HomeMy WebLinkAboutSubsoil Study for Foundation Design 10.18.2021Kumar & Associates, Inc. 5020 County Road 154
Geotechnical and Materials Engineers Glenwood Springs, co BIBI
and Environmental Scientists p5ol'lt3; (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
An Employee Owned Company www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
RECEIVED
GARFIELD COUNTY
COMMUNITY DEVELOPMENT
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 28, IRONBRIDGE, PHASE 3
1486 RIVER BEND WAY
GARFIELD COUNTY, COLORADO
PROJECT N0. 21-7-686
OCTOBER 18, 2021
PREPARED FOR:
SCIB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUITE 5201B
ASPEN, COLORADO 81611
luke.gosda@sunriseco.com
TAJ3LE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION.
SITE CONDITIONS
SUBSIDENCE POTENTIAL.
FIELD EXPLORATION .
SUBSURFACE CONDITIONS
FOUNDATION BEARnSJG CONDITIONS.
DESIGN RECOMMENDATIONS
FOUNDATIONS ......
FLOOR SLABS
UNDERDRAIN SYSTEM .........
SURFACE DRAINAGE.
LIMIT ATIONS.
FIGURE l- LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS
TABLE l- SUMMARY OF LAJE30RATORY TEST RESULTS
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l
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.- 3
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Kumar & Associates, Inc.Project No. 21-7-686
PTJRPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 28, Ironbridge, Phase 3, 1486 River Bend Way in Garfield County, Colorado. The project
site is shown on Figure 1. The purpose of the study was to develop recoini'nendations for the
foundation design. The study was conducted in accordance with our agreement for geotechnical
engineenng services to SCIB, LLC dated August 2, 2021.
A field exploration program consisting of two exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils obtained during the field
exploration were tested in tlie laboratory to determine their classification, compressibility or
swell potential, and other engineering characteristics. The results of the field exploration and
laboratory testing were analyzed to develop recoinmendations for foundation types, depths and
allowable pressures for the proposed building foundation. This report sun'u'narizes the data
obtained during this study and presents our conclusions, design recommendations and other
geotechnical engineeig considerations based on the proposed constnuction 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 within the building envelope shown on Figure 1. For the purposes of our analysis, we
assume the proposed residence will be a one- or two- story wood-frame stnicture over a
crawlspace with an attached slab-on-grade garage. Grading for the structure is assumed to be
relatively minor with cut depths between about 2 to 4 feet. We assume relatively light
foundation loadings, typical of the proposed type of constniction
If building loadings, location or grading plans change significantly from those described above,
we should be notified to re-evaluate the recointnendations contained in this report.
SITE CONDITIONS
The lot was vacant and appeared to have had minor cut and fill grading, likely during the
subdivision development. The surface of the lot slopes gently down to tlie northeast with about
4 feet of elevation difference across the building envelope area. A detention basin is located in
the nordi corner of the lot and a trail traverses the lot from the south and trends north around the
detention basin. Vegetation consists of grasses and weeds
Kumar & Associates, Inc.Project No. 21-7-686
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SUBSIDENCE POTENTIAL
The geologic conditions were described in a previous report conducted for planning and
preliminary design of the overall subdivision development by Hepworth-Pawlak Geotechnical
(now Kumar & Associates) dated October 29, 1997, Job No. 197 327. The natural soils on the
lot mainly consist of saiidy silty clay alluvial fan deposits overlying gravel terrace alluvium of
the Roaring Fork River. The river alluvium is mainly a clast-supported deposit of rounded
gravel, cobbles, and boulders typically up to about 2 to 3 feet in size in a silty sand matrix and
overlies siltstone/claystone bedrock.
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge subdivision
These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some
massive beds of gypsum and limestone. Dissolution of the gypsum under certain conditions can
cause sinkholes to develop and caii produce areas of localized subsidence. A sinkhole occurred
in the parking lot adjoining the golf cart storage tent in January 2005 which was backfilled and
compaction grouted To our knowledge, that sinkhole has not shown signs of reactivation such
as ground subsidence since the remediation. Sinkholes possibly related to the Evaporite were not
observed in the in'u'nediate area of the subject lot. Based on our present knowledge of the
subsurface conditions at tlie site, it cannot be said for certain that sinkholes related to the
underlying Evaporite will not develop. The risk of future ground subsidence on Lot 28
throughout tlie service life of the proposed building, 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 tlie project was conducted on September 1, 2021. Two exploratory
borings were drilled at the approximate locations sliown on Figure l to evaluate the subsurface
conditions. The borings were advanced with 4-inch diameter continuous flight augers powered
by a tnuck-mounted CME-45B drill rig. The bomgs were logged by a representative of Kumar
& Associates, Inc.
Samples of the subsoils were taken witli 1%- and 2-inch I.D. California or split-spoon samplers.
