HomeMy WebLinkAboutSubsoil Studyrcrt Kumar & Assoclates, lnc. @
Geotechnical and Materials Engineers
and Environmental Scientists
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, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOI]NDATION DESIGN
PROPOSED RESIDENCE
LOT 40, PHASE 3, TRONBRTDGE
BLUE HERON DRIVE
GARFTELD COUNTY, COLORADO
PROJECT NO. 21-7-236
APRrL 14,2021
PREPARED FOR
scrB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD, SUITE 52018
ASPEN' COLORADO 81611
luke.gosda@sunriseco.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION
SITE CONDITIONS....
GEOLOGY
FIELD EXPLORATION
SIIBSURFACE CONDITIONS ..
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS
FOLTNDATIONS
FLOOR SI,ARS
UNDERDRAIN SYSTEM
SURFACE DRAINAGE...............
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1
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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.21-7-236
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 40, Phase 3, konbridge, Blue Heron Drive, Garf,reld 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 2,202I.
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 f,reld 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
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 1. For the purposes of our analysis, we
assume the proposed residence will be a wood frame structure over a crawlspace with an
attached slab-on-grade garage. Grading for the structure will be relatively minor with cut depths
up to about 2 to 3 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 site was vacarrt at the time of our field exploration. The site is split in two relatively
flat benches separated by a relatively steep 6 foot tall slope. The borings were drilled on the
Kumar & Associates, lnc. o Project No. 21-7-236
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upper bench as shown on Figure L Vegetation consists of grass and weecls. The downhill sicle
of Blue Heron Drive appears to be a fill bench for resiclence construction placecl during the
subdivisiort tleveloprnent. The Roaring Fork River is located downhill about Yc mile to the north.
GEOLOGY
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) clatecl October 29,1997, Job No. 191 327. 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, fîne-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 40 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
Â-¿.-..^ I^-,t-^il^--^^ ^,-I ^L^^L1-,-----l-- Lal ' l'¡ î'1 II 'll'rurule Bruuilu ¡iuusruËilçç uil r-ur +u rrlrougnout tne servlce ltle oI me proposeo Dullqlng, ln our
opinion, is low; however, the owner should be made aware of the potential for sinlchole
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 conclucted on March 22,2021. Two exploratory
borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions.
The borings were advancerl with 4 inch diameter continuous flight augers powered by a tnrck-
Kumar & Associates, lnc. @ Project No. 21.7.236
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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 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 about I foot of topsoil/root zone overlying silt and sand fill tobetweenlYz
and 8 feet deep where up to 5 feet of very stiff, sandy silt and clay soils was encountered. Dense,
silty sandy gravel with cobbles was encountered below the fill or silt and clay soils to the
maximum explored depth of 21 feet.
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
relatively undisturbed drive samples of the fill and natural clay soils, presented on Figure 4,
indicate low compressibility under existing low moisture conditions and light loading. The fill
sample showed minor expansion potential and the clay sample showed a low to moderate
expansion potential when wetted under constant light surcharge. Results of gradation analyses
performed on small diameter drive samples (minus l%-inch fraction) of the fill and underlying
granular soils are shown on Figure 5. 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 footing foundations placed on the relatively dense fill soil above the natural silt and clay
soils or underlying dense gravel soils should be adequate for support of the proposed residence
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with relatively low settlement potential. Although the tested soil samples showed a minor
expansìon potential whcn wcttecl, onr experience in this area is that the existing fill and clay soils
aro lypically not expansive. Footings bearing entirely on the dense gravel soils should have a
low settlement risk. The bearing condition of the soils exposed in the excavation should be
further evaluated at the time of construction.
DESIGN RECOMMENDATIONS
FOUNDATIONS
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 if encountered.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the relatively dense fill soils or the underlying natural soils
should be designed for an allowable bearing pressure of 1,500 psf. Based on
experience, we expect settlement of footings designed and constructed as
discussed in this secl"ion will be about 1 inch or less. Post-construction settlement
could be around 1 inch mainly if the 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
atea.
4) Continuous foundation walls should be heavily rcinforccd 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 fbr the onsite soils as backfill.
5) Any topsoil and loose disturbed soils should be removed and the footing bearing
lcvcl extended down to the relatively dense fill soils or natural soil bcncath thc
Kumar & Assoclates, lnc. o Project No. 21.7.236
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fill. The exposed soils in footing area should then be moistened and compacted.
Additional structural fill can consist of the onsite soils compacted to atleast9So/o
of standard Proctor density at near 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 on-site fill 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 road base
should be placed beneath garage level slabs. This material should consist of minus 2-inch
aggregate with at least 50% retained on the No. 4 sieve and less than L2o/o passing the No. 200
s1eve.
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 filI soils devoid of vegetation, topsoil and oversized rock.
I-INDERDRAIN 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)
Kumar & Associates, lnc, @ Project No.21-7'236
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placed at each level of excavation and at least 1 fcrot below lowest acljacent finish grade and
sloped at a minimum lYo to a suitablc gravity outlct or clrywell based in the gravel soils. Free-
draining granular matcrial 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 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.
SURT'ACE DRAINAGE
Proper surface grading and drainage will be critical to prevent wetting of the bearing soils and
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) lnundation of the foundation excavations and underslab areas should be avoided
during construction.
2) Exterior backfill should be adjustecl [o 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 90Yo 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 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 the on-site 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
backfill.
5) Landscaping which requires regular heavy inigation 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 inigation.
I,IMITATIONS
This study has been conductcd in accordance with generally accepted geotechnical engineering
principles attd practices in this area at this time. We make no warranty either express or implied.
