HomeMy WebLinkAboutSubsoil StudyIC iiffi¡fi:i:'i¡tj'*''rÊ;'n""
An Employcc Owncd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
wrvu,.kunrarusa.com
Offrce Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
Lo-ï 76, PINYON MESA
PINYON MESA DRIVE
GARFTELD COUNTY, COLORADO
PROJECT NO.21-7-363
JUNE 15,2021
PREPARED FOR:
BRIAN HOLMES
5960 FIELD STREET
ARVADA, COLORADO 8OOO4
tr rian5. h olm es5@ gm ail.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION
SITE CONDITIONS....
SUBSIDENCE POTENTIAL
FIELD EXPLORATION
SUBSURFACE CONDITIONS
FOLINDATION BEARING CONDITIONS ..
DESIGN RECOMMENDATIONS ....................
FOUNDATIONS
FOUNDATION AND RETAINING WALLS
FLOOR SLABS
TINDERDRAIN SYSTEM .............
SURFACE DRAINAGE...............
LIMITATIONS
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURE 3 - SWELL-CONSOLIDATION TEST RESULTS
TABLE I- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc. @ Project No. 21-7-363
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot76, Pinyon Mesa, Pinyon Mesa Drive, Garfield County, Colorado. The project site is shown
on Figure 1. The purpose of the study was to develop recommendations for the foundation
design. The study was conducted in accordance with our agreement for geotechnical engineering
services to Brian Holmes dated April 16, 202I.
An exploratory boring was drilled to obtain information on the subsurface conditions. Samples
of the subsoils obtained during the field exploration were tested in the laboratory to determine
their classification, compressibility or swell and other engineering characteristics. The results of
the field exploration and laboratory testing were analyzedfo develop recommendations for
foundation types, depths and allowable pressures for the proposed building foundation. This
report summarizes the data obtained during this study and presents our conclusions, design
recommendations and other geotechnical engineering considerations based on the proposed
construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
Plans for the proposed residence were not available at the time of our study. The proposed
residence is assumed to be a one or two-story structure over a lower walkout basement level with
attached garage at the main level. Ground floors will likely be a combination of structural over
crawlspace and slab-on-grade. Grading for the structure is assumed to be relatively minor with
cut depths between about 3 to 8 feet. We assume relatively light foundation loadings, typical of
the proposed type of construction.
If building loadings, location or grading plans change significantly from those described above,
we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The subject site was vacant at the time of our field exploration. The ground surface is sloping
down to the west at a grade of about 15 percent with about 10 feet of elevation difference acloss
the building envelope. Pinyon Mesa Drive borders the north side of the lot and is approximately
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I feet above the center of the building site. The ground surface is mostly barren with sparse
grass and scattered sage brush at the rear ofthe lot.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian Age Eagle Valley Evaporite underlies the Pinyon Mesa
Development. These rocks are a sequence of gypsiferious shale, fine-grained sandstone/siltstone
and limestone with some massive beds of gypsum. There is a possibility that massive gypsum
deposits associated with the Eagle Valley Evaporite underlie portions of the property.
Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can
produce areas of localized subsidence. During previous work in the area, sinkholes have been
observed scattered throughout the lower Roaring Fork River valley.
No evidence of subsidence or sinkholes was observed on the property or encountered in the
subsurface materials, however, the exploratory boring was relatively shallow, for foundation
design only. Based on our present knowledge of the subsurface conditions at the site, it cannot
be said for certain that sinkholes will not develop. The risk of future ground subsidence at the
site throughout the service life of the proposed structure, in our opinion is low, however the
owner should be aware of the potential for sinkhole development. If further investigation of
possible cavities in the bedrock below the site is desired, we should be contacted.
FIELD EXPLORATION
The field exploration for the project was conducted on May 13, 2021. One exploratory boring
was drilled at the location shown on Figure 1 to evaluate the subsurface conditions. The boring
was advanced with 4-inch diameter continuous flight augers powered by a truck-mounted CME-
458 drill rig. The boring was logged by a representative of Kumar & Associates, Inc.
Samples of the subsoils were taken with 1%-inch and 2-inch LD. spoon samplers. The samplers
were driven into the subsoils at various depths with blows from a 140-pound hammer falling 30
inches. This test is similar to the standard penetration test described by ASTM Method D-1586.