The samplers were driven into the subsoils at various depths with blows from a 140-pound
liaini'ner falling 30 inches. This test is similar to the standard penetration test described by
ASTMMethodD-1586. Thepenetrationresistancevaluesareanindicationoftherelative
density or consistency of the subsoils. Depths at wliich the samples were taken and the
Kumar & Associates, Inc.Project No. 21-7-686
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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. Below
about 6 inches of topsoil, the subsoils consist of about 5'/2 to 8!/2 feet of stiff to very stiff, slightly
sandy to sandy clay and silt overlying dense to very dense, silty gravel and cobbles down to the
maximum explored depths of 9 and 14 feet. Drilling in the dense granular materials with auger
equipment was difficult due to tbe cobbles and probable boulders and practical drilling refusal
was encountered in the deposit.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density, swell-consolidation and percent silt and clay-sized particles passing the No
200 sieve. Results of swell-consolidation testing performed on relatively undisturbed drive
samples of silt and clay soils, presented on Figure 4, indicate low compressibility under existing
low moisture conditions and light loading and variable low compression and low expansion
potential when wetted under loading The laboratoi'y testing is surninarized in Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist.
FOUNDATION BEARING CONDITIONS
The upper clay and silt soils encountered in tl'ie borings possess low bearing capacity and
typically a low to moderate settlement potential if wetted. Testing indicates the clay and silt
soils are slightly expansive or con'ipressible when wetted. Our experience in the area indicates
the swell potential is minor (if any) and can be discounted in foundation design. We should
observe the soil conditions exposed at the time of excavation and evaluate them for swell-
compression potential and possible mitigation such as sub-excavation to a certain depth and
replacement with compacted structural fill or extending the foundation bearing level down to the
underlying coarse granular soils. Shallow spread footings placed on the clay and silt soils can be
used for support of the proposed residence with a risk of foundation movement mainly if the
bearing soils become wetted. Proper surface drainage as described in this report will be critical
to the long-tenn perfonnance of the structure. A low settlement risk can be achieved by
extending the bearing level down to the relatively dense, coarse granular soils.
Kumar & Associates, Inc.Project No. 21-7-686
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DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed constniction, tlie building can be founded with l) spread footings bearing on the
natural clay and silt soils with a settlement risk and potentially an expansion risk or, 2) spread
footings bearing on the underlying nahiral, dense granular soils with a comparatively lower risk
of settlement.
The design and constniction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural clay and silt soils sliould be designed
for an allowable bearing pressure of 1,500 psf. Footings placed on the
undisturbed natural granular soils should be designed for an allowable bearing
pressure of 3,000 psf. Based on experience, we expect initial settlement of
footings designed and constnicted as discussed in this section will be about 1 inch
or less. Additional differential movement up to about 1 inch could occur if the
clay and silt bearing soils are wetted.
2)The footings should have a minimumwidth of 18 inches for continuous walls and
2 feet for isolated pads.
3)Exterior footings and footings beneath 11nheated 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 tliis
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 12 feet.
Foundation walls acting as retaining structures should also be designed to resist a
lateral earth pressure corresponding to an equivalent fluid unit weight of at least
55 pcf for the on-site clay and silt soil as backfill.
5)Topsoil, fill and any loose disturbed soils sliould be removed and the footing
bearing level extended down to the firm natural soils. The exposed soils in
footing areas sliould tlien be moistened and compacted. Structural fill, if used,
should consist of relatively well-graded granular material such as road base
compacted to at least 98% of standard Proctor density at near optimum moisture
Kumar & Associates, Inc.Project No. 21-7-686
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6)
content. The fill should extend beyond the footing edges a distance at least equal
to one-half the fill depth below the footing.
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
constniction with a risk of movement mainly if the bearing soils are wetted. To reduce the
effects of some differential movement, floor slabs should be separated from all bearing walls and
columns witli 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 reinforcenyent should be established by the designer based on experience and
the intended slab use. A minimum 4-inch layer of relatively well graded sand and gravel such as
road base should be placed beneath interior slabs for support. This material should consist of
minus 2-inch aggregate with at least 50% retained on the No. 4 sieve and less than 12% passing
the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95% of maxinium
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 or a suitable imported material such as
road base.
UNDERDRAIN SYSTEM
It is our understanduig the ground level, finislied floor elevation of the residence is at or above
the surrounding grade Therefore, a foundation drain system is not recommended. It has been
our experience in the area and where clay soils are present 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 recomn'iend below-grade construction, SIICII as
retaining walls and basement areas, if provided, be protected from wetting and hydrostatic
pressure buildup by an underdrain and wall drain system. An underdrain is not recomi'nended
around the shallow crawlspace area to help limit the potential for wetting below the sliallow
footings.
Kumar & Associates, Inc.Project No. 21-7-686
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Ifthe finished floor elevation of the proposed stnicture has a floor level below the surrounding
grade or a taller crawlspace is constnicted, we should be contacted to provide recoendations
for an underdrain system. All earth retaining stnictures should be properly drained.
