HomeMy WebLinkAboutSubsoil Study for Foundation Design 05.11.2022l(+rt Kurmr & Assoclatss, lnc.o
Geotechnical and Materials Engineers
and Environmental Scientists
An Empþyoc oltrfþd Compony
5020 Cotutty Road 154
Glenrvood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
rvrvrv.krunarusa.com
Ofïìce Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Splings. and Summit Cotmty, Coloraclo
RËGËNEÐ
P*',äiiåhYi-v-iH#J'SUBSOIL STUDY
FOR f,'OT'NDATION DESIGN
PROPOSED RESIDENCE
LOT E-7, ASPEN GLEN
38 PUMA LANE
GARFTELD COUNTY, COLORADO
PROJECT NO.22-7-199
MAY 11,2022
PREPARED F'OR:
GIARD HOMES
ATTN: ROGER GIARI)
1431 AIRPORT ROAD
RIFLE, COLORADO 81650
Roser. giard@ einrdhomes.com
TABLE OF'CONTENTS
PURPOSE AND SCOPE OF STI"IDY ...............- 1 -
PROPOSED CONSTRUCTION .I
SITE CONDITIONS I
SUBSIDENCE POTENTIAL 2-
SUBSURFACE CONDITIONS .......- 3 -
FOI.INDATION BEARING CONDITIONS ....... 3 .
DESIGN RECOMMENDATIONS ..................
FOUNDATIONS
FOUNDATION AND RETAINING WALLS
FLOOR SLABS
SURFACE DRAINAGE................
LIMITATIONS
FIGURE I - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL-CONSOLIDATION TEST REST]LTS
FIGURE 5 _ GRADATION TES'T RESULTS
ITABLE 1- SUMMARY OF LABORATORY TEST RESULTS
J
J
4
5
6
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Kumar & Associates, lnc. @ Project No.22-7-199
PURPOSE AND SCOPE OF'STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
LotE-7, Aspen Glen, 38 Puma Lane, Garfield County, Colorado. The project site is shown on
Figure 1. The puiposÇ of the study was to develop recommendations for the foundation design.
The study was conducted in accordance with our agreement for geoteqhnical engineering
services to Giard Homes dated February 23,2022. Chen-Northern,Inc. previously conducted a
preliminary geotechnical study for preliminary plat design of the Aspen Glen development under
their Job No. 4 ll2 92, reports dated December 20, l99l and May 28, 1993.
A field exploration program consisting of exploratory borings \ryas 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 baöed on the proposed construction and the subsurface conditions
encountered.
PROPOSED CONSTRUCTION
The proposed residence will be a two-story wood-framed structure with an attached garage and
located on the lot as shown on Figure 1. Ground floors will be structural above crawlspace in the
living areas and slab-on-grade in the attached garage. Grading for the structure is assumed to be
relatively minor with cut depths between 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.
STrn CONDITIoNS
The lot is located on the southwest side of PumaLarrc as shown on Figure 1. Topography at the
site is valley bottom. The ground surface is relatively flat and slightly sloping down to the
northwest with about 1 foot of elevation difference across the proposed building footprint.
Vegetation consists of native grass and weeds.
Kumar & Associates, lnc. @ Project No.22-7-199
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SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Vallcy Evaporitc underlies the Aspen Glen
development. These roclcs u'e & sequence of gypsiferous shale, fine-grained sandstone and
siltstone with some massive beds of gypsum and limestone. There is a possibility that massive
g)¡psum deposits associated with the Eagle Valley Evaporite underlie portions of the lot.
Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can
produce areas of localized subsidence. Several sinkholes were observed by Chen-Northern
(1992 and 1993) scattered throughout the Aspen Glen property during the subdivision
development. These sinkholes appear similar to others associated with the Eagle Valley
Evaporite in areas of the Roaring Fork River valley.
The lot is located along the southeast perimeter of a broad subsidence area but sinkholes were
not observed in the immediate area of the subject lot. The closest mapped sinkhole within the
broad subsidence area is located about 550 feet northwest of the subject lot. No evidence of
cavities was encountered in the subsurface materials; however, our exploratory borings were
relatively shallow, for foundation design only. Basecl on our present knowleclge of the
subsurface conditions at the site, it cannot be said for certain that sinkholes will not develop.
The risk of future ground subsidence on Lot E-7 throughout the service life of the proposed
residenee from ground subsidence due to subsurface voids, in our opinion, is low and similar to
other nearby platted lots but the owner should be made aware of the potential for sinkhole
clcvclopmcnt. 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 February 24,2022. Three exploratory
borings were drilled at the locations shown on Figurc I to cvaluatc 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.
Samples of the subsoils were taken with l%-inch ancl 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 hy 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.
