +3
percentage points of optimum for predominantly clay fill materials, and at a moisture
content within 2 percentage points of the optimum moisture content for predominantly
granular fill materials. The contractor should be aware that the clayey soils may become
somewhat unstable and deform under wheel loads if placed near the upper end of the
moisture range.
Degree of Compaction: The following compaction criteria should be followed during
construction:
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AREA
MINIMUM PERCENTAGE OF
MAXIMUM STANDARD PROCTOR
DENSITY (ASTM D-698)
Fills placed shallower than 8 feet 95%
Fills beneath exterior slabs and pavement
structures 95%
Fills placed deeper than 8' 98%
Utility Trenches 95%
Foundation Wall Backfill 95%
A representative of the geotechnical engineer should observe fill placement on a full-time
basis.
SURFACE DRAINAGE
Proper surface drainage is important for acceptable performance of the structures during
construction and after the construction has been completed. Drainage recommendations
provided by local, state and national entities should be followed based on the intended use of the
structures. The following recommendations should be used as guidelines and changes should
be made only after consultation with the geotechnical engineer.
1. Excessive wetting or drying of the foundation and slab subgrades should be avoided
during construction.
2. Exterior backfill should be adjusted to near optimum moisture content (generally ±2% of
optimum unless indicated otherwise in the report) and compacted to at least 95% of the
ASTM D698 (standard Proctor) maximum dry density.
3. The ground surface surrounding the exterior of the building should be positively sloped to
drain away from the foundation in all directions. We recommend a minimum slope of 12
inches in the first 10 feet in unpaved areas. Site drainage beyond the 10-foot zone should
be designed to promote runoff and reduce infiltration. These slopes may be changed to
comply with the ADA Act. (Special Publication 43 of the Colorado Geologic Survey, “A
Guide to Swelling Soils for Colorado Homebuyers and Homeowners,” provides useful
information on drainage design and landscaping on expansive soil sites.) Surface
diversion features should be provided around parking areas to prevent surface runoff from
flowing across the paved surfaces.
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4. The upper 1 to 2 feet of the backfill should be relatively impervious material compacted as
above to limit infiltration of surface runoff.
5. Ponding of water should not be allowed in backfill material of in a zone within 10 feet of
the foundation walls, whichever is greater.
6. Roof downspouts and drains should discharge well beyond the limits of all backfill.
7. Landscaping adjacent to buildings underlain by moisture-sensitive soils should be
designed to avoid irrigation requirements that would significantly increase soil moisture
and potential infiltration of water within at least 10 feet of foundation walls. Landscaping
located within 10 feet of foundation walls should be designed for irrigation rates that do
not significantly exceed evapotranspiration rates. Use of vegetation with low water
demand and/or drip irrigation systems are frequently used methods for limiting irrigation
quantities.
Lawn sprinkler heads and landscape vegetation that requires relatively heavy irrigation
should be located at least 10 feet from foundation walls. Even in areas away from
buildings, it is important to provide good drainage to promote runoff and reduce infiltration.
Main pressurized zone supply lines, including those supplying drip systems, should be
located more than 10 feet from buildings in the event that leaks occur. All irrigation
systems, including zone supply lines, drip lines, and sprinkler heads should be routinely
inspected for leaks, damage, and improper operation.
SEISMIC DESIGN CRITERIA
The soil profile generally consists of 15 to 25 feet of stiff to very stiff clayey overburden overlying
firm to very hard claystone and sandstone bedrock. According to International Building Code
(IBC) 2015, the overburden soils encountered at the site generally classify as IBC Site Class D,
and the bedrock encountered classifies as Site Class C. Based on the soil profile encountered in
our borings and the standard penetration testing from the field exploration, the estimated weighted
average of shear wave velocity in the upper 100 feet indicates that IBC Site Class D should be
used in the design. Based on the subsurface profile, site seismicity, and the anticipated ground
water conditions, liquefaction is not a design consideration.
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CLUBHOUSE SWIMMING POOL
The boring drilled in the approximate area of the swimming pool (Boring 15) encountered pre-
existing fill and natural overburden soil to a depth of about 20 feet, overlying bedrock.
Groundwater was measured at approximately 11 feet below the surface 23 days after drilling.
Proper design and construction of below-ground pool structures is critical to their satisfactory
performance when expansive materials are present. All swimming pools have a tendency to leak.
A small amount of leakage can cause the subsoils to swell and result in pool or slab movement
which widens existing cracks and introduces more water into the subsoils, thereby compounding
the problem.
