HomeMy WebLinkAboutSoils Report 12.16.2020Ilt+A
Kumar & Associates, Inc.®
Geolerhnical and Materials Engineers
and Environmental Scien!ists
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
An Employee Owned Company www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
December 16, 2020
Crawford Design Build, LLC
Attn: Simon Bentley
P.O. Box 1236
Carbondale, Colorado 81623
(cdbsim on@colrlcast.net)
Project No. 20-7-664
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 5, Wooden Deer
Subdivision, TBD Wooden Deer Road, Garfield County, Colorado
Dear Simon:
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 Crawford Design Build, dated October 30, 2020. The data obtained and
our recommendations based on the proposed construction and subsurface conditions encountered
are presented in this report.
Proposed Construction: The proposed residence will be a two-story wood frame structure with
a slab -on -grade main floor, located on the site in the area of the pits shown on Figure 1. Cut
depths are expected to range between about 3 to 4 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 building area is located on the south side of the lot in a relatively flat area
with steeper slopes down to the south along the property line. The area is near the top of a broad
ridge. There is a knoll of basalt rocks just southwest of the proposed house and south of the
proposed septic area. The area of the lot north of the building area slopes strongly down to the
north at about 10 to 12 percent. Vegetation consists of relatively dense pinon and juniper forest
with an understory of mostly dead sage brush and cactus. Basalt rocks were observed on the
ground surface in the building area.
Subsidence Potential: Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the
lot. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with
some massive beds of gypsum and limestone. There is a possibility that massive gypsum
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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. During previous work in the area, several sinkholes were observed
scattered throughout the Missouri Heights area. These sinkholes appear similar to others
associated with the Eagle Valley Evaporite in areas of the lower Roaring Fork 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 pits 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 5 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.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating
three exploratory pits at the approximate locations shown on Figure 1. The logs of the pits are
presented on Figure 2. The subsoils encountered, below about 10 to 12 inches of topsoil, consist
of basalt rocks up to boulder size in a white, calcareous, gravelly sandy clayey silt matrix.
Refusal to digging with a small backhoe was encountered at 3 to 4 feet in the pits in the building
area and at 5 feet in the profile pit in the septic area. No free water was observed in the pits at
the time of excavation and the soils were slightly moist.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend spread footings
placed on the undisturbed natural basalt rock soil designed for an allowable soil bearing pressure
of 2,000 psf for support of the proposed residence. The matrix soils tend to compress after
wetting and there could be some post -construction foundation settlement. Footings should be a
minimum width of 18 inches for continuous walls and 2 feet for columns. Loose and 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 soils. 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 (if any) should be designed to resist a lateral earth pressure based on an equivalent
Kumar & Associates, Inc. ® Project No. 20-7-664
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fluid unit weight of at least 45 pcf for the on -site soil as backfill excluding organics and rock
larger than 6 inches.
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 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 gravel should be placed beneath slabs to provide support. This material
should consist of minus 2-inch aggregate with less than 50% passing 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 or a suitable imported gravel devoid of vegetation, topsoil and oversized rock.
Underdrain System: Although free water was not encountered during our exploration, it has
been our experience in the area that 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, deep crawlspace
and basement areas, be protected from wetting and hydrostatic pressure buildup by an underdrain
system. The proposed slab -on -grade construction should not require an underdrain.
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
placed at each level of excavation and at least 1 foot below lowest adjacent finish grade and
sloped at a minimum 1% to a suitable gravity outlet. Free -draining granular material 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 1'/2 feet deep.
Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the residence has been completed:
11 Inundation of the foundation excavations and underslab areas should be avoided
during construction.
Kumar & Associates, Inc. ® Project No. 20.7.664
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2) Exterior hackfiill should be adjusted to near optimum moisture and compacted to
at least 95% of the maximum standard Proctor density in pavement and slab areas
and to at least 90% of the maximum standard Proctor density in landscape areas.
Free -draining wall backfill (if any) should be 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 6 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.