Kumar & Associates, lnc. @ Project No.22-7-199
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SUBSURFACE CONDITIONS
T
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils encountered, below about Yz-foot of topsoil, consist of about 4 to 6% feet of stiff, sandy
to very sandy silty clay overlying relatively dense, silty s#dy gravel and cobbles with small
boulders. Drilling in the dense coarse granular soils with auger equipment was difficult due to
the cobbles and boulders and drilling refusal was encountered in the deposit in all three borings.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density, and percent finer than sand size gradation analyses. Results of swell-
consolidation testing performed on relatively undisturbed drive samples of the silty clay soils,
presented on Figure 4, indicate low compressibility under light loading and natural low moisture
condition and low expansion potential when wetted. The samples showed moderate
compressibility under additional loading after wetting. The laboratory testing is summarized in
Table l.
No free water was encountered in the borings at the time of drilling and the soils were typically
slightly moist.
FOUNDATION BEARING CONDITIONS
The soils encountered at proposed excavation depths typically consist of low bearing capacity
silty clay with variable compressibility/heave potential mainly when wetted. The underlying
dense coarse granular soils have moderately high bearing capacity and relatively low settlement
potential. Lightly loaded spread footings can be used for building support and accepting a risk of
differential movement mainly if the silty clay soils become wetted. Extending the foundation
down to bear on the dense coarse granular soils is a way to mitigate the differential foundation
movement potential.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we believe the building can be founded with spread footings bearing
on the natural soils with some risk of movement as described below. 'We should review the
footinf subgrade conditions for possible sub-excavation of potentially expansive soils.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
Kumar & Associates, Inc. o Project N0.22-7-199
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1)Footings placed on the undisturbed natural soils should be designed fcrr an
allowable bearing pressure of 1,500 psf. Based on experience, we expect initial
settlementtlffotltings.l",ign"Mtructeclasc1iscusseclinthissectionwill
be about 1 inch or less. Additional settlement on the order of %to lYz-inches is
possible if the silty clay bearing soils are wetted. Footings extended down to bear
entirely on the coarse granular soils can be designed for an allowable bearing
pressure of 3,000 psf with settlement potential of about I inch or less.
The footingffiıuldfãrr, a minimum width of 18 inches for continuous walls and
2 feet for isolated pads.
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. \---
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 rcsist
lateral earth pressures as discussed in the "Foundation and Retaining'Walls"
section of this report.
The topsoil and loose or disturbed soils should be romoveel down to the
undisturbed natural soils. The exposed soils in footing area should then be
moistened and compacted. The exposed silty clay soils should be further
evaluated for expansion/compression potential and the need for sub-excavation
and replacement with compacted structural fill at the time of excavation.
Structural fill placed below footing areas (if any) should extend horizontally out
from the edge of the footing to a distance equal to at least %the depth of fill
below the footing and be compacted to at least 98olo of standard Proctor density
at near optimum moisture content.
A representative of the geotechnical engineer should evaluate structural fill for
compaction on a regular basis and observe all footing excavations for bearing
conditions prior to concrete placement.
3)
4)
FOUNDATION AND RETAINING WALLS
Foundal.ion walls ancl retaining structures which are laterally supported and can be expected to
undergo only a slight amount of deflection should be designed for a lateral earth pressure
computed on the basis of an equivalent fluid unit weight of at least 55 pcf for backfill consisting
2)
s)
6)
Kumar & Associates, lnc. @ Project No.22.7-199
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of the on-site soils. Cantilevered retaining structures (if any) which are separate from the
residence and can be expected to deflect sufficiently to mobilize the full active earth pressure
condition should be designed for a lateral earth pressure computed on the basis of an equivalent
fluid unit weight of at least 45 pcf for backfill consisting of the on-site soils.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, constrlction materials and equipment. The
pressures recommended above assume drained conditions behind the walls and a horizontal
backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will
increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain
should be provided to pro$ent hydrostatic pressure buildup behind walls.
Backfill should be placed in uniform lifts anä compacted to at least 90% of the maximum
standard Proctor density at near optim¡rm moisture content. Backfill placed in pavement and
walkway areas should be compacted to at least 95o/o of the maximum standard Proctor density.
Care should be taken not to overcompact the backfill or use large equipment near the wall, since
this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall
backfill should be expected, even if the material is placed correctly, and could result in distress to
facilities constructed on the bSkfill.
r
The lateral resistance of foundftion or retaining wall footings will be a combination of the
sliding resistance of the footing on the foundation materials and passive earth pressure against
the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated
based on a coefficient of friction of 0.35. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 300 p"f. fï,.
coeffrcient of friction and passive pressure values recommended above assume ultimate soil
strength. Suitable factors of safety should be included in the design to limit the strain which will
occur at the ultimate strength, particularly in the case of passive resistance. Fill placed against
the sides of the footings to resist lateral loads should be compacted to at least 95% of the
maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab-on-grade
construction with some risk of movement mainly if the subgrade 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
Kumar & Associates, lnc. o Project N0.22-7-199
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spacing and slab reinforcement should be established by the designer based on experience and
the intended slab use. A minimuur 4-inch layer of relatively well gradetl sand ancl gravel such as
road base should be placed beneath interior slabs for support. 'I'his material should consist of
minus 2-inch aggregate with at least 50% retained on the No. 4 sieve and less than 12% passing
the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95% of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site soils devoid of topsoil and oversized rocks.
