HomeMy WebLinkAboutObservation of Excavation 09.15.2014Gtech
HEPWORTH-PAWLAK GEOTECHNICAL
September 15, 2015
Ralph Young
621 S. 2"d Street
Carbondale, Colorado 81623
(fooffsopris. net)
Hepworth-Pawlak Geotechnical, Inc.
5020 Conry Road 154
Glenwood Springs, Colorado 81601
Phone: 970-945-7988
Fax: 970-945-8454
email: hpgeo@hpgcotech.com
Job No. 115 425A
Subject: Observation of Excavation, Proposed Young Residence, 27 Cedar
Street, Satank, Garfield County, Colorado
Dear Ralph:
As requested, the undersigned representative of Hepworth-Pawlak Geotechnical, Inc.
observed the excavation at the subject site on September 10, 2015 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 September 10, 2015.
The proposed residence will be a one story wood frame structure over a full basement
level. Footings were designed for an assumed allowable soil bearing pressure of
1,500 psf.
At the time of our visit to the site, the foundation excavation had been cut in one level
from 61/2 to 81/2 feet below the adjacent ground surface. The soils exposed in the
bottom of the excavation consisted of dense, silty sandy gravel with cobbles and small
boulders. No free water was encountered in the excavation and the soils were slightly
moist. The spread footings had been formed and concrete was in place at the time of
our site visit.
Considering the conditions exposed in the excavation and the nature of the proposed
construction, spread footings placed on the undisturbed natural gravel soil designed
for an allowable soil bearing pressure of 1,500 psf can be used for support of the
proposed residence. Footings should be a minimum width of 16 inches for continuous
walls and 2 feet for columns. Exterior footings should be provided with adequate soil
cover above their bearing elevations for frost protection. 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 also be designed to resist a lateral earth pressure based on an equivalent fluid
unit weight of at least 45 pcf for on-site soil as backfill. A perimeter foundation drain
should be provided to prevent temporary buildup of hydrostatic pressure behind the
Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989
Ralph Young
September 15, 2015
Page 2
basement walls and prevent wetting of the lower level. Structural fill placed within
floor slab areas can consist of the on-site gravel soils (less than 5 -inch rock) or a
suitable imported gravel 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.
Subsidence Potential: Satank is underlain by Pennsylvania Age Eagle Valley
Evaporite bedrock. The evaporite contains gypsum deposits. Dissolution of the
gypsum under certain conditions can cause sinkholes to develop and can produce
areas of localized subsidence. During previous work in the area, a sinkhole was
observed south of the Community School. Sinkholes were not observed in the
immediate area of the subject lot. Based on our present knowledge of the site, it
cannot be said for certain that sinkholes will not develop. In our opinion, the risk of
ground subsidence at this site is low and similar to other lots in the area not containing
subsurface voids or sinkholes but the owner should be aware of the potential for
sinkhole development.
The recommendations submitted in this letter are based on our observation of the soils
exposed within the foundation excavation and do not include subsurface exploration
to evaluate the subsurface conditions within the loaded depth of foundation influence.
This study is based on the assumption that soils beneath the footings have equal or
better support than those exposed. The risk of foundation movement may be greater
than indicated in this report because of possible variations in the subsurface
conditions. In order to reveal the nature and extent of variations in the subsurface
conditions below the excavation, drilling would be required. 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,
HEPWORTH — PAWLAK GEOTECHNICAL, INC.
.00 Li
Daniel E. Hardin, P.E.
Rev. by: SLP
DEH/ksw
24443
V 2
9-175.74,
Job No. 115 425A
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