HomeMy WebLinkAboutSubsoil StudyT(t'T Xumar & Associates, lnc. 5020 County Road 1S4
Geotechnical and Materials Engineers Glenwood Springs, CO g1601
and Env¡ronmentalScientists phonä: (g7O) g45_7ggg
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
An Employcc Owncd Compony www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 26, IRONBRTDGE' PHASE lrr
RIVER BEND WAY
GARFIELD COUNTY, COLORADO
PROJECT NO. 21-7-304
MAY 7,2021
PREPARED FOR:
SCIB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUITE 52018
ASPEN' COLORADO 81611
luke.gosda@,sun riseco.com
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
UNDERDRAIN SYSTEM ..........
SURFACE DRAINAGE..............
LIMITATIONS
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - SWELL-CONSOLIDATION TEST RESULTS
FIGURE 4 - GRADATION TEST RESULTS
TABLE 1 - SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc.Project No. 21-7-304
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot26,Ironbridge, Phase III, River Bend V/ay, 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 accordance with our agreement for geotechnical
engineering services to SCIB, LLC dated March 29,2A2L
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 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, design recommendations and other geotechnical
engineering considerations based on the proposed construction and the subsurface conditions
encountered.
PROPOSED CONSTRUCTION
The design of the proposed residence was preliminary at the time of our study and will be
located between the exploratory borings shown on Figure 1. In general, we assume a one or
two-story wood frame structure over a crawlspace in the living area and slab-on-grade floor in
the garage. Grading for the structure is assumed to be relatively minor with cut depths between
about 3 to 5 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 reporl.
SITE CONDITIONS
The lot was vacant, with utilities located at the street. The surface of the lot was smooth, slightly
moist, and sloped gently down to the north-northwest toward the Roaring Fork River with about
2 feet of elevation difference in the general building area. Vegetation consists of grass and
weeds.
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SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Ironbridge 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 thc
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 Ironbridge development. These sinkholes appear similar to others
associated with the Eagle Valley Evaporite in areas of the Roaring Fork 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 26 throughout the service life of the proposed residence, in our
opinion, is low and similar to other lots in the area; however, the owner should be made aware of
the potential for sinkhole development. If further investigation of possible cavities in the
bcdrock bclow the site is desired, we should be contacted.
FIELD EXPLORATION
The field exploration for the project was conducted on March 31,202I. 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, Inc.
Samples of the subsoils were taken with T% 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-1586
The penetration resistance values are an indication of the relative density or consistency of the
subsoils. Dcpths at which thc samplcs wcrc takcn ond thc pcnctration resistonce vnlues 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.
SIIRST}RFACE CONDITTONS
Graphic logs of the subsurface conditions encounterecl at the site are shown on Figure 2, Below
about 6 inches of topsoil, the subsoils consist of between llz and 4/z feet of stiff to very stiff
Kumar & Associates, lnc.Project No.21-7-304
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sandy silty clay. Dense, slightly silty sandy gravel with cobbles and probable small boulders was
encountered below the sandy silty clay soil and continued to the explored depths of I I feet in
Boring I andS% feet in Boring 2. Drilling in the dense granular soils with auger equipment was
diff,rcult due to the cobbles and boulders and drilling refusal was encountered in the deposit in
Boring 2.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and gradation analyses. Results of swell-consolidation testing performed on
a relatively undisturbed drive sample of the sandy silty clay, presented on Figure 3, indicate low
compressibility under light loading and low expansion when wetted. The laboratory testing is
summarized in Table 1. Results of gradation analyses performed or small diameter drive
samples of the underlying granular soils (minus llz-inch fraction) are shown on Figure 4.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist.
FOUNDATION BEARING CONDITIONS
The clay soils encountered on the lot have an expansion potential and could heave shallow
footings and slab-on-grade. Footings can be used for support of the building and should be
deepened below the clay soils and placed on the underlying relatively dense sandy gravel and
cobble soil to achieve a low movement risk. We should evaluate the subgrade conditions at the
time of construction for expansion potential and the need to lower the bearing elevation.
DESIGN RECOMMENDATIONS
FOTINDATIONS
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 gravel and cobble soils with a low settlement risk. Structural fill can be used to
reestablish design bearing level and should consist of relatively well graded granular material
compacted to at least 98o/o 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.
l) Footings placed on the undisturbed natural gravel and cobble soil or compacted
structural fill should be designed for an allowable bearing pressure of 2,500 psf.
Based on experience, we expect settlement of footings designed and constructed
as discussed in this section will be about % to I inch or less.
Kumar & Associates, lnc.Project No. 21-7-304
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2)The footings should have a minimum width of 16 inches for continuous walls and
2 feet for isolated pads.
Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above their bearing elevation for frost protection. Placement
of foundations a[ least 36 inches below exterìor grade ìs typically used in this
area.
Continuous foundation walls should be 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 coffesponding to an equivalent fluid unit weight of at least
5 5 pcf for the onsite sandy silty clay soil, or 45 pcf for the onsite sandy gravel
soil, as backfill.
The topsoil, clay soil and loose or disturbed soils should be removed in footing
areas. The exposed soils in footing areas should then be moistened and
compacted. The soils should be protected from frost and concrete should not be
placed on frozen soils. Structural fill placed to reestablish design bearing level
should extend laterally beyond the footing edge a distance equal to at least one-
half the depth of fill below the footing.
A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
3)
4)
FLOOR SLABS
The natural granular soils are suitable to support lightly loaded slab-on-grade construction. Slabs
underlain by clay soils will have a risk of movement due to expansion potential. Structural fill at
least2 feet deep consisting of the on-site granular soils or tá-incÀroad base can be used to limit
the settlement risk. 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.
