HomeMy WebLinkAboutSubsoil StudyIC fl'#1fi'ff:nrcirn[i3;å*'"
An Employcc Owncd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fott Collins, Glenwood Springs, and Summit County, Colorado
RECEIVED
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GARFIELD COUNTY
COMMUNITY DEVELOPMENT
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 360 TRONBRTDGE PHASE 3
BLUE HERON DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 21-7-433
JUNE t0,2021
PREPARED FOR:
SCIB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUrTE 52018
ASPEN' COLORADO 81611
lu ke. gosda@,su nriseco.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ...
PROPOSED CONSTRUCTION
SITE CONDITIONS
GEOLOGY...
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
FIGURES 3 and 4 - SV/ELL-CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc, @ Project No.21-7-433
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot3í,Ironbridge Phase 3, Blue Heron Drive, 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 May 10, 2021.
A f,reld 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
Plans for the proposed residence were not available at the time of our study. For the purposes of
our study the proposed residence is assumed to be a one- or two-story structure possibly over a
walkout basement level with attached garage. Ground floors are assumed to be a combination of
slab-on-grade and structural over crawlspace. Grading for the structure is assumed to be
relatively minor with cut depths between about 2 to 6 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 subject lot was vacarrt at the time of our field exploration and had been fill graded during the
subdivision development. Vegetation consists of grass and weeds. The ground surface in the
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southern area of the lot is sloping down to the northeast at a grade of about 7 percent
transitioning to a steeper north-facing slope near the north and west side of the lot. The Roaring
Fork River is located downhill about %mile to the north.
GDOLOGY
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. I97 321. The natural soils on the
lot mainly consist of sandy silt and 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.
Bcdrock of thc Pcnnsylvanian agc Eaglc 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 can produce areas of localized subsidence. A sinkhole occurred
in the parking lot adjoining the golf cart storage tent in January, 2005 located about lo mile to the
south of Lot 36 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
grouürl subsideäce on Lot 36 iiuoughout the service life of the proposed buiiding, in our opinion,
is low; however, the owner should be made aware of the potential for sinkhole developmcnt. If
further investigation of possible cavities in the bedrock below the site is desired, we should be
contacted.
FIELD EXPLORATION
The fielcl exploration for the project was conducted on May 18, 2021. Two explorctory borings
were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions. The
horings were advancecl with 4-inch cliameter continuotrs flight augers powered by a truck-
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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 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. Depths at which the samples wers taken and the penetration resistance values are
shown on the Logs of Exploratory Borings, Figure 2. The samples were refurned 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
subsoils consist of about 2t/z feet of clay fill generally overlying dense, slightly silty sandy gravel
and cobbles. A layer of stiff, silty sandy clay was encountered in Boring 2 from 2Yz to 6 feet
deep. Drilling in the dense granular soils with auger equipment was difficult due to the cobbles
and boulders and drilling refusal was encountered in the deposit.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and percent finer than sand grain size gradation analyses. Results of swell-
consolidation testing performed on relatively undisturbed drive samples, presented on Figures 3
and 4, indicate low to moderate compressibility under conditions of loading and wetting. The
shallow sample of clay fîlltested showed a minor expansion potential when wetted under
constant light surcharge andthe natural clay soil showed a low collapse potential when wetted.
The laboratory testing is summarizedin Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist.
FOUNDATION BEARING CONDITIONS
Spread footings placed on the relatively dense fill soil or underlying natural soils should be
adequate for support of the proposed residence with relatively low settlement potential. The
expansion potential exhibited by the tested sample of clay fill is likely an anomaly. We believe
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the swell potential can be discounted in the foundation design. Footings bcaring entirely on the
undcrlying dense gravel soils will have a low risk of settlement. ConLinuous l'oundation walls
that transition between bearing material types will have a risk of differential settlement due to the
varied bearing conditions. Providing heavy reinforcement in foundation walls especially across
material transitions will help to reduce the risk of differential foundation settlement. The bearing
condition of the soils exposed in the excavation should be fuither evaluated at the time of
construction.
DESIGN RECOMMENDATIONS
FOLINDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the building be founded with spread footings bearing
on the relatively dense fill soils or the underlying natural soils.
The design and construction criteria presented below should be observed for a spreacl footing
foundation system.
1) Footings placed on the relatively dense fill soils or underlying natural soils should
be designed fcrr an allowable bearing pressure of 1,500 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. Post-construction settlement could be up
to around I inch mainly if the f,rll or clay bearing soils become wet.
2) The footings should have a minimr¡m width of I8 inches for continnous rvalls 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 leeL.
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 onsitc soils as backfill.
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5)Any topsoil and loose disturbed soils in footing areas should be removed. The
exposed soils in footing area should then be moistened and compacted.
