HomeMy WebLinkAboutSubsoil Study for Foundation Design 05.11.2021rclt $;,ffilfi#ir'å'Ëtrr :iÍå'
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Ân Employcc Owncd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
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Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STTJDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 2, DI.INLAP MINOR SUBDIVISION
JE\ilELL LANE (COTJNTY ROAD 2s9)
GARFTELD COT.INTY, COLORADO
PROJECT NO.2l-7-222
MAY tt,202t
PRE,PARED FOR
JOSE GONZALES
P.O.B,O){2s44
GLEN\ilOOD SPRINGS, COLORADO 81602
dspincorporated@gmail.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION ...
SITE CONDITIONS
FIELD EXPLORATION...
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS ..................,.
FOTINDATIONS
FOLTNDATION AND RETAINING WALLS
FLOOR SLABS
UNDERDRAIN SYSTEM .............
SURFACE DR4IN4G8...............
LIMITATIONS
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc. @ Project No.21-7-222
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located onLot 2,
Dunlap Minor Subdivision, east of Jewell Lane, Garfield County, Colorado. The project site 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 agreement for geotechnical engineering
services to Jose Gonzales, dated February 22,202I.
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,
compressibility or swell and other engineering characteristics. 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 foundation. This report
summarizes the data obtained during this study and presents our conclusions, recommendations
and other geotechnical engineering considerations based on the proposed construction and the
subsurface conditions encountered.
PROPOSED CONSTRUCTION
The residence is proposed in the area of Boring I as shown on Figure 1. The house will be about
2,400 square feet and will be one story wood frame construction over a crawlspace. We assume
excavation for the building will have a maximum cut depth of about 3 to 5 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.
The proposed garage will be in the area of Boring2. We assume the garage will be one-story
wood frame structure with a slab-on-grade floor.
SITE CONDITIONS
The site is vacant pasture and is vegetated with grass and weeds. The site has had minor grading
up to 2 feet of cut/fill to flatten building areas. The site slopes generally down to the east with a
small hill immediately west of the building area.
Kumar & Associates, lnc. @ Project No.21-7-222
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FIELD EXPLORATION
The field exploration for the project was conducted on March 26,202I. Two exploratory
borings were drilled at the locations shown on Figure I to evaluate the subsurface conditions.
The borings were advanced with 4 inch diameter continuous flight auger powered by a truck-
mounted CME-458 drill rig. The borings were logged by a representative of Kumar &
Associates, Inc.
Samples of the subsoils were taken with a 2 inch I.D. spoon sampler. The sampler was 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 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 engineer and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface prof,rles encountered at the site are shown on Figure 2. Below
about I foot of organic topsoil, the subsoils in the proposed house area (Boring 1) consist of
about 2 feet of very stiff sandy clayey silt overlying2 feet of weathered siltstone bedrock which
transitions to very hard sandstone bedro ck at 51/z feet deep down to the bottom of Boring I at
1 1 feet.
In the proposed garage area (Boring 2), below I foot of topsoil, the subsoils consist of 3 feet of
sandy silty clay overlying 2 feet of weathered claystone bedrock. At a depth of about 6 feet in
Boring 1, claystone bedrock was encountered down to the bottom of the boring at 10 feet.
Laboratory testing performed on samples obtained during the field exploration included natural
moisture content, density and percent finer than sand size analyses. Swell-consolidation testing
was performed on relatively undisturbed drive samples of the clay and claystone from Boring 2.
The swell-consolidation test results, presented on Figure 4, indicate low compressibility under
relatively light surcharge loading and a low to moderate expansion potential when wetted under a
constant light surcharge. The laboratory testing is summarized in Table 1.
Kumar & Associates, lnc, @ Project No.21-7-222
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No free water was encountered in the borings at time of drilling. The subsoils were slightly
moist to moist.
FOUNDATION BEARING CONDITIONS
The subsoils/bedrock materials encountered at the garage site (Boring 2) possess moderate to
high expansion potential when wetted. The expansion potential can probably be partly mitigated
by sub-excavation and replacement with structural fill and load concentration to reduce or
prevent swelling in the event of wetting below the foundation bearing level. Surface runofi
landscape irrigation, and utility leakage are possible sources of water which could cause wetting.
DESIGN RECOMMENDATIONS
FOIINDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the buildings be founded with spread footings placed
on undisturbed natural soils or bedrock or structural fill compacted to at least 98Yo of standard
Proctor density at near optimum moisture content.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural soils/bedrock can be designed for an
allowable bearing pressure of 4,000 psf. The footings should also be designed for
a minimum dead load pressure of 1,000 psf. In order to satisfy the minimum dead
load pressure under lightly loaded areas, it may be necessary to concentrate loads
by using a grade beam and pad system. 'Wall-on-grade construction is not
recommended at this site to achieve the minimum dead load.