Septic Area: The subsoil information obtained in the proposed septic area (Profile Pit 1) can be
used for a properly designed infiltration septic system. The soil type is R-0 per state design
criteria due to the basalt rocks encountered, and a sand filter will be required below the
infiltrators.
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 pits excavated at the locations indicated on Figure 1
and to the depths shown on Figure 2, 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 pits and variations in the subsurface conditions may not become evident until
excavation is performed. if conditions encountered during construction appcar 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 verify that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
Kumar & Associates, Inc. ® Project No. 20-7-664
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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 & Associates, inc.
Daniel E. Hardin, P.E.
Rev. by: SLP
DEH/kac
attachments Figure 1— Location of Exploratory Pits
Figure 2 — Logs of Exploratory Pits
Figure 3 — Gradation Test Results
Kumar & Associates, Inc. Project No. 20-7-664
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APPROXIMATE SCALE —FEET
PROFfLE PIT 1
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20-7-664
Kumar & Associates
LOCATION OF EXPLORATORY PITS
Fig. 1
4
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1-
w
I
2
H
d
w
D
0
— 5
LEGEND
PIT 1
EL. 6721'
PIT 2
EL. 6718'
PROFILE PIT 1
EL. 6721'
0
—w
L
2
1-
d
GRAVEL=48 D
SAND=35 5 —
SILT=15
CLAY=2
TOPSOIL; ORGANIC SANDY SILTY CLAY WITH ROOTS AND BASALT ROCKS, LOOSE, SLIGHTLY
MOIST, DARK BROWN.
BASALT ROCKS (GP); GRAVEL TO BOULDER SIZE IN A SANDY CLAYEY SILT MATRIX,
CALCAREOUS, DENSE, SLIGHTLY MOIST, WHITE.
DISTURBED BULK SAMPLE.
t PRACTICAL AUGER REFUSAL.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON NOVEMBER 4, 2020.
2. THE LOCATIONS OF THE EXPLORATORY PITS WERE MEASURED APPROXIMATELY BY PACING FROM
BUILDING CORNERS STAKED IN THE FIELD.
3. THE ELEVATIONS OF THE EXPLORATORY PITS WERE OBTAINED BY INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED.
4. THE EXPLORATORY PIT 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 PIT LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE PITS AT THE TIME OF EXCAVATION. PITS WERE
BACKFILLED SUBSEQUENT TO SAMPLING.
7. LABORATORY TEST RESULTS:
GRAVEL = PERCENT RETAINED ON NO. 10 SIEVE;
SAND = PERCENT PASSING NO. 10 SIEVE AND RETAINED ON NO. 325 SIEVE
SILT = PERCENT PASSING NO. 325 SIEVE TO PARTICLE SIZE .002MM;
CLAY = PERCENT SMALLER THAN PARTICLE SIZE .002MM.
20-7-664
Kumar & Associates
LOGS OF EXPLORATORY PITS
Fig. 2
HYDROMETER ANALYSIS
SIEVE ANALYSIS
}
24
, 56I11.
TIME READINGS
HR. 7 HR 1 MN.
151Nb, (t71ki wt. 4uw.. 032T
U.S. STANDARD SERIES I CLEAR SQUARE OPENINGS
#140 #60 #35 #1B #10 #4 31$ 3N' 11ik' 3" 5"6" 8"
`
.,,,,
PERCENT RETAINED
3 0 0 0 0 0 0 0 0 0
r
r
N m ro
O O O O J O O O O
PERCENT PASSING
.001 .002 .005 .009 019 ,045 .106 .025 .500 1.00 2.00 4.75 9.5 190 37.5 76.2 152 203
DIAMETER OF PARTICI FS IN MII I RAFTERS
CLAY SILT SAND
I1 GRAVEL
Y.rw MED
I_._._I Ec�xIV.
:nwl. i raeoM., f ustm
COBBLES
GRAVEL 48 % SAND 35 % SILT 15 % CLAY 2 %
USDA SOIL TYPE: Very Gravelly Loamy Sand FROM: Protile Pit 1 @ 4'-5'
20-7-664
Kumar &Associates
GRADATION TEST RESULTS
Fig. 3