HomeMy WebLinkAboutSubsoils Report for Foundation DesignlGrtffi,ffiffi1r':F'i'*"
An Employca otmcd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970)94s-8454
ernai I : kaglenwood@kumarusa.com
www.kumarusa.coul
Offrce Locations: Denver (HQ), Parker, Colorado Springs, Fort Collirrs, Glenwood Springs, and Summit Counfy, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
TRACT 28, ANTLERS ORCHARI)
EAST OF COUNTY ROAD 237 (HARVEY GAp ROAD)
GARFTELD COUNTY, COLORADO
PROJECT NO.20-7-190
AUGUST 7,2020
PREPARED FOR:
GILBERTO MONTENEZ
C/O JESUS MONTENEZ
719 BURNING MOUNTAIN AVENUE
NEW CASTLE' COLORADO 81647
tonymontenez 1 60 I @gmail . com
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TABLE OF CONTDNTS
PURPOSE AND SCOPE OF STUDY ..
PROPOSED CONSTRUCTION
SITE CONDITIONS..
FIELD EXPLORATION .....
SUBSURFACE CONDITIONS
FOI.INDATION BEARING CONDITIONS ...
DESIGN RECOMMENDATIONS ....................
FOUNDATIONS
FOTINDATION AND RETAINING WALLS
FLOOR SLABS
UNDERDRAIN SYSTEM...
SURFACE DRAINAGE......
LIMITATIONS
FIGURE I - LOCATION OF EXPLORATORY BORINGS
FIGURE 2. LOGS OF EXPLORATORY BORINGS
FIGURE 3 . LEGEND AND NOTES
FIGURES 4 & 5 - SWELL.CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESI-TI,TS
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2-
5
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4
5
5
6
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Kumar & Associates, lnc. a Project No.20-7.190
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FIELD EXPLORATION
The field exploration for the project was conducted on June 17, 2020. 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 auger powered by a truck-
mounted CME-45B drill rig. The borings were logged by a representative of Kumar &
Associates, Inc.
Samples of the subsoils were taken with 1% 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-i586.
The penetration resistance values are an indication of the relative density or consistency of the
subsoils and hardness of the bedrock. 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 returned to our laboratory for review by the project gngineer and testing.
SUBSURFACE CONDITIONS
Graphfc logs of the subsurface profiles encountered at the site are shown on Figure 2. Below a
thin organic topsoil layer, the subsoils consist of very stiff to hard sandy clay down to depths of 3
to 6 feet. Below the clay, weathered to very hard siltstone bedrock was encountered down to the
maximum depth explored, 2l feet.
Laboratory testing performed on samples obtained during the field exploration included natural
moisture content and density. Swell-consolidation testing was performed on relatively
undisturbed drive samples of the clay subsoils and underlying siltstone. The swell-consolidation
test results for the clay soils, presented on Figure 4, indicate low compressibility under relatively
light surcharge loading and a high expansion potential when wetted under a constant light
surcharge. Swell-consolidation test results for the weathered siltstone, presented on Figure 5,
indicate low compressibility under relatively light surcharge loading and moderate
compressibility under increased loading after wetting. The laboratory testing is summarizedin
Table 1.
No free water was encountered in the borings at time of drilling. The subsoils and siltstone were
slightly moist.
FOUNDATION BEARING CONDITIONS
The clay soils encountered at the site possess high expansion potential when wetted. Surface
runoff, landscape irrigation, and utility leakage are possible sources of water which could cause
Kumar & Associates, lnc. @ Project No,20.7.190
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on Tract
28, Antlers Orohard, oast of Harvey Cap Road, Garfield County, Colorado. Thc projcct sitc is
shown on Figure 1. The purpose of the study was to develop recommendations for foundation
design. The study was conducted in accordance with our agteement for geotechnical engineering
services to Gilberto Montenoz, dated March 17,2020.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils and bedrock obtained during
the field exploration were tested in the laboratory to determine their classification,
compressihility 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, recofirmendations
and other geotechnical engineering considerations based on the proposed construction and the
subsurface conditions encountered.
PROPOSED CONSTRUCTION
At the time of our study, design plans for the residence had not been developed. The building is
proposed in the area roughly between exploratory boring locations shown on Figure 1 or,
possibly, just to the east of there. We assume excavation for the building will have cut depths of
about 3 to 6 feet below the existing ground surface. For the purpose of our analysis, foundation
loadings for the structure were assumed to be relatively light and typical of the proposed type of
construction.
If building loadings, location or grading plans arc significantly different from those described
above, we should be notified to re-evaluate the recorrunendations contained in this report.
SITE CONDITIONS
The site is vacant and appears to be a former pasture and vegetated with grass and weeds with
some trees. Cobbles were observed scattered on the ground surface. The lot slopes moderately
down to the north at grades of 5 to 10 percent.
