HomeMy WebLinkAboutSubsoil Studyl(trt l(utmr & Assoclates, Inc.6
Geotechnical and Materials Engineers 5020 County Road 154
and Environmental Scientists Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
An Employcc o,wmd compony www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
RECEIVED
,rilt íl rl"¿lll'i.
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED DAVIS RESIDENCE
LOT H21, ASPEN GLEN
75 HORSESHOE LANE
GARFIELD COIAITY, COLORADO
PROJECT NO. 2l-7-123
FEBRUARY 1.1,2021
PREPARED FOR¡
RESORT CONCEPTS
ATTN: RICK HERMES
P. O. BOX 5127
EDWARDS, COLORADO 81632
rickh@reso rtconcentsco.com
GARFIELD COUNTY
COMMUNITY DEVELOPMENT
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION
SITE CONDITIONS.
SUBSIDENCE POTENTIAL
FIELD EXPLORATION .......
SUBSURFACE CONDITIONS ..
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS
FOUNDATIONS
FLOOR SLABS.
T]NDERDRAIN SYSTEM
SURFACE DRAINAGE...
LIMITATIONS
FIGURES 1A and 18 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 . LEGEND AND NOTES
FIGURES 4 through 6 - SWELL-CONSOLIDATION TEST RESULTS
TABLE I - SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc. @ Project No. 20-7-123
PURPOSE AND SCOPE OF'STUDY
This report presents the results of a subsoil study for the proposed Davis residence to be located
on Lot H21, Aspen Glen, 75 Horseshoe Trail, Garfield County, Colorado. The project site is
shown on Figures lA and 18. The purpose of the study was to develop reconìmendations for the
foundation design. The study was conducted in accordance with our agreement for geotechnical
engineering services to Resort Concepts dated January 12,2021.
A ñeld exploration program consisting of exploratory borings 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 analyzed to develop recoÍìmendations for foundation types, depths and allowable
pressgres 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 residence will be located on the lot as shown on Figure lB. The building will be a one and
two story wood frame structure over a partial basement level. Ground floors will be structural
over crawlspace in the living areas and slab-on-grade in the basement and attached garage.
Grading for the structure is assumed to be relatively minor with cut depths between about 2 to I
feet. \Me 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 recoÍìmendations contained in this report.
SITE CONDITIONS
The lot was vacant at the time of the field exploration and the ground surface appeared mostly
natural. The terrain was gently sloping down to the east at grades from about 3 to 6Yo. Elevation
difference across the proposed building foot-print is about 3 feet and across the lot is about 8 to
10 feet. Vegetation consisted of grass and weeds. There is a pond adjacent the east side of the
lot.
Kumar & As¡ociates, lnc. @ Project No.20-7123
1
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvaniaîage Eagle Valley Evaporite underlies the Aspen Glen
development. 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 of localized subsidence. During previous work in the area, several sinkholes were
observed scattered throughout the development, mostþ east of the Roaring Fork River. These
sinkholes appear similar to others associated with the Eagle Valley Evaporite in other areas of
the Roaring Fork River Valley.
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 borings were 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 H21 throughout the service life of the proposed residence, in
our opinion, is low and similar to other nearby platted lots; 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 January 19,2021. Two exploratory
borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions.
The borings were advanced with 4 inch diameter continuous flight augers powered by a truck-
mounted CME-458 drill rig. The borings were logged by a representative of Kumar &
Associates.
Samples of the subsoils were taken with l% inch and 2inchl.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-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 Logs of Exploratory Borings, Figure 2. The samples were retumed to our
laboratory for review by the project engineer and testing.
Kumar & Associates, lnc, @ Projec{ No. 20-7-123
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SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils encountered, below about Yzfootoftopsoil, consisted offrom about 10 to 13 feet of
very stiff, sandy siþ clay underlain by relatively dense, siþ sandy gravel and cobbles with
small boulders that extended down to the maximum drilled depth of 21 feet. Drilling in the
dense coarse granular soils with auger equipment was difficult due to the cobbles and boulders
and drilling refusal was encountered in Boring 1 in the deposit.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and percent finer than sand size gradation analyses. Results of swell-
consolidation testing performed on relatively undisturbed drive samples of the siþ clay soils,
presented on Figures 4 through 6, indicate low to moderate compressibility under conditions of
loading and wetting. Several of the samples showed a low swell potential when wetted under a
constant 1,000 psf surcharge. The laboratory testing is summarizedinTable 1.