The penetration resistance values are an indication of the relative density or consistency of the
subsoils. Depths at which the samples were taken and the penetration resistance values are
shown on the Log of Exploratory Boring, Figure 2. The samples were returned to our laboratory
for review by the project engineer and testing.
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SUBSURFACE CONDITIONS
A graphic log of the subsurface conditions encountered at the site is shown on Figure 2. The
subsoils consist of about 25 feet of very stiff to hard, sandy clay with scattered gravel overlying
hard/medium dense, clay and sand with gravel down to the maximum explored depth of 36 feet.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and density and finer than sand grain size gradation analyses. Results of swell-
consolidation testing performed on relatively undisturbed drive samples of the clay soils,
presented on Figure 3, indicate low compressibility under natural low moisture conditions and
low expansion potential when wetted. The laboratory testing is summarized in Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils were slightly
moist.
FOUNDATION BEARING CONDITIONS
The natural sandy clay soils possess relatively low bearing capacity and variable swell or
settlement potential mainly when wetted. A shallow foundation placed on these soils will have a
risk of movement if the soils become wetted and care should be taken in the surface and
subsurface drainage around the house to prevent the soils from becoming wet. It will be critical
to the long-term performance of the structure that the recommendations for surface grading and
drainage contained in this report be followed. The amount of movement, if the bearing soils
become wet, will mainly be related to the depth and extent of subsurface wetting but may result
in settlements of around I to 2 inches which could cause building distress. Mitigation methods
such as removing and replacing the bearing soils as compacted structural fill or micro-piles down
into the more gravelly soils could be used to support the proposed house with a lower risk of
movement.
DESIGN RECOMMENDATIONS
FOLINDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the nature of
the proposed construction, we recommend the building be founded with spread footings bearing
on a minimum of 3 feet of compacted structural fill below garage and crawlspace footings. We
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should observe the soils for use of compacted structural fill below basement level footings. We
should be contacted for additional recommendations if a deep foundation is desired.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the compacted structural fill should be designed for an
allowable bearing pressure of 2,000 psf. Based on experience, we expect initial
settlement of footings designed and constructed as discussed in this section will
be about 1 inch or less. Additional differential movements of about %to I inch
could occur if the bearing soils are wetted.
2) The footings should have a minimum width of 16 inches for continuous walls and
2 feet for isolated pads.
3) 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 exterior grade is typically used in this
area.
4)Continuous foundation walls should be heavily reinforced top and bottom to span
local anomalies such as by assuming an unsupported length of at least 14 feet.
Foundation walls acting as retaining structures should also be designed to resist
lateral earth pressures as discussed in the "Foundation and Retaining Walls"
section of this report.
The topsoil, sub-excavated depth and any loose disturbed soils should be removed
below the foundation area. The exposed soils in footing areas after sub-
excavation should then be moistened and compacted. Structural fill should
consist of low permeable soil (such as the on-site sandy clay soils) compacted to
at least 98%o of standard Proctor density within 2o/o of optimum moisture content.
The structural fill should extend laterally beyond the footing edges equal to at
least/z the fill depth below the footing.
A representative of the geotechnical engineer should evaluate the fill placement
for compaction and observe all footing excavations prior to concrete placement to
evaluate bearing conditions.
5)
6)
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FOUNDATION AND RETAINING WALLS
Foundation walls and retaining structures which are laterally supported and can be expected to
undergo only a slight amount of deflection should be designed for a laterul earth pressure
computed on the basis of an equivalent fluid unit weight of at least 55 pcf for backfill consisting
of the on-site fine-grained soils. Cantilevered retaining structures which are separate from the
residence and can be expected to deflect sufficiently to mobilize the full active earth pressure
condition should be designed for a lateral earth pressure computed on the basis of an equivalent
fluid unit weight of at least 45 pcf for backfill consisting of the on-site fine-grained soils.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The
pressures recommended above assume drained conditions behind the walls 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 foundation wall or retaining structure. An underdrain
should be provided to prevent hydrostatic pressure buildup behind walls.
