HomeMy WebLinkAboutObservation of Excavation 11.16.2015HEPWORTH-PAWLAK GEOTECHNICAL
November 16, 2015
David Iuele
P.O. Box 67
Glenwood Springs, Colorado 81602
(aucdave r[msn.com)
Hepworth-Paw1 ik Geotechnical, Inc.
5020 County Road 154
Glcn aot1 Springs, Colorado 8160!
Phone. 970-945-7988
F.uv 970-947-845.1
email hpgco@hpgcotcch.com
Job No. 115 534A
Subject: Observation of Excavation, Proposed Residence, 1007 County Road 110,
Garfield County, Colorado
Dear Mr. Iuele:
As requested, a representative of Hepworth-Pawlak Geotechnical, Inc. observed the
excavation at the subject site on November 7, 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
November 6, 2015.
The proposed residence will be a one to two story wood frame structure supported on
spread footings with a structural ground floor above crawlspace and possibly some slab
on grade floor areas. We understand the building area was sub -excavated down to
remove soft upper soils and it is proposed to re-establish the foundation bearing level on
the order of a few feet using compacted aggregate base course or similar granular
backfill.
At the time of our visit to the site, the building excavation bottom was irregular and from
about 7 to 10 feet below the adjacent grades. The materials exposed in the bottom of the
excavation consisted of weathered to hard siltstone bedrock and gypsum of the Eagle
Valley Evaporite. Results of swell -consolidation testing performed on a sample of the
weathered siltstone taken from the site, shown on Figure 1, indicate the materials are
slightly compressible under conditions of loading and wetting. No free water was
encountered in the excavation and the materials were slightly moist to moist. The bottom
of the excavation was blanketed and the subgrade was not frozen.
Considering the conditions exposed in the excavations and the nature of the proposed
construction, spread footings placed on the undisturbed weathered to hard bedrock or
compacted granular backfill should be designed for an allowable maximum soil bearing
Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989
David Iuele
November 16, 2015
Page 2
pressure of 2,000 psf for support of the proposed residence. The weathered bedrock and
granular backfill may compress if wetted under load and result in relatively small post -
construction settlement, mainly if the bearing materials become wet. Backfill placed
below footing areas should be reasonably well -graded granular material such as aggregate
base course, free of frozen materials, placed at near optimum moisture in 8 inch
maximum lifts and compacted to at least 98 percent of standard Proctor density. Backfill
placed below footings should extend laterally beyond the edge of footings a distance at
least half the depth of the backfill. Strip footings should be a minimum width of 16
inches and isolated footing pads should be a minimum of 24 by 24 inches. Soils in
footing areas that become loose or disturbed should be removed or compacted prior to
concrete placement. The subgrade, backfill and bearing soils should be protected against
freezing 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.
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 on-site soil or
granular soil as backfill. An underdrain should be provided to prevent hydrostatic
pressure buildup behind walls acting as retaining structures and to prevent wetting of
lower areas, such as the crawlspace. Structural fill placed within floor slab areas should
consist of granular 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.
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the site. These rocks
are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some
massive beds of gypsum and limestone. There is a possibility that massive gypsum
deposits associated with the Eagle Valley Evaporite underlie portions of the property.
Dissolution of the gypsum under certain conditions can cause sinkholes to develop and
can produce areas of localized subsidence. Sinkholes were not observed in the immediate
area of the subject building site. Based on our present knowledge of the subsurface
conditions at the site, it cannot be said for certain that sinkholes will not develop. The
risk of future ground subsidence at the site throughout the service life of the proposed
residence, in our opinion, is low; however, the owner should be made aware of the
potential for sinkhole development. If further investigation of possible cavities in the
bedrock below the site is desired, we should be contacted.
The recommendations submitted in this letter are based on our observation of the
materials exposed within the foundation excavations 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 materials 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
Job No. 115 534A
David Iuele
November 16, 2015
Page 3
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.
Thomas J. Westhoff, CET
Reviewed by:
Steven L. Pawlak, P.E.
TJW/ksw
Attachment: Figure 1 -- Swell -Consolidation Test Results
Job No, 115534A
Compression - Expansion %
1
0
1
2
3
4
5
Moisture Content 12.8 percent
Dry Density = 101 pcf
Sample of: Weathered Siltstone
From: Bottom of excavation
•
IN\
Compression
-upon
wetting
0,1
10
APPLIED PRESSURE = ks(
10
100