All fill 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 fill can consist of the
on-site gravel soils devoicl of oversizecl rock, vegetation, ancl topsoil.
UNDERDRAIN 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
5)
6)
Kumar & Associates, lnc Project No.21.7.304
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seasonal runoff. Frozen ground during spring runoff can also create a perched condition. We
recommend below-grade construction, such as retaining walls, crawlspace and basement areas (if
any), be protected from wetting and hydrostatic pressure buildup by an underdrain system.
If required, the drains should consist of 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 with the drain invert elevation at least 1 foot below lowest
adjacent finish grade and sloped at a minimum lo/o to a suitable gravity outlet or sump. Free-
draining granular material used in the underdrain system should contain less than 2o/opassingthe
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 lYz feet deep.
SURFACE DRAINAGE
Providing proper surface grading and drainage is very important to the satisfactory performance
of the foundation. 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 95Yo of the maximum standard Proctor density in pavement and slab areas
and to at least 90o/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 6 inches in the first 10 feet in unpaved areas and a minimum slope of
3 inches in the first l0 feet in paved areas. Free-draining wall backfill should be
covered with filter fabric and capped with about 2 feet of the on-site finer grained
soils to reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy inigation 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 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.
Kumar & Associates, lnc.Project No, 21-7-304
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The conclusions and recommendations submitted in this report are based upon the tlata 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) cleveloping
in the future. If the client is conccmcd about MOBC, thcn a professional in this special field of
practicc should bc consultcd. Our findings include interpolation and extrapolation of thc
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 appeff different from those described in this report, we should be notified so
that re-evaluation of the iecommendations 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 veriff that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. Vy'e 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 & Associateso Inc.
David A. Noteboom, Staff Engineer
Reviewed by
Steven L. Pawlak. P.
ST,P/kac
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Kumar & Associates, lnc.Project No, 21-7.304
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APPROXIMATE SCALE-FEET
LOCATION OF TXPLORATORY BORINGS Fig. 121 -7 -304 Kumar & Associates
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BORING 2
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LEGEND
TOPSOIL; CLAY, SANDY, SILTY, ORGANIC, FIRM, SLIGHTLY MOIST, BROWN.
CLAY (cL); SANDY, SILTY, STIFF T0 VERY STIFF, SLIGHTLY MOIST, BROWN
GRAVEL (GP_GM); SANDY, SLIGHTLY SILTY, COBBLES, PROBABLE BOULDERS, DENSE,
SLIGHTLY MOIST, LIGHTLY BROWN, ROUND ROCK.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
i DRTVE SAMPLE, 1 3/9-|NCH r.D. SPL|T SPOON STANDARD PENETRATToN TEST
j^/1? DRIVE SAMPLE BLOW COUNT. INDICATES THAT 14 BLOWS OF A 140-POUND HAMMER''l'- FALLING 50 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
I eucrrcAL AUcER REFUSAL.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON MARCH 31, 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 MEASURED BY INSTRUMENT LEVEL AND
REFER TO THE BENCHMARK ON FIG. 1.
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 DENsrTy (¡rcr) (lSrV D2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D6915);
-2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM Dl140);
21 -7 -304 Kumar & Associates LOGS OF IXPLORATORY BORINGS Fig. 2
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SAMPLE OF: Sondy Silty Cloy
FROM: Boring 1 @ 1'
WC = 16.7 %, DD = 105 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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Kumar & Associates SWELL-CONSOLIDATION TTST RESULTS Fig.321 -7 -304
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HYDROMETER ANALYSIS SIEVE ANALYSIS
TIME READINOS
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U.S. STANDARD SERIES CLEAR SQUARE OPENINGS
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DIAMETER OF
CLAY TO SILT COBBLES
GRAVEL 54 % SAND
LIQUID LIMIT
SAMPLE OF: Slighlly Silty Sondy Grovel
35%
PLASTICITY INDEX
SILT AND CLAY 11 %
FROM: Boring 1 @ 7' ond 10' (combined)
=
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90
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DIAMETER OF PARTICLES IN MILLIMETERS
CLAY TO SILT COBBLES
GRAVEL 58 % SAND
LIQUID LIMIT
SAMPLE OF: Slightly Sllty Sondy Grovel
31 %
PLASTICITY INDEX
SILÏ AND CLAY 11 %
FROM: Borlng 2 @ 2.5' ond 5'
(comblned)
Thqss losl rssulls opply only lo lh6
sompl6s which w€r6 16slod, Thg
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ASÍM C136 ond/or ASTM Dr14O.
SAND GRAVEL
FINE MEDIUM COARSE FINE COARSE
HYDROMETER ANALYSIS SIEVE ANALYSIS
TIME REAÐINGS
6OMIN igMIN ¡MIN IUIN24 HRS 7 HRS.5 MIN i5 MIN
U.S. STANDARD SERIES CLEAR SQUARE OPENINGS
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21 -7 -304 Kumar & Associates GRADATION TEST RESULTS Fig. 4
K+lI iiffilfi'ff#l:ffiini'ii å *' "TABLE ISUMMARY OF LABORATORY TEST RESULTSNo. 2l-7-304Slightly Silty Sandy GravelSOIL TYPESandy Silty ClaySlightly Silty Sandy GravellosflUNCONFINEDCOMPRESSIVESTRENGTHP/olPLASTICINDEXATTERBERG LIMITS("/"1LIQUID LIMITPERCENTPASSING NO.200 stEvEII11SAND(%)351J5458GRADATION(f/"1GRAVELlocflNATURALDRYDENS]TY1052.0PlolNATURALMOISTURECONTENT16.70.4(ft)DEPÍHI7 &.t0Combined2%and 5CombinedSAMPLE LOCATIONBORING12