HomeMy WebLinkAboutSubsoil Study for Foundation Design 06.27.19l(+rlHffiiffiffiåiú-*
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¡in Employco Orngd Compony
502û County Road 154
Glenwood Springs, C0 81601
phone: (970) 945-7988
fax: (970) 945-8454
emai I : kaglenwood@)kumarusa.com
www.kulnarusa.com
Office Locations: Denver (l{Q), Par}<er, Colorado Springs, Fort Collins, Glenrvood Springs, and Sumnit Counry Colorado
SUBSOIL STUDY
F'OR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 1, SILWAI\ SUBDTVISTON
TBD COTINTY ROAD 335
WEST OF APPLE TREE PARK
GARFTELD COUNTY, COLORADO
RECE¡VED
teT 3 1 2t1$
GARF¡ELD COUNTY
COMMUNITY DEVELOPMENT
PROJECT NO. 19-7-291
JIINE 27,2019
PREPARED FOR:
JOHN GOSS
4726 COANTY ROAD 33s
NE\il CASTLE, COLORADO 81647
i ohn goss06 ldcomcast.net
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION
SITE CONDITIONS
FIELD EXPLORATION
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS
FOLINDATIONS
FOTINDATION AND RETAINING W
FLOOR SLABS
UNDERDRAIN SYSTEM
LIMITATrONS.................
FIGURE 1 . LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 and 5 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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ALLS
Kumar & Associates, lnc. 6 Project No. 19-7-291
PURPOSE A¡{D SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on Lot 1,
Sillivan subdivision, TBD County Road 335, west of Apple Tree Park, 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 general accordance
with our agreement for geotechnical engineering services to John Goss dated I|l4.ay 9,2019.
A field 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 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 modular, single-story structure over a basement level with a
slab-on-grade floor and located as shown on Figure 1. Grading for the structure is assumed to be
relatively minor with cut depths between about 3 to 7 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 building site was vacant at the time of our field exploration. There is driveway access off
County Road 335 into the building site and temporary trailers just northeast of the proposed
residence site. The ground surface through the building area is relatively flat then drops steeply
down around 80 feet to the Colorado River located at the toe of the steep slope as indicated by
Kumar & Associates, lnc.6 Project No, 19-7-291
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lhe oontour lincs on Figure 1. Vegetation consists of sparse gfðs, weeds and brush ifi the
building area and pinon, juniper and sage brush on the steep slope.
FIELD EXPLORATION
Tlre lield cxpkrration for thc pruject was conducted on May 20,2019. 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 I% inch and 2-inch I.D. spoon samplers. The samplers
were ddven 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 returned to our
laboratory for review by the project engineer and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsurface profile encountered in the borings was relatively uniform and consists of interbedded,
medium dense/very stitI, silty sand and clay with scattered gravel. Similar soils are expected to
extend down considerable depth, possibly to near the river level.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density, and finer than sand size gradatíon analyses. Results of swell-consolidation
testing performed on relatively undisturbed drive samples of the sand and clay soils, presented
on Figures 4 and 5, indicate low compressibility under natural low moisture content and light
loading and moderate to high compressibility with a low collapse potential (settlernent under
constant load) when wetted and additionally loaded. The laboratory testing is summarized in
Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils were moist
to slightlymoist with depth.
Kuma¡ & Associates, lnc. c'Projecl No. 19.7-29'l
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FOUNDATION BEA.RING CONDITIONS
The silty sand and clay soils have low bearing capacity and moderate compressibility potential
under light loading. Shallow spread footings placed on the natural soils can be used for
foundation with a risk of settlement and distress mainly if the bearing soils are wetted. A deep
foundation such as micro-piles could be used to achieve a low settlement risk foundation and
could be 50 feet or more in depth to reach zuitable dense gravel soils or bedrock. If a deep
foundation is desired, additional deep exploration will be needed to develop design
recommendations.
The foundation should be set back from the steep slope adequale distance to not adversely
impact the slope stability. It appears a setback of 12 feet should be adequate provided
construction activity does not disturb the slope. Surface water from the development should not
be directed to the steep slope near the residence and be by sheet flow rather than concentrated.
DESIGN RECOMMENDATIONS
FOLINDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, the building can be founded with spread footings bearing on the
natural soils with a risk of settlement and distress. Precautions should be taken to prevent
wetting of the bearing soils.
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 1,200 psf. Based on experience, we expect initial
'settlement of footings designed and constructed as discussed in this section will
be about I inch or less. Additional differential settlement up to around i inch or
more could occur depending on the depth and extent of wetting.
2) The footings should have a minimum width of 24 inches.
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 gtade is typically used in this
area.
Kumar & Associates, lnc. Ù Project No. 19-7-291
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4)Continuous foundation walls should be heavily reirrforced top and bottom to span
local anomalies such as by assuming an unsupported length of at least 14 feet and
built in a box-like configuration. Foundation walls acting as retaining structures
should also be designed to resist lateral earth pressures as discussed in the
"Foundation and Retaining ÏValls" section of this report.
The topsoil and any loose disturbed soils should be removed in the footing areas.
The exposed soils should then be moistened and compacted.
