HomeMy WebLinkAboutSubsoils Report for foundation DesignIUA
Kumar & Associates, Inc.
Geotechnical and Materials Engineers
and Environmental Scientists
email
An Employee Owned Company
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
kaglenwood@kumarusa.com
www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
February 21, 2025
Offroad Design
Attn: James Watson
484 County Road 113
Carbondale, Colorado 81623
jwatson soprls.net
Project No. 20-7-346.A
Subject: Presumptive Bearing Conditions, Proposed Residence, 484 County Road 113,
Garfield County, Colorado
Dear James:
As requested, we are providing presumptive bearing conditions for the proposed residence
foundation design based on assumed subsurface conditions at the subject site. The services were
performed supplemental to our agreement for professional engineering services to you, dated
February 20, 2025. We previously provided foundation design recommendations for a shop
building at another location on the property, report dated August 4, 2020, Project No. 20-7-346.
The proposed residence will be located on the east side of the existing driveway. The proposed
residence is assumed to be a one- or two-story wood -frame structure. Ground floor could be
structural over crawlspace or slab -on -grade. Excavation depth for this type of construction is
expected to be relatively shallow, about 3 to 5 feet.
The subject site was developed with a one and two-story residence and steel frame outbuildings
at the time of our field visit on February 21, 2025. The ground surface was graded to near level
benches generally sloping down to the west. Vegetation was mostly baren in the access and
parking areas with scattered trees bushes and weeds between access drives.
Our previous study of the subject site consisted of drilling two exploratory borings west of the
main driveway. The subsoils encountered, below topsoil and fill to between 1 and 2 feet deep,
consisted of very stiff, sandy silt and sandy clay to between 25 and 36 feet deep where dense,
silty sandy gravel was encountered to the maximum drilled depth of 41 feet. Fill depths are
likely variable and could be deeper in the proposed building area. Groundwater was encountered
at depths of 20 and 32 feet deep and is not expected to be encountered in the assumed shallow
excavation depth at the subject site.
At assumed foundation depth we expect the excavation to expose fill or natural sandy silt soils.
The fill soils are not suitable for support of the proposed residence and should be removed from
below foundation areas. The sandy silt soils possess low bearing capacity and typically low to
moderate settlement potential primarily when wetted. The sandy silt can be used for support of
Offroad Design
February 21, 2025
Page 2
spread footings with a risk of foundation settlement primarily if the bearing soils become wetted.
Precautions should be taken to prevent wetting of the bearing soils during and after construction.
Foundation Recommendations: Based on our review of the previous subsoil study and
expected soils for building support, spread footings placed on the undisturbed natural soil
designed for an allowable soil bearing pressure of 1,500 psf can be aas med for support of
the proposed building foundation with a risk of foundation settlement. Footings should be a
minimum width of 18 inches for continuous walls anted 2 feet for columns. We should observe
the completed foundation excavation for bearing con itions. The four ion excavation may
need to sub -excavated below the minimum design bearing elevation if fill soils are encountered.
Sub -excavated depths to remove fill soils can be backt711cd with com acted str al fill
consisting on the onsite natural or a suitable imported granular material such as CDOT Class 2 or
6 base course. Structural fill should be spread in thin horizontal lifts, moisture conditioned to
near optimum and compacted to at least WM=ummMTFUr density. A
representative of the geotechnical engineer should test structural fill for compaction on a regular
basis at the time of placement. 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 well
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 55 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.
Kumar & Associates, Inc. Project No. 20-7-346.A
Offroad Design
February 21, 2025
Page 2
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 4 feet, be protected from wetting and hydrostatic pressure buildup by
an underdrain system. For the assumed shallow crawlspace area an underdrain system is not
required.
If installed, the drains should consist of rigid perforated PVC 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 '/2% 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 Meet deep and covered with filter fabric such as Mirafi 140n or 160n. An
impervious membrane such as 20 mil PVC should be placed beneath the drain gravel in a trough
shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils.
Surface Drainage: Providing and maintain proper surface drainage will be critical to the long-
term satisfactory performance of the proposed residence. 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 covered with filter fabric and 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.
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.
Kumar & Associates, Inc. Project No. 20-7-346.A
Offroad Design
February 21, 2025
Page 2
Limitations: The findings and recommendations submitted in this letter are based on the
proposed type of construction, our previous subsurface exploration and does not include
subsurface exploration within the proposed building site to evaluate the subsurface conditions
within the loaded depth of foundation influence. The risk of foundation movement may be
greater than indicated in this report because of possible variations in the subsurface conditions.
It is possible the data obtained by subsurface exploration could change the recommendations
contained in this letter. 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.
If you have any questions or need further assistance, please call our office.
Sincerely,
Kumar & Associate
James H. Parsons,
Rev. By: SLP
JHP/kac
Kumar & Associates, Inc. Project No. 20-7-346.A