HomeMy WebLinkAboutSubsoil Studyl(trt l0mar&Associates, lnc. 5020 County Road 154
Geotechnical and Materials Engineers Glenwood Springs, CO g1601
and Environmenrat Scientists pnonä: (g70) g4s_7gg8
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
An Employcc Ownod 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 27, TRONBRTDGE, PHASE IrI
RIVER BEND WAY
GARFTELD COUNTY, COLORADO
PROJECT NO.21-7-337
MAY 7,2021
PREPARED FOR:
scrB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUITE 52018
ASPEN, COLORADO 81611
Iu ke. gosda@sunriseco.com
TABLE OF'CONTENTS
PURPOSE AND SCOPE OF STIJDY....
PROPOSED CONSTRUCTION
STTE CONDITTONS
SUBSIDENCE POTENTIAL...
FIELD EXPLORATION...
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS ....
DESIGN RECOMMENDATIONS
FOUNDATIONS
FLC)OR SLABS
UNDERDRAIN SYSTEM
SURFACE DRAINAGE.............
LIMITATIONS..
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS
FIGURE 5 - GRADATION TEST RESULTS
TABLE 1, SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc.Project No. 21-7-337
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot2T,Ironbridge, Phase III, River Bend \ü/ay, Garfield County, Colorado. The project site is
shown on Figure 1. The pu{pose 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 April 8,202L
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 had not been determined at the time of our study. For the
purpose of analysis, we assume a one or two-story wood frame structure over a crawlspace in the
living area and a 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 notihed to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The lot was vacant, with utilities located at the street. The surface of the lot was smooth, slightly
moist, and slopes gently down to the northeast toward the Roaring Fork River with around 3 to
4 feet of elevation difference across the buildin g area. Vegetation consists of grass and weeds.
Kumar & Associates, lnc.Project No. 21-7-337
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SUBSIDtrNCtr POTBNTIAL
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 gypsuur and lirnestone. There is a possibility that massive gypsurl deposits
associated with the Eagle Valley Evaporitc undcrlic portions of the lot. Dissolution of thc
gypsum under certain conditions can cause sinkholes to develop and can produce areas of
lLrcalizecl 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 irnmediate 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 fbr certain that sinkholes will not develop. The risk of
future ground subsidence on Lot 27 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
bedrock below the site is desired, we should be contacted.
FIELD EXPLORATION
The field exploration for the project was conducted on April 14, 202I. Two exploratory borings
were drilled at the 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, lnc.
Samples of the subsoils were taken with I% 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 stanclarcl penetration test described by ASTM Mcthod D-1586
The penetration resistance values are an indication of the relative density or consistency of the
subsoils. Depths at which the samples were takcn and thc pcnctration rcsistancc valucs arc
shown on the Logs of Exploratory Borings, Figurc 2. Thc samplcs were returned to our
laboratory for review by the project engineer and testing.
Kumar & Associates, lnc.Project No. 21-7-337
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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 between 2 feet (in Boring 1) and 9 feet (in
Boring 2) of stiff to very stiff, sandy silt and clay. Dense, slightly silty sandy gravel with
cobbles and probable small boulders was encountered below the silt and clay soil and continued
to the explored depth of 14 feet in Boring I andl feet in Boring 2. Drilling in the dense granular
soils with auger equipment was difficult due to the cobbles and boulders and practical drilling
refusal was encountered in the deposit in both borings.
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 silt and clay soil, presented on Figures 4, indicate
low to moderate compressibility under conditions of loading and wetting. The silt and clay soils
in this area can have a low swell or collapse potential (settlement under constant load) when
wetted. The laboratory testing is summarizedinTable 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 silt and clay soils have low bearing capacity and generally low to moderate
compressibility when wetted. Shallow spread footings placed on the natural silt and clay soils
can be used with a risk of settlement as described below. The footing bearing level on Lot 2l
should be deepened below existing ground surface so there is no more than about 5 feet of silt
and clay soils below the bearing level as a foundation settlement rnitigation measure. In sub-
excavated areas below design footing level, the on-site soils or imported granular material such
as3/¿-inch base course could be placed as compacted structural fill for footing support.
Extending the footing bearing level down to the underlying gravel and cobble soil (such as near
Boring 2) could also be used to achieve a low settlement risk.
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 spread footings bearing on the
Kumar & Associates, lnc.Project No.21-7-337
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naturâl silt and clay or granular soils or compacted structural fill with a settlemcnt risk. The
design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placcd on the undisturbed natural silt and clay or granular soils or
stnrctural fill should be designed for an allowable bearing pressure of 1,500 psf.
Based otr experierìcs, ws expect initial seLllement of footings designetl antl
constructed as discussed in this section will be about 1 inch or less. Additional
difl'erential settlement of around Y, to 1 inch coulrl occr¡r if the hearing 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 cxtcrior gradc is typically uscd in this
arca.
4) Continuous foundation walls should be reinforced top and bottom to span local
anomalies such as by assuming an unsupported length of at least 14 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
55 pcf for the onsite silt and clay soil as backfill.
