HomeMy WebLinkAboutCARRIAGE HOUSE APARTMENTS - PDP - PDP120035 - SUBMITTAL DOCUMENTS - ROUND 1 - RECOMMENDATION/REPORTTABLE OF CONTENTS
SUMMARY ............................................................................................................................ - 1 -
PURPOSE AND SCOPE OF WORK ...................................................................................... - 3 -
PROPOSED CONSTRUCTION ............................................................................................. - 3 -
SITE CONDITIONS ............................................................................................................... - 4 -
SUBSURFACE CONDITIONS ............................................................................................... - 4 -
LABORATORY TESTING ...................................................................................................... - 5 -
FOUNDATION AND FLOOR SLAB CONSIDERATIONS ....................................................... - 6 -
FOUNDATION RECOMMENDATIONS.................................................................................. - 8 -
FLOOR SLABS .................................................................................................................... - 12 -
SEISMIC DESIGN CRITERIA .............................................................................................. - 15 -
FOUNDATION WALLS AND RETAINING STRUCTURES................................................... - 15 -
EXTERIOR FLATWORK ...................................................................................................... - 16 -
UNDERDRAIN SYSTEM ..................................................................................................... - 16 -
SURFACE DRAINAGE ........................................................................................................ - 17 -
TEMPORARY EXCAVATIONS ............................................................................................ - 18 -
WATER SOLUBLE SULFATES ........................................................................................... - 18 -
PAVEMENT DESIGN ........................................................................................................... - 18 -
DESIGN AND CONSTRUCTION SUPPORT SERVICES .................................................... - 20 -
LIMITATIONS ...................................................................................................................... - 21 -
FIG. 1 – LOCATIONS OF EXPLORATORY BORINGS
FIG. 2– LOGS OF EXPLORATORY BORINGS
FIG. 3 – LEGEND AND NOTES
FIGS. 4 and 5 – SWELL-CONSOLIDATION TEST RESULTS
TABLE I – SUMMARY OF LABORATORY TEST RESULTS
APPENDIX – DARWIN PAVEMENT SOFTWARE PRINTOUTS
SUMMARY
1. The subsurface conditions encountered at the site were evaluated by drilling 8
exploratory borings at the site. The borings encountered 4 to 6 inches of topsoil
overlying nil to 5 feet of man-placed fill. The fill was composed of fine to coarse-grained
sandy lean clay with occasional small gravels. The natural soil underlying the fill
material consisted of fine-grained sandy lean clay with occasional zones of lean clay. In
the deeper foundation borings, fine to coarse-grained clayey sand was encountered
below the clayey soil and continued to the overburden soil/bedrock interface. The
natural overburden clay soils were slightly moist to moist, and generally very stiff to hard.
The natural clayey sands were slightly moist to wet below ground water, and medium
dense to dense.
A combination of sandstone and claystone bedrock was encountered in the deeper
foundation borings at depths of approximately 18 to 23 feet below the existing ground
surface and continued to the explored depths of 25 feet in the borings. Fine to coarse-
grained sandstone was encountered in Borings 2 through 5, claystone bedrock was
encountered in Boring 6, and interbedded sandstone and claystone bedrock was
encountered in Boring 1. The sandstone and claystone bedrock was slightly moist to
moist, and hard to very hard.
Ground water was encountered in the borings at the time of drilling at depths between
16 and 19 feet. A follow-up measurement made 10 days later indicated that the
presence of ground water had stabilized in the borings at depths of approximately 15.5
to 16 feet below the existing ground surface.
2. Based on the results of laboratory testing, the on-site clay overburden soils are moisture
sensitive, generally exhibiting low to high potential for expansion upon wetting. The
most positive method to limit potential foundation movement due to potential moisture
related expansion is to support the building on straight-shaft piers drilled into bedrock.
However, if the risk of some potential movement posed by a shallow foundation
alternative is accepted by the owner, a shallow foundation system such as spread
footings or Post Tensioned Slabs (PT-Slabs) would be feasible for the proposed
structures at the site. If selected, spread footings should be placed on a minimum of 3
feet of properly compacted structural fill and should be designed for an allowable soil
bearing pressure of 2,500 psf.