Based on these considerations and the subsurface conditions, we suggest the following
precautions be taken in the design and construction of the proposed pool.
1. The pool should be designed and constructed to withstand differential movement without
serious cracking.
2. Natural material below the pool should be removed to a depth of 12 inches and replaced
with a non-expansive, impervious fill material compacted to 95% of the standard Proctor
(ASTM D698) maximum dry density at or above optimum moisture content.
3. A perimeter drain should be constructed at the base of the pool and sloped at a minimum
1% to an outlet where water can be removed by gravity flow or pumped. The drains should
consist of perforated pipe surrounded by a minimum of 12 inches of free-draining granular
material.
A minimum 4-inch-thick free-draining gravel layer should be placed beneath the pool deck
and swimming pool floor. The drainage layer under the pool should slope to a drain line
or collection point from which water can be removed by pumping or gravity drainage. The
drainage layer under the deck should slope to a perimeter drain or be connected to the
under-pool layer by free-draining backfill wrapped in a filter fabric.
Free-draining aggregate should conform to the requirements of CDOT Class B or Class
C Filter Material, unless a filter geotextile is used on the slab subgrade and around
underdrain trenches; in that case a coarser free-draining gravel not necessarily meeting
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graded filter criteria, such as AASHTO No. 57 or No. 67 Aggregate, may be used. Pipe
slots or perforations should be sized in accordance with the type of free-draining material
surrounding the pipe. We are available to assist in the underdrain system design if
requested.
4. A tight joint should be provided between the pool and deck so water splashed from the
pool will not infiltrate the subsoils. Cracks which develop on the deck while the pool is in
service should be caulked to prevent water infiltration.
5. The pool deck and adjoining area should be sloped to minimize ponding and infiltration of
moisture into the subsoils. Lawn irrigation should be kept to a minimum adjacent to the
pool.
6. We recommend the bottom of the pool be kept at least 3 feet above groundwater to
prevent hydrostatic uplift on the pool should groundwater levels rise.
These precautions will not eliminate the risk of damage to the pool and deck due to expansive
materials, but should reduce the chances of subsurface materials becoming wetted and
subsequent movement due to changes in moisture content.
LATERAL EARTH PRESSURES
Earth retaining structures should be designed for the lateral earth pressure based on the degree
of rigidity of the retaining structure and the type of backfill material used. Retaining structures
that are laterally supported and can be expected to undergo only a moderate amount of deflection
should be designed for earth pressures based on the following equivalent fluid unit weights:
CDOT Class 1 (<20% passing No. 200 Sieve) ........................................................... 50 pcf
On-site, moisture-conditioned cohesive soil backfills ................................................ 65 pcf
Cantilevered retaining structures that can be expected to deflect sufficiently to mobilize the full
active earth pressure condition should be designed for the following equivalent fluid unit weights:
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CDOT Class 1 (<20% passing No. 200 Sieve) ........................................................... 40 pcf
On-site, moisture-conditioned cohesive soil backfills ................................................ 55 pcf
The pressures recommended above assume drained conditions behind the structures and a
horizontal backfill surface. The buildup of water behind a wall or an upward sloping backfill
surface will increase the lateral pressure imposed on a retaining structure.
PAVEMENT DESIGN
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. Soils are represented for pavement design purposes by
means of a soil support value for flexible pavements and a modulus of subgrade reaction for rigid
pavements. Both values are empirically related to strength.
Subgrade Materials: Based on the results of the field and laboratory studies, the majority
subgrade materials at the site classify as A-6 and A-7-6 soils with group indices between 3 and
32 in accordance with the American Association of State Highway and Transportation Officials
(AASHTO) classification. A sample of sandy lean clay classified as A-4 with a group index of 3,
and samples of poorly-graded sand with silt and gravel and silty clayey sand classified as A-2-4
with a group index of 0. Based on the subsurface soil properties, and on our experience with
similar soils, a design resilient modulus of 3,025 psi was selected for flexible pavements and a
modulus of subgrade reaction of 70 pci was selected for rigid pavements.
Design Traffic: Since anticipated traffic loading information was not available at the time of report
preparation, we are assuming that traffic to the site will consist primarily of automobile traffic and
small moving vehicles that will utilize the facility on a routine basis. The following 18-kip equivalent
single axle loading (ESAL) values were assumed as indicated below:
Paved Area 18-kip
ESALS
Parking Stalls 36,500
Fire Access and
Drive Lanes within
the Parking Lots
109,500
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The ESAL assumptions above are typical values we have used in the past for similar projects
based on our experience. We should be contacted to reevaluate the recommendations provided
herein if the any of the traffic distribution assumptions are found to be different than those
described above.
