HomeMy WebLinkAboutSubsoil Study for Foundation Design 07.09.2021I.* ii+ilfi'ffi:'fffiifr"Ê;;'**
An Emfloycc Owncd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
www.kumarusa.colrl
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Sumrnit County, Colorado
SCAilIIED
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 263, TRONBRIDGE
RIVER VISTA
GARFTELD COUNTY' COLORADO
PROJECT NO. 21-7-514
JULY 9,2021
PREPARED FOR:
SCIB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD' SUrTE 52018
ASPEN, COLORADO 81611
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
BACKGROLIND INFORMATION ...
PROPOSED CON S'I'RU C'I'ION
SITE CONDITIONS
SUBSIDENPE POTENTIAL]":
FIELD EXPLGRÄTTOT{
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS ....
FOUNDATIONS
FOUNDATION AND RETAINING WALLS.
NONSTRUCTURAL FLOOR SLABS
LTNDERDRAIN SYSTEM
SITE GRADING...............
SURFACE DRAINAGE...
LIMITATIONS
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURES 3 &,4 _ SWELL-CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc, @ Project No.21-7-514
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot263,Ironbridge, River Vista, 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 June 6, 202T.
An exploratory boring was drilled 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 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.
BACKGROUND INFORMATION
The proposed residence is located in the existing Ironbridge development. Hepworth-Pawlak
Geotechnical, Inc. (now Kumar & Associates) previously conducted subsurface exploration and
geotechnical evaluation for the development of Villas North and Villas South parcels, Job No.
105 1 15-6, report dated September 14, 2005, and performed observation and testing services
during the infrastructure construction, Job No. 106 0367, between April 2006 and April 2007.
The information provided in these previous reports has been considered in the current study of
Lot263.
PROPOSED CONSTRUCTION
At the time of our study, design plans for the residence had not been developed. The residence
will likely be a one or two-story, wood-frame structure with structural slab foundation and no
basement or crawlspace. 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.
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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 rcport.
SITE CONDITIONS
The subject site was vacant at the time of our field exploration. The lot is located in the
northeastern part of the Villas South Parcel. The natural terrain prior to development in 2006
sloped down to the east at about 5Yo grade. The subdivision area was elevated by hlling above
the original ground surface to create a relatively flat building site off River Vista. 'l'he eastern
edge of the lot slopes steeply down to the adjacent pedestrian path. Vegetation consists of grass
and weeds with scattered sage brush and one shrub bush.
SUBSIDENCE POTENTIAL
Eagle Valley Evaporite underlies the project area which is known to be associated with sinkholes
andlocalized ground subsidence in the Roaring Fork Valley. A sinkhole opened in the cart
storage parking lot located east of the Pro Shop and north of the Villas South parcel in January
2005. Irregular surface features were not observed in the Villas South parcel that could indicate
an unusual risk of future ground subsidence andlocalized variable depths of the debris fan soils
were generally not encountered by the previous September 14,2005 geotechnical study. The
current subsurface exploration performed in the area of the proposed residence on Lot 263 did
not encounter voids. In our opinion, the risk of future ground subsidence on Lot 263 throughout
the service life of the proposed residence is low and similar to other areas of the Roaring Fork
Valley where there have not been indications of ground subsidence.
FIELD EXPLORATION
The field exploration for the project was conducted on June 14, 202I. One exploratory boring
was drilled at the location shown on Figure 1 to evaluate the subsurface conditions. The boring
was advanced with 4-inch diameter continuous flight augers powered by a truck-mounted CME-
458 drill rig. The boring was logged by a representative of Kumar & Associates, Inc.
Samples of the subsoils were taken with 1%-inch and 2-inch LD. California liner and split-spoon
samplers. The samplers were drivcn into thc subsoils at various depths with blows from a 140
pouttd hammçr lalling 30 inches. This test is similar to the standard penetration test clescribect by
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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 Log of Exploratory Boring, Figure 2. The
samples were returned to our laboratory for review by the project engineer and testing.