3. 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 space. However, due to the high
cost of a structural floor system, slab-on-grade construction is feasible provided the
owner understands that some slab movement may take place even though mitigation
measures would be used to reduce the potential for slab distress resulting from ground
heave. If a slab on grade approach is selected, floor slabs constructed at or near the
existing grade should be underlain by a minimum 7-foot thick zone of properly
compacted and moisture conditioned structural fill, and basement slabs that are
constructed 8 to 10 feet below grade should be underlain by a minimum of 3 feet of
properly compacted and moisture-conditioned structural fill. If the proposed elevations
of the floor slabs are different than the ones assumed above, the minimum depth of
structural fill will need to be adjusted proportionally. A deep perimeter underdrain
system extending below the base of the sub-slab fill is recommended.
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4. We believe PT-slab foundations could be a feasible foundation support provided that the
PT-slabs built at or near the existing grade are underlain by a minimum 4-foot thick zone
of properly compacted and moisture conditioned structural fill, and basement slabs
constructed 8 to 10 feet below the existing grade are underlain by a minimum of 2 feet of
properly compacted and moisture conditioned structural fill. If the proposed elevations of
the PT-Slabs are different than the ones assumed above, the minimum depth of
structural fill will need to be adjusted proportionally. If PT slabs should be designed for
an allowable soil bearing pressure of 2,500 psf. A deep perimeter underdrain system
extending below the base of the fill zone is also recommended for this alternative.
5. Areas of pavement restricted to automobile parking should be paved with a minimum of
6.0 inches of full-depth asphalt. Driveways and fire lanes should be paved using a
minimum of 7.0 inches of full-depth asphalt. As an alternative to the full depth
recommendation, a composite section consisting of 4.0 inches of asphalt over 7.0 inches
of high quality aggregate base course may be used for parking areas, and a section
consisting of 5.0 inches of asphalt over 8.0 inches of aggregate base course may be
used for the driveways and fire lanes. Truck loading areas, dumpster pads, and other
areas where truck turning movements are concentrated should be paved using a
minimum of 6.0 inches of Portland cement concrete.
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PURPOSE AND SCOPE OF WORK
This report presents the results of a geotechnical engineering study for the proposed student
apartment development to be located at 1035 and 1319 South Shields Street in Fort Collins,
Colorado. The project site is shown on Fig. 1. This geotechnical engineering study was
conducted for the purpose of developing foundation, site grading and paving recommendations.
The information and conclusions presented herein are based on data obtained from exploratory
borings drilled for this study at the site. This study has been performed in general accordance
with our Proposal No. P-12-533 to Catamount Properties, LTD dated October 16, 2012, and
revised November 9, 2012.
A field exploration program consisting of 8 exploratory borings was conducted to obtain
information on subsurface conditions. Samples of the soils obtained during the field exploration
program were tested in the laboratory to determine their engineering properties, compressibility
or swell characteristics, and classification. The results of the field exploration and laboratory
testing were analyzed to develop recommendations for foundation types, depths and allowable
pressures for the proposed building foundations, and pavement recommendations. The results
of the field exploration and laboratory testing program are presented herein.
This report has been prepared to summarize the data obtained during the 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 development are included in the report.
PROPOSED CONSTRUCTION
We understand that the proposed development will consist of a student residential complex with
5 multi-unit buildings with associated paved parking areas. Four of the planned buildings will
have full or partial basements. We assume that construction will be typical of multi-family
residential structures with light to moderate foundation loads. Planned site development will
also include on site storm water detention and water quality features, and may include the use
of permeable pavers and other hardscape areas.
If the proposed construction is significantly different than described above or depicted in this
report, we should be notified to reevaluate the recommendations provided in this report.
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SITE CONDITIONS
At the time of the field exploration, the site was occupied by single-family residences with
associated yards and semi-rural areas. The project site currently sits on an approximately 1.5
acre, nearly rectangular parcel that is bounded on the north by Springfield Drive, on the east by
South Shields Street, on the south by residential properties that front South Bennett Road, and
on the west by single-family residential properties. The site is covered by deciduous, fruit
bearing trees, and grasses that are associated with the residential properties on the site. The
site is generally flat and has a gentle slope down to the east. The maximum elevation
difference across the site is on the order of approximately 2 to 3 feet.
SUBSURFACE CONDITIONS
The subsurface conditions encountered at the site were evaluated by drilling 5 exploratory
borings near and within the proposed building footprints, and 3 borings within the limits of the
proposed parking and drive lane areas. The borings were logged by a representative of Kumar
& Associates. The approximate locations of the borings are presented on Fig. 1. The logs of
the borings along with explanatory notes are presented on Figs. 2 and 3.