Pavement Design: The following table presents the recommended pavement thicknesses:
Area
Full Depth
Asphalt
(inches)
Composite Section
(Asphalt/ABC) inches
Parking Stalls 6.0 4.0 / 7
Fire Access and Drive
Lanes within the Parking
Lots
7.0 4.5 / 9
ABC – Aggregate Base Course
Dumpster pads and other areas where truck turning movements are concentrated should be
paved with a minimum of 6.0 inches of Portland cement concrete. The concrete pavement should
contain sawed or formed joints to ¼ of the depth of the slab at a maximum distance of 15 feet on
center.
Pavement Material Recommendations: Pavement Materials: The following are recommended
material and placement requirements for pavement construction for this project site.
1. Aggregate Base Course: The aggregate base course used in the flexible composite
alternative should meet the requirements Class 6 aggregate base course in accordance
with CDOT specifications. The aggregate base course should be compacted to a
minimum of 95 percent of the maximum modified Proctor dry density (AASHTO T180) at
a moisture content within 2 percentage points of optimum.
2. Hot Mix Asphalt: Hot mix asphalt (HMA) materials and mix designs should meet the
applicable requirements indicated in the current CDOT “Standard Specifications for Road
and Bridge Construction”. We recommend that the HMA used for this project is designed
in accordance with the SuperPave gyratory mix design method. The mix should meet
Grading S specifications with a SuperPave gyratory design revolution (NDESIGN) of 75.
A mix meeting Grading SX specification can be used for the top lift wearing course,
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however, this is optional. The mix design(s) for the HMA should use a performance grade
(PG) asphalt binder of PG 58-28 or PG 64-22. However, we recommend the PG 58-28
binder which tends to perform better under relatively low traffic volumes. Placement and
compaction of HMA should follow current CDOT standards and specifications.
Subgrade Preparation: Prior to placing the pavement section, the subgrade beneath pavements
should be thoroughly scarified and well-mixed to a depth of 12 inches, adjusted to a moisture
content between -1 to +3 percentage points of the optimum moisture content for clayey
subgrades, and within 2 percentage points of the optimum moisture content for granular
subgrades and compacted to at least 95% of the standard Proctor (ASTM D698) maximum dry
density.
Proof rolling should be performed after the specified compaction is obtained. Proof rolling should
be performed with heavy rubber-tired equipment with a tire pressure of at least 100 psi capable
of applying a minimum load of 18-kips per axle. Areas where excessive deflection occurs should
be ripped, scarified, wetted or dried if necessary, and recompacted to the required moisture and
density specifications.
Drainage: The collection and diversion of surface drainage away from paved areas is extremely
important to the satisfactory performance of pavement. Drainage design should provide for the
removal of water from paved areas and prevent the wetting of the subgrade soils.
DESIGN AND CONSTRUCTION SUPPORT SERVICES
Kumar & Associates, Inc. should be retained to review the project plans and specifications for
conformance with the recommendations provided in our report. We are also available to assist
the design team in preparing specifications for geotechnical aspects of the project, and performing
additional studies if necessary, to accommodate possible changes in the proposed construction.
We recommend that Kumar & Associates, Inc. be retained to provide construction observation
and testing services to document that the intent of this report and the requirements of the plans
and specifications are being followed during construction. This will allow us to identify possible
variations in subsurface conditions from those encountered during this study and to allow us to
re-evaluate our recommendations, if needed. We will not be responsible for implementation of
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the recommendations presented in this report by others, if we are not retained to provide
construction observation and testing services.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
practices in this area for exclusive use by the client for design purposes. The conclusions and
recommendations submitted in this report are based upon the data obtained from the exploratory
borings at the locations indicated on Fig. 1, and the proposed type of construction. This report
may not reflect subsurface variations that occur between the exploratory borings, and the nature
and extent of variations across the site may not become evident until site grading and excavations
are performed. If during construction, fill, soil, rock or water conditions appear to be different from
those described herein, Kumar & Associates, Inc. should be advised at once so that a re-
evaluation of the recommendations presented in this report can be made. Kumar & Associates,
Inc. is not responsible for liability associated with interpretation of subsurface data by others.
The scope of services for this project does not include any environmental assessment of the site
or identification of contaminated or hazardous materials or conditions.