SUBSURFACE CONDITIONS
A graphic log of the subsurface conditions encountered at the site is shown on Figure 2. The
subsoils consist of about 9 feet of compacted fill soils overlying loose to medium dense (sand) or
medium stiff to very stiff (silt) soils with gravel and gravelly layers (alluvial fan deposits)
underlain by dense, slightly silty to silty, sandy gravel with cobbles at a depth of about 47.5 feet
to the maximum explored depth of 51 feet. The fill materials were mainly placed in 2006 and
consist of relatively dense, mixed silt, sand and gravel.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and density and swell-consolidation tests. Results of swell-consolidation testing
performed on relatively undisturbed drive samples of the sand and silt soils, presented on Figures
3 and 4, indicate low to moderate compressibility under loading and low collapse potential when
wetted under light load. The laboratory testing is summarizedin Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils were slightly
moist to moist with depth.
FOUNDATION BEARING CONDITIONS
The upper 9 feet of soils encountered in the boring consist of fill placed mainly in 2006 as part of
the subdivision development. The field penetration tests and laboratory tests performed for the
study, and review of the field density tests performed during the fill construction indicate the
structural fill was placed and compacted to the project specified minimum 95Yo of standard
Proctor density. Alluvial fan soils which tend to collapse (settle under constant load) when
wetted were encountered below the fill. The amount of settlement will depend on the thickness
of the compressible soils due to potential collapse when wetted, and the future compression of
the wetted soils following construction. Relatively deep structural fill as encountered will also
have some potential for long-term settlement but should be significantly less than the alluvial fan
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deposits. Proper grading, drainage and compaction as presented in the Surfoce Drainage section
will help to keep the subsoils dry and reduce the settlement risks. A heavily reinforced structural
slab or post-tensioned slab foundation designed for significant differential settlements is
recommended for the building support. As an alternative, a deep foundation that extends down
into the underlying dense, river gravel could be used to reduce the building settlement risk.
DESIGN RECOMMENDATIONS
FOLINDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the nature of
the proposed construction, we recommend the building be founded with a heavily reinforced
structural slab or post-tensioned slab f'oundation bearing on at least 9 feet of the existing
compacted structural fill. If a deep foundation system is considered for building support, we
should be contacted for additional recommendations.
The design and construction criteria presented below should be observed for a heavily reinforced
structural slab or post-tensioned slab foundation system.
1) A heavily reinforced structural slab or post-tensioned slab placed on compacted
structural fill should be designed for an allowable bearing pressure of1 ,500 psf.
The post-tensioned slab placed on structural fill should be designed for a wetted
distance of 10 feet or at least half of the slab width, whichever is greater.
Settlement of the foundation is estimated to be about 1 inch based on the long-
term compressibility of the fill. Additional settlement of about 2 inches is
estimated if the underlying debris fan soils were to become wet. Settlement from
the deep wettirrg would tend io 'oe uniftrrm across ihe buiiding area ancl the
settlement potential of the fill section should control the design.
2) The thickened sections of the slab for support of concentrated loads should have a
minimum width of 20 inches.
3) The perimeter turn-down section of the slab 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. If a fiost-
t('"--
protected foundation is used, the perimeter turn-down section should have at least
18 inches ofsoil cover.
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4)The foundation should be constructed in a "box-like" conftguration rather than
with irregular extensions which can settle differentially to the main buildingarca.
The foundation walls, where provided, should be heavily 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, if any, should also
be designed to resist lateral earth pressures as discussed in the "Foundation and
Retaining Walls" section of this report.
The root zone and any loose or disturbed soils should be removed. Additional
structural fill placed below the slab should be compacted to at least 98Yo of the
maximum standard Proctor density within 2 percentage points of the optimum
moisture content.
A representative of the geotechnical engineer should evaluate the compaction of
the fill materials and observe all footing excavations prior to concrete placement
to evaluate bearing conditions.
5)
FOUNDATION AND RETAINING WALLS
Foundation walls and retaining structures (if any) 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 50 pcf for backfill
consisting of the on-site soils. Cantilevered retaining structures 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 40 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, construction materials and equipment. The
pressures recommended above assume drained conditions behind the walls and ahorizontal
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 prevent hydrostatic pressure buildup behind walls.
Backfill should be placed in uniform lifts and compacted to at least 90Vó of the maximum
standard Proctor density at a moisture content near optimum. Backfill placed in pavement and
6)
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walkway areas should bc compacted to at least 95Yo of the maximum standard Proctor density.
Care should be taken not to ovcrcompact the baoktill or use large equipment near the wall, since
this coulcl oause 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 backfill.
The lateral resistance of foundation 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 325 pcf. The
coefficient 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
occrtr at the ultimate strength, particularly in the case of passive resistance. Fill placecl against
the sides of the footings to resist lateral loads should be compacted to at least 95o/o of the
maximum standard Proctor density at a moisture content near optimum.