The borings encountered approximately 4 to 6 inches of rooted topsoil overlying nil to 5 feet of
man-placed fill. The fill was generally composed of fine to coarse-grained sandy lean clay with
occasional small gravels. The natural soil underlying the fill material consisted of fine-grained
sandy lean clay with some zones of lean clay. In the deeper foundation borings, fine to coarse-
grained clayey sand was encountered below the natural lean clay soil and continued to the
overburden soil/bedrock interface.
Based on penetration resistance obtained during the field exploration, the upper natural lean
clay soils were very stiff to hard in consistency, and ranged from slightly moist to moist. The
clayey sand soils were medium dense, and slightly moist to wet below the water table.
A combination of sandstone and claystone bedrock was encountered in the deeper foundation
borings at depths of approximately 18 to 23 feet and continued to the explored depth of
approximately 25 feet in those borings. Fine to coarse-grained sandstone was encountered in
Borings 2 through 5. The sandstone was uncemented and hard to very hard. Claystone
bedrock was encountered in Boring 6 and was hard to very hard. Interbedded claystone and
sandstone bedrock was encountered in Boring 1.
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Ground water was encountered in 3 of the borings at the time of drilling at depths ranging
between 16 and 19 feet. Follow-up measurements made at the site 10 days later indicated the
presence of ground water in all of the deeper borings at depths of approximately 15.5 to 16 feet
below the existing ground surface. Fluctuations in the ground water level may occur with time
and in response to seasonal changes and precipitation events.
LABORATORY TESTING
Laboratory testing was performed on selected soil samples obtained from the borings to
determine in situ soil moisture content and dry density, Atterberg limits, swell-consolidation
characteristics, and concentration of water soluble sulfates. The results of the laboratory tests
are shown to the right of the logs on Fig. 2 and summarized in Table I. The results of specific
tests are graphically plotted on Figs. 4 and 5. The testing was conducted in general accordance
with ASTM testing procedures.
Swell-Consolidation: Swell-consolidation tests were conducted on samples of the existing fill
and the natural lean clay overburden soils in order to determine their compressibility and swell
characteristics under loading and when submerged in water. Each sample was prepared and
placed in a confining ring between porous discs, subjected to a surcharge pressure of 1,000 psf,
and allowed to consolidate before being submerged. The sample height was monitored until
deformation practically ceased under each load increment.
Results of the swell-consolidation tests are presented on Figs. 4 and 5. Based on the results of
the laboratory swell-consolidation testing, a sample of the natural sandy lean clay fill exhibited
moderate consolidation potential when wetted under a surcharge pressure of 1,000 psf. A
sample of the natural lean clay also exhibited moderate to high swell potential when wetted
under a similar surcharge pressure. We believe the moderate to high swell potential exhibited
by the tested samples was generally the result of the overburden clayey soils having relatively
low moisture contents and high dry densities.
A sample of natural lean clay exhibited minor additional compression when wetted. Based on
the relatively dry and sandy condition of that sample, the additional compression is likely the
result of sample disturbance.
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FOUNDATION AND FLOOR SLAB CONSIDERATIONS
Drilled Piers: The existing on-site clay overburden soils are moisture sensitive, generally
exhibiting moderate to high potential for expansion upon wetting. The most positive method to
limit potentially excessive foundation movement due to potential moisture related expansion is
to support the buildings on straight-shaft piers drilled into bedrock. Using a drilled pier
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: Soil-supported floor slabs present a difficult problem where expansive materials
are present because sufficient dead load cannot be imposed on them to resist the uplift
pressure generated when the underlying expansive 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 space. The structural floor would be supported on grade beams
and piers the same as the main structure.
We understand that a structural floor system may result in relatively high cost to the project.
Slab-on-grade construction may be an alternative for the building floor slabs provided the owner
understands that some slab movement may occur even though mitigation measures are used to
reduce the potential for slab distress resulting from ground heave. The use of a slab-on-grade
floor instead of a structural floor will require sub-excavation beneath the slab subgrade elevation
and backfilling with a zone of compacted fill. Subexcavation requirements for the planned
buildings are presented in the “Floor Slabs” section of this report.
