HomeMy WebLinkAboutSubsoil Studyrcrt Kumar&Associates,lnc. 5020 County Road 154
Geotechnical and Materials Engineers Glenwood Springs, CO 91601
and Environmentat scientists pnone: 1o7o¡ oas-zsaa
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
Ân Employee O\ryned Compcny wwwkumarusa'com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 77, FILTNG 2, PINYON MESA
PINYON MESA DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO.21-7-665
ocToBER lt,202l
PREPARED FOR:
CAMILLA LANGENFELD
10532 HILLROSE STREET
PARr(ER, COLORADO 80134
(çiamilla.langen feld@smail.com)
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION
SITE CONDITIONS.
SUBSIDENCE POTENTIAL
FIELD EXPLORATION
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS .
DESIGN RECOMMENDATIONS
FOUNDATIONS
FOUNDATION AND RETAINING WALLS
FLOOR SLABS
LIMITATIONS
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURES 3 and 4 - SWELL-CONSOLIDATION TEST RESULTS
FIGURE 5 _ GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc.Project No. 21-7-665
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot77, Filing 2, 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 Carnilla Langenfeld, dated August 12,2021.
A field exploration program consisting of an exploratory boring was conducted 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 analyzedto 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
The proposed residence will be a single-story structure over a walkout basement with an attached
garage and located on the site as shown on Figure 1. Garage and basement floors will be slab-
on-grade. Grading for the structure is assumed to be relatively minor with cut depths between
about 3 to 10 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 repoft.
SITE CONDITIONS
The subject site was vacant at the time of our field exploration. The ground surface slopes down
to the northwest with a fairly consistent grade of 12 to 150lo across the lot. Elevation difference
across the building area is about 8 feet. Vegetation consists of grass and weeds with sage brush
in about the back half of the lot.
Kumar & Associates, lnc.Project No. 21-7-665
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SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the subject site. These rocks
are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some massive beds
of gypsum and limestone. There is a possibility that massive gypsum deposits associated with
the Eagle Valley Evaporite underlie portions of the lot. Dissolution of the gypsum under certain
conditions can cause sinkholes to develop and can produce areas oflocalized subsidence.
During previous work in the area, sinkholes have been observed scattered throughout the lower
Roaring Fork Valley. These sinkholes appear similar to others associated with the Eagle Valley
Evaporite in this area.
Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities
was 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 on Lot 77 throughout the service life of the proposed residence, in our
opinion, is low; however, the owner should be made 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 August 3I,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.
Samples of the subsoils were taken with I% inch and 2-inch I.D. 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-l586.
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.
Kumar & Associates, lnc.Project No.2l-7-665
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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 ofabout l0 feet ofstiff, sandy silt and clay overlying very stiffto hard, sandy
clay with scattered gravel to a depth of 22 feef. Silty, slightly clayey sand with gravel was
encountered from22 feet to the maximum drilled depth of 46 feet.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and density and finer than sand size gradation analyses. Results of swell-consolidation
testing performed on relatively undisturbed drive samples of the upper soils, presented on
Figures 3 and 4, showed high compressibility (sample at2Yz feet) and low to moderate
compressibility with expansion potential (sample at 10 feet) under conditions of loading and
wetting. Results of gradation analysis performed on the underlying coarse granular soils are
shown on Figure 5. The laboratory testing is summarizedin 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 sandy silt and clay soils within about the upper 10 feet are low density and moderately to
highly compressible when wetted under load. The underlying sandy clay soils exhibit relatively
low compressibility under light loading and low expansion potential when wetted. At assumed
excavation depths we expect the subgrade will be mainly sandy silt and clay soils and transition
into sandy clay soils at basement depth. Excavations of less than about 7 feet below existing
ground surface should be sub-excavated as needed to provide at least 3 feet ofcompacted
structural fill below design bearing level. Spread footings placed on compacted structural fill or
the deeper sandy clay soils should be feasible for foundation support of the residence with a risk
of differential movement due to variable bearing conditions. A low settlement risk option would
be to extend the foundation bearing level down to natural granular soils encountered at 22 feet
with a deep foundation system such as micro-piles.
DESIGN RECOMMENDATIONS
FOUNDATIONS
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
Kumar & Associates, lnc.Project No.21-7'665
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on at least 3 feet of compacted structural fill or the deeper sandy clay soils. If a lower settlement
risk, deep foundation system is desired, we should be contacted to provide design
recomrnendations.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural sandy clay soils or structural fill
should be designed for an allowable bearing pressure of 1,500 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 with about t/zto I inch of
additional differential movement if the bearing soils are wetted.
2) The footings should have a minimum width of 20 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.
5) The topsoil, low density material (upper 5 to I feet) and any loose or disturbed
soils should be removed and the footing bearing level extended down to the firm
natural soils. The exposed soils in footing area should then be moistened and
compacted. Structural fill placed below footing areas can consist of the onsite
soils or imported 34-inchroad base compacted to at least 98% of standard Proctor
density at near optimum moisture content and extend to at least Ilz feet beyond
the footing edges.
6) A representative of the geotechnical engineer should perform compaction testing
on structural fill during placement and observe all footing excavations prior to
concrete placement to evaluate bearing conditions.
Kumar & Associates, lnc.Project No. 21-7-665
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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 lateral 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 up\Ã/ard 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 90Yo of the maximum
standard Proctor density at a moisture content near optimum. Backfill placed in pavernent and
walkway areas should be compacted to at least 95o/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 conectly, 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 fiiction of 0.35. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 350 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
Kumar & Associates, lnc.Project No. 21-7-665
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the sides of the footings to resist lateral loads should be compacted to at least 95o/o 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 risk of settlement similar to footings described above. The clay soils at
basement level should be evaluated for expansion potential and the need for sub-excavation and
replacement with structural fill for floor slab heave mitigation. 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 50o/o retained on the No. 4 sieve and less than 2Yo passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95Yo of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the
onsite soils devoid of vegetation, topsoil and oversized rock.
LINDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
the area and where there are clay soils 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 and
basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain
system. An underdrain should not be placed around slab-at-grade garage and shallow crawlspace
areas to help limit the potential for wetting the bearing soils.
The drains should consist of drainpipe placed in the bottom ofthe wall backfill surrounded 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 l%o to
a suitable gravity outlet. Free-draining granular material used in the underdrain system should
contain less than 2Yo passing the No. 200 sieve, less than 50% passing the No. 4 sieve and have a
Kumar & Associates, lnc.Project No. 21-7-665
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maximum size of 2 inches. The drain gravel backfill should be at least lYzfeet 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
potential differential foundation settlements. 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 95Yo 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 heavy inigation should be located at least
10 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. We make no warranty 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
construction and our experience in the area. Our services do not include determining the
presence, prevention or possibility of mold or other biological contaminants (MOBC) developing
in the future. If the client is concemed about MOBC, then a professional in this special field of
Kumar & Associates, lnc.Project No. 21-7-665
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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 performed. 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. We 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 veriS' 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,
Kue*:*r & Á"sç*eisåes, Iæe"
Steven L. Pawlak, P
Reviewed by:
Daniel E. Hardin, P.E.
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BORING 1
EL. 61 91'LEGEND
0 TOPSO|L; ORGANIC SANDY SILT AND CLAY, SIGHTLY MOIST, BROWN.
7 /12
WC=4.9
DD=97 SILT AND CLAY
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CLAY (CL); S|LTY, SANDY, SCATTERED CRAVEL, VERY STTFF T0 HARD,
SLIGHTLY MOIST, MIXED BROWN, CALCAREOUS & GYPSUM CRYSTALS.
sAND (SM); STLTY, SLTGHTLY CLAYEY, SCATTERED GRAVEL T0 GRAVELLY,
MEDIUM DENSE, SLIGHTLY MOIST, MIXED GRAY-BROWN.
10 28/12
WC= 1 0.6
DD= I 04 !
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DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
DRTVE SAMPLE, 1 3/8-|NCH t.D. SpLrT Sp00N STANDARD PENETRATTON
fEST.
15 7712DR|VE SAMPLE BLOW COUNT. INDICATES THAT 7 BLOWS 0F A
"'-14o-pouND HAMMER FALLINc J0 tNcHES WERE REQUTRED To DRtvE THE
SAMPLER 12 INCHES.
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20 NOTES
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WC=6.7
DD= 1 07
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THE EXPLORATORY BORING WAS DRILLED ON AUGUST 31, 2021 WITH A
4-INCH DIAMETER CONTINUOUS FLIGHT POWER AUGER,
2, THE LOCATION OF THE EXPLORATORY BORING WAS MEASURED
APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE SITE PLAN
PROVIDED.
25 5. THE ELEVATION OF THE EXPLORATORY BORING WAS OBTAINED BY
INTERPOLATION BTTWEEN CONTOURS ON THE SITE PLAN PROVIDED.
2e/12
WC=2.8
+4=32
-2OO=25 4, THE EXPLORATORY BORING LOCATION AND ELEVATION SHOULD BE
CONSIDERED ACCURATE ONLY TO THE DEGREE IMPLIED BY THE METHOD
USED.
50 5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING
LOG REPRESENT THE APPROXIMATE BOUNDARIES BETWEEN MATERIAL
ÏYPES AND THE TRANSITIONS MAY BE GRADUAL.
35/ 12
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORING AT THE TIME OF
DRILLING.
zÊ 7, LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216);
DD = DRY DENSITY (PCT) (ISTU D 2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D 6913);
-200 = PERCENIAGT PASSING N0. 200 SIEVE (ASTM D 1140).
40
45
51 /6
21 -7 -665 Kumar & Associates LOG OF EXPLORATORY BORING Fig. 2
SAMPLE OF: Sondy Silt ond Cloy
FROM:Boringl@2.5'
WC = 4.9 %, DD = 97 pcf
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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21 -7 -665 Kumar & Associates SWELL_CONSOLIDATION TEST RESULTS Fig. 3
SAMPLE OF: Sondy Silty Cloy
FROM: Boring 1 @ 10'
WC = 10.6 %, DD = 104 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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SIEVE ANALYSISHYDROMEIER ÀNALYSIS
TIME RÊADINGS
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DIAMETER OF
CLAY TO SILT COBBLES
GRAVEL 32 % SAND
LIQUID LIMIT
SAMPLE OF: Silty Sond with Grovel
43%
PLASTICITY INDEX
SILT AND CLAY 25 %
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21 -7 -665 Kumar & Associates GRADATION TEST RESULTS Fig. 5
l(t I iå'p3,ffiiffËtrf,YÊü,**TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.21-7-665SOIL TYPELIQUID LIMITLIMITSUNCONFINEDCOMPRESS¡VESTRENGTHPERCENTPASSING NO.200 sIEVEPLASTICINDEXSandy Silt and ClaySandy Silt and ClaySandy Silty ClaySandy Silt and Clay withGypsum CrystalsSilty Sand with Gravel877325SAND(%)43GRADATION(/")GRAVELJ¿NATURALDRYDENSITYlpcf)974.910st04r0710.66.72.8(%)NATURALMOISTURECONTENT8.6SAMPLE LOCATION12%2552010DEPTHBORING