HomeMy WebLinkAboutObservation of Excavation Report 05.07.2013HEPWORTH- PAWLAK GEOTECHNICAL
May 7, 2013
Richard Splain
rsplain(yahoo.com
-S020 154
L11-1.1117 ;
Job No. 112 434A
Subject: Observation of Excavation, Proposed Dry Storage Building, 556 County
Road 352, Garfield County, Colorado
Dear Mr. Splain:
As requested, a representative of Hepworth-Pawlak Geotechnical, Inc. observed the
excavation at the subject site on April 29, 2013 to evaluate the materials exposed for
foundation support. The findings of our observations and recommendations for the
foundation design are presented in this report. We previously conducted a subsoil study
for design of foundations at the site and presented our find ngs in a report dated January
8, 2013, Job No. 112 434A.
We understand that the building is located to the west of the area shown on Figure 1 of
our previous report.
We visited the site twice on April 29th. At the time of our initial visit to the site, the
exterior footing trench excavation had been cut in one level about 3 feet below the
adjacent ground surface. The materials exposed in the bottom of the excavation consisted
of hard sandstone in the eastern half of the excavation and slightly sandy clay in the
western half of the excavation. Based on our previous report, the clays on this site are
expansive if wetted and we recommended that the clays be removed from below footing
grade. Pits were dug and revealed that the sandstone was about 7 feet below design
footing grade at the northwest corner of the building, at about 2 feet below footing grade
at the southwest corner and at about 1 foot below design footing grade near the middle of
the south side. Three alternatives were discussed: lower the footing bearing level down to
the underlying sandstone, backfill the trench back up to design footing grade with "flow
fill" or lean concrete, or place compacted 3/ -inch road base back up to design footing
grade.
We visited the site again later in the day and observed that the clay had been removed
down to the sandstone bedrock in the western half of the building area. No free water
was encountered in the excavation and the exposed bedrock was slightly moist. We
understand that flow fill will be used to backfill the 1 to 7 foot deep trench back up to
design footing grade.
Parker 3034841-7119 Coloradu Sprin s 719-633-5562 Sit erLhume 970-468-1989
Richard Splain
May 7, 2013
Page 2
The subsurface conditions exposed in the excavation are consistent with those previously
encountered on the site with the top of the sandstone dropping off in the western part of
the site. The sandstone or flow fill should be suitable for support of spread footings
designed for the recommended allowable bearing pressure of 3,000 psf. Placement of the
interior slab on the expansive clay should be feasible for the proposed dry storage
building provided the clays do not become wetted. The clays tend to swell if wetted and
the floor slab could heave up to several inches depending on the depth and extent of
wetting. We assume the risk of wetting is low. If the use of the building is such that the
risk of wetting is higher, then the risk of floor slab movement must be accepted by the
owner. If this risk is not acceptable, then the clay should be at least partially removed to a
depth of at least 3 feet and replaced with compacted structural fill such as %-inch road
base. Other recommendation,, presented in our previous report which are applicable
should also be observed.
The recommendations submitted in this letter are based on our observation of the soils
exposed within the foundation excavation and the previous 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 GEOTCHNICAL, INC.
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cc: Kurtz & Associates — Brain Kurtz kurtsena sopris.net
Boundaries Unlimited — Phil Harris phil(bu-inc.corn
Job No. 112 434A
Ge tech