5) The topsoil, silt and clay soils greater than 5 feet deep and any loose or disturbed
soils should be removed in footing areas. 'l'he exposed soils should then be
moistened and compacted. Structural fill should extend laterally beyond the
footing edges at least %the fill depth below the footing and be compacted to at
least9So/o of the standard Proctor density atnear optimum moisture content. The
soils should be protected from frost and concrete should not be placed on frozen
soils.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrctc placcmcnt to cr¡aluatc bcaring 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 settlement if the bearing soils are wetted. Structural fill about 2 feet
Kumar & Associates, lnc.Project No. 21-7-337
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deep consisting of imported granular material such as 3/o-inch base course 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 silt and clay soils devoid of vegetation and topsoil.
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, 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 Io/o to a suitable gravity outlet or sump. Free-
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 I% feet deep. An impervious membrane such as 20 mil
PVC should be placed beneath the pipe and drain gravel in a trough shape and attached to the
foundation wall with mastic to prevent wetting of the bearing soils.
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 andunderslab areas shouldbe avoided
during construction.
Kumar & Associates, lnc.Project No. 21-7-337
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2)Exterior backfill shoulcl be acljusted to near optimum moisture and compacted to
at least 95o/o of the maximum standard Proctor density in pavement and slab areas
ancl to at least 909á of the maxiinum starrdard Proctor density in landscape areas.
The ground surface surrounding the exterior of the building should be sloped to
drain away from the foundation in all directions. \üy'e 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
covererl with filter fabric ancl capped rvith about 2 feet of the on-site soils to
reduce surface water infiltration.
Roof downspouts and drains should discharge well beyond the limits of all
backfill.
Landscaping which requires regular heavy irrigation 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.
3)
4)
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) 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 identihed 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 lo revicw ancl
monitor the implementation of our recomnìendations, and to verifir that the recommendations
5)
Kumar & Associates, lnc.Project No. 21.7.337
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. 'We recommend on-site observation
¡
- the geotechnfcal engineer.
Respectfirlly Submitted,
Kumar & Associates, Inc.
David A. Noteboom, Staff Engineer
Reviewed by:
Stwen L. Pawlak. P.E.
SLP/kac
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Kumar & Associates, lnc.Project No.21-7-337
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BORING 2
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BENCMARK: MANHOLE
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21 -7 -337 Kumar & Associates LOCATION OF TXPLORATORY BORINGS Fig. 1
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BORING 1
97'
BORING 2
94'
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21 -7 -337 Kumar & Associates LOGS OF TXPLORATORY BORINGS Fig. 2
LEGEND
T0PSOIL; CLAY, SANDY, SILTY, ORGANIC, FIRM, MOIST, BROWN.
CLAY AND SILT (CL-ML); SANOY, SOME ROOTS, STIFF TO VERY STIFF, SLIGHTLY
MOIST, LIGHT BROWN, CALCAREOUS, LOW PLASTICITY.
GRAVEL (GM-GP); SANDY, SLIGHTLY SILTY, COBBLES AND PROBABLE BOULDERS, VERY DENSE,
SLIGHTLY MOIST, LIGHT BROWN WITH GRAY, ROUND ROCK.
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DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
DRIVE SAMPLE, 1 3/8-|NCH t.D. SPLIT SPOON STANDARD PENETRATTON TEST
1a/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 14 BLOWS 0F A 14O-POUND HAMMER..,.- FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
f enacrrcAL AUGER REFUSAL.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON APRIL 1 4, 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 HARD 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 DENSITY (PCt) (NSTV D2216);+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D6913);
_2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D1140);
21 -7 -337 Kumar & Associates LEGEND AND NOTES Fí9. 3
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SAMPLE OFr Sondy Silt ond Cloy
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SWELL-CONSOLIDATION TEST RESULTS Fig. 421 -7 -337 Kumar & Associates
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HYDROMETER ANALYSIS SIEVE ANALYSIS
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foo.o75 .150 .300 ¡ .600 t.t8
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DIAMETER OF PARTICLES IN MILLI
r 2.36
2.O
METERS
1.54.Í
152
CLAY TO SILT COBBLES
GRAVEL 64 % SAND 26
LIQUID LIMIT
SAMPLE OF: Slighlly Silly Sondy Grovel
PLASTICITY INDEX
SILT AND CLAY 10 %
FROM: Boring 2 @ 2.5' ond 5' (oombinod)
fhoss losl rosulls opply only lo lhê
sompl€s which w6re l€sl€d. Thql.sllng r6porl shqll nol bs r€producod,
oxcopl ln tull, wlthoul lh€ wrlll€nqpprovol of Kumor & A66oolol6s, lno.
Siovo onolysls l6sllng ls porformed ln
occordonco wllh ASTM D6915, ASÍM D7928,
ASTM C156 ond/or ASTM 01140.
SAND GRAVEL
FINE MEDIUM COARSE FIN E COARSE
21 -7 -337 Kumar & Associates GRADATION TEST RESULTS Fig.5
rcnKumar & Asmciates, lnc.'Geotechnical and Materials Engineersand Environmental ScientistsTABLE 1SUMMARY OF LABORATORY TEST RESULTSSOIL TYPESandy Silt and ClaySandy Silt and ClaySlightly Silty Sandy Gravel8210t.1266447I2ALIQUID LIMITGRADATION101t076.39.2SAMPLE LOCATIONDEPTHBORINGUNCONFINEDCOMPRESSIVESTRENGTHPLASTICINDEXPERCENTPASSING NO,200 stEvENATURALDRYDENSI'IYNATURALMOISTURECONTENT2/z and 5CombinedSAND("/"1GRAVELf/"1No.21'7-337