SURFACE DRAINAGE
It will be critical to the building perfornnance to keep the bearing soils dry. The following
drainage precautions should be observed during constniction and maintained at all times after the
residence has been completed:
l) Inundationofthefoundationexcavationsandunderslabareasshouldbeavoided
during constnuction.
2) Exterior backfill should be adjusted to near optimuin moisture and compacted to
at least 95% of the maximiun standard Proctor density in pavement and slab areas
and to at least 90% 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. Graded swales sliould liave a
4)
minimum slope of 3%.
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
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 ii'rigation
LIMIT ATIONS
This study lias been conducted in accordance witli 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
consti'uction 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 tlie client is concerned about MOBC, tlien 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
Kumar & Associates, Inc.Project No. 21-7-686
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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 interpretattons by others of our information. As the project evolves, we
should provide continued consultation and field services during consttuction to review and
monitor the implementation of our recommendations, and to verify that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the fecominendations 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,
Kumar & Associates, Inc.
Mark Gayeski, E.I.T.
Reviewed by:
Steven L. Paw/l .4"'- '!' !22
Kumar & Associates, Inc.Proiect No. :?1-7-686
S
BORING 1 BORING 2
EL. 5934.5' El. 5937.5'
0 0
13/12
WC=3.8
DD=106 14/12
5 5
17/12 18/12
,., WC=9.5 wc='7.g
.:. DD=91 DD=101
', -200=91
h 2
LJ LJ
Lu LJ
I.a_ 1_4
I I
I 10 ... 10 I
s .' 30/6, 50/2 hCL CL
LJ 1_J
0 0
15 15
20 20
21-7-686 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
LEGEND
TOPSOILi CLAY AND SILT, SANDY WITH ORGANICS, FIRM, SLIGHTLY MOIST, DARK BROWN.
€C,LxAYscsA,NDsrSFl>Tr(oCL;cMhLy);srSiLrlrG,HTsLuYicsSAnNyDYvoTOs_.SAuNcDYm:lTpHowVxE.RY SANDY AND GRAVELLY
[;ceava AND COEIBLES (GM); SANDY, SILTY, DENSE TO VERY DENSE, SLIGHTLY MOIST, MIXED
[EIROWN. ROUNDED ROCK.
f DRIVE SAMPLE, 2-INCH 1.D. CALIFORNIA LINER SAMPLE.
!DRIVE SAMPLE, 1 3/8-INCH 1.D. SPLIT SPOON STANDARD PENETRATION TEST.
13 Il 2 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 13 BLOWS OF A 140-POUND HAMMERFALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON SEPTEMEIER 1, 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 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 AT THE TIME OF DRILLING.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSITY (pcf) (ASTM D2216);
-200= PERCENTAGE PASSING N0. 200 SIEVE (ASTM D1140).
21-7-686 Kumar & Associates LEGEND AND NOTES Fig. 3
X
1
J
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qn
SAMPLE OF: Sandy CIC)Y and Silt
FROM: Boring 1 @ 2.5'
WC = 3.8 %, DD = 106 pcf
ADDITIONAL COMPRESSION
%UNDE%,CO%TA:TrPiRESSURE
I
z
8 i
<
2_
o
; _ 2
0
-3
-4
%
.1 10 APPLIEDPRESSURE- KSF TO 100
X
2
J
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LJ
gl
I
z
0_ n
SAMPLE OF: Clay and Sift
FROM: Boring 2 @ 5'
WC = 7.9 %, DD = 101 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
h-
<
Cl
J
o
; _ I
o
0
-2
)
These lest results apply only to the
samples tested. The lasting report
shall not be iepnoduced. except in
lull. without the wdllen approval oT
Kumar and Ilgsoeiales. Inc. Swell
Caniolidakioii legtlng pwlarmad in
accomanca with ASTM D-4548.
.1 10 APPLIED PRESSURE - KSF i0 100
21-7-686 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 4
l Kumar & Associates, Inc.o( + ffi Geotechnical and Materials Engineers ffl and Environmental Scientists
TABLEI
SUMMARY OF LABORATORY TEST RESULTS
Project No. 21-7-686
SAMPLE LOCATION NATURAL
MOISTURE
CONTENT
(%)
NATURAL
DRY
DENSITY
(pcf)
GRADATION
PERCENT
PASSING NO.
200 SIEVE
ATTERBERG LIMITS uNCONFINED
COMPRESSIVE
STRENGTH
(psf)
SOIL TYPEBORINGDEPTH
(ff)
GRAVEL
(%)
SAND
(%)
LIQUID LIMIT
(%)
PLASTIC
INDEX
(%)
1 2!/i 3.8 106 Sandy Clay and Silt
5 9.5 91 91 Clay and Silt
2 5 7.9 101 Clay and Silt