No groundwater was encountered in the borings at the time of drilling and the subsoils were
slightly moist.
F'OUNDATION BEARING CONDITIONS
The silty clay soils possess low bearing capacity and may tend to settle or heave some when
wetted. The underlying dense coarse granular soils possess moderate bearing capacity and
relatively low settlement potential. At assumed excavation depths, the subgrade soils are
expected to consist of the siþ clay. Spread footing bearing on these soils should be feasible for
foundation support with some risk of movement. The risk of movement is primarily if the
bearing soils were to become wetted, and precautions should be taken to prevent wetting. Based
on our experience in the area, the siþ clay soils typically do not possess an expansive potential
and the potential for expansion can be neglected in the foundation design but should further be
evaluated at the time of construction. A lower risk of foundation movement would be to bear the
footings entirely on the dense coarse granular soils'
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we believe the building can be founded with spread footings bearing
on the natural soils with some risk of movement.
Kumar & Associates, lnc. @ Project No.20-7-123
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The design and construction criteria presented below should be observed for a spread footing
foundation system,
1) Footings placed on the undisturbed natural soils should be designed for an
allowable bearing pressure of 1,500 psf. Based on experience, we expect
settlement of footings designed and constructed as discussed in this section will
be about I inch or less. There could be some additional movement if the bearing
soils if the bearing soils were to become wetted. The magnitude of the additional
movement would depend on the bearing conditions and depth and extent of the
wetting, but may be on the order of Yzto 1 inch.
2) Footings placed on the undisturbed natural coarse granular soils should be
designed for an allowable bearing pressure of 3,000 psf. Based on experience, we
expect settlement of footings designed and constructed as discussed in this section
will be about I inch or less.
3) The footings should have a minimum width of 16 inches for continuous walls and
2 feet for isolated pads.
4) Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above their bearing elevation for frost protection. Placement
of foundations at least 36 inches below exterior grade is typically used in this
area.
5) Continuous foundation walls should be heavily reinforced top and bottom to span
local anomalies and better withstand the effects of some differential movement
such as by assuming an unsupported length of at least 14 feet. Foundation walls
acting as retaining structures should also be designed to a lateral earth pressure
corresponding to an equivalent fluid unit weight of at least 50 pcf.
6) All existing fill, topsoil and any loose disturbed soils should be removed and the
footing bearing level extended down to the firm natural soils, and the subgrade
should then be moistened and compacted.
7) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade
construction. There could be some slab movement if the silty clay subgrade were to become
wetted. Providing a depth, typically I%to 2 feet, of aggregate base course below floor slabs
Kumar & Associates, lnc. @ Project No.20-7-123
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could be done to reduce the risk of movement. The need for base course below floor slabs
should be further evaluated at the time of construction.
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 inchlayer of free-draining gravel
should be placed immediately beneath basement level slabs to facilitate drainage. This material
should consist of minus 2-inchaggregate with at least 50% retained on the No. 4 sieve and less
than2o/o passing the No. 200 sieve.
All filt 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 fiIl can consist of the on-
site soils devoid of topsoil and oversized (plus 6-inch) rocks, or of CDOT Class 5 or 6 aggregate
base course.
LTNDERDRAIN SYSTEM
Although groundwater was not encountered during our exploration, it has been our experience in
the area and lvhere clay soils are present that local perched groundwater can develop during
times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can also
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 around shallow crawlspace areas (less than 4 feet deep) may not be
needed with adequate compaction of foundation backfill and positive surface slope away from
foundation walls.
The drains should consist of 4 inch diameter PVC drainpipe placed in the bottom of the 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 1 foot below lowest adjacent finish grade and
sloped at a minimum YzYoto a suitable gravity outlet, a sump and pump system or to a properly
constructed drywell. Free-draining granular material used in the underdrain system should
contain less than 2Yo passingthe 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 lYzfeet deep and be
covered by filter fabric such as Mirafi 140N.
Kumar & Associates, lnc, @ Project No.20-7-123
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SURFACE DRAINAGE
Positive surface drainage is a very important aspect of the project to prevent wetting of the
bearing soils. 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 95% 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 recoÍtmend 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.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfïll.
5) Landscaping which requires regular heavy irrigation, such as sod, and lawn
sprinkler heads 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 inigation.