A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
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
cornputed on the basis of an equivalent fluid unit weight of at least 55 pcf for backfill consisting
of the on-site 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 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% of the maximum
standard Proctor density at near optimum moisture content. Backfill placed in pavement and
walkway areas should be compacted to at leastg5Yo 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 expcctcd, cvcn if thc matcrial is placcd corrcctly, and could result in distress to
facilities constructed on the backfill. Backfill should not contain organics, debris or rocks larger
than about 6 inches.
5)
6)
FOI-INDATION AND RETAINING WALLS
Kumar & Associates, lnc. r Project No. 19.7-291
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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 backfrll 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 compacted to at least95o/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 like that for footing foundations. 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 less than?o/o passing the No. 200
sieve.
All filImaterials for support of floor slabs should be compacted to at least95o/o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site 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 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 areaso be protected
from wetting and hydrostatic pressure buildup by an underdrain system. If a shallow crawlspace
Kumar & Associates, lnc. {r Project No. 19-7-291
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is used (and around a garcge if built), an underdrain should not be provided to help keep the
shallow footings dry.
The drains should consist of 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 ofexcavation and at least 1 foot below lowest adjacent finish grade and sloped at a
minimum t/zo/oto a suitable sump pit (do not trench outlet to the steep slope). Free-draining
granular material used in the underdrain system should contain less than 2o/o passingthe No. 200
sieve, less than 50o/o passing the No. 4 sieve and have a maximum size of 2 inches. The drain
gravel backfill should be at least l% 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
Development of proper surface grading and drainage will be critical to keeping the bearing soils
dry and limiting building settlement and distress througlrout the building life. The following
drainage precautions should be observed during construction and maintained at all times after the
residence has been completed:
1) Inundation of the founclation 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 90%o 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
covcrcd with filter fabric and capped with at least 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) Londscaping which rcquircs rcgular hcavy irrigation should be located at least l0
feet from foundation walls. Consideration should bc givcn to use of xeriscape to
reduce the potential for wetting of soils below the building caused by inigation.
Kumar & Aesociates, lnc.0 Project No. 19-7-291
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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) deveioping
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 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 verify that the recotnmendations
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,
Kurnar' & Associates, Inc.
Steven L. Pawlak, P.E
Reviewed
David A. Young,
SLP/kac
cc: Fisher
Boundaries
(@-hiehs{g.sas)
Kumar & Associales, lnc. e
Lewis -lnc.com
Project No. 19-7-291
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19-7-291 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
LEGEND
SAND AND CLAY (SC-CL); SILIY, SCATTERED GRAVEL, LOOSE/STIFF To MEDIUM DENSE/VERY
ST F WITH DEPTH, MOIST TO SLIGHTLY MOIST WITH DEPTH, SROWN.
DRIVE SAMPLE, 2_INCH I,D. CALIFORNIA LINER SAMPLE.
i DR|VE SAMPLE, 1 3/8-tNCH t.D. SPLTT SPOON STANDARD PENETRATTON TEST
q,71 2 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 5 BLOWS OF A 1 4O-POUND HAMMER"/'- FALLTNG s0 TNcHES wERE REQUIRED To DRIvE THE SAMpLER 12 tNcHES.
NOTES
I THE EXPLORATORY BORINGS WERT DRILLED ON MAY 2A, 2019 WITH À 4_INCH-DIAMETER
CONTINUOUS_FLIGHT POWER AUGER.
2. THE LOCAT¡ONS OF THE EXPLORATORY BOR|NGS WERE MEASURED APPROXIMATELY BY PACING
FROM THE STAKED BUILDING CORNERS.
3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED.
4. THË ËXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULÞ BE CÔNSIDERTD ACCURATÊ
ONLY ÏO 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. GROUND}VATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE
'IME
OF DRILL'NG
7, LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);
DÐ = DRY DENSTTY (pcf) (nSrU 0ZZr0);
-20A= PERCENTAGE PASSING No. 200 SIEVE (ASTM D1140)
19-7 -291 Kumar & Associates LEGEND AND NOTES Fig. 3
SAMPLE OF: Silty Cloyey Sond
FROM:Boringl@5'
WC = 8.6 %, DD = 105 pcf
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SAMPLE OF: Silty Gloyey Sond
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Kumar & Associates SWELL-CONSOLIDAÏION TEST RTSULTS Fî9. 419-7-291
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SAMPLE OF: Very Sondy Silty Cloy
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19-7-291 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 5
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I G rt åitiåTi':'fr:ffinii$'*"ÏABLE 1SUMMARY OF LABORATOHY TEST RESULTSNo.19-7-291SOIL TYPESilty Clayey SandSilty Clayey SandVery Sand Silty ClayUNCONFINËDCOMPRESSIVESTRENGÌHSilty Clayey SandVery Sand Silty Claylo/àPLASTICINDEXATTËRBÊRG LIMITSlo/"1LIQUID LIMITPERCE¡ITPASSING NO.200 stgvEs635SAND("ôGRADATION(7")GRAVELlpcûNATURALDRYDENSITY10599t07JII103/9r\NATURALMOISTURECONTENT8.86.44.3I 1.94.50I2A2Y,01lltìDEPTH5SAMPLE LOCATIONBORING12