SURFACE DRAINAGE
Proper grading and drainage will be very important to keeping the bearing soils dry and limiting
the building movement and potential distress. A perimeter foundation drain around shallow
crawlspace areas (less than4 feet deep) should not be needed with adequate compaction of
foundation backfill and positive surface drainage away from foundation walls. 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.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95Yo of the maximum stanclard Proctor density in pavement and slab areas
and to at least 90o/o 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.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irrigation, such as sod, and sprinkler
heads 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 ge$erally accepted geotechnical engineering
principles and practices in this arca atthis time. '!Ve make no warranty either express or implied.
Kumar & Associates, lnc. @ Project No.22-7.199
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The conclusions and recoflrmendations submitted in this report are based upon the data obtained
from the exploratory borings drilled at the locations indicated on Figure l, 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¡ecommendations may be made.
This re,port 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 recofirmendations, and to veriry that the recommendations
have bee,n appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. Vy'e recommend on-site observation
of excavations and foundation bearing strata and testing of structural fill by a representative of
the geotechnical engineer.
Respectfu lly Submitted,
Kumar & Associates, Inc.
.t
: ti "'. .,n ,.¡ *:.]- *--'y'r',r+iu"r"'' { },-
r{obert r. ouran, plPl
Reviewed by:
David A. Young, P.E.
RLD/kac
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Kumar & Associates, lnc. ri Project No. 22.7-199
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PROPOSED
RESIDENCE
EORING 5
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APPROXIMATE SCALE-FEET
22-7 -199 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Flg. 1
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:
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BORING 1
EL. 6075.5'
BORING 2
EL. 6075.5'
BORING 5
EL. 6074.5'
0 0
14/12
23/12
WC=8.5
DD=1 12
50/12
18/12
5
16/12
WC=9.9
DD=1 O8
51s/12
WC=7.5
-200=51
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15 lt
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20 20
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22-7 -199 Kumar & Associates ,t t'n' 2LOGS OF EXPLORATORY BORINGS
I
LEgEND
N
TOPSOIL; SANDY SILTY CLAY, FIRM, MO|ST, BROWN, ROOT ZONE.
CLAY (CL); SILTY, SANDY TO VERY SANDY, STIFF, SLIGHTLY MOlSt, REDDlSl.l BROWN,
SLIGHTLY CALCAREOUS.
GRAVEL AND COBBLES
MOIST, MIXED BROWN,
(0u);
ROCK
WITH SMALL BOULDERS, SANDY, SILTY, DENSE, SLIGHTLY
S ARE PRIMARILY ROUNDED TO SUBROUNDED.
DRIVE SAMPLE, 2-INCI{ I.D. CALIFORNIA LINER SAMPLE
I DRIVE SAMPLE, 1 5/9-|NCH t.D. SPLTT SPOON STANDARD PENETRATTON TEST.
,\/1, DRIVE SAMPLE BLOW COUNT. INDICATES THAT 25 BLOWS 0F A 140-POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
f enacrrcAL AuGER DRTLLTNc REFUSAL.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON FEBRUARY 24, 2022 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 TIIE SITE PLAN PROVIDED.
5. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE APPROXIMATED BY INTERPOLATION
BETWEEN CONTOURS O¡¡ 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) (ASTU D2216)I
-200= PERCENTAGE PASSING N0. 200 SIEVE (ASTM 01140). .
22-7 -199 Kumar & Associates LEGEND AND NOTES Fig. 5
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IF
SAMPLE OF: Sondy Silty Cloy
FROM:Borlng1@2'
WC = E.5 75, DD = 112 pcl
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EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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SAMPLE OF: Sondy Silty Cloy
FROM: Boring 5 @.4'
WC = 9.9 %, DD = 108 pcf
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EXPANSION UNDER CONSTANT
PRESSURE UPON WEÏTING#
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Fig. 422-7-199 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS
,
l$rt:ffiniffiiffin'1'å;'**'TABLE 1SUMMARY OF LABORATORY TEST RESULTSSOIL TYPESandy Siþ ClayVery Sandy Siþ ClaySandy Silt"v Clay(psf)UI.ICONFINEDcouPREsstvESTRENGTH(ololPLASTICINDEXATTERBERG LITITS(%lLIQUID LIfT]fPERCENTPASSING NO,200 stEvEI5SANDP/"1GRADATIONf/"1GRAVELlocf)NATURALDRYDENS]TYrt2108(o/ |NATURAT]IOISTURECONTE¡¡T8.57.59.9fftlDEPTH254SAIIPLE LOCATIONBORINGI2aJNo. 22-7-199