Backfill should be placed in uniform lifts and compacted to at least 90%o of the maximum
standard Proctor density at a moisture content near optimum. Backf,rll placed in pavement and
walkway areas should be compacted to at least 95%o of the maximum standard Proctor density.
Care should be taken not to overcompact the backfill or use large equipment near the wall, since
this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall
backfill should be expected, even if the material is placed correctly, and could result in distress to
facilities constructed on the backfill. Backfill should not contain organics, debris or rock larger
than about 6 inches.
The lateral resistance of foundation or retaining wall footings 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 0.35. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 325 pcf. The
coefficient of friction and passive pressure values recommended above assume ultimate soil
strength. Suitable factors of safety should be included in the design to limit the strain which will
occur at the ultimate strength, particularly in the case of passive resistance. Fill placed against
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the sides of the footings to resist lateral loads should be compacted to at least 95Yo of the
maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab-on-grade
construction with a movement risk similar to the foundation if the underlying soils are wetted.
To reduce the effects of some differential movement, floor slabs should be separated from all
bearing walls and columns with expansion joints which allow unrestrained vertical movement.
Floor slab control joints should be used to reduce damage due to shrinkage cracking. The
requirements for joint spacing and slab reinforcement should be established by the designer
based on experience and the intended slab use. A minimum 4-inch layer of free-draining gravel
should be placed beneath basement level slabs to facilitate drainage. This material should
consist of minus 2-inch aggregate with at least 50% retained on the No. 4 sieve and Iessthan2o/o
passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95o/o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the
onsite soils or imported granular soils devoid of vegetation, topsoil and oversized rock.
TINDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
the area and where clay soils are present that local perched groundwater can develop during
times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can create a
perched condition. We recommend below-grade construction, such as retaining walls,
crawlspace and basement areas, be protected from wetting and hydrostatic pressure buildup by
an underdrain system. An underdrain should not be provided around slab-at-grade garage and
shallow crawlspace areas to help limit potential wetting of bearing soils from shallow water
sources.
The drains should consist of drainpipe placed in the bottom of the wall backfill sunounded above
the invert level with free-draining granular material. The drain should be placed at each level of
excavation and at least I foot below lowest adjacent finish grade and sloped at a minimum 7o/o to
a suitable gravity outlet or sump and pump. Free-draining granular material used in the
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underdrain system should contain less than 2Yo passing the No. 200 sieve, less than 50% passing
the No. 4 sieve and have a maximum size of 2 inches. The drain gravel backfill should be at
least 1lz feet deep. An impervious membrane such as 20 mil PVC should be placed beneath the
drain gravel in a trough shape and attached to the foundation wall with mastic to prevent wetting
of the bearing soils.
SURFACE DRAINAGE
Proper surface grading and drainage will be critical to keeping the bearing soils dry and limiting
building movement. The following drainage precautions should be observed during construction
and maintained at all times after the residence has been completed:
1) Inundation ofthe foundation excavations and underslab areas should be avoided
during construction.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95o/o of the maximum standard Proctor density in pavement and slab areas
and to at least 90Yo of the maximum standard Proctor density in landscape areas.
3) The ground surface surrounding the exterior of the building should be 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 and a minimum slope of
3 inches in the first 10 feet in paved areas. Free-draining wall backfill should be
covered with filter fabric and capped with about 2 feet of the on-site soils to
reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular hear,y inigation should be located at least
5 feet from foundation walls. Consideration should be given to use of xeriscape
to reduce the potential for wetting of soils below the building caused by irrigation.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this area at this time. V/e make no waranty either express or implied
The conclusions and recommendations submitted in this report are based upon the data obtained
from the exploratory boring drilled at the location indicated on Figure 1, the proposed type of
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construction and our experience in the area. Our services do not include determining the
presence, prevention or possibiiity of mold or other biological contaminants (MOBC) developing
in the future. If the client is concerned about MOBC, then a professional in this special field of
practice should be consulted. Our findings include interpolation and extrapolation of the
subsurface conditions identified at the exploratory boring and variations in the subsurface
conditions may not become evident until excavation is perfonned. If conditions encountered
during construction appear different from those described in this report, we should be notified so
that re-evaluation of the recommendations may be made.
