HomeMy WebLinkAboutSubsoil Study for Foundation Design 09.01.2020rcrf *iffififfir:É:l;1fiiË;u
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An Employcc Ownpd Cempony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
wwwkumarusa,cottt
Ofüce Locations: Denver (HQ), Pa¡ker, Colorado Springs, Fort Collins, Glenwood Springs, and Surnmit County, Colorado
RECEryED
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,r,îfr,i,lFlP,,Ê?#ilåiSUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 2, ENGLUND/MOORE SUBDIVISION
1666I HIGHWAY 82
GARFIELD COUNTY, COLORADO
PROJECT NO.20-7-471
SEPTEMBER 1,2020
PREPARED FOR:
NATE SIMMONS
C/O BACKBONE MEDIA
65 NORTH 4rH STREET, SUITE ONE
CARBONDALE, COLORADO 81623
nate.simm ons@backbonemedia.net
TABLE OF CONTENTS
PROPOSED CONSTRUCTION 1
SITE CONDITIONS -1-
SUBSIDENCE POTENTIAL I
FIELD EXPLORATION .|
-L-
SUBSURFACE CONDITIONS a
FOUNDATION BEARING CONDITIONS ......- 3 -
DESIGN RECOMMENDATIONS .........
HELICAL PIERS.......
MAT FOUNDATION ALTERNATIVE
FLOOR SLABS
SURFACE DRAINAGE......
LIMITATIONS 5
FIGURE I - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & A¡sociates, lnc, @ Project No.20-7-471
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on Lot 2,
Englund/Moore Subdivision, 16661 Highway 82, Garfield 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 proposal for geotechnical
engineering services to Nate Simmons dated August 12,2020.
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 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
Design plans for the residence have not been developed. We assume the proposed residence will
be a one or two story structure with a slab-on-grade floor. A separate swimming pool is also
being considered. 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.
When building location, grading and loading information have been developed, we should be
notified to re-evaluate the recommendations presented in this report.
SITE CONDITIONS
The proposed building area is graded relatively flat. The former "River Pond" at this site where
the residence will be located was backfilled about 27 yearc ago. The backfill consisted of
dredged and excavated material from "S Pond", located east of Lot I and was placed without
compaction control.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the site. These rocks are a
sequence of gypsiferous shale, fine-grained sandstone and siltstone with some massive beds of
Kumar & Associates, lnc. o Project No. 20-7-471
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gypsum and limestone. There is a possibility that massive gypsum 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.
Sinkholes have been observed scattered throughout the Roaring Fork River Valley.
Sinkholes were not observed in the immediate 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 for certain that sinkholes will not develop. The risk of
future ground subsidence on Lot 2 throughout the service life of the proposed residence, 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 freld exploration for the project was conducted on August 20,2020. Two exploratory
borings were drilled at the locations shown on Figure 1 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, 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 3 to 5% feet of sandy clay fill overlying 9Yz to I I feet of loose, clayey
silty sand and medium stiff, sandy silty clay pond sediment flrll. Below depths of 14 to 15 feet,
relatively dense, silty sand and gravel was encountered down to the maximum depth drilled of
2l feet. Drilling in the dense granular soils with auger equipment was difficult due to cobbles
within the gravel subsoils.
Kumar & Aesociates, lnc. @ Project No. 20-7-471
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Laboratory testing performed on samples obtained from the borings included natural moisture
content, density and gradation analyses. Results of gradation analyses performed on small
diameter drive samples (minus l%-inch fraction) of the coarse granular subsoils are shown on
Figure 4. The laboratory testing is summarized in Table 1.
Free water was encountered in the borings at depths of ll% to l3 feet at the time of drilling. The
fill soils and subsoils were very moist to wet.
FOUNDATION BEARING CONDITIONS
The existing fill at the site is variable quality and generally compressible. Conventional spread
footings bearing on these materials will have a risk of excessive foundation settlement and
building distress.We recommend a relatively deep foundatron system, such as driven or drill
piles or helical piers, bearing ln the underlying natural gravel and cobble subsoils, for support of
the proposed residence. Allowable load capacities of aboutl0 to 40 kips should be feasible
depending on the pile type and size. The pile should have relatively low settlement potential.
Cobbles and boulders in the fiil lif any¡ could cause installation difficulties for the piles.
A monolithic slab (mat) foundation can be considered as a foundation alternative with a higher
settlement risk. The slab should be heavily reinforced to help limit the effects of potential
differential settlement. A soil surcharge could be placed on the building site for several months
prior to the slab construction to further reduce the settlement potential.
An on-grade concrete feasible due to the for
differential settlements at this site. A concrete ppq[on piles may be feasible.
DESIGN RECOMMENDATIONS
HELICAL PIERS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the building be founded on piles that bear in the
natural granular soils. Helical piers can be considered for building support. The following
recommendations are presented below for a helical pier system. If recommendations for other
pile types are desired, we should be contacted.
1) Allowable bearing capacity of a helicalpier drilled to refusal into the underlying
natural gravel and cobble subsoils should be about 20 to 25 kips. Settlements for
properly installed helical piers area expected to beffiıÏr%"ì:nch.
Kumar & Associates, lnc. o Project No.20-7-471
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2) Difficult installation could be encountered due to cobbles and boulders in the fill.
