HomeMy WebLinkAboutSubsoils Report for Foundation DesignI (*rt f,'iffil;i:,ffi,#:;l''l;; ;' *^
An Employcc €hrncd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
June21,202I
Carmen and
3841 SE way
Oregon 97080
pedrad6@aol.com
Project No.21,-7-477
Subject:Subsoil Study for Foundation Design, Proposed Residence, Lot74, Filing 9,
Elk Springs,2955 Elk Springs Drive, Garfield County, Colorado
Dear Carmen and Ann:
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 May 2I,2021. 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 design was preliminary atthe time of our
study. In general, the residence will be a one and two-story structure over a crawlspace or with
slab-on-grade floor located on the site in the area of the pits shown on Figure 1. Cut depths
could range between about 2 to 5 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 lot was vacant and vegetated with grass, weeds and sagebrush. The lot is
on a natural hilltop and slopes moderately down to the south with around 3 to 4 feet of elevation
difference across the assumed building site. The ground surface is natural with basalt cobbles
and boulders visible on the surface. The lot is bordered on the north by Elk Springs Drive.
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. Below about t/z to I foot of topsoil, the subsoils typically consist of basalt
gravel and cobbles with boulders in a pale-tan, highly calcareous sand and silt matrix. InPit2,
below the topsoil, about IYz feet of very stiff sandy silty clay was encountered overlying the
basalt rocks. Digging was difficult due to the highly calcareous matrix and basalt cobbles and
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boulders and practical digging refusal was encountered in the deposit in all three pits. Results of
swell-consolidation testing performed on a relatively undisturbed sample of the clay soil,
presented on Figure 3, indicate low compressibility under existing moisture conditions and light
londing nnd a minor expansion potcntial when wcttcd. No frce water was observed in the pits nt
thc tinrc uf sxcavatiou aud the suils wuru sliglrtly nruist.
Foundation Recommendations: Considering the slrbsoil conditions encountered in the
cxploratory pits and thc naturc of thc proposcd construction, wc rccommcnd sprcad footings
placed on the undisturbed natural highly calcareous sand and silt or basalt rock soil encountered
at depths of about %to2% feet and designed for an allowable soil bearing pressure of 2,000 psf
for support of the proposed residence. The sand and silt soils tend to compress after wetting and
there could be some post-construction foundation settlement. Footings should be a minimum
width of 16 inches for continuous walls and2 feet for columns. The topsoil, clay soils 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 soils. We
should observe the completed excavation for bearing conditions. 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 alateral 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.
Floor Slabs: The natural on-site soils, exclusive of topsoil and expansive clay, 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 relatively well graded sand and gravel such as road base should
be placed beneath slabs for support. This material should consist of minus 2-inch aggregate with
less than 50% passing the No. 4 sieve and less than l2o/o passing the No. 200 sieve. If a
basement is proposed, the gravel layer should be relatively free draining with less than2o/o
passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95o/o of maximum
standard Proctor density at a moisture content near optimum. Required filI can consist of the
Kumar & Associates, lnc. @ Project No. 21-7-477
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on-site soils devoid of vegetation, topsoil, clay and oversized rock or a suitable imported sand
and gravel such as 3A-inchroad base.
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, 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 1 foot below lowest adjacent finish grade and sloped at a minimum Io/oto
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 IYzfeet 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 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 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 should be capped with about 2 feetof the on-site, finer
graded soils to reduce surface water infiltration.
3) The ground surface surounding 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 l0 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 irigation 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 inigation.
Limitations: This study has been conducted in accordance with generally accepted geotechnical
engineering principles and practices in this area atthis time. We make no warranty either
Kumar & Aseociates, lnc. @ Project No. 21-7417
4
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 assumed type of construction, and ow experience in the
area- Our services do not include dete,nnining 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 exfrapolation 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 appeur 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 prepmed for the exclusive use by our client for design purposes. We me 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 v€riry 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 shata and testing of structural fiIl 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.
Steven L. Pawlak, P.E.
Reviewed by:
!
Daniel E. Hardin, P.E.
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Attachments: Figure 1 - Location of Exploratory Pits
Figure 2 - Logs of Exploratory Pits
Figure 3 - Swell-Consolidation Test Results
Table I - Summary of Laboratory Test Results
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Kumar & Associates, lnc. o Profec.tNo. 21-74n
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PIT 1
EL. 1 00'
PIT 2
EL. 1 01'
PIT 5
EL. 97,
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LEGEND
TOPSOIL; ORGANIC SANDY SILT AND CLAY WITH SCATTERED GRAVEL AND COBBLES, BROWN
CLAY (CL); SILTY, SANDY, VERY STIFF, SLIGHTLY MOIST, BROWN
BASALT GRAVEL, COBBLES AND BOULDERS (GM); HIGHLY CALCAREOUS SAND AND SILT
MATRIX, DENSE, SLIGHTLY MOIST, GRAY TO WHITE.
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HAND DRIVEN 2-INCH DIAMETER SAMPLE
DISTURBED BULK SAMPLE.
PRACTICAL DIGGING REFUSAL.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON MAY 26, 2021.
2. THE LOCATIONS OF THE EXPLORATORY PITS WERE MEASURED APPROXIMATELY BY PACING FROM
FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY PITS WERE MEASURED BY HAND LEVEL AND REFER TO
PIT 1 GROUND SURFACE AS EL. 1 OO" ASSUMED.
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
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216):
DD = DRY DENSITY (PCt) (ISTU D 2216).
21-7 -477 Kumar & Associates LOGS OF EXPLORATORY PITS Fig. 2
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SAMPLE OF: Sondy Cloy
FROM: Pii 2 @ 1.5'
WC = 9.9 %, DD = 103 pcf
to
Codi&lid tating p.rfomd in
dccordond dth m D-4il4.
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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21-7 -477 Kumar & Associates SWELL-CONSOLIDATION TEST RESULT Fig. 3
I(JlA Kunnr & Associales, lnc."
Geotechnical and Materials Engrneets
and Environnental Scientisls
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
No.21-7-477
SOIL TYPE
Highly Ca-careous Silt
Sandy Clay
UNCONFINED
COMPRESSIVE
STRENGTH
Iolol
PLASTIC
INDEXLIQUID LI1'TT
Iohl
PERCENT
PASSING NO,
200 stEVEf/"1
SAND
GMDATION
r:hl
GRAVEL
NATURAL
DRY
DENSlTY
locfl
103
lol
NAT!IRAL
MOF]TURE
CONTENT
11.0
9.9
(ftl
EEPTH
2t/r-3
t%
SAMPLE LOCA-ION
PIT
1
2