HomeMy WebLinkAboutSubsoils Report for Foundation Designrcn Kumar & Associates, lnc, u'
Geotechnical and Materials Engineers
and Environmental Scientists
An Employee Owned Compcny
5020 County Road 154
Glenwood Springs, CO 8160 I
phone: (970) 945-7988
fax: (970) 945-8454
enrai I : kaglenrvood@kurnarusa.cou'r
t{tl,ut. l< Ltntartt sa. cot.tt
Offlce Locations: Denver (FIQ). Parker, Colorado Splings, Fort Collins, Glenwood Springs, arrd Sutnurit County, Colorado
October 21,2021
Lisa McPhsrson
78 River Glen
Carbondale, Colorado 81623
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Project No.2l-7-164
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot2I, Ranch at
Coulter Creek, Saddle Drive, Garfield County, Colorado
Dear Ms. McPherson:
As requested, Kumar & Associates, Inc. performed a subsoil study for design of foundations at
the subject site. The study was conducted in accordance with our agreement for geotechnical
engineering services to you dated September 16,202L The data obtained and our
recommendations based on the proposed construction and subsurface conditions encountered are
presented in this report.
Proposed Construction: Residence building plans were not available at the time of our study.
In general, a single-family residence is proposed in the northeast part of the lot roughly between
the exploratory borings located as shown on Figure 1. Ground floors could be a combination of
structural over crawlspace and slab-on-grade. Cut depths are assumed to range between about2
to 6 feet. Foundation loadings for this type of construction are assumed to be relatively light and
typical of the proposed type of construction.
If building conditions or foundation loadings are significantly different from those described
above, we should be notified to re-evaluate the recommendations presented in this report.
Site Conditions: The subject site was vacant atthe time of our field exploration. The proposed
building site is a broad hilltop with gentle to moderate side slopes down away from Boring 2
shown on Figure 1. Vegetation consists of sagebrush, grass and scrub oak with basalt rocks
exposed on the ground surface throughout the lot including the proposed building area.
Subsurface Conditions: The subsurface conditions atthe site were evaluated by drilling
2 exploratory borings at the approximate locations shown on Figure 1 The logs of the borings
are presented on Figure 2. The subsoils encountered, below about 4 inches of topsoil, consist of
dense basalt gravel, cobbles and boulders in a calcareous sand and silt matrix to the maximum
boring depth of 7 feet. Drilling in the dense cobbles and boulders was difficult and practical
1
auger refusal was encountered in the borings. Results of gradation analyses performed on small
diameter drive samples of the sand and silt matrix soils (minus llz-inch fraction) of the coarse
granular subsoils are presented on Figure 3. The laboratory test results are summarizedin
Table 1. No free water was encountered in the borings at the time of drilling and the soils were
slightly moist.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory borings and the nature of the proposed construction, we recommend spread footings
placed on the undisturbed natural rocky soil designed for an allowable bearing pressure of 2,500
psf for support of the proposed residence. Foundation settlement potential should be low, up to
around 1 inch. Footings should be a minimum width of 16 inches for continuous walls and2 feet
for columns. The topsoil and loose disturbed soils encountered at the foundation bearing level
within the excavation should be removed and the footing bearing level extended down to the
undisturbed natural rocky soils. Voids created by boulder removal should be backfilled with
compacted road base or concrete. We should observe the completed building excavation for
bearing conditions prior to forming footings. Exterior footings should be provided with adequate
cover above their bearing elevations for frost protection. Placement of footings at least 36 inches
below the exterior grade is typically used in this area. Continuous foundation walls should be
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 be designed to resist a
lateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for the on-site
soil as backfill, excluding organics and rock larger than 6 inches. A sliding coefficient of 0.45
and equivalent fluid lateral passive earth pressure of 375 pcf can be used to resist lateral loading
on the foundation.
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 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 forjoint 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 basement level slabs to facilitate drainage.
This material should consist of minus 2-inchaggregate with less than 50Yo passing the No. 4
sieve and less than 2o/o passingthe No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95Yo of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the
onsite soils devoid of vegetation, topsoil and oversized rock or imported gravel such as road
base.
Kumar & Associates, lnc, @ Project No. 21-7-764
a-J-
Underdrain System: Although free water was not encountered during our exploration, 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, crawlspace and
basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain
system.
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 placed at each level of
excavation and at least I foot below lowest adjacent finish grade and sloped at a minimum I%o to
a suitable gravity outlet. Free-draining granular material used in the underdrain system should
contain less than 2Yo passingthe 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 IYz feet deep.
Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the residence has been completed:
1) Inundation of the 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 95oh 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.
Free-draining wall backfill should be covered with filter fabric and capped with
about 2 feet of the on-site, finer graded soils to reduce surface water infiltration.