2) Based on experience, we expect initial settlement of footings designed and
constructed as discussed in this section will be up to about 1 inch. There could be
additional movement of around I to 2 inches if the expansive 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
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to design the foundation wall to span an unsupported length of at least 14 feet.
Foundation walls acting as retaining strucfures should also be designed to resist a
Iateral earth pressure as discussed in the "Foundation and Retaining Vy'alls"
section of this report.
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 finish grade is typically used
in this area.
Prior to the footing construction, any existing fill, topsoil and loose or disturbed
soils should be removed and the footing bearing level extended down to
competent bearing soils.
A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
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 and at least 45 pcf for backf,rll consisting of imported granular
materials. 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 and at least 3 5 pcf for
backfill consisting of imported granular materials.
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.
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Kumar & Associates, lnc. @ Project No.21-7-222
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Backfill should be placed in uniform lifts and compacted to at least 90o/o of the maximum
standard Proctor density at a moisture content slightly above 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 backf,tll.
'We recommend imported granular soils for backfilling foundation walls and retaining structures
because their use results in lower lateral earth pressures. Imported granular wall backfill should
contain less than 15% passing the No. 200 sieve and have a maximum size of 5 inches. Granular
materials should be placed up to within2 feet of the ground surface and to a minimum of 3 feet
beyond the walls. The upper 2 feet of the wall backfill should be a relatively impervious on-site
soil or a pavement structure should be provided to prevent surface water infiltration into 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 0.30. 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
the sides of the footings to resist lateral loads should be a nonexpansive material compacted to at
least95o/o of the maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The on-site 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.
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To reduce the effects of some differential movement, nonstructural floor slabs should be
separated from all bearing walls and columns with expansion joints which allow unrestrained
vertical movement. Interior non-bearing partitions resting on floor slabs should be provided with
a slip joint at the bottom of the wall so that, if the slab moves, the movement cannot be
transmitted to the upper structure. This detail is also important for wallboards, stairways and
door frames. Slip joints which will allow at least Ilz-inches of vertical movement are
recommended. Floor slab control joints should be used to reduce damage due to shrinkage
cracking. Slab reinforcement and control joints should be established by the designer based on
experience and the intended slab use.
A minimum 4 inch layer of road base gravel should be placed immediately beneath slabs-on-
grade. This material should consist of minus 2-inch aggregate with less than 50o/o passing the
No. 4 sieve and less than I2o/o passing the No. 200 sieve.
Required fill beneath slabs can consist of a suitable imported granular material, 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 loose or disturbed soil should be removed prior to fill
placement.
The above recommendations will not prevent slab heave if the expansive soils underlying slabs-
on-grade become wet. However, the recommendations will reduce the effects if slab heave
occurs. All plumbing lines should be pressure tested before backfilling to help reduce the
potential for wetting.
UNDERDRAIN SYSTEM
Although groundwater was not encountered during our exploration, it has been our experience in
mountainous areas and where clay soils are present and bedrock is shallow that local perched
groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground
during spring runoff can create a perched condition. Therefore, we recommend below-grade
construction, such as crawlspace areas, be protected from wetting by an underdrain system. The
drain should also act to prevent buildup of hydrostatic pressures behind foundation walls.
Kumar & Associates, lnc. @ Project No.2'l-7-222
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The underdrain system should consist of a drainpipe surrounded by free-draining granular
material placed at the bottom of the wall backfill. The drain lines should be placed at each level
of excavation and at least I foot below lowest adjacent finish grade, and sloped at a minimum
Io/o grade to a suitable gravity outlet. Free-draining granular material used in the drain system
should consist of minus 2-inch aggregate with less than 50%o passing the No. 4 sieve and less
than2o/o passing the No. 200 sieve. The drain gravel should be at least llz feet deep. Void form
below the foundation can act as a conduit for water flow. An impervious liner such as 20 mil
PVC should be placed below the drain gravel in a trough shape and attached to the foundation
wall above the void form with mastic to keep drain water from flowing beneath the wall and to
other areas of the building.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the residence and garage have been completed:
1) Excessive wetting or drying of the foundation excavations and underslab areas
should be avoided during construction. Drying could increase the expansion
potential of the soils.
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 areas and to at
least90o/o of the maximum standard Proctor density in landscape areas. Free-
draining wall backfill 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 for wetting of soils below the building caused by inigation.
Kumar & Associates, lnc. o Project No.21-7-222
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LIMITATIONS
This study has been conclucted in acoordance with generally accepted geotechnical engineering
principles and practices in tiris area at this time, We make no warranty either express or implied.