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 borings drilled at the locations 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 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 borings 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 reporto we should be notified so
that re-evaluation of the recommendations may be made.
Kumar & Associates, lnc.6 Project No. 20'7-123
TO HORSESHOE LANE
PROPOSED SITF PI AN
2 20 40
APPROXIMATE SCALE-FEET
21 -7 -123 Kumar & Associates LOCATION OF IXPLORATORY BORINGS Fis. 1B
BORING 1
EL. 6051.5'
BORING 2
EL. 6053.5'
0 0
23/ 12
DD=5.1
-2OO=73
42/ 12
5
15/12
WC=8.0
DD=1 1 2
25/ 12
WC=5.9
DD=1 1 4
5
1s/12 22/12
WC=5.9
DD=1 1 5
'10 10
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DD=113
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15 15
82/12
20 20
82/12
25 25
Fig. 2LOGS OF EXPLORATORY BORINGS21 -7 -123 Kumar & Associates
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TOPSOIL; ORGANIC SILTY CLAY, FIRM, MOIST, DARK BROWN.
CLAY (CL); SILTY TO SILT AND CLAY WITH DEPTH, VERY STIFF, SLIGHTLY MOIST, BROWN
GRAVEL AND COBBLES
MOIST, MIXED BROWN,
(GU); WlrU SMALL BOULDERS, SANDY, SILTY, DENSE, SLIGHTLY
SUBROUNDED TO ROUNDED ROCKS.
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DRIVE SAMPLE, 2*INCH I.D. CALIFORNIA LINER SAMPLE.
DR|VE SAMPLE, 1 3/8*INCH l.D. SPLIT SPOON STANDARD PENETRATION TEST
,z/.c DRTVE SAMPLE BLOW COUNT. |ND|CATES THAT 25 BLoWS OF A 14o-POUND HAMMERL!/ IA FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
I pn¡cncAL AUGER DRILLING REFUSAL.
I
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON JANUARY 19,2021 WITH A 4-INCH-DIAMETER
CONTINUOUS-FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED.
4. THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATE
ONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SIIOWN ON THE EXPLORATORY BORING LOGS REPRESENT ÏHE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER LEVELS SHOWN ON THE LOGS WERE MEASURED AT THE TIME AND UNDER
CONDITIONS INDICATED. FLUCTUATIONS IN THE WATER LEVEL MAY OCCUR WITH TIME.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSITY (PCf) (ASTM D2216):
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D6913);
-200= PERCENTAGE PASSING N0. 200 SIEVE (ASTM Dl140).
21 -7 -123 Kumar & Associates LEGEND AND NOTTS Fig. 3
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SAMPLE OF: Sondy Silty Cloy
FROM:Boringl@4'
WC = 8.0 %, Dù = 112 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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21-7-123 Kumar & Associates SWTLL_CONSOLIDATION TEST RESULTS Fig. 4
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SAMPLE OF: Sondy Silty ond Cloy
FROM:Boringl@10'
WC = 5.6 %, DD = 115 pcf
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1.0 PRESSURE - KSF t0 100
21-7-123 Kumar & Associates SWTLL_CONSOLIDATION TEST RESULTS Fig. 5
SAMPLE OF: Sondy Silly Cloy
FROM:Boring2@4'
WC = 5.9 %, DD = 114 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
(
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t,0 PRESSURE - KSF 10
PRESSURE - KSF
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SAMPLE OF: Sondy Silty Cloy
FROM:Borlng29^7'
WC = 5,9 %, DD = 115 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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emplð tótad, lhr tËtìng r6pod
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21 -7 -123 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 6
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lftrtiåtr'¡.['ffiî:ffi1,'rÊü*.*TABLE 1SUMMARY OF LABORATORY TEST RESULTSSandy Silty ClaySandy Silty ClaySandy Siþ ClaySandy Silty ClaySandy Silty ClaySOIL TYPE(osfìUNCONFINEDCOMPRESSI\ÆSTRENGTHLIQUID LIilFPI-ASTICINDEXATTERBERG LIMITSPERCEI{TPASSn{G NO.200 stEvE73(/,\SAND(Y"lGRA\IELGRADATION113tt41r5locf)}IAfURALDRYDEilSTY5.1t125.95.98.03.6(oltlNATURALMOISTURÊCONTENT471410ffr)DEPTH2BORING1SAMPLE LOCATIONNo.21-7-123