NONSTRUCTURAL FLOOR SLABS
Compacted structural fill can be used to support lightly loaded slab-on-grade construction
separate from the building foundation. The fill soils can be compressible when wetted and can
result in some post-construction settlement. To reduce the effects of some differential
movement, nonstructural floor slabs should be separated from buildings to allow unrestrained
vertical movement. Floor slab control joints should be used to reduce damage due to shrinkage
cracking. The requirements for joint spacing and siab reinforcement shouici be established by the
designer based on experience and the intended slab use. A minimum 4-inch laycr of rclativcly
well-graded sand and gravel, such as road base, should be placed beneath slabs as subgrade
support. This material should consist of minus 2-inchaggregate with at least 50% retained on
the No. 4 sieve and less than I2Yo passing the No. 200 sieve.
All fill materials fbr support of tloor 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 predominantly granular soils devoid of vegetation, topsoil and oversized rock.
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UNDERDRAIN SYSTEM
It is our understanding the finished floor elevation at the lowest level is at or above the
surrounding grade. Therefore, a foundation drain system is not required. It has been our
experience in the areathat 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, be protected from
wetting and hydrostatic pressure buildup by an underdrain and wall drain system.
If the finished floor elevation of the proposed structure has a floor level below the surrounding
grade, we should be contacted to provide recommendations for an underdrain system. All earth
retaining structures should be properly drained.
SITE GRADING
Extensive grading was performed as part of the existing Villas South development. Additional
placement and compaction of the debris fan soils could be needed to elevate the site to design
grades and reduce the risk of excessive differential settlements and building distress. In addition,
the water and sewer pipe joints should be mechanically restrained to reduce the risk ofjoint
separation in the event of excessive differential settlement. Additional structural fill placed
below foundation bearing level should be compacted to at least 98% of the maximum standard
Proctor density within 2o/o of optimum moisture content. Prior to fill placement, the subgrade
should be carefully prepared by removing any vegetation, organic soils, and any other
deleterious materials and compacted to at least 95Yo of the maximum standard Proctor density at
near optimum moisture content. The fill should be benched into slopes that exceed 20Yo grade.
Permanent unretained cut and fill slopes should be graded at 2 horizontal to 1 vertical (2H: lV) or
flafter and protected against erosion by revegetation or other means. This office should review
site grading plans for the project prior to construction.
SURFACE DRAINAGE
Precautions to prevent wetting of the bearing soils, such as proper backfill construction, positive
backfill slopes, restricting landscape irrigation and use of roof gutters, need to be taken to help
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limit settlement and building distress. The following drainage precautions should be observed
during construction and maintained at all times after the residencc has bccn complctcd:
1) Inundation of the building structural slab foundation excavations should be
avoided during construction.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95Yo of the maximum standard Proctor density in pavement and
nonstructural slab areas and to at least 90%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 6 inches in the f,rrst 5 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%.
4) Roof downspouts and drains should discharge at least 5 feet beyond the
foundation and preferably into a subsurface solid drainpipe.
5) Landscaping which requires regular heavy irrigation should be located at least
10 feet from lountlation 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
prirruiples altl prauliues in l.]ris area aL this [ime. 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 boring drilled at the location indicated on Figure 1, the proposed type of
construction and our experience in the area. Our services do not include determining the
prcscncc, prcvcntion or possibility of mold or other biological contarninants (MOBC) developing
in the future. If the client is concemed about MOBC, then a professional in this special held of
practice should be consulted. Our findings include interpolation and extrapolation of the
subsurface conditions identified at the exploratory boring and variations in the subsurface
conditions may not become evident until excavation is performed. If conditions encountered
during construction appear clilferent from those described in this report, we should be notified so
that re-evaluation of thc rccomrncndations may be made.
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This report has been prepared for the exclusive use by our client for design pu{poses. 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 veriÛr 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,'
Kumar & Associates" Inc.
Mark E.I.T.
Reviewed by:
Steven L. Pawlak,
MGlkac
1â222
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Kumar & Associates, lnc. €Project No. 21.7.514
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BORING I
LOr t63
LOT 264
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PROPERIY
LrNE
LOl 262
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APPROXIMATE SCALE_FEET
21 -7 -51 4 Kumar & Associates LOCATION OF EXPLORATORY BORING Fig. 1
BORING 1
EL. 5970'
LEGEND
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FILL; SAND, SILTY T0 VERY SILTY, SLIGHTLY CLAYEY, GRAVELLY,
DENSE, SLIGHTLY MOIST ÏO MOIST, LIGHT TO MEDIUM BROWN.
