HomeMy WebLinkAboutSubsoils Report for Foundation DesignlGrtåiffififfiffinii;-'"
An Employce Owncd 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 Comç Colorado
April18,2023
Chris Morrow
4190 County Road 103
Carbondale, Colorado 81623
cmcconstruction I 23 @smail.com
Project No. 23-7-158
Subject: Subsoil Study for Foundation Design, Proposed Shop/ADU , lg}County
Road 103, Carbondale, Colorado
Dear Chris:
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 February 16,2023. The data obtained and our
recommendations based on the proposed construction and subsurface conditions encountered
are presented in this report.
Proposed Construction: The proposed shop/ADU will be a single-story wood-frame building
with attached garage located on the site in the area of the pits shown on Figure l. Ground floor
will be slab-on-grade. Cut depths are expected to range between about2 to 4 feet. Foundation
loadings for this type of construction are assumed to be relatively light to moderate and typical
of the proposed type of construction.
If building conditions or foundation loadings are significantly different from those desuibed
above, we should be notified to re-evaluate the recommendations presented in this report.
Site Conditions: The subject site was developed with an A-frame residence and single-story
outbuildings at the time of our field exploration. The ground surface was gently sloping down
tothesouthatgradesofb@t.Vegetationconsistsofgrassandcottonwood
trees.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating
two exploratory pits at the approximate locations shown on Figure l The logs of the pits are
presented on Figure 2. The subsoils encountered, below about I %to 3Yz feet of topsoil, consist
of very stiff to hard, sandy to very sandy silt with scattered basalt fragments to 4 feet deep in
Pit 1 and weathered, very stiff to hard, highly calcareous very sandy silt (caliche)to 6Yz feet deep
in Pit 2. Results of swell-consolidation testing performed on relatively undisturbed samples of
the silt and caliche, presented on Figures 3 and 4, indicate low to moderate compressibility under
-2
existing moisture conditions and light loading and minor collapse potential (settlement under
constant load) when wetted. The sample of silt from Pit I exhibited a moderate settlement
potential when subjected to increased loading after wetting, No free water was observed in
the pits at the time of excavation and the soils were slightly moist to 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 soil designed for an allowable soil bearing pressure of
*50,q-psfferyupport of the proposed residence. The 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 and2 feet for columns. Loose disturbed soils and
existing fill 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 u1 1s¿st 36 inches.below the exterior grade is typically used in this area.
Continuous foundation walls should be well reinforced top and bottom to span local anomalies
and better resist any differential movement 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 fluid unit weight of at least 50 pcf for the on-site
soil as backfill.
X'loor 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 road base gravel should be placed beneath slabs-on-grade to provide support. This
material should consist of minus 2-inch aggregate with less than 50Yo passing the No. 4 sieve
and less than lt9'o passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least95%o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the
on-site soils devoid ofvegetation, topsoil and oversized rock.
Underdrain System: It is our understanding the proposed finished floor elevation at the lowest
level is at or above the surrounding grade. Therefore, a foundation drain system is not required.
Kumar & Associates, Inc. o Project No. 23-7-158
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Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the building 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 95%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.
Free-draining wall backfill should be covered with filter fabric and capped with
about2 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 l0 feet in pavement and walkway areas. A swale may be
needed uphill to direct surface runoff around the building.
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 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 concemed
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 evolveso we
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to veri$, that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
Kumar & Associates, lnc. o Project No. 23-7-158
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ormodifications to the recomme,ndations presented herein. IVe recommend on site observation
of excavations and foundation bearing strata and testing of structural filI by a representative of
the geotechnical engineer.
Ifyou have any qucstions or if we may be of further assistance, please let us know.
Respectfulþ Submitted,
Kumar & Associates, lnc,
James H. Parsons, P.E.
Reviewedby:
Daniel E. Hadin, P.E.
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attachme,nts Figure I - Location of Exploratory Pits
Figure 2 - Logs of Exploratory Pits
Figures 3 and 4 - Swell-Consolidation Test Results
Table I - Summary of Laboratory Test Results
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23-7 -158 Kumar & Associates LOCATION OF EXPLORATORY PITS Fig. 1
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LEGEND
10
TOPSOIL SILT AND CLAY, ORGANICS, BROWN, MOIST, FIRM
srLT (ML); CLAYEY, SANDY TO VERY SANDY, SLTGHTLY CALCAREOUS, SCATTERED BASALT
FRAGMENTS, HARD, SLIGHTLY MOIST, BROWN.
SILT (ML); SANDY TO VERY SANDY HIGHLY CALCAREOUS, CALICHE, VERY STIFF TO HARD,
LESS WEATHERED WITH DEPTH, SANDY LENSES.
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HAND DRIVE SAMPLE
DISTURBED BULK SAMPLE.
PRACTICAL DIGGING REFUSAL.
NOTES
I. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON APRIL 13,2023.
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 AS A 1OO' ASSUMED BENCHMARK.
4. THE EXPLORATORY PIT LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATE ONLY
TO TF|E DEGREE |MPLIED 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:
WC = WATER CONTENT (%) (ASTM Ð 2216);
DD = DRY DENSITY (pcf) (ASTM D 2216);
-2oo= PERCENTAGE PASSING N0. 200 SIEVE (ASTM D 1140).
23-7-158 Kumar & Associates LOGS OF EXPLORATORY PITS Fig. 2
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SAMPLE OF: Sondy Cloyey Silt
FROM:Boringl@2'
WC = 1O.9 %, DD = 93 pcf
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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23-7-158 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 3
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SAMPLE OF: Colcoreous Sondy Silt (Coliche)
FROM: Boring 2 GD 5.5'
WC = 33.0 "t6, DD = 93 pcf
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23-7 -158 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 4
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
No.23-7-158
ATTERBERG LIIIITSsaltPtLOCATTO]{
SOIL TYPE
f0Aì
LIQUID LITIT
t%t
PLASTIC
It{DÊX
lDll
UNCONFII{ED
corPRESstvE
STRENGII{
aq1t
ITATURAL
il0tsTuRE
COI{TENT
IIATURAL
DRY
DENSITY
fôcll
GRAVEL
(96)
SAI{D
(%)
PERCÊI{T
PASSING NO.
200 stEvE
BORING
rñl
DEPTH
Sandy Clayey Silt210.9 93I
Sandy Clayey Silt5l4ts.4
Calcareous Sandy Silt
lCaliche)2 3Y2 33.0 93