HomeMy WebLinkAboutSubsoil Study for Foundation Design 12.29.16H-P~KUMAR
Geotechnical Engineering I Engineering Geology
Materials Testing I Environmental
5020 County Road 154
Gl•nwood Springs, CO 81601
Phone: (970) 94!5-7988
Fax: (970) 945-8454
Email: hpkglenwood@kumarusa.com
Office Locations: Parker, Glenwood Springs, and Silverthorne, Colorado
December 29, 2016
Tara Oak.ides
1927 Panorama Drive
Carbondale, Colorado 81623
(tdakides@gmail.com)
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Project No.16-7-649
Subject: Subsoil Study for Foundation Design, Proposed Barn/ADU, Lot C, Panorama
Reserve, Garfield County, Colorado
Dear Ms. Dakides:
As requested, H-P/Kumar 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 December 20, 2016. The data obtained and our recommendations based on
the proposed construction and subsurface conditions encountered are presented in this report.
Proposed Construction: The proposed barn/ADU will be two story wood frame construction
with a partial slab-on-grade floor and located on the site as 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 vacant lot was covered with about 4 to 6 inches of snow at the time of our
exploration. The property is vegetated with grass and weeds. The site is located on a rolling
upland mesa and the ground surface slopes down to the northwest at a grade of about 12 percent
in the building area.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two
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 6 inches of topsoil, consist of
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sandy silty clay. Results of swell-consolidation testing performed on relatively undisturbed
samples of sandy silty clay, presented on Figure 3, indicate low compressibility under existing
moisture conditions and light loading. 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 1,500
psffor support of the proposed barn/ADU. The 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 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
42 inches below the exterior grade is typically used in this area. As an alternative, shallow, frost-
protected foundations, including thickened slabs, can be used provided the foundations are
insulated in accordance with the International Residential Code. 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.
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 free-draining gravel should be placed beneath basement level slabs to facilitate drainage.
This material should consist of minus 2 inch aggregate with less than 50% passing the No. 4
sieve and less than 2% 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 devoid of vegetation and topsoil.
Undcrdrain 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 also create a
perched condition. We recommend below-grade construction, such as retaining walls or
H-P'l::iKUMAR Project No. 16·7-649
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crawlspace areas, be protected from wetting and hydrostatic pressure buildup by an underdrain
system. If the finished floor elevation at the lowest level is at or above the surrounding grade a
foundation drain system may not be required.
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 I% 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 barn/ ADU 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 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 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 12 inches in the first IO feet in unpaved areas and a minimum slope of3
inches in the first 10 feet in pavement and walkway areas. A swale will be
needed uphill to direct surface runoff around the structure.
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 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.
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 detennining 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
H-P~KUMAR Project No. 16-7-649
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exploratory pits and variations in the subsurface conditions may not become evident until
excavation is perfonned. 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 verify 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,
Louis Eller
\
Daniel E. Hardin, P.E.
DEH/kac
Attachments Figure 1 -Location of Exploratory Pits
Figure 2 -Logs of Exploratory Pits
Figure 3 -Swell-Consolidation Test Results
cc: Greenline Architects -Attn: Steve Novy (snovy@s..7feenlinearchitects .com )
H-P~KUMAR Project No . 16-7-649
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:~t-----.....--------.--------------4 J} 16-7-649 H-P~KUMAR LOCATION OF EXPLORATORY PITS Fig. 1
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LEGEND
PIT 1
EL. 7195'
WC=B.9
00=101
PIT 2
EL. 7186'
WC=13.4
D0=95
WC=12.3
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00=99 5
-200=7
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~TOPSOIL; ORGANIC SANDY SILT ANO CLAY, F'IRM, MOIST, DARK REDDISH BROWN.
EJCLAY (CL): SANDY, SILTY, VERY STlrF', MOIST, BROWN, POROUS, BLOCKY.
~ HAND DRIVEN LINER SAMPLE .
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON DECEMBER 22, 2016.
2. THE LOCATIONS OF' THE EXPLORATORY PITS WERE MEASURED APPROXIMATELY BY PACING FROM
F'EATURES SHOWN ON THE SITE Pt.AN PROVIDED.
3. THE ELEVATIONS or THE EXPLORATORY PITS WERE OBTAINED BY INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED.
4. THE EXPLORATORY PIT LOCATIONS 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 or EXCAVATING. PITS WERE
BACKFILLED SUBSEQUENT TO SAMPLING.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (") (ASTM 0 2216);
DD = DRY DENSITY (pcf) (ASTM D 2216):
-200 = PERCENTAGE PASSING NO. 200 SIEVE (ASTM D 1140).
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SAMPLE OF: Sandy Siity Clay
FROM: Boring 1 0 3'
we = S.9 "· DD = 101 pcf
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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APPLIED PAtSSURE -kSF 10
SAMPLE OF: Sandy Siity Clay
FROM: Boring 2 0 2.5'
WC = 13.4 X, OD = 95 pcf
APPLIED PRtSSVRE -kSF 10
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
100
100
iii--1-6---7---6-49~..----Ku_m_a-r&~As-s-oc-la-te-s~..--~S-W-EL_L ___ C_O_N-SO_L_ID_A_T_IO-N--TE_S_T_R_E_S_U_LT_S _____ Fl-g.--3--i
H-P~KUMAR
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 16-7-649
SAMPLE LOCATION NATURAL GRADATION ATTERB£RG LIMITS UNCONFINED
MOISTURE NATURAL PERCENT COMPRESSIVE ORV DENSITY GRAVEL SAND PLASTIC PIT DEPTH CONTENT PASSING NO. LIQUID LIMfT INDEX STRENGTH SOil TYPE
(%) 1%1 200SIEVE
lftl ("l (pef) l"I 19'1 (pSf)
1 3 8.9 101 Sandy Silty Clay
2 21/1 13.4 95 Sandy Silty Clay
31/i 12.3 99 78 Sandy Silty Clay