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November 2, 2020
Carla Ostberg
33 Four Wheel Drive Road
Carbondale, Colorado 81623
Project No.20-7-659
Subject: Observation of Backhoe Pits, Proposed Residence and ADU/Pole Barn, 129 Cains
Lane, Lot 10, Pinyon Peaks Subdivision, Garfield County, Colorado
Dear Carla:
As requested, Kumar & Associates, Inc. observed several backhoe pits 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 29, 2020. 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 will be a two-story wood frame structure over
a crawlspace. The attached garage will have a slab -on -grade floor. The ADU will be one story
over crawlspace. The pole barn will be one story with gravel floor. Cut depths are expected to
range between about 2 to 6 feet. We understand that 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 site is vacant and slopes down to the north at 10% to 15% grade.
Vegetation consists of scattered to moderately dense juniper and pinon trees with an understory
of sagebrush, grass and weeds. The house and ADU/pole barn corners were staked in the field.
The lot is accessed from Cains Lane, located along the south side of the property.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating
four exploratory pits, two in the proposed house area and two in the ADU/pole barn area. The
subsoils encountered, below about 8 to 10 inches of topsoil, consist of about 1 foot of sandy silty
clay with basalt rocks overlying basalt rocks up to large boulder size in a sandy clayey silt matrix
starting at 1'A to 2 feet deep down to digging refusal at the bottom of the pits at 4 to 4'A feet
-2 -
deep. The clay soils overlying the basalt rocks were slightly deeper in the pole barn area. 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 clay or basalt rock soil designed for an allowable soil bearing
pressure of 1,500 psf for support of the proposed residence and ADU/pole barn. The clayey
portion of the soils tends 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 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 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, topsoil and oversized rock.
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 also create a
Kumar & Asso:.iales, Inc. ProIesl Ho. 0-7.6%
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perched condition. We recommend below -grade construction, such as retaining walls, deep
crawlspace and basement areas, be protected from wetting and hydrostatic pressure buildup by
an underdrain system.
The drains should consist of rigid 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 1% 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 11/2 feet
deep.
Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the residence and ADU/pole barn have 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 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
express or implied. The conclusions and recommendations submitted in this report are based
upon the data obtained from the exploratory pits excavated at the site, 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 concerned about MOBC, then a professional in this special field of
practice should be consulted. Our findings include interpolation and extrapolation of the
Prooct No_ 204.559
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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 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,
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Project No, 20-7-659