Kumar & Associates, lnc. @ Project No. 2l-7-236
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 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 reconÌmendations 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 verifu that the recommendations
have been appropriately interpreted. Signifrcant 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.
James H. Parsons, P.E.
Reviewed by:
Steven L.
JHPlkac
Kumar & Associates, lnc, i'Project No. 21-7-236
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11.0'
106 .2'
LOT 4I
126 .9 '
'7 3 ..j
LOT 39
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APPROXIMATE SCALE-FEET
\ e8.0'
LOT 40
S¿!'BÃCK
BORING f\o
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PF.CPERi'Y
LrNE
21 -7 -236 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig.1
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BORING 1
EL.= 1 00'
BORING 2
EL.=101'
0 0
21 /12
WC=4.9
DD= 1 07
2e/12
q 50/12 q
e/6, 17 /6
WC=3.8
14=41
-200=33
26/12
10 10
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35/ 12 15/ 12
WC= 1 0.9
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DD= 1 00
15 15
50/ 4 53/ 12
20 2063/ 12
25 25
33/12
WC=7.3
DD=1 1 9
+4=13
-ZQO=62
21 -7 -236 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
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I
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LEGEND
TOPSOIL: SILT, SANDY, SCATTERED GRAVEL, ORGANICS, ROoT ZONE, FIRM, MOIST, BROWN
FILL: SILT, SANDY TO VERY SANDY, CLAYEY, SCATTERED GRAVEL WITH DEPTH, VERY
ST|FF/MED|UM DENSE, SL|GHTLY MO|ST, RED-BROWN.
SILT AND CLAY (ML-CL): SANDY, SCATTERED GRAVEL, VERY STIFF, SLIGHTLY MOIST, TAN.
GRAVEL (GM): SANDY, SILTY, COBBLES, DENSE, SLIGHTLY MoIST, RED-BRoWN, RoUNDED
ROCK.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
I DR|VE SAMPLE, 1 3/9-|NCH t.D. SpLtT SPOoN STANDARD pENETRATTON TEST.
21 /12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 21 BLOWS OF A 14o-POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
NOTES
1 . THE EXPLORATORY BORINGS WERE DRILLED ON MARCH 22, 2021 WITH A 4-INCH DIAMETER
CONTINUOUS-FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY TAPING
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.
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) (ISTV D2216);+4 = PERCENTAGE RETAINED ON N0. 4 SIEVE (ASTM D6915);
-2OO= PERCENTAGE PASSING NO.2OO SIEVE (ASTM 01140);
21 -7 -236 Kumar & Associates LTGEND AND NOTES Fig.3
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SAMPLE OF: Sondy Silt (Fill)
FROM: Boring 1 @ 1'
WC = 4.9 %, DD = 107 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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1.0 ED PRESSURE - KSF 100
SAMPLE OF: Sondy Silty Cloy
FROM:Boring2@10'
WC = 1 O.9 %, DD = 100 pcf
innot b6 rôproduc€d,
w¡thout th€ vr¡tl€n opprovol of
ond k6ociolcs, lnc. Swell
only to th€
EXPANSION UNDER CONSÏANT
PRESSURE UPON WETTING
21 -7 -236 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 4
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70
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,oof .o75 .t 50 .500 !
DIAMETER OF
2.O
MILLIMETERS
CLAY TO SILT COBBLES
GRAVEL 41 % SAND
LIQUID LIMIT
SAMPLE OF: Sondy Silty Grovel (Fill)
26%
PLASTICITY INDEX
SILT AND CLAY 33 %
FROM: Boring 1 @7.5'
too
90
ao
70
60
50
40
50
10
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10
20
30
40
50
60
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=
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I l..ot 9 .o37 150 5A.t 7 ã.2 1 27 200
DIA
,125
OF PARTICLES IN M S
CLAY TO SILT COBBLES
GRAVEL 13 % SAND 25
LIQUID LIMIT
SAMPLE OF: Grovelly Sondy Silt (Fîll)
%
PLASTICITY INDEX
SILT AND CLAY 62 %
FROM:Borlng2(}5'
lhsso lesl rô3ulls qpply only lo lh6
sqmplôs whlch wsr€ l€slsd, Thsl€sllng r€porl sholl nol b6 roproduced,
.xcspl ln full, wllhoul lhr wrlllrn
opprovol of Kumor & Assoclql€s, Inc.
Slsvo onqlysls l€sllñg ls p€rformed lh
occordoncÊ wlth ASTM 06913, ASTM D7928,
ASTM C136 ond/or ASTM Dll¡l0.
HYDROMETER ANALYSIS SIEVE ÀNÀLYSIS
lIMÊ REÂOINGS u.s.
60MIN
I
l
HRS 7 HRS
urN t 5 MtN
SAND GRAVEL
FIN E MEDIUM COARSE FINE COARSE
SIEVE ANALYSIS
CLEAR SOUARE OPENINGS
L
u.s.
HYDROMETER ANALYSIS
TIME READINGS
7 HRS
l l
SAND GRAVEL
FIN E MEDTUM lCOrnSe FINE COARSE
21 -7 -236 Kumar & Associates GRADATION TTST RESULTS Fig. 5
rc iitih:.ffiffÉtrriliä*."TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.21-7-236NATURALMOISTURECONTENTNATURALDRYDENSITYGRADATIONATTERBERG LIMITSUNCONFINEDCOMPRESSIVESTRENGTHBORINGDEPTHGRAVEL(:/"1SAND(%)PERCENTPASSING NO.200 stEVELIQUID LIMITPLASTICINDEXSOIL TYPEI14.9t07Sandy Silt (Fill)7y,3.84I26JJSandy Silty Gravel (Fill)257.3119I32562Gravelly Sandy Silt (Fill)1010.9100Sandy Silty Clay