Kumar & Associates, inc, d Project No, 20-7-190
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wetting. The expansion potential can't be mitigated by load concentration of the light residential
construction and the clay soils should be removed from below footing and slab-on-grade areas.
Spread footing foundations can be placed on the underlying siltstone bedrock to avoid potential
heave of the clay soils. Recommendations for design of spread footing foundations placed on the
siltstone bedrock are presented below.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the residence be founded with spread footings placed
on undisturbed siltstone bedrock. All sandy clay soils should be removed from below footing
and slab-on-grade areas to expose the siltstone bedrock. A limited depth of imported structural
fill, such as 3/a-inch road base, can be placed over the siltstone bedrock, if needed.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed siltstone bedrock or up to 2 feet of compacted
structural fill can be designed for an allowable bearing pressure of 3,000 psf. The
depth of structural fill below footings should be limited to about 2 feet. Structural
filI should consist of 3/e-inch road base compacted to at least 98% of the maximum
standard Proctor density.
2) Based on experience, we expect settlement or heave of footings designed and
constructed as discussed in this section will be up to about 1 inch. There could be
some additional movement if the bearing soils were to become wet.
3) The footings should have a minimum width of 16 inches for continuous footings
and24 inches for isolated pads.
4) Continuous foundation walls should be reinforced top and bottom to span local
anomalies and limit the risk of differential movement. One method of analysis is
to design the foundation wall to span an unsupported length of at least 10 feet.
Foundation walls acting as retaining structures should also be designed to resist a
lateral earth pressure as discussed in the "Foundation and Retaining Walls"
section of this report.
5) Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above their bearing elevation for frost protection. Placement
of foundations at least 36 inches below the exterior grade is typically used in this
area.
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Prior to the footing construction, any existing clay, topsoil and loose or disturbed
soils should be removed and the footing bearing level extended down to the
siltstr.rne bedrock.
A represcul.ative uf the geoteuhnical engineer should observe all footing
excavations prior to concrete placement to evaluate bcaring conditions.
FOTINDATION AND RETATNING WALLS
Foundation walls and retaining strucfures 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 45 pcf for backfill consisting
of the imported granular soils or on-site well-broken weathered bedrook. 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 tbr a lateral earth pressure
computed on the basis of an equivalent fluid unit weight of at least 35 pcf for backfill consisting
of the on-site well-broken siltstone or imported granular soils and at least 50 pcf for backfill
consisting of on-site clay 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 retaining walls.
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density at a moisture content near optimum. Backfill in pavement 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 correctly and could result in distress to facilities
constructed on the backfill.
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 0.40. Passive pressure of compacted backfill against the
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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
the sides of the footings to resist lateral loads should be a non-expansive material compacted to
at least 950/o of the maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The on-site clay soils possess an expansion potential and slab heave could occur if the subgrade
soils were to become wet. Slab-on-grade construction may be used provided precautions are
taken to limit potential movement and the risk of distress to the building is accepted by the
owner. A positive way to reduce the risk of slab movement, which is commonly used in the
area, is to construct structurally supported floors over crawlspace. As an alternative, all clay
soils should be removed from below slab-on-grade areas and replaced with compacted structural
filI.
A minimum 4 inch layer of free-draining gravel should be placed immediately beneath basement
level slabs-on-grade. This material should consist of minus 2-inch aggregate with less than 50oh
passing the No. 4 sieve and less than2o/o passing the No. 200 sieve. The free-draining gravel
will aid in drainage below the slabs and should be connected to the perimeter underdrain system.
Required fill beneath slabs can consist of a suitable imported granular material such as % inch
road base, excluding topsoil and oversized rocks. The fill should be spread in thin horizontal
lifts, adjusted to at or above optimum moisture content, and compacted to at least 95o/o of the
maximum standard Proctor density. All vegetation, topsoil and clay soils should be removed
prior to fill placement.
LINDERDRAIN SYSTEM
Although gtoundwater was not encountered during our exploration, it has been our experience in
mountainous areas and where bedrock is shallow, that local perched gtoundwater can develop
during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can
also create a perched condition. Therefore, we reconrmend below-grade construction, such as
crawlspace and basement areas, be protected from wetting by an underdrain system. The drain
should also act to prevent buildup of hydrostatic pressures behind foundation walls.
The underdrain system should consist of a drainpipe surrounded by free-draining gtanular
material placed at the bottom of the wall backfill. The drain lines should be placed at each level
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of excavation and at least I foot below lowest adjacent finish grade, and sloped at a minimum
l%o gtade to a suitable gravity outlet. Free-draining granular material used in the drain system
should consist of minus 2-inch aggregate with less than 50Yo passing the No. 4 sieve and less
than 2o./o passing the Nu. 200 sieve. The drain gravel should be at least 17i feet deep.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the residence has been completed:
1) All clay soils should be removed from within the building area below footing and
slabs-on-gradc. The clay soils can be left in place below crawlspace areas where
there are no footings or slabs.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95% of the maximum standard Proctor density in pavement areas and to at
least 90% of the maximum standard Proctor density in landscape areas. Free-
draining wall backfill (if any) should be capped with about 2 to 3 feet of the on-
site soils to reduce surface water infiltration.