Additional structural fill can consist of the onsite soils compacted to atleasf 98o/o
of the maximum standard Proctor density atnear optimum moisture content. New
structural fill should extend laterally beyond the footing edges a distance equal to
at least 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
construction. 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 below slabs-on-grade for
support. This material should consist of minus 2-inch aggregate with at least 50Yo retained on
the No. 4 sieve and less than I2Yo passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95Yo 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.
UNDERDRAIN SYSTEM
The proposed shallow (less than 4 feet) crawlspace and slab-on-grade garage should not require a
perimeter underdrain system provided that the site grading recommendations contained in this
report are followed. We recommend that below-grade construction, such as retaining walls, deep
crawlspace and basement areas, be protected from wetting and hydrostatic pressure buildup by
an underdrain system.
If installed, 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
6)
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placed at each level of cxcavation and at least 1 foot below lowest adjacent finish grade and
sloped at a minitnurn 1olo to a suitable gravity outlet or drywell based in the gravel soils. I,ree-
draining granular material used in the underdrain system should contain less than 2% passing the
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 l% feet deep. An impervious membrane such as 30 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
Proper surface grading and drainage will be critical to prevent wetting of the bearing soils and
satisfactory performance of the founclation. 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 bc adjustcd 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%o of the maximum standard Proctor density in landscape areas.
3) The grouncl snrface srrrounding the exterior of the building should be slopecl 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 (if any)
should be capped with about 2 feet of thç on-sitç finer graded soils to reduce
surface water infiltration. Graded swales should have a minimum slope of 3%.
4) Roof downspouts and drains should discharge well beyond the limits of all
backtìll.
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 irrigation.
LIMITATIONS
This sfudy has bccn conducted in accordance with generally accepted geoteclurical engineering
principles and practices in this area at this time. We make no warranty either express or implied.
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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) developing
in the future. If the client is concemed 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 recoÍrmendations, and to veriff that the recommendations
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,
Kuruar &,4sseciates, Inc.
9n-11t2.T. P¿¡*<¿aat¿
James H. Parsons, P.E
Reviewed by:
Steven L. Pawlak,
JHP/kac
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Kumar & Asscciates, lnc. ''Project No. 21"7-433
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APPROXIMATE SCALE-FEET
21 -7 -433 Kumar & Associates LOCATION OF EXPLORATORY BORINGS 1Fig.
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BORING 1
EL. 102.5'
BORING 2
EL. I 10.5'
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LEGEND
FILL: SILTY SANDY CLAY WITH GRAVEL, MEDIUM DENSE, SLIGHTLY MOIST, DARK BROWN.
CLAY (CL); SILTY SANDY, STIFF, SLIGHTLY MOIST, BROWN, SLIGHTLY CALCAREOUS, LOS
PLASTICITY.
GRAVEL AND COBBLES (GM-GP); SLIGHTLY SILTY, SANDY' PROBABLE BOULDERS, DENSE'
SLIGHTLY MOIST, BROWN, ROUNDED ROCK.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
i DRTVE SAMPLE, 1 3/8-INCH l.D. SPLIT SPOON STANDARD PENETRATIoN TEST
17/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 17 BLOWS OF A 14o-POUND HAMMER
FALLING 50 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
PRACTICAL AUGER REFUSAL.I
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON MAY 18,2021 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 (WITH ASSUMED ELEVATION DATUM).
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 (PCI) (ISTV D2216);
-ZQQ= PERCENTAGE PASSING No. 200 SIEVE (ASTM 01140).
21 -7 -433 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
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SAMPLE 0F: Sondy Silty Cloy (Flll)
FRÔM: Boring 1 @ 1'
WC = 11.8 %, DD = 115 pcfI
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EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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21-7 -433 Kumar & Associates SWTLL-CONSOLIDATION TEST RESULTS Fig. 3
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SAMPLE OF: Sondy Silty Cloy
FROM:Boring2@4'
WC = 7.6 %, DD = 95 pcf
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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21 -7 -433 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 4
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I (+rt irffi ['ffi:Ë::*liiiå*. "TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.21-7-433SOIL TYPEUNCONFINEDCOMPRESSIVESTRENGTHSandy Silty Clay fFill)Sandy Silty Clay (Fill)Sandy Silty ClaySlightly Silty Sandy Gravelt%lPI.ASTICINDEXATTERBERG LIMITS(ololLISUID LIMIT11575106RADATIONPERCENTPASSING I.TO.200 stEvENATURALDRYDENSITYSAND%tGRAVELl:/"111895lololNATURALMOISTURECONTENT1 1.8tt.27.60.7I14112SAIUPLE LOCATIONDEPTHBORING