HomeMy WebLinkAboutSubsoil Studyl(tn Kumar & Assoclatas, lnc. 5020 County Road 154
Geotechn¡cal and Materials Engineers Glenwood Springs, CO 81601
and Environmenratscientists phonä: (g7d) g+s_zggg
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
An Employcc Owncd Gompony www.kumarusa.com
Ofüce Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
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SUBSOIL STTJDY
FOR FOTINDATION DESIGN
PROPOSED RESIDENCE
LOT 59, TRONBRTDGE, PHASE 3
63 BLUE HERON DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 2r-7-657
SEPTEMBER 29,2021
PREPARED FOR:
scrB, LLc
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUITE 52018
ASPEN, COLORADO 8161I
luke.gosda@sunriseco.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF SruDY ..
PROPOSED CONSTRUCTION
SITE CONDITIONS....
SUBSIDENCE POTENTIAL...
FIELD EXPLORATION
SUBSURFACE CONDITIONS ......
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS .......
FOUNDATIONS .....
FLOOR SLABS
UNDERDRAIN SYSTEM
SURFACE DRAIN4GE.............. -
LIMITATIONS...
FIGURE I - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 . LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SV/ELL.CONSOLIDATION TEST RESULTS
TABLE I - SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, Inc.Project No.21-7-657
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 59, Ironbridge, Phase 3, 63 Blue Heron Drive in Garfield County, Colorado. The project site
is shown on Figure l. The purpose of the study \Mas to develop recommendations for the
foundation design. The study was conducted in accordance with our agreement for geotechnical
engineering services to SCIB, LLC dated August 2,202I.
A field exploration program consisting of trryo exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils obtained during the fïeld
exploration were tested in the 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 recommendations for foundation types, depths and
allowable pressures for the proposed building foundation. This report summarizes the data
obtained'during this study and þresents our conclusions, design recommendâtions 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 within the building envelope shown on Figure l. For the purposes of our analysis, we
assume the proposed residence will be a one- or two- story wood-frame structure 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. VVe assume relatively light
foundation loadings, fypical 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 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 the northeast with about
4 feet of elevation difference across the building envelope area. A steep slope about 6 to 8 feet
high follows the southern part of the lot and an asphalt paved path follows the north and east
sides of the lot. Vegetation consists of sparse grasses and weeds.
Kumar & Associates, lnc.Project No.2l-7-657
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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 sandy 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 about2 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 honbridge 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 can produce areas of localized subsidence. A sinkhole occurred
in the parking lot adjoining the golf cart storage tent in January, 2005 located several hundred
feet south of Lot 59 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 immediate area of the
subject lot. Based on our present knowledge of the subsurface conditions at the 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 59 throughout the 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 fïeld exploration for the project was conducted on August 31,2021. Two exploratory
borings were drilled at the approximate locations shown on Figure I 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 l%-inch and Z-inch I.D. California or split-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
Kumar & Associates, Inc.Project No. 21-7-657
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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. Below
about 6 inches of topsoil, the subsoils consist of about 7t/zto 8 feet of stiff to very stiff, slightly
sandy to sandy silty clay overlying medium dense to dense, slightly silty to silty sandy gravel
with cobbles and possible boulders down to the maximum explored depths of 13 and 16 feet.
Drilling in the granular materials with auger equipment was difficult due to the cobbles and
probable boulders and practical 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, 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 clay soils, presented on Figure 4, indicate low compressibility under existing low
moisture conditions and light loading and minor expansion potential when wetted under loading.
The laboratory testing is summarized in Table l.
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 soils encountered in the borings possess low bearing capacity and typically a low
to moderate settlement potential if wetted. Testing indicates the clay is slightly expansive. 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. Shallow spread footings placed
on the clay 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-term performance 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, lnc.Project No.21-7-657
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DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, the building can be founded with 1) spread footings bearing on the
natural clay soils with a settlement risk or, 2) spread footings bearing on the underlying natural,
dense granular soils with a comparatively lower risk of settlement.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
l) Footings placed on the undisturbed natural clay soils should 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
aqd constructed as discussed in this section will be about I inch or less.
Additional differential movement up to about I inch could occur if the clay
bearing soils are wetted.
2) The footings should have a minimum width of 18 inches for continuous walls and
2 feet for isolated pads.