Foundation Alternatives: Considering the sub-excavation and backfilling requirements for slab-
on-grade construction, we believe shallow foundations, such as spread footings or post-
tensioned slabs (PT-slabs) may be feasible foundation support alternatives provided they are
also underlain by a zone of compacted fill. Although there would still be a possibility of some
foundation movement due to the potential swelling of the clay overburden soil beneath the
compacted fill zone, the potential for foundation or slab movement would be mitigated by
implementing the recommended over-excavation and compacted fill placement, constructing
and maintaining good surface drainage, installing an underdrain system, and minimizing
landscape irrigation. If the risk of some potential movement posed by a shallow foundation
alternative system is not accepted by the owner, we should be contacted to provide
recommendations for a drilled pier and structural floor system.
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Discussion of Foundation and Floor Slab Movement: The following discussion presents
estimates of ground heave for different wetting depth scenarios to aid in the decision making
process for foundation and floor support systems. The risk of ground heave beneath the
building can be reduced to a certain degree by providing a zone of non- to low-swelling,
relatively impervious, compacted fill directly beneath foundations and floor slabs. Heave
estimate calculations can be useful in evaluating the relative effectiveness of varying the
thickness of this prepared fill zone. However, such calculations cannot address the uncertainty
in the potential depth and degree of wetting that may occur under beneath the building or the
variability of swell potential across the site, which may be erratic at the site.
We have performed calculations to demonstrate the potential for ground heave if the clay
overburden soils beneath the building should be thoroughly wetted to significant depth, including
below the depth of the compacted fill zone. The following table presents estimates of potential
heave based on the results of swell-consolidation tests using test and analysis methods
generally accepted in the Colorado Front Range. Both depth of wetting and depth of the
prepared fill zone were considered as variables in the analysis.
Alternative
Ground Heave in Inches
10 feet of
wetting
15 feet of
wetting
20 feet of
wetting
No moisture treatment 4.9 6.4 7.8
3 feet of moisture-conditioned on-site soils 2.9 4.1 5.7
5 feet of moisture conditioned on-site soils 2.0 3.6 4.9
7 feet of on-site moisture conditioned soils 1.0 2.5 3.9
The heave estimate calculations demonstrate that significant heave should be expected if
thorough wetting of the natural soils beneath the building occurs to significant depth below the
bottom of the prepared fill zone. However, our experience indicates that the large majority of
similar structures underlain by similar subsoils do not experience extreme moisture increases in
the underlying soils to significant depth provided that good surface and subsurface drainage is
designed, constructed, and maintained, and that good irrigation practices are followed. Wetting
can also occur as a result of unforeseeable influences such as plumbing leaks or breaks, or, in
some cases, even due to off-site influences depending on geologic conditions.
It should be noted that the heave estimates presented above are conservative. Also, the heave
estimates presented above are for floor slabs, which are generally lightly loaded and are larger
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than the amount of heave that would be experienced by a more heavily loaded PT-slab
foundation.
We understand that 4 of the 5 buildings planned for the site will have basements that may
extend 8 to 10 feet below grade. In terms of heave potential, the mitigating factors at the site
include the presence of granular soils below depths of about 12 to 15 feet, as well as ground
water at depths of about 15 feet. These conditions would be expected to reduce the
contribution of the natural soils below approximately 15 feet to the potential of heave-related
foundation and slab movement. Based on this, we believe that the heave experienced by
basement slabs would be considerably less than the estimates presented in the above table.
Considering the above discussion, we believe PT-slab foundations or soil-supported floor slabs
and spread footings may be considered for the project, provided that the potential for foundation
or floor slab movement due to ground heave and associated possible distress is recognized by
the owner. The intent of our recommendations for PT-slab foundations and soil-supported floor
slabs is to provide for conditions where there is a good chance ground heave beneath the
building will not exceed amounts acceptable to the owner. The recommendations should result
in heave movements that do not exceed 1 inch and are unlikely to significantly exceed 2 inches
unless extreme wetting is allowed. Barring unforeseen events, we do not believe extreme
wetting is likely to occur if the surface and subsurface drainage and irrigation recommendations
presented in this report are followed.
FOUNDATION RECOMMENDATIONS
Spread Footing Foundations: The design and construction criteria presented below should be
used for a spread footing foundation system. The construction details should be considered
when preparing the project documents.
1. Footings should be placed a minimum of 3 feet of properly compacted structural fill and
should be designed for an allowable soil bearing pressure of 2,500 psf. The footings
should also be designed for a minimum dead load pressure of 800 psf. In order to
satisfy the minimum dead load pressure and minimum footing width criteria, it may be
necessary to concentrate loads by using a grade beam and pad footing. If this system is
used, a void should be provided beneath the grade beams between footings.
