HomeMy WebLinkAboutSubsoils Report for Foundation DesignI(tA ltumar&Associates,lnc. 5020 County Road 154
Geotechnical and Materials Engineers Glenwood Springs, CO 81601
and Environmentatscientists phone: (g7o) 945_7ggg
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
An Employcc Owncd Compony wwwkumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 57, PHASE 3,IRONBRTDGE
BLUE HERON DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO.21-7-439
JULY 21,2021
PREPARED FOR:
SCIB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUITE 52018
ASPEN, COLORADO 81601
luke. gosda@sunriseco.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 - 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. 21.7.439
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot 57, Ironbridge, Phase 3, Rlue Heron f)rive, 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 May 10, 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 the laboratory to determine their classification, compressibility or
swell potential, 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 recomrnendations and other
geotechnical engineering considerations based on the proposed construction and the subsurface
conditions encountered.
PROPOSED CONSTRUCTION
At the time of our qtudy, design plans for the residence had not been developed. The building is
proposed within the building envelope shown on Figure 1. For tle purposes of our analysis, we
assume the proposed residence will be a one- and two- story wood-frame structure over a
crawlspace with an attached slab-on-grade garage. Grading for the structure is assuined 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 construction.
If building loadings, location or grading plans change significantly from those described above,
we should be notif,red to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The lot was vacant and appeared to have had some minor overlot grading, likely during the
subdivision development. The surface of the lot slopes gently down to the north with about
2 feet of elevation difference across the building envelope area. A steep slope about 8 to 10 feet
high follows the southwest part of the lot. Vegetation consists of sparse grasses and weeds.
Kumar & Associates, lnc.Project No. 21-7-439
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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 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.
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 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 57 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 57 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 field exploration for the project was conducted on June 16, 2021. Two exploratory borings
were drilled at the approximate locations shown on Figure 1 to evaluatc thc subsurface
conclitiorrs. The borings were advanced with 4-inch diameter continuous flight augers powered
by a truck-mounted CME-45B drill rig. The borings were logged by a representative of Kumar
& Associales, Inc.
Samples of the subsoils were taken with 1%-inch 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 hammer falling 30 inches. This test is similar to the standard penetration test described by
Kumar & Associates, lnc.Project No.21.7.439
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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 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. The
subsoils consist of 7 to 8 feet of stiff to very stiff, sandy silty clay overlying dense, slightly silty
sandy gravel and cobbles with possible boulders down to the maximum explored depth of 16
feet. A 1 -foot-thick layer of sandy gravelly fill was encountered at the surface of Boring I . This
fill was probably placed as part of initial subdivision development.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density, swell-consolidation and gradation analyses. 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 a
variable minor expansion or collapse potential on the sample when wetted under loading.
Results of gradation analyses performed on small diameter drive samples (minus I%-inch
fraction) of the coarse granular subsoils are shown on Figure 5. The laboratory testing is
summarized 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 soils encountered in the borings possess low bearing capacity and typically a low
to moderate settlement potential if wetted. Our experience in the arca 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. 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.
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DESIGN RECOMMENDATIONS
FOLINDATIONS
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 clay soils with a settlement risk.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural clay soils should be designed for an
allowable bearing pressure of 1500 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. Additional differential movement up to about 1 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
Z-&g-f"t itolated 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 uscd 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 coffesponding to an equivalent fluid unit weight of at least
55 pcf for the on-site clay soil as backfill.
5) Topsoil 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.
of the geotechnical engineer should observe all footing
excavations prior to concrete acement to evaluate
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
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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 I2Yo passing the No. 200 sieve.
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 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 crawlspace areato help lirnit the potential for wetting below the shallow footings.
If the finished floor elevation of the proposed structure has a floor level below the surrounding
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.
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 ofthe foundation excavations and underslab areas should be avoided
during construction.
Kumar & Associates, lnc.Project No.21-7-439
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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 90Yo of the maximum standard 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. 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 should have a
minimum slope of 3%.
Roof downspouts and drains should discharge well beyond the limits of all
backfill.
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 study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this area at this time. We make no warranty eithcr cxprcss or implied.
The conulusions 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 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 recommendations, and to veriS that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
Kumar & Associates, lnc.Project No.21.7.439
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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.
James H. Parsons, P
Reviewed by:
Steven L. Pawlak, P.E.