The conclusions and recoÍtmendations submitted in this report are based upon the data obtained
from the exploratory borings drilled /pits excavated at the locations indicated on Figure 1, the
proposecl type of construction and our experience in the area. Our senices do not include
detennining the presence. prevention or possibiiity cf mold or other biological contaminants
{MOBC) developing iir tire future. If the client is concerned about MOBC, then a professionai in
this special field of praciice shouid be consulted. Our findings include interpolation and
extrapoiation of the subsurface conditions identified at the exploratory borings and variations in
the subsurface conctitions may not become evident until excavation is performed. If conditions
encountered during construction appeff to be different from those described in this report.'*,e
should be notifiecl at once sc re-evaluation of the recorumendations may be made.
This repoft has been prepared for the exclusive use by our client for design purposes. \\¡e are not
responsibie for technical interpretations by others of our information. As the proìect errolves" we
should provide continued consultation and field sen'ices during construction to tevierv and
monitor the implementation of our reconmendations, and to verifu that the recommendations
have been appropriately inteipreted. Signifìcant design c,hanges rnay require additionai anal-vsis
or modifications of the recommendations presented herein. We recommend on-site observation
of excavations and founclation bearing strata and testing of structural fìll iry a represeutative of
the geotechnical engineer.
Respectftitrl5" Subrnittecl,
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21 -7 -222 Kumar & Associates LOCATION OF IXPLORATORY BORINGS Fig.1
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BORING 1
EL. 102.5'
HOUSE
BORING 2
EL. I 00'
GARAGE
0 26/12
WC=7.0
DD=117
-200=78
22/12
WC=7.5
DD=1 13
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WC=7.4
DD=131
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10 50/2
50/6
WC=8.0
DD= 1 29
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26/ 12
WC=5.7
DD=110
-200=63
21 -7 -222 Kumar & Associates LOGS OF TXPLORATORY BORINGS Fig.2
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LEGEND
AZÃ'ülÃ-ùlN
TOPSOIL: ORGANIC SANDY SILT, FIRM, SLIGHTLY MOIST, DARK BROWN.
SILT (ML): cLAYEY, SANDY, VERY STIFF, SLIGHTLY MolST, LIGHT BROWN (B0RlNG 1)
CLAY (CL): SILTY, SANDY, WITH SANDSTONE FRAGMENTS, VERY STIFF, SLIGHTLY MOIST, LIGHT
BROWN (BoRrNc 2).
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WEATHERED STLTSTONE/CLAYSTONE: HARD, SLTGHTLY MO|ST, L|GHT BROWN AND GRAY (BORTNG 1)
WEATHERED CLAYSTONE; HARD, SLTGHTLY MOIST, LIGHT BROWN AND GRAY (BORTNG 2).
SANDSTONE BEDROCK; VERY HARD, SLIGHTLY MOIST, GRAY (BORING 1)
CLAYSTONE BEDROCK; HARD, SLIGHTLY MOTST, LIGHT BROWN AND GRAY (BORTNG 2)
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
26/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 26 BLOWS OF A f40-POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
NOTES
THE EXPLORATORY BORINGS WERE DRILLED ON MARCH 26, 2021 WITH A 4_INCH DIAMETER
CONTINUOUS_FLIGHT POWER AUGER.
2. THE LOCAÏIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. ÏHE ELEVATIONS OF THE EXPLORATORY BORINGS WERE MEASURED BY HAND LEVEL AND REFER
TO THE GROUND ELEVATION AT BORING 2 AS 1OO FEEÏ.
4. THE EXPLORATORY BORING LOCATIONS AND ELEVAÏIONS SHOULD BE CONSIDERED ACCURATE
ONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AÏ THE TIME OF DRILLING
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSITY (PCt) (NSTV D2216);
-2oo= PERCENTAGE PASSING N0. 200 SIEVE (ASTM D1140);
21 -7 -222 Kumar & Associates LEGEND AND NOTIS Fig.3
SAMPLE 0F: Sondy Silty Cloy
FROM:Boring2@1'
WC = 7.3 %, DD = 113 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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SAMPLE 0F: Cloystone
FROM:Boring2@9'
WC = 8.0 %, DD = 129 pcf
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Consolidot¡oñ tesl¡ng pllorm"d in
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EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
21 -7 -222 Kumar & Associates SWELL_CONSOLIDATION TEST RESULTS Fig. 4
K+lI *iffih:*äfffiir ilää'*"TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.21-7-222Weathered Siltstone/Sandy Silty ClaySOIL TYPESandy SiltCSandy Silty ClayWeathered ClaystoneClaystone(%lEXPANSION2.24.9ATTERBERG LIMITSLIQUID LIMITEXPANSIONPRESSUREPERCENTPASSING NO.200 srEVEPLASTICINDEX788,00020,00063GRADATIONNATURALDRYDENSITYSAND(%)GRAVEL(%)t177.0128113110IJIt29f/"1NATURALMOISTURECONTENT1.4/-J5.77.48.01J592SAMPLE LOCATION14DEPTH1BORING