HomeMy WebLinkAboutObservation of Excavation 02.01.2010Gtech
HEPWORTH - PAWLAK GEOTECHNICAL
February 1, 2010
Andrew Beuter
County Road 109
Carbondale, Colorado 81623
Hepworth-Pawlak Geotechnical, Inc.
5020 County Road 154
Glenwood Springs, Colorado 81601
Phone: 970-945-7988
Fax: 970-945-8454
email: hpgeo@hpgeotech.com
Job No. 110 020A
Subject: Observation of Excavation, Proposed Residence, Lot C, Beulah Wilson
Exemption, 6195 County Road 109, Garfield County, Colorado
Dear Mr. Beuter:
As requested, a representative of Hepworth-Pawlak Geotechnical, Inc. observed the
excavation at the subject site on January 29, 2009 to evaluate the soils exposed for
foundation support. The findings of our observations and recommendations for the
foundation design are presented in this report. The services were performed in
accordance with our agreement for professional engineering services to you, dated
February 1, 2010. We previously conducted a subsoil study for design of foundations at
the site and presented our findings in a report dated May 24, 2001, Job No. 101 338C.
The proposed residence will be one and two story wood frame construction above a
partial basement and partial crawlspace with an attached garage. Basement and garage
floors will be slab -on -grade. A spread footing foundation imposing a maximum soil
bearing pressure of 1,200 psf and a minimum continuous footing width of 18 inches was
used for design based on our previous report.
At the time of our visit to the site, the footings were in place and the foundation
excavation had been cut in three levels from 3%2 to 9 feet below the adjacent ground
surface. The soil conditions were variable. The soils exposed in the bottom of the lowest
level of the excavation consisted of silty sandy gravel with cobbles. The soils exposed in
shallower cut areas consisted of silty sandy clay. The clay layer appeared to be 1 to 3 feet
thick below the footings. The footings were being protected from frost with insulated
blankets. Some snow and ice was observed in the excavation bases and the client was
advised to remove the snow and ice and continue to protect the excavation bases from
frost. No free water was encountered in the excavation and the soils were slightly moist to
moist.
Considering the conditions exposed in the excavation and the nature of the proposed
construction, spread footings placed on the undisturbed natural soil designed for an
allowable soil bearing pressure of 1,200 psf should be adequate for support of the
proposed residence. The upper silty sandy clay soils tend to compress when wetted and
there could be some post -construction settlement of the foundation if the bearing soils
become wet. The settlement would be differential between footings bearing on the clay
and footings bearing on the gravel soils. Loose and disturbed soils in footing areas should
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Andrew Beuter
February 1, 2010
Page 2
be removed and the bearing level extended down to the undisturbed natural soils. The
bearing soils should be protected against frost and concrete should not be placed on
frozen soils. Exterior footings should be provided with adequate soil cover above their
bearing elevations for frost protection. Continuous foundation walls should be reinforccd
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 also be designed to
resist a lateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for
on-site soil as backfill. A perimeter foundation drain should be provided to prevent
temporary buildup of hydrostatic pressure behind the basement walls and prevent wetting
of the lower level. Structural fill placed within floor slab areas can consist of the on-site
soils compacted to at least 95% of standard Proctor density at a moisture content near
optimum. Backfill placed around the structure should be compacted and the surface
graded to prevent ponding within at least 10 feet of the building. Landscaping that
requires regular heavy irrigation, such as sod, and sprinkler heads should not be located
within 10 feet of the foundation.
The recommendations submitted in this letter are based on our observation of the soils
exposed within the foundation excavation and the previous limited subsurface exploration
at the site. Variations in the subsurface conditions below the excavation could increase
the risk of foundation movement. We should be advised of any variations encountered in
the excavation conditions for possible changes to 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,
HEPWORTH — PAWLAK GEOTECHNICAL, INC.
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Louis E. Eller
Reviewed by:
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