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2. The on-site overburden soils if properly moisture conditioned and compacted, should be
suitable for use as structural fill beneath the building areas. Imported structural fill, if
required should consist of low permeability, non-expansive material meeting the
following requirements:
Percent Passing No. 200 Sieve Minimum 25
Liquid Limit Maximum 30
Plasticity Index Maximum 10
Fill source materials not meeting the above liquid limit and plasticity index criteria may
be acceptable (provided the minimum percentage passing the No. 200 sieve is satisfied)
if the swell potential when remolded to 98% of the standard Proctor (ASTM D 698)
maximum dry density at optimum moisture content and wetted under a 200 psf
surcharge pressure does not exceed ½%.
Structural fill beneath foundations should be placed and compacted to at least 98% of
the standard Proctor (ASTM D 698) maximum dry density at moisture contents within 0
to +3 percentage points of optimum moisture content. Prior to placing the structural fill,
the exposed subgrade surface at the base of the sub-excavation should be scarified to a
depth of 12 inches, adjusted to a moisture content between 0 and 3 percentage points
above optimum, and re-compacted to provide a firm, uniform base for subsequent fill
placement. New fill should extend down from the edges of the footings at a 1 horizontal
to 1 vertical projection.
3. Spread footings placed on properly compacted structural fill should have a minimum
footing width of 24 inches for isolated pads, and 16 inches for continuous footings.
4. Exterior footings 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 on properly compacted structural fill
material 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 185 pcf.
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6. Compacted fill placed against the sides of the footings to resist lateral loads should be
non-to low-swelling, and be free of claystone fragments, organics or other deleterious
material.
7. Excessive wetting or drying of the foundation excavations should be avoided during
construction.
8. A representative of the geotechnical engineer should observe all footing excavations
prior to concrete placement.
Post-Tensioned Slab Foundations: 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 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.
The International Building Code (IBC) permits designing PT-slabs in accordance with the
methods outlined in either the Second or Third Editions. Of the two, we recommend using the
guidelines provided in PTI’s Second Edition for designing PT-slabs at the site. The design
method presented in both editions is empirical and was developed in other parts of the country
based on assumptions relating clay mineralogy and climate to soil swell characteristics. The
PTI method does not take into account direct measurements of a soil’s swell-consolidation
characteristics, which are routinely used for foundation design in the Denver area.
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 to be constructed at or near the existing grade
be supported on a minimum of 4 feet of properly moisture-conditioned and compacted
structural fill extending to natural soil. As indicated in the “Discussion of Heave Potential
and Floor Slab Movement” section of this report, we estimate the potential for heave
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under basement level slabs placed 8 to 10 feet below existing grade to be significantly
less. Therefore, we recommend that basement slabs constructed 8 to 10 feet below the
existing grade be supported on a minimum of 2-feet of properly moisture-conditioned
compacted structural fill extending to undisturbed natural soil. If the proposed elevations
of the PT-Slabs are different that the ones assumed above, the minimum depth of
structural fill will need to be adjusted proportionally. The minimum depths recommended
above should be measured from the slab subgrade level. The overexcavation for the
compacted fill zone should extend beyond the limits of the PT-slab foundation to a
minimum distance equal to the depth of overexcavation. Loose or soft material
encountered within the foundation excavation should be removed and replaced with
compacted structural fill.
Prior to placing the sub-slab fill, the upper 12 inches of the subgrade materials at the
base of the sub-excavation should be scarified, moisture conditioned, and recompacted
to at least 95% of the standard Proctor maximum dry density at moisture contents within
0 to 3 percentage points above optimum.
2. The on-site overburden soils are suitable for use as structural fill beneath the building
areas provided that they satisfy the material requirements, and are placed and
compacted according to Item 2 of the “Spread Footing Foundations” section of this
report.
3. PT-slab foundations bearing on compacted structural fill material placed as
recommended herein should be designed for a maximum allowable bearing pressure of
2,500 psf.
4. Based on the procedures outlined in the PTI Manual (Second Edition), post tensioned
slab foundations should be designed based upon differential swell (ym) of 3.43 inches for
the center lift condition, and 0.93 inches for the edge lift condition. The differential swell
is derived from an edge moisture variation (em) of 5.3 feet for the center lift condition, 2.5
feet for the edge lift condition, a depth to constant soil suction of 7 feet, a soil suction
(pF) of 3.6 feet, a moisture velocity of 0.7 inches per month, and a montmorillonite clay
soil.