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 grade is typically used in this
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 soil as backfill.
5) Topsoil, fill and any loose disturbed soils should be removed and the footing
bearing level extended down to the firm natural soils. The exposed soils in
footing area should then be moistened and compacted. Structural fill should
consist of relatively well-graded granular material such as road base compacted to
at least 98oá of standard Proctor density at near optimum moisture content. The
fill should extend beyond the footing edges a distance at least equal to one-half
the fill depth below the footing.
Kumar & Associates, lnc.Project No.2l-7-657
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6)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
construction 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 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 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 l2% passing
the No- 200 sieve.
Al1fill 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 soils devoid of vegetation, topsoil and oversized rock or a suitable imported material such as
road base.
UNDERDRAIN SYSTEM
It is our understanding the ground level, finished 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 recommend below-grade construction, such 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 recommended
around the shallow crawlspace area to help limit the potential for wetting below the shallow
footings.
If the finished floor elevation of the proposed structure has a floor level below the sunounding
grade or a taller crawlspace is constructed, we should be contacted to provide recommendations
for an underdrain system. All earth retaining structures should be properly drained.
Kumar & Associates, Inc.Project No. 2l-7-657
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SURFACE DRAINAGE
It will be critical to the building performance to keep the bearing soils dry. The following
drainage precautions should be observed during construction and maintained at all times after the
residence has been completed:
l) Inundation of the 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 95o/o of the maximum 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 l0 feet in unpaved areas and a minimum slope of
3 inches in the first 10 feet in paved areas. Graded swales should have a
minimum slope of 3o/o.
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
l0 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 areaat 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 I, 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 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.
Kumar & Associates, Inc.Project No.21-7-657
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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 verifu that the recornmendations
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,
Kumar & Associates,Inc.
Mark Gayeski,
Reviewed by:
I.T.
Steven L. Pawlak, P
SLPlkac
Kumar & Associates, lnc,Project No. 21-7-657
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21 -7 -657 Kumar & Associates LOCATION OF TXPLORATORY BORINGS Fig. 1
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21 -7 -657 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
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LEGEND
TOPSOIL; SAND AND SILT, CLAYEY WITH GRAVEL AND ORGANICS, FIRM, SLIGHTLY MOIST,
TAN.
cLAy (cL), stLTy, sLtcHTLy SANDY TO SANDY, STTFF TO VERY ST|FF, SLIGHTLY MO|ST, TAN,
TRACE POROSITY, TRACE CALCAREOUSNESS, ROOTS.
GRAVEL (GM); SANDY, SLIGHTLY SILTY TO SILTY WITH COBBLES AND PROBABLE SMALL
BOULDERS, MEDIUM DENSE TO DENSE, SLIGHTLY MOIST, MIXED LIGHT GRAY, BROWN AND TAN.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
i DRTVE SAMPLE, 1 S/9-|NCH l.D. SPLIT SPOON STANDARD PENETRATION TEST
16/ 12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 16 BLOWS OF A 14o-POUND HAMMER
FALLING 50 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
I PRACTICAL AUGER REFUSAL.
NOTES
1, THE EXPLORATORY BORINGS WERE DRILLED ON AUGUST 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.
5. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INÏERPOLATION 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 ÏRANSITIONS 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 (PCt) (ASTU D2216);
-200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM 01140).
21 -7 -657 Kumar & Associates LEGEND AND NOTES Fig. 3
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SAMPLE OF: Silty Cloy
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WC = 7.8 %, DD = 100 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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21 -7 -657 Kumar & Associates SWELL_CONSOLIDATION TEST RESULTS Fig. 4
I(+rtiffihiffifffini':i.Ëü-*TABLE 1SUMMARY OF LABORATORY TEST RESULTSSOIL TYPESilty ClaySilty ClaySilty ClaylosflUNCONFINEDCOMPRESSIVESTRENGTHP|-ASTtCINDEX(ololATTERBERG LIMITS(ololLIOUID LIMTÍPERCENTPASSING NO,200 stEvE85SAND("/"1GRADATIOill"/"1GRAVEL(pcf)NATURALDRYDENSITYr0210097l:/"1NATURATMOISTUREcot{TEt¡T6.27.811.0lft)DEPTH2y,52y,SAIIPLÊ LOCATIONBORING12No.2l-7-657