HomeMy WebLinkAboutSubsoils Study for Foundation DesignI (}rl [1ffi1f,##o'nHff;,','3;n'*,,,
An Employcc Owncd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
emai I : kaglenwood@kumarusa.com
www.kumarusa.com
Offrce Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
January 11,2022
Brent Buchanan
1061 Park West Drive
Glenwood Springs, Colorado 81601
brent.audioed ge@aol.com
Project No. 21-7-844
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 40, Filing 9,
Elk Springs,2950 Elk Springs Drive, Garfield County, Colorado
Dear Brent:
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 October 25,2021. The data obtained and our
recommendations based on the proposed construstion and subsurface sonditions encountered are
presented in this report.
Proposed Construction: Development plans for the lot were not available at the time of our
study. The proposed residence will generally be located in the area of the exploratory pits shown
on Figure 1. Ground floors could be structural over crawlspace in living areas and slab-on-grade
in the garage. Cut depths are expected to range between about 2 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 at the time of our field exploration. The ground
surfbce slope is relatively uniform at about 10% down to the south with about 8 to 10 feet of
elevation difference across the general building area. Vegetation consists of grass, weeds and
sagebrush with scattered juniper trees.
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 I to I% feet of topsoil, consist of
I to 1% feet of very stiff, sandy clay with scattered gravel overlying dense/hard basalt cobbles
and boulders in a highly calcareous, sandy silt matrix. Results of a gradation analysis performed
on a sample of the highly calcareous silt matrix (minus 3-inch fraction) obtained from the site are
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presented on Figure 3. The laboratory test results are summarized in Table 1. No free water was
observed in the pits at the time of excavation and the soils were slightly moist.
Foundation Bearing Conditions: The upper clay soils encountered to a depth of up to ZYz feet
possess low bearing capacity and typically variable compression/expansion potential. The
underlying basalt rock/silt soils possess moderate bearing capacity and typically low
compressibilify potential. At assumed bearing depths, the foundation excavation could expose
the upper clay and underlying gravel soils. Spread footings that transition between the clay and
basalt rock soil will have an increased risk of differential foundation movement possibly
resulting in distress to the residence. To limit the movement risk, the upper clay soils should be
removed from below footing areas and replaced with compacted structural fill or the footing
bearing level extended down to the underlying granular soils.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, the upper clay soils should be
removed and spread footings placed entirely on the undisturbed natural granular soil designed for
an allowable soil bearing pressure of 2,000 psf. The matrix soils tend to compress after wetting
and there could be some post-constffi[dundation settlement. Footings should be a
minimum width of l6 inches for continuous walls and2 feet for columns. 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 granular soils. We should
observe the completed 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 shdild-nforced 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 55 pcf for the on-site soil as backfill. Structural fill placed
for foundation support should consist of relatively well graded granular material such as road
base compacted to at least 98% of standard Proctor density at near optimum moisture content.
Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded
slab-on-grade construction with a risk of movement due to the variable soil conditions. 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 free-draining gravel
should be placed beneath basement level slabs to facilitate drainage. This material should
Kumar & Associates, lnc. @ Project No. 21-7-844
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consist of minus 2-inch aggregate with less than 5AYo passing 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 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 granular material such
as road base.
Underdrain System: Although free water was not encountered'during our exploration, it has
been our experience in the area and where clay soils are present, 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 (if any), 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 surounded 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 lo/o 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 I% feet deep.
Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the residence has been completed:
l) 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%o 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 l2 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 irigation should be located at least
10 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.
Kumar & Associates, lnc. @ Project No. 21-7-84/'
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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
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 I
and to the depths shown on Figure 2othe 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 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 verifu 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 furttrer assistance, please let us know.
Respectfu lly Submitted,
Kumar & Associates,
Steven L. Pawlak,
Reviewed by:
Daniel E. Hardin, P.E.
SLP/kac
Attachments Figure I - Location of Exploratory Pits
Figure 2 - Logs of Exploratory Pits
Figure 3 - Gradation Test Results
Table 1 - Summary of Laboratory Test Results
F
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Kumar & Associates, lnc,6 Project No. 21-7-844
21 -7-844 Kumar & Associates LOCATION OF EXPLORATORY PITS Fig. 1
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PIT 1
EL. 701 I'PIT 2
EL. 7020'
PIT 5
E1.7018'
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I +4=32
-200= 1 8
5
LEGEND
TOPSOIL; ORGANIC SANDY SILTY CLAY, SLIGHTLY MOIST, BROWN, FROZEN
CLAY (CL); SILTY, SANDY, SCATTERED BASALT GRAVEL, VERY STIFF, SLIGHTLY MOIST,
BROWN, LOW PLASTICITY.
HIGHLY CALCAREOUS SILT AND BASALT ROCK (ML-GM); SANDY, HARD/DENSE, SLIGHTLY
MOIST, PALE_WHITE
DISTURBED BULK SAMPLE
t PRACTICAL DIGGING' REFUSAI-.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON DECEMBER 14,2021
2. THE LOCATIONS OF THE EXPLORATORY PJTS WERE MEASURED APPROXJMATELY BY PACING FROM
FEATURES SHOWN ON THE SITE PLAN PROVIDED.
5. 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.
7. LABORATORY TEST RESULTS:+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D A22);
-2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D 1 1 4O).
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21 -7 -844 Kumar & Associates LOGS OF EXPLORATORY PITS Fig. 2
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CLAY TO SILT COBBLES
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LIQUID LIMIT - PLASTICITY INDEX
SAMPLE OF: hlghly Colcdroous Sond ond Sllt
SILT AND CLAY 1A %
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21 -7 -844 Kumar & Associates GRADATION TEST RESULTS Fig. 3
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SUMMARY OF LABORATORY TEST RESULTS
No.2l-7-844
1 3%to 4
PIT DEPTH
SAMPLE LOCATION
Pl
NATURAL
MOISTURE
CONTENT
NATURAL
DRY
DENSITY
(pcfl
32
SAND
f/,1t:/,1
GRAVEL
50
GRADATION
I 8
PERCENT
PASSING,NO.
200 stEVE
UNCONFINED
COMPRESSIVE
STRENGTHLIQUID LIMIT PLASIIC
INDEX
Highly Calcareous Sand
and Silt
SOIL TYPE