JHP/kac
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Kumar & Associates, lnc.Project No. 21-7-439
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Fig. 1LOCATION OF TXPLORATORY BORINGSKumar & Associates21 -7 -439
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PROPERTY
I,TNE
/,6.01
LOT 58
SETBACK
1 oq tr |
7'
L21- .'7 |
APPROXIMATE SCALE_FEET
102.0'
102
BORING 2
BORI
100.0'
SETBACK 1,,
o
O
.).-80
LOT 51
/
1
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03
PROPERTY
LTNE
r02 .0,1,,
0'19
LOT 56
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PROPERTY
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Fig. 2LOGS OF EXPLORATORY BORINGSKumar & Associates21 -7 -439
WC=0.6
+4=67
-200=6
BORING 1
EL. 10'l .5'
BORING 2
EL. 101.5'
0 0
14/12
WC=7.0
DD=94
11/12
WC=5.7
DD=94
-200=56
5 16/ 12
WC=5.1
DD= 1 02
8/6, 26/6
47/12
E
1s/12
F
trJl!lL
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o_
trJo
27/12 F
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10 10
74/12
15 1552/6
20 20
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Fig. 3LEGEND AND NOTESKumar & Associates21-7 -439
LEGEND
FILL: CLAY, SANDY, GRAVELLY, SLIGHTLY MOIST, BROWN AND TAN.
CLAY (CL); SANDY, SLTGHTLY POROUS, STTFF TO VERY ST|FF, SL|GHTLY MO|ST, BROWN
AND TAN. LOW PLASTICITY.
GRAVEL (GM_GP); SLIGHTLY SILTY TO SILTY, SANDY, COBBLES, PROBABLE BOULDERS,
DENSE, SLIGHTLY MOIST, BROWN, ROUNDED ROCK.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
i DRTVE SAMPLE, 1 3/8-tNCH t.D. SpLtT SPOON STANDARD pENETRATTON TEST
14/ 12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 14 BLOWS OF A 14o-POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
-.> DEPTH AT WHICH BORING CAVED FOLLOWING DRILLING
NOTES
1 . THE EXPLORATORY BORINGS WERE DRILTED ON JULY 1 6, 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 INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED AND THE ASSUMED CONTOUR ELEVATIONS.
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) (ISTU D2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D6913);
-20O= PERCENTAGE PASSING No. 200 SIEVE (ASTM D1140).
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Fig. 4SWELL-CONSOLIDATION TEST RESULTSKumar & Associates21-7 -439
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1 r.0 APPLIED PRESSURE -10
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2
1,0 APPLIED - KSF l0 00
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
1
:
1
l
\
\
l
l
l
l
l
SAMPLE OF: Sondy Silty Cloy
FROM:Boringl@2.5'
WC = 7.0 %, DD = 94 pcf
l
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Ih* l63t rdults opply only to th6
sompl63 t$16d, Thc tGtlng r€pod
sholl not be r6prcducad, €xc.pt in
tull, rithout th€ orittcn opproval of
Kumor ond A8aoclota8, lnc, sv€ll
Consolidotion t.stlng p.dorm.d in
occordonc€ wlth ffi D-4546.
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EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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SAMPLE OF: Sondy Siliy Cloy
FROM:Boring2@5'
WC = 5.1 %, DD = 1O2 pcl
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Fig. 5GRADATION TEST RESULTSKumar & Associates21 -7 -439
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||rr r r rlr--- i l r r,--t,.r1,I'-.t I I
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CLSAR SQUARE OPENINGS
\/An\/^r11/t6
U.S. SIANDARD SERIES
{50 440 r30 4i6 ltd 4r)0 4100
TIME READINGS
24 HRS 7 HRS
SIEVE ANALYSISHYDROMETER ANALYSIS
0
to
20
30
,to
50
60
70
ao
9o
100
=
90
ao
70
50
50
10
20
10
o
=
,1 50 .500 76,2 27
I
200.425
PARTICLES
2,O
DIAMETER OF IN ETERS
CLAY TO SILT COBBLES
GRAVEL 67 % SAND
LIQUID LIMIT
SAMPLE OF: Slightly Sllty, Sondy Grovel
27%
PLASTICITY INDEX
SILT AND CLAY 6 %
FROM: Boringl CD 7.5' & 10' (Combined)
Thos€ losl resulls opply only lo lh6
somplcs which wrrr loslod. Ih6
lqsling roporl sholl nol bs r6produc6d,
cxcrpl ln full, wllhoul lhe wrltlenqpprovol of Kumqr & Assoqlolos, lnc.
Slovc onolysls tosllng ls psrformsd ln
occordonce wlth ASTM D6913, ASTM 07928,
ASTM Ct36 ondlor ASTM 01140.
COARSEFINECOARSEMEDIUMFIN E
GRAVELSAND
I(+A Kumar & Associates, lnc.@
Geotechnical and Materials Engineers
and Environmental Scientists
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
No.21-7-439
2
5
2y
5 I
3.7
r02
94 56
67 3t
1 94
6
A
LIQUID LIMIT
SAMPLE LOCATION
BORING DEPTH
71/, & rc 0.6
7.0
GRADATIONNATURAL
MOISTURE
CONTENT
NATURAL
DRY
DENSITY
PERCENT
PASSING NO.
200 srEvE
PLASTIC
INDEX
2Y,
GRAVEL
(%)
SAND
%t
(psfl
UNCONFINED
COMPRESSIVE
STRENGTH
Sandy Silty Clay
Slightly Silty, Sandy
Gravel
Very Sandy, Silty Clay
Sandy Silty Clay
SOIL TYPE