Piles that refuse short of the bearing gravels should be re-drilled until adequate
bearing has been achieved.
3) The pile installation and bearing depths should be observed by a representative of
the geotechnical engineer.
MAT FOUNDATION ALTERNATIVE
The design and construction criteria presented below should be used for a mat foundation
system:
1) The mat can be designed for an allowable bearing pressure of 800 psf or a
modulus of subgrade reaction of 50 pci. Settlement could be on the order of 2 to
3 inches, depending on the depth of compressible soils, rigidity of the mat and
loading conditions.
2) Removal and replacement of 2 to 3 feet of the existing fill compacted would help
provide more uniform bearing conditions. The fill should be a granular material
devoid of vegetation and topsoil and rock larger than I inches. Subgrade
stabilization may be needed if soft soil conditions are encountered.
3) The soil surcharge should be placed on the prepared subgrade and have a depth on
the order of6 to 8 feet.
4) The perimeter of the mat should be protected against frost penetration by soil
cover or insulation.
5) Loose or disturbed soils at the bearing level, and structural fill within the building
area should be compacted to at least95o/o of the maximum standard Proctor
density near optimum moisture content.
6) A representative of the geotechnical engineer should observe the mat excavation
and test structural fill prior to concrete placement.
FLOOR SLABS
Non-structural floor slabs should be placed on compacted structural frll or be structurally
supported such as on a pile foundation or a mat foundation. To reduce the effects of some
differential movement, non-structural 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 free-draining gravel should be placed beneath
the slabs to act as a leveling course and provide a break for capillary moisture rise. This material
Kumar & Associates, lnc, o Project No. 20-7-471
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should consist of minus 2-inch aggregate with at least 50% retained on the No. 4 sieve and less
than2o/o passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95%o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the
imported granular soils devoid of vegetation, topsoil and oversized rock.
SURFACE DRAINAGE
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 and underslab areas should be avoided
during construction.
2) Exterior backfill should be adjusted to 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 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 6 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.
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 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 recommendations may be made.
Kumar & Associates, lnc. @ Project No.20-7-471
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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 veriff that the recommendations
have been 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,
Kumnr & Associntes, fnc.
Daniel E. Hardin, P.E.
DEH/kac
cc: Divide Creek
Kumar & Associates, lnc. ô Project No. 20-7-471
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Fig. 1LOCATION OF EXPLORATORY BORINGS20-7 -471 Kumar & Associates
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Fig. 2LOGS OF EXPLORATORY BORINGS20-7 -471 Kumar & Associates
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FILL: GRAVELLY SANDY CLAY, SILTY, MEDIUM STIFF, SLIGHTLY MOIST TO MOIST, BROWN TO
BLACK.
POND SEDIMENT FILL; ORGANIC SANDY SILTY CLAY TO ORGANIC CLAYEY GRAVELLY SAND,
SOFT TO VERY LOOSE, VERY MOIST TO WET, BLACK.
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SAND AND GRAVEL (SV-CU); SLTGHTLY S|LTY, COBBLES W|TH DEPTH, VERY DENSE, WET,
BROWN.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
DRIVE SAMPLE, 1 3,/8-INCH I.D. SPLIT SPOON STANDARD PENETRATION TEST
DRIVE SAMPLE BLOW COUNT. INDICATES THAT 6 BLOWS OF A 1 4O-POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCIIES.
4 oEprx To wATER LEVEL ENcouNTERED AT THE TIME oF DRTLLTNG.
_> DEPTH AT WHICH BORING CAVED FOLLOWING DRILLING.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON AUGUST 20,2O2O 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 lHE SAME AND THE LOGS OF THE
EXPLORATORY BORINGS ARE PLOTTED TO DEPTH.
4. THE EXPLORATORY BORING LOCATIONS 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 LEVELS SHOWN ON THE LOGS WERE MEASURED AÏ THE TIME AND UNDER
CONDITIONS INDICATED. FLUCTUATIONS IN THE WATER LEVEL MAY OCCUR WITH TIME.
7. LABORATORY TEST RESULTS:
wc = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSITY (pct) (Asru D2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (¡STV OOSIS);
-200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D1 140).
LEGEND AND NOTES Fig. 320-7 -471 Kumar & Associates
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HYDROMETER ANALYSIS SIEVE ANALYSIS
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SAND GRAVEL
FINE MEDIUM COARSE FINE COARSE
I{YDROMETER ANALYSIS SIEVE ANALYSIS
U.S. STANDARO SERIES CLEAR SOUARE OPENINOS
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20-7 -471 Kumar & Associates GRADATION TESÏ RESULTS Fig. 4
lC iffiåffifffi*r:-å**TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo. 20-7-4712II584l5I052%NATURALMOISTURECONTENTDEPTHBORINGSAMPLE LOCA.61140.934.8I4I46.s34.412.279locflNATURALDRYDENSITY475344319456616646959LIQUID LIMITGRADATIONPERCENTPASSING NO,200 stEvEPLASTICINDEXGRAVELlf/"|SAND(:/"1losfìUNCONFINEDCOMPRESSIVESTRENGTHOrganic Clayey GravellySandOrganic Sandy Silty ClaySilty Sandy GravelOrganic Sandy Silty ClayOrganic Sandy Silty ClayGravelly Sandy ClaySOIL TYPESlightly Silty Sand andGravel