5. 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.
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6. 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.
7. 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.
We recommend that an underdrain system be constructed at the base of the sub-slab fill zone
to prevent development of perched water in the fill. Inclusion of a properly designed and
constructed underdrain system will be a critical component in reducing potential slab heave.
This underdrain system should be designed in accordance with recommendations in the
“Underdrain System” section of this report.
The precautions and recommendations itemized above will not prevent the movement of PT-
Slabs if the underlying materials are subjected to alternate wetting and drying cycles. However,
the precautions should reduce the damage if such movement occurs.
FLOOR SLABS
As mentioned in the “Foundations and Floor Slab Considerations” section of this report, the
most positive method to avoid damage as a result of floor slab movements is to construct a
structural floor above a well-ventilated crawl space. Due to the potential for a structural floor
system to be very costly to the project, slab-on-grade construction may be considered as an
alternative to both the structural floor system and PT-slab foundation provided the increased risk
of distress resulting from floor slab movement is accepted by the owner. If a slab-on-grade
approach is selected, the following measures should be taken to mitigate or reduce slab
movements, and reduce the potential for damage which could result from movement should the
underslab materials be subjected to moisture changes. If a structural floor system is desired,
we should be notified to provide structural floor recommendations.
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1. Floor slabs constructed at or near existing grade should be placed on a subslab fill zone
consisting of minimum of 7 feet of properly compacted and moisture conditioned
structural fill. As indicated in the “Discussion of Heave Potential and Floor Slab
Movement” section of this report, we estimate the potential for heave under basement
level slabs placed 8 to 10 feet below existing grade to be significantly less. Therefore,
the subexcavation requirement for basement floor slabs constructed 8 to 10 feet below
the existing grade may be reduced to a minimum zone of 3 feet of properly compacted
and moisture conditioned structural fill. If the proposed elevations of the floor slabs are
different than assumed above, the minimum depth of structural fill will need to be
adjusted proportionally. The on-site overburden material is acceptable for use as
structural fill beneath floor slabs.
Structural fill placed beneath floor slabs should meet the material type and placement
requirements recommended in the Spread Footing, and Foundations section of this
report. However, structural fill beneath floor slabs should be compacted to at least 95%
of the standard Proctor (ASTM D 698) maximum dry density. Prior to placing the
subslab fill, the upper 12 inches of the subgrade materials at the base of the sub-
excavation should be scarified, moisture conditioned, and recompacted to at least 95%
of the standard Proctor maximum dry density at moisture contents within 0 to 3
percentage points above optimum.
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 won’t 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 grade beams and piers and to construct the slabs
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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.
Floor slabs should not extend beneath exterior doors or over foundation grade beams,
unless saw cut at the beam after construction.
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.
5. If moisture-sensitive floor coverings will be used, additional 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.
6. 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.
7. The geotechnical engineer should evaluate the suitability of proposed underslab fill
material. Evaluation of potential replacement fill sources will require determination of
laboratory moisture-density relationships and swell consolidation tests on remolded
samples.
We recommend that an underdrain system be constructed at the base of the subslab fill zone to
prevent development of perched water in the fill. Inclusion of a properly designed and
constructed underdrain system will be a critical component in reducing potential slab heave.
This underdrain system should be designed in accordance with recommendations in the
“Underdrain System” section of this report.
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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.
SEISMIC DESIGN CRITERIA
The Colorado Front Range is located in an area of low seismic activity. The overburden soils
generally classify as International Building Code (IBC) Site Class D, and the claystone and
sandstone bedrock generally classify as IBC Site Class C. Based on the standard penetration
testing from the field exploration, the 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.
FOUNDATION WALLS AND RETAINING STRUCTURES
Earth retaining structures should be designed for the lateral earth pressure generated by the
backfill. Rigid earth retaining structures that are restrained from lateral deflection should be
designed for the at-rest condition. Cantilevered retaining structures that are capable of
deflecting under lateral loads will allow mobilization of the backfill. These types of walls may be
designed for the reduced earth pressure represented by the active condition. Materials that
meet CDOT Class 1 Structure Backfill can result in lower earth pressure on walls. The table
below provides design equivalent fluid pressure criteria for drained conditions (where drainage
is provided behind the wall) and undrained conditions.
