HomeMy WebLinkAboutSubsoil Study for Foundation Design 01.13.2021rcrf iiffilfl'ffiffffi*iTiå*"'
An Employcc Olrncd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com
Offrce Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
January 13,2021
Barbara Dills
2646Dolores Way
Carbondale, Colorado 81 623
barbara. dills@ gmail. com
Subject:
Project No. 20-7-709
Subsoil Study for Foundation Design, Proposed House, ADU, Garage/Hay Barn
and Horse Bam, Southeast of 2050 County Road 102, Garfreld County, Colorado
Dear Barbara:
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 datedNovember 16, 2020. The data obtained and our
recommend¿tions based on the proposed construction and subsurface conditions encountered are
presented in this report.
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ProposedConstruction:Theproposedæresidencewillbeinthesoutheastpart
of the building envelope, the horse barn will be in the northwest corner and the garage/hay shed
will be near the middle of the north side of the envelope, as shown on Figure 1. Ground floor in
the ADU and house will be structural over crawlspace or slab-on-grade. Cut depths are expected
to range between about 3 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 building area slopes down to the south at about l0%o grade. Vegetation
consists of mostly sage brush, grass and weeds with scattered juniper and pinon trees. Many
basalt rocks to boulder size were observed on the ground surface.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating
four 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 oftopsoil, consist ofup
to about I foot of stift sandy silty clay with basalt rocks overlying relatively dense, basalt rocks
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from gravel to boulder size in a calcareous, sandy clay and silt matrix. Results of Atterberg
limits testing and percent finer than sand size gradation analysis performed on disturbed samples
of the clay and basalt rock matrix soils obtained from the site indicate the clays have medium
plasticity. The test results are shown on Table 1. No free water was observed in the pits at the
time of excavation and the soils were slightþ moist to moist.
tr'oundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recoÍtmend spread footings
placed on the undisturbed natural soil designed for an allowable soil bearing pressure of
2,000 psf for support of the proposed ADU, residence, gùrage and bam structures. If needed,
up to two feet of structural fill could be placed below footings. The structural fill should consist
of 3/+-tnchroad base compacted in 6 to 8-inch lifts to at least 98% of the maximum standard
Proctor density. 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 shouldbe removed and the footing bearing level extended down to the
undisturbed natural soils. We should observe the completed excavation of each building 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
10 feet. Foundation walls acting as retaining structures should be designed to resist alatenl
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, 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 slabs to facilitate drainage. This material
should consist of minus 2-inch aggregate with less than50Yo passing the No. 4 sieve and less
than2%o passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least95o/o of maximum
standard Proctor density at a moisture content near optimum. Required fiIl can consist of
Kumar & Associates, lnc. @ Project No. 20-7-709
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imported 3/q-inchroad base or onsite predominantþ granular soils devoid of vegetation, topsoil
and oversized rock.
Underdrain System: Structures where slab grade is at or above exterior grade should not need
an 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 and crawlspace
areas deeper than about 3 feet, 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 1olo to
a suitable gravity outlet. Free-draining granular material used in the underdrain system should
contain less than 2o/opassrngthe 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 lYzfeetdeep.
Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the proposed structures have 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 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 covered with filter fabric and capped with
abott2 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 found¿tion in all directions. We recommend a minimum
slope of 12 inches in the fust 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.
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
Kumar & Associates, lnc. @ Project No. 20-7-709
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express or implied. The conclusions and rec,ommendations 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 conskuction, 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 fi.rture. 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 re,presentative of
the geotechnical engineer.
If youhave any questions or if we may be of further assistance, please let us know.
Respectfully Submitted,
Kumar & Associates,
Daniel E. Hardin,
Rev. by: SLP
DEHlkac
attachments Figure I - Location of Exploratory Pits
Figure 2 - Logs of Exploratory Pits
Table I - Summary of Laboratory Test Results
Kumar & Associates, lnc. o Project No. 20-7-709
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APPROXIMATE SCALE-FEET
Fig. 1Kumar & Associates LOCATION OF EXPLORATORY PITS20-7 -709
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4PIT
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55
PIT I
EL. 7156'
Pfi2
EL.7150'
PIT 5
EL. 7132'EL. 7120'
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) WC=7.7
-200=90
LL=36
Pl=18
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-l WC=12.6,' -2oo=40
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BARN ADU SEPTIC
SILTY, SLIGHTLY SANDY TO SANDY WITH BASALT COBBLES, MEDIUM STIFF TO
, SLIGHTLY MOIST, BROWN.
LEGEND
TOPSOIL. SLIGHTLY SANDY SILTY CLAY, ROOTS ÀND ORGANICS WITH COBBLES ON THE
SURFACE. LOOSE, SLIGHTLY MOIST TO MOIST, BROWN.
CLAY
VERY
(cL),
STIFF
GRAVEL (GM-GC), BASALT GRAVEL AND COBBLES lN A WHITE SANDY, SILTY TO CLAYEY
CALCAREOUS MATRIX, DENSE TO VERY DENSE, SLIGHTLY MOIST, WHITE AND BROWN.
DISTURBED BULK SAMPLE.
PRACTICAL DIGGING REFUSAL.
NOTES
f. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON NOVEMBER 20, 2020.
2. THE LOCATIONS OF THE EXPLORATORY PITS WERE LOCATED BY THE CLIENT.
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:
wc = WATER CoNTENT (%) (ASTM D 2216);
-2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D
LL = LIQUID LIMIT (ASTM D 4518);
PI = PLASTICITY INDEX (ASTM D 4518).
I 1 a0);
Ir
I
Fis. 2LOGS OF EXPLORATORY PITSKumar & Associates20-7-709
lGrtiffiifimffil:'.";-."TABLE ISUMMARY OF LABORATORYTEST RESULTSNo. 20-7-709Slightly Sandy ClayVery Clayey Sandy GravelSOIL TYPEloslìUNCONFINEDCOMPRESSIVESTRENGTH51PLASTICINDEXPll81ATTERBERG LIMITS(ololLIQUID LIMIT364t9046PERCENTPASSING NO.200 slEvEt%lSANDGRADATION(:/"1GRAVELlncflNATURALDRYDENSITY7.712.6lololNATURALMOISTURECONTENTr-r%3-4fftlDEPTH1JSAMPLE LOCATIONPIT