Swelling soils and bedrock are present at this site. Such materials are stable at their natural
moisture content but will undergo high volume changes with changes in moisture content. The
extent and amount of perched water beneath the building site as a result of area precipitation and
irrigation, and inadequate surface drainage, is difficult, if not impossible, to foresee.
The recommendations presented in this report are based on current theories and experience of
our engineers on the behavior of swelling soil and bedrock in this area. The Owner should be
aware that there is a risk in constructing buildings and pavements in an area of highly expansive
soil and bedrock. Following the recommendations given by a geotechnical engineer, careful
construction practice and prudent maintenance by the owner can, however, decrease the risk of
foundation, slab and pavement movement due to expansive materials.
RRK/js
Rev. by: NK
cc: file
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TABLE I SUMMARY OF LABORATORY TEST RESULTS PROJECT NO.: 20-1-671 PROJECT NAME: The Quarry Multi-Family Development DATE SAMPLED: 11-13-2020 and 11-17-2020 DATE RECEIVED: 11-18-2020 SAMPLE LOCATION DATE TESTED NATURAL MOISTURE CONTENT (%) NATURAL DRY DENSITY (pcf) GRADATION PERCENT PASSING NO. 200 SIEVE ATTERBERG LIMITS WATER SOLUBLE SULFATES (%) MDD* (pcf) OMC* (%) AASHTO CLASSIFICATION (group index) SOIL OR BEDROCK TYPE BORING DEPTH (feet) GRAVEL (%) SAND (%) LIQUID LIMIT (%)PLASTICITY INDEX (%)1 4 11-18-20 16.6 112.3 0 20 80 35 20 0.07 A-6 (14) Lean Clay with Sand (CL) 2 4 11-27-20 18.1 107.7 74 37 21 A-6 (14) Lean Clay with Sand (CL) 3 4 11-27-20 13.0 115.0 77 38 20 A-6 (14) Lean Clay with Sand (CL) 4 4 11-27-20 9.5 119.5 58 26 10 A-4 (3) Sandy Lean Clay (CL) 5 1 11-27-20 11.0 118.9 90 44 28 A-7-6 (26) Lean Clay (CL) 6 9 11-27-20 26.2 97.8 96 49 31 A-7-6 (32) Lean Clay (CL) 7 4 11-18-20 15.7 108.2 0 18 82 34 20 0.07 A-6 (15) Lean Clay with Sand (CL) 8 9 11-27-20 27.5 96.5 94 42 24 A-7-6 (23) Lean Clay (CL) 9 4 11-27-20 8.4 105.0 52 26 11 A-6 (3) Sandy Lean Clay (CL) 10 4 11-27-20 8.1 113.6 6 38 56 34 20 A-6 (8) Sandy Lean Clay (CL) 11 1 11-27-20 6.8 96.5 49 30 16 A-6 (4) Clayey Sand (SC) 12 9 11-27-20 28.0 95.2 82 43 26 A-7-6 (21) Lean Clay with Sand (CL) 13 4 11-27-20 11.9 116.7 69 37 22 A-6 (13) Sandy Lean Clay (CL) 14 4 11-27-20 13.8 119.6 73 30 17 0.19 A-6 (10) Lean Clay with Sand (CL) 15 14 11-27-20 7.9 128.6 48 42 10 NV NP A-2-4 (0) Poorly-Graded Sand with Silt and Gravel (SP-SM) 16 4 11-27-20 9.9 118.2 55 34 18 A-6 (7) Sandy Lean Clay (CL) 17 4 11-27-20 9.2 124.5 62 35 20 A-6 (9) Fill: Sandy Lean Clay (CL) P-1 1 11-27-20 7.1 120.3 10 36 54 36 20 A-6 (7) Sandy Lean Clay (CL) P-3 1 11-27-20 3.3 110.8 4 71 25 22 6 A-2-4 (0) Silty Clayey Sand (SC-SM) P-5 4 11-27-20 9.2 108.2 64 35 20 A-6 (10) Sandy Lean Clay (CL) P-6 1 11-27-20 9.7 117.2 75 35 20 A-6 (13) Lean Clay with Sand (CL) P-7 1 11-27-20 6.0 110.6 16 38 46 33 18 A-6 (4) Clayey Sand with Gravel (SC) P-9 1 11-27-20 8.1 109.4 56 32 16 A-6 (6) Sandy Lean Clay (CL) B-5 4 – 9 71 35 20 105.7 / 15.3 A-6 (12) Lean Clay with Sand (CL) *Maximum Dry Density (MDD) and Optimum Moisture Content (OMC) as determined by ASTM D698