Material Drained At-
Rest (pcf)
Drained Active
(pcf)
Undrained At-
Rest (pcf)
Undrained Active
(pcf)
On-Site
Overburden Clay
65 45 95 85
CDOT Class 1 60 40 90 85
All foundation and retaining structures should be designed for surcharge pressures such as
adjacent buildings, traffic, construction materials, and equipment. The pressures recommended
above are given for drained and undrained conditions behind the walls and a horizontal backfill
surface. The buildup of water behind a wall or an upward sloping backfill surface will increase
pressure imposed upon a foundation wall or retaining structure.
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Backfill placed against the sides of the below grade structure should be placed in uniform lifts
and compacted to 95% of the standard Proctor (ASTM D 698) maximum dry density at a
moisture content within 2 percentage points of optimum moisture content. Care should be taken
not to over-compact the backfill since this could cause excessive lateral pressure on the walls.
It should be noted that some settlement of deep foundation wall backfills will occur even if the
backfill material is placed correctly.
EXTERIOR FLATWORK
To limit potential movement due to swelling soils and frost conditions, subgrade preparation
beneath exterior flatwork, including sidewalks and patio areas, where reduction of heave
potential is considered important, flatwork should placed on a minimum of 3 feet of moisture
conditioned fill compacted to at least 95% percent of the standard Proctor (ASTM D 698)
maximum dry density and placed between 0 and +3 percentage points of optimum moisture
content. Where reduction of heave potential is less of a concern, the depth of subexcavation
and backfilling could be reduced to a minimum of 2 feet provided that the risk of distress is
accepted by the owner.
UNDERDRAIN SYSTEM
An underdrain should be constructed at the base of the subslab fill zone to prevent development
of perched water in the fill. This recommendation is for both the slab-on-grade and PT-slab
foundation alternatives. The underdrain system should consist of drain lines extending along
the perimeter of the overexcavated zone.
The drain lines should consist of perforated PVC drain pipe placed in the bottom of a trench
excavated at least 1 foot below the base of the overexcavated zone. The drain pipe should be
surrounded above the invert level with free-draining granular material. Drainage aggregate
used in the underdrain systems should consist of a material with a gradation meeting
requirements for a No. 67 coarse aggregate in accordance with ASTM D 488. The drain pipe
trench and free-draining gravel zone should be wrapped with a geotextile fabric to prevent
migration of fines from the surrounding soil into the drainage material.
The base of the overexcavation should be graded to slope towards the drain lines with a
minimum slope of ½%. The overall underdrain system should be sloped at a minimum slope of
½% to a sump where water can be removed by pumping. The sump should be provided with
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alarms in the event the pumping equipment malfunctions. In addition, the drain lines should be
provided with appropriately spaced cleanouts for maintenance and inspection, which we
recommend be performed on a routine basis.
SURFACE DRAINAGE
Proper surface drainage is very important for acceptable performance of 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 structure. 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
standard Proctor (ASTM D 698) maximum dry density.
3. The ground surface surrounding the exterior of buildings should be sloped to drain away
from the structures or foundations 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. A minimum slope of 3
inches in the first 10 feet is recommended in paved or flatwork areas. These slopes may
be changed as required for handicap access points in accordance with the Americans
with Disabilities Act.
4. To promote runoff, the upper 1 to 2 feet of the backfill adjacent to buildings should be a
relatively impervious on-site soil or be covered by flatwork or a pavement structure.
5. Ponding of water should not be allowed in foundation backfill material or in a zone within
10 feet of the building.
6. Roof downspouts and drains should discharge well beyond the limits of all backfill or be
tight-lined to planned storm water facilities.
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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 ten feet of foundation walls.
TEMPORARY EXCAVATIONS
We assume that the temporary excavations will be constructed by over-excavating the slopes to
a stable configuration where enough space is available. All excavations should be constructed
in accordance with OSHA requirements, as well as state, local and other applicable
requirements. Depending on the depth of the excavation, site excavations will generally
encounter existing clay fills, natural clay soils, and clayey sand soils. Existing fills and natural
overburden soils will generally classify as OSHA Type C soils.