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.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irrigation should be located at least
5 feet from foundation walls. Consideration should be given to use of xeriscape
to reduce the potential tbr 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
frotn 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 concemed about MOBC, then a professional in this special field of
practice should be consulted. Our findings include interpolation and extrapolation of the
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APPROXIMATE SCALI-FEET
2A-7 -19A Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
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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 to be different from those described in this report, we should be
notificd al. ttnce stt re-evaluation of the recoruuendatious may tre nrade.
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 constructinn to re;view and
monitor the implementation of our recommendations, and to verifu that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications of 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 geoteclurical engineer.
Respectfully Submitted,
Kumar & Associates, fnc.
Daniel E. Hardin, P.E.
Reviewed by:
Steven L. Pawlak, P.E
DEH/kac
Kumar & Associates, lnc. e Project No.20.7-190
SAMPLE OF: Weoihered Sillslone
FROM: Boring 2 @ 10'
WC = 6.3 %, DD = 117 pcf
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ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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1.0 APPLIED PRESSU - KSF 10 100
20-7 -190 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 5
SAMPLE OF: Sondy Cloy
FROM:Boringl@2.5
WC = 7.3 %, DD = 1ZS pcl
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EXPANS]ON UNDER CONSTANT
PRESSURE UPON WETTING
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t.0 PRESSURE -l0 100
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EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
3
2
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APPLIED - KSF t0
SAMPLE OF: Sondy Cloy
FROM:Boring2@5'
WC = 5.3 %, DD = 130 pcf
ont to
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20-7 -190 Kumar & Associates SWELL-CONSOLIDATION TEST RTSULTS Fig. 4
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LEGEND
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TOPSOIL; SILT, SAND, CLAY,
BROWN TO TAN.
ROOTS AND ORGANIC MATERIAL, DRY TO SLIGHTLY MOIST, LIGHT
CLAY (CL-CH); SANOY, VERY STIFF TO HARD, SLIGHTLY MOIST, LIGHT BROWN AND GRAY.
ffi wearu.RED stLTSToNE; MEDTUM HARD To HARD,
'LTGHTL'
Morsr, cRAy wrrH BRowN
YI
SILTSTONE BEDROCK; HARD TO VERY HARD, SLIGHTLY MOIST, GRAY
DRIVE SAMPLE, 2_INCH I.D. CALIFORNIA LINER SAMPLE
DRIVE SAMPLE, 1 3/8_INCH I.D. SPLIT SPOON STANDARD PENETRATION TESTi
37/12 ?RIVE SAIVIPLE BLOW COUNT. |ND|CATES THAT J7 BLOWS oF, FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER
--> DEPTH AT WHICH BORING CAVED.
4O-POUND HAMMER
INCHES.
(-) upoN WETT|NG UNDER CONSTANT LOAD
A1
12
NAIES
2.
3.
4.
5.
THE EXPLORATORY BORINGS WERE DRILLED ON JUNE 17,2020 WITH A 4-INCH_DIAMETERCONTINUOUS-FLIGHT POWER AUGER.
THE LOCATIONS OF THE EXPLORATORY BORINGS WERE LOCATED BY THE CLIENT.
THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE MEASURED BY HAND LEVEL AND REFERTO BORING 1 AS ELEVATION 1OO' ASSUMED.
THE EXPTORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATEONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING IOGS REPRESENT THEAPPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND iHE TRANSITIoNS MAY BE GRADUAL.
GROUNDWAIER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING.6
7 LABORATORY TEST RESULTS:wc = wATER CONTENT (%) (ASTM D2216);DD = DRY DENSTTY (pcr) (nsru D2216);sc = pERCENT SWELL (+) OR CONSOLTDATTON
(ASTM D 4s46, METHoD B).
20-7 -190 Kumar & Associates LEGEND AND NOTES Fig. 3
BORING 1
El.100'BORING 2
EL. 95.4'
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23/ 12
37/12
WC-7.3
DD= 1 23
SC=*3.3
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40/6, sO/4.5
WC=4.5
DD= 1 32
43/ 12
WC=5.3
DD= 1 30
SC=+4.8
q
so/3
WC=6.2
DD=1 15
5a/5
WC=6.5
DO=117
SC=--0.7
0
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snlo s 550/?
so/3 50/2.s 20 - ,1.
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20-7 -190 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2