This report has been prepared for the exclusive use by our client for design purposes. \ùy'e are not
responsible for technical interpretations by others of our information. As the project evolves, we
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to veriff that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. We recommend on-site observation
of excavations and foundation bearing strata and testing of structural fill by a representative of
the geotechnical engineer.
Respectfully Submitted,
Kutum"*w & .AçsCIciæ{es" truc.
W F. F¿a*<¿l,t*f
James H. Parsons, P.E.
Reviewed by:
Steven L, Pawlak,
JHPlkac
Cc: Kurt Peterson
Kumar & Associates, lnc, ?"Frojeet Nc.21.7-363
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21 -7 -363 Kumar & Associates LOCAÏION OF EXPLORATORY BORING Fig. 1
Eå:
BORING 1
EL. 6f 86'
LEGEND
0
CLAY (CL); SILTY, SANDY, SCATTERED GRAVEL AND COBBLES, SLIGHTLY
CALCAREOUS, VERY STIFF TO HARD, SLIGHTLY MOIST, LIGHT BROWN TO
TAN AND WHITE, LOW PLASTICITY,
20/12
SAND AND CLAY (SC-CL); GRAVEILY, MEDTUM DENSE/HARD, SLTGHTLY
MOIST, TAN AND WHITE.
25/12
WC=8.4
DD= 1 08
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
36/ 12
WC=7.9
DD=117
-200=82
i DR|VE SAMPLE, 1 5/8-|NCH t.D. SPL|T SP00N STANDARD
PINETRAÏION TEST.
10 1e/12
WC=6,5
DD= 1 07
n" rn n DRIVE SAMPLE BLOW COUNT, INDICATES THAT 20 BLOWS 0FLv/ tL A 140-pouND HAMMER FALLTNG J0 lNcHEs WERE RE0U|RED
TO DRIVE THE SAMPLER 12 INCHES.
NOTES
15
3t /12 1 THE EXPLORAÏORY BORING WAS DRILLED ON MAY 13, 2021
WITH A 4-INCH DIAMETER CONTINUOUS FLIGHT POWER AUGER.
2 ÏHE LOCATION OF ÏHE EXPLORATORY BORING WAS MIASURTD
APPROXIMATETY BY PACING FROM FEATURES SHOWN ON THE
SITE PLAN PROVIDED.F-
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5 THE ELEVATION OF ÏHE EXPLORAÏORY BORING WAS OBTAINED
BY INTIRPOLATION BETWEEN CONÏOURS ON THE SITE PLAN
PROVIDID.
20
22/12
4. THE IXPLORATORY BORING LOCAÏION AND ELEVATION SHOULD BE
CONSIDTRID ACCURATE ONLY ÏO THE DEGREE IMPLIID BY THE
METHOD USID.
25
47 /12
5. THE LINIS BETWEEN MATERIALS SHOWN ON THE EXPLORATORY
BORING LOG REPRESENT THE APPROXIMAÏE BOUNDARITS
BETWEEN MATERIAL TYPES AND ÏHE ÏRANSITIONS MAY BE
GRADUAL.
6. GROUNDWAÏER WAS NOÏ ENCOUNTERED IN THE BORING AT THE
TIME OF DRILLING.
30
36/12 7 LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216);
DD = DRY DENSTTY (pcf) (lSrV O ZZ1O);
_2OO = PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D 1140).
35
58/ 12
40
21 -7 -363 Kumar & Associates LOG OF EXPLORATORY BORING Fig. 2
SAMPLE OF: Sondy Cloy
FROM:Boring|@4'
WC = 8.4 %, DD = 108 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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SAMPLE OF: Sondy Cloy
FROM: Boring 1 @ 10'
WC = 6.5 %, DD = 1O7 pc't
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
21 -7 -363 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 5
I(+rtirffiifi'trtffËtr*$Ë;å,*^TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.21-7-363SOIL TYPESandy ClaySandy ClaySandy ClayUNCONFINEDCOMPRESSIVESTRENGTHPLASTICINDEX(%lATTERBERG LIMITS(%lLIQUID LIMI'ÍPERCENTPASSING NO.200 stEVE82SAND(%)GRADATION(%)GRAVELNATURALMOISTURECONTENTNATURALDRYDENSITY108TT7r078.47.96.5(ft)DEPTH4710SAMPLE LOCATIONBORING1