WATER SOLUBLE SULFATES
The concentration of water-soluble sulfates measured in samples of the natural overburden clay
soils were less than 0.02%. These concentrations of water soluble sulfates represent a Class 0
severity exposure to sulfate attack on concrete exposed to these materials. The degree of
attack is based on a range of Class 0, Class 1, Class 2, and Class 3 severity exposure as
presented in ACI 201.2R. Based on the laboratory test results, we believe special sulfate
resistant cement will not be required for concrete exposed to the on-site soils.
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.
Subgrade Materials: Based on the results of the field and laboratory test data, the pavement
subgrade materials at the site classify between A-4 and A-6 soils with group indices ranging
from 0 to 9 in accordance with the American Association of State Highway and Transportation
Officials (AASHTO) classification system. Soils classifying as A-4 would generally be
considered to provide fair to poor subgrade support for pavements, while soils classifying as A-6
would generally be considered to provide poor subgrade support. For design purposes, a
resilient modulus value of 3,025 psi was selected for flexible pavements.
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Design Traffic: It appears that daily traffic at the site will be limited to automobile traffic that will
utilize the facility along with occasional truck traffic on an intermittent basis. At the time of report
preparation, traffic data was not available. Therefore, we have assumed as 18-kip equivalent
single axle loading (ESAL) of 36,500 for areas restricted to automobile parking and an ESAL of
109,500 for the driveways and fire lanes. If anticipated traffic is significantly different from that
assumed above, we should be contacted to reevaluate our recommendations.
Pavement Sections: The pavement sections were developed using the DARWinTM computer
software that solves the AASHTO pavement design equations. Areas of pavement restricted to
automobile parking areas should be paved with a minimum of 6.0 inches of full-depth asphalt.
Driveways and fire lanes should be paved using a minimum of 7.0 inches of full-depth asphalt.
As an alternative to the full-depth recommendation, a composite section consisting of 4.0 inches
of asphalt over 7.0 inches of high quality aggregate base course may be used for the parking
areas, and a section of 5.0 inches of asphalt over 8.0 inches of high quality aggregate base
course may be used for the driveways and fire lanes.
Truck loading areas, 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. All
concrete pavement areas on the site should contain sawed or formed joints to ¼ of the depth of
the slab and a maximum distance of 12 to 15 feet on center.
Pavement Materials: Hot mix asphalt (HMA) and Portland cement concrete (PCCP) pavement
should meet the latest applicable requirements, including the CDOT Standard Specifications for
Road and Bridge Construction. We recommend that the asphalt placed for the project is
designed in accordance with the SuperPave gyratory mix design method. The mix should
generally meet Grading S or SX requirements with a SuperPave gyratory design revolution
(NDESIGN) of 75. A PG 58-28 asphalt binder should be used for the mix.
Subgrade Preparation: Existing fills present a problem where present beneath pavements,
particularly when the existing fill was not placed under controlled conditions. Fills of poor or
unknown quality could result in potentially excessive short- and long-term settlements when
subjected to traffic loads, or increases in moisture.
The most positive method for limiting pavement movements caused by settlement of existing fill
of unknown quality is to completely excavate and replace the existing fill with properly
compacted and moisture conditioned fill. A cost-saving alternative is to remove a portion of the
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existing fill beneath pavements, provided the risk of movement in excess of normally accepted
settlement tolerances is acceptable to the owner. Provided that this alternative is acceptable,
we recommend that the upper 2 feet of existing fill underlying pavements, be excavated and
replaced with properly compacted and moisture conditioned fill.
The owner should be aware that subexcavation and replacement will reduce but not eliminate
potential movement of pavements should moisture levels increase within existing fills beneath
the replacement fill and/or pavement. Prior to placing compacted fill, the exposed subgrade
soils should be scarified to a depth of 12 inches, adjusted to a moisture content between 0 and
+3 percentage points of the optimum moisture content and recompacted to at least 95% of the
standard Proctor maximum dry density (ASTM D 698).
The pavement subgrade should be proofrolled with a heavily loaded pneumatic-tired vehicle or
a heavy, smooth drum roller compactor. Pavement design procedures assume a stable
subgrade. Areas that deform excessively under heavy wheel loads are not stable and should
be removed and replaced to achieve a stable subgrade prior to paving. The contractor should
be aware that the clay soils, including on-site and imported materials, may become somewhat
unstable and deform under wheel loads if placed near the upper end of the moisture range.
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.
RRK/jw
cc: book, file
Kumar & Associates, Inc.
APPENDIX
DARWin™ PAVEMENT DESIGN CALCULATIONS