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 pavement and walkway areas.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irrigation should be located at least
5 feet from the building. Consideration should be given to the use of xeriscape to
limit potential wetting of soils below the foundation caused by irrigation.
Limitations: This sfudy 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 1
and to the depths shown on Figure 2,the proposed type of construction, and our experience in
Kumar & Associates, lnc. o Project No. 21-7-764
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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 at once so 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 veriff 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.
If you have any questions or if we may be of further assistance, please let us know.
Respectfully Submitted,
Kumar & Associateso Inc.
Steven L. Pawlak, P.E.
Reviewed by:
Daniel E. Hardin, P.E.
SLP/kac
Attachments: Figure 1 - Location of Exploratory Borings
Figure 2 -Logs of Exploratory Borings
Figure 3 - Gradation Test Results
Table 1 - Summary of Laboratory Test Results
Kumar & Associates, lnc. @ Project No. 21-7-764
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21 -7 -764 Kumar & Associates LOCATION OF TXPLORATORY BORINGS Fig.1
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LEGEND
TOPSOIL; ORGANIC SAND AND SILT WITH BASALT GRAVEL, COBBLES AND BOULDERS, BROWN
m
BASALT COBBLES AND BOULDERS (GM); CALCAREOUS SAND AND SILT MATRIX, DENSE,
SLIGHTLY MOIST, GRAY.
I DRTVE SAMPLE, 1 3/8-|NCH t.D. SPLTT SPOON STANDARD PENETRATION TEST
28/6 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 28 BLOWS OF A 1 4O-POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 6 INCHES.
t PRACTICAL AUGER REFUSAL.
NOTES
1 . THE EXPLORATORY BORINGS WERE DRILLED ON SEPTEMBER 23, 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 NOT MEASURED 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 WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTU POSIS);
_2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM Dl140).
Fig. 221 -7 -764 Kumar & Associates LOGS OF EXPLORATORY BORINGS
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IIME REAOINOS u.s.
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100
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LES IN M
CLAY TO SILT COBBLES
GRAVEL 19 % SAND 41 %
LIQUID LIMIT PLASTICITY INDEX
SAMPLE OF: Silty Sond wiih Grovel Molrlx
SILT AND CLAY 37 %
FROM: Borlng 1 @ 2.5' & 5' (Combined)
=
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Li_ l I 100127 2001 .14 9.5 l9
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DIAMETER OF PARTICLES IN MI
CLAY TO SILT COBBLES
GRAVEL 21 % SAND
LIQUID LIMIT
SAMPLE OF: Very Silty Sond wllh Grovel
Molrix
31 %
PLASTICITY INDEX
FROM:Boring2@5'
SILT AND CLAY 4A %
Theso l€sl rasulls qpply only lo lh€
sompl€s whlch w€r€ l€slsd. The
l€sllng reporl shqll nol bo rsproduced,
€xc€pl ln full, wllhoul ihs wrlllonqpprovql of Kumqr & Assoclol€s, lnc.
Slev€ onolysls losllng ls prrform€d In
qccordqnce wlth ASTM D6913, ASTM 07928,
ASTM C136 ond/or ASTM 01140,
HYDROMETER ANALYSIS SIEVE ANALYSIS
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SAND GRAVEL
FINE MEDIUM ICOARSE FINE COARSE
SIEVE ANALYSISHYDROMETER ANALYSIS
IIME READINGS
24 HRS 7 HRS 4t
U.S. STANDARD SERIES
4Ed 4fd ttd 4t6 3to 4a 5'6'ta
CLEAR SOUARE OPENINGS
t/a' t/ar 1 1/2'
,/
SAND GRAVEL
FIN E MEDTUM lCOrnSe FINE COARSE
21 -7 -7 64 Kumar & Associates GRADATION TEST RESULTS Fig. 3
K+n Kumat & Associates, lnc.'u
Geoiecnnrcal and Maleriais Engrneers
and Environmental Scientists
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
SOIL TYPE
Silty Sand with Gravel
Matrix
Very Silty Sand with
Gravel Matrix
(osfl
UNCONFINED
COMPRESSIVE
STRENGTH
(ol
PLASTIC
INDEX
ATTERBERG LIMITS
(olol
LIQUID LIMIT
PERCENT
PASSING NO.
200 slEvE
37T91 1.s 44
48
SAND
l:/")
1
aJ
GRADATION
(%)
GRAVEL
I2
(pcfl
NATURAL
DRY
DENSTTY
("/rl
NATURAL
MOISTURE
CONTENT
2% and 5
combined
{ftt
DEPTH
SAMPLE LOCATION
BORING
1
12.852
No.2l-7-764