HomeMy WebLinkAboutSubsoil Study1(+AKawr & Associates, Inc.®
Geotechnical and Materials Engineers 5020 County Road 154
and Environmental Scientists Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
An Employee Owned Company www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
August 18, 2020
Don Click
P.O. Box 1508
Glenwood Springs, Colorado 81602
(dclick cr„rfpcorp.com) Project No. 20-7-464
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 47, Filing 6, Elk
Springs, 0047 Woodruff Road, Garfield County, Colorado
Dear Mr. Click:
As requested, Kumar & Associates 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 August 14, 2020. The data obtained and our recommendations
based on the proposed construction and subsurface conditions encountered are presented in this
report. Hepworth-Pawlak Geotechnical (now Kumar & Associates) previously performed a
preliminary geotechnical study for Filings 6 through 9, Elk Springs (formerly Los Amigos Ranch
PUD) and reported the findings on February 14, 1997, Job No. 197 617. Hepworth-Pawlak
Geotechnical also performed a subsoil study on this lot dated April 20, 2006, Job No. 106 317.
Proposed Construction: Plans for the proposed residence are conceptual and the current report
was prepared for purchase of the property. Typical construction in the area consists of one and
two story wood -frame construction with an attached garage over a basement, crawlspace or slab -
on -grade. Cut depths are assumed to range between about 2 to 8 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 lot was vacant and the property corners and building envelope corners
were staked at the time of our site visit. Vegetation consists of scattered sage brush, cactus, grass
and weeds. The ground surface is relatively flat and gently to moderately sloping down to the
west in the building area. Numerous basalt cobbles and boulders were observed on the ground
surface of the lot.
Subsurface Conditions: The subsurface conditions at the site were evaluated by observing an
exploratory pit recently dug near the southeast building envelope corner. The pit was excavated
about 6 feet deep and the subsoils encountered, below about % foot of topsoil, consist of basalt
gravel, cobbles and boulders in a white, sandy silt matrix down to the maximum depth excavated
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of 6 feet. No free water was observed in the pit and the soils were slightly moist. These soils are
similar to those previously encountered on this site in pits excavated to depths of 4 to 5 feet, near
the northeast and southwest building envelope corners. Our experience in this area is that a
typical large trackhoe can usually excavate at least two feet deeper than the bottom of the
exploratory backhoe pits.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend spread footings or
thickened edge slab -on -grade placed on the undisturbed natural granular soil designed for an
allowable soil bearing pressure of 2,000 psf for support of the proposed residence. Footings
should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Utility
trenches and cuts deeper than about 7 feet may require rock excavating techniques such as
chipping or blasting. Topsoil, sandy silty clay and loose or 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. Voids created from boulder removal at
footing grade should be backfilled with a structural material such as road base compacted to at
least 98 percent standard Proctor density at a moisture content near optimum or with concrete.
Exterior footings or tuned -down edges should be provided with adequate cover above their
bearing elevations for frost protection or protected with rigid foam insulation. Placement of
footings at least 36 inches below the exterior grade is typically used in this area. Continuous
foundation walls and turn -down sections 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 (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, excluding
organics and rock larger than about 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
separated from bearing walls and columns should have expansion joints which allow
unrestrained vertical movement. This would not apply to thickened edge slabs. 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 relatively well graded sand and gravel
such as road base should be placed beneath slabs for support. This material should consist of
minus 2 -inch aggregate with less than 50% passing the No. 4 sieve and less than 12% 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.
Kumar & Associates, Inc. ® Project No. 20-7-464
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Underdrain System: Although free water was not encountered in the exploratory pits, 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,
crawlspace and basement areas, 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 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 1'/2 feet deep.
Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the residence 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 6 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. A swale may be
needed uphill to direct surface runoff around the residence.
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 5
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 conditions observed in the exploratory pits previously excavated at the site, the
expected 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
Kumar & Associates, Inc. ;� Project No. 20-7-464
(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 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.
Sincerely,
Kumar & Associates, Inc.
Daniel E. Hardin, P.
DEH/kac
Cc: Scott Dillard (scottdillardrealtornagmail.com)
Kumar & Associates, Inc. � Project No. 20-7-464