53/6
SAND AND SILT (SM-ML); WITH GRAVEL & SCATTERED C0BBLES,
RELATIVELY THIN CLAY LAYERS AT DEPTH, TRACE TO SLIGHT
CALCAREoUSNESS, L00SE T0 MEDTUM DENSE (SAND) 0R MEDTUM STTFF T0
VERY STTFF (S|LT), SLTGHTLY MoIST T0 Mo|ST, LTGHT T0 MEDTUM BRoWN.
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50/5 SAND AND GRAVEL (SM-GM); SILTY, MEDIUM DENSE, SLIGHTLY M0lsT
TO MOIST, LIGHT TO MEDIUM BROWN.
60/12
WC=6.8
DD=1 14
-200=50
GRAVEL (GM-GP); SLIGHTLY SILTY TO SILTY, SANDY, COBBLES, POSSIBLE
BOULDERS, ROUNDED ROCK, DENSE, SLIGHTLY MOIST TO MOIST, LIGHT
GRAY TO BROWN.
10 35/ 12
WC=6.8
DD= 1 03
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
i DRTVE SAMPLE, 1 3/8-|NCH t.D. SpLrT Sp00N STANDARD
PENETRATION TEST.
15
15/ 12 56712 DRIVE SAMPLE BL0W COUNT. INDICATES THAT 60 BLOWS 0F--I'- A 14o-POUND HAMMER FALLING 30 INCHES WERE REQUIRED
TO DRIVE THE SAMPLER 12 INCHES.
20
7/12 NOTES
THE EXPLORATORY BORING WAS DRILLED ON JUNE 14, 2021
WITH A 4-INCH DIAMETTR CONTINUOUS FLIGHT POWER AUGER.
17 /12
WC= 1 0.0
DD= 1 06
2. THE LOCATION OF THE EXPLORATORY BORING WAS MEASURED
APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE SITE
PLAN PROVIDED.
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3. THE ELEVATION OF THE EXPLORATORY BORING WAS OBTAINED
BY INTERPOLATION BETWEEN CONTOURS ON THE SITE PLAN
PROVIDED.
30
40
4. THE EXPLORATORY BORING LOCATION AND ELEVATION SHOULD BE
CONSIDERED ACCURATE ONLY TO THE DEGREE IMPLIED BY THE
METHOD USED.
17 /12
1o/12
WC= 1 8.6
-200=76
5. THE LINES BITWEEN MATERIALS SHOWN ON ÏHE EXPLORATORY
BORING LOG REPRESENT THE APPROXIMATE BOUNDARIES BETWEEN
MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL,
6 GROUNDWATER WAS NOT ENCOUNTERED IN THE BORING AT THE
TIME OF DRILLING.
7 LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216);
DD = DRY DENSITY (PCf) (ASTM D 2216);
-200 = PERCENTAGE PASSING N0. 200 SIEVE (ASTM D 1140)
45
50
50/.75
LOG OF EXPLORATORY BORING fig. 221 -7 -51 4 Kumar & Associates
SAMPLE 0F: Sond ond Silt wilh Grovel
FROM: Boring 1 @ 10'
WC = 6.8 %, DD = 105 pcf
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21 -7 -51 4 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 3
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SAMPLE OF: Sqnd ond Silt
FROM:Boringl@25'
WC = 1 O.0 %, DD = 106 pcf
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21 -7 -51 4 Kumar & Associates SWELL_CONSOLIDATION TEST RESULTS Fig. 4
I
rc 5;,çl,¡'ffi¡':Ëii'"nÊ;ä'*'*TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.21-7-514SOIL TY"ESlightly Gravelly SlightlyClayey Sandy Silt (Fill)Sand and Silt with GravelSand and SiltSandy SiltlosflUNCONFINEDCOMPRESSIVESTRENGTHlololPLASTICINDEXATTERBERG LIMITS(ololLIQUID LIMITPERCENTPASSING NO.2()f) SIEVE5016%tSANDGRADATION$tGRAVELlocflNATURALDRYDENSITYrr4103106P/"1NATURALMOISÏURECONTENT6.86.810.018.6fft)DEPTH7Y,012540SAIIPLE LOCATIONBORING1