HomeMy WebLinkAboutEngineer's Observation of Excavation 04.17.2015HEPWORTH—PAWLAK GEOTECHNICAL
April 17, 2015
Jeff Nalley
10918 County Road 245
New Castle, Colorado 8I647
CIIIc;IIIC'', i;a'L IU.ill.(OIII
Job No. 114 527A
Subject: Observation of Excavation, Proposed ADU Residence, 10918 County
Road 245 (Buford/West Elk) Road, Garfield County, Colorado
Dear Jeff
As requested, a representative of Hepworth-Pawlak Geotechnical, Inc. observed the
excavation at the subject site on April 3, 20I5 to evaluate the soils 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 findings in a report dated December 19, 2014,
and reviewed the cut slope condition above the building site in a letter dated February 27,
2015, under our Job No. 114 527A.
The proposed construction is similar to that discussed in our previous report. The
building has been designed to be supported on spread footings using an allowable soil
bearing pressure of 1,500 psf as recommended with some risk of settlement. The lowest
level terrace cut on the slope above the house will be retained by the building foundation.
At the time of our site visit, the foundation excavation which was essentially complete
had been cut in one level from about 3':1 to 8 feet below the adjacent ground surface. The
soils exposed in the bottom of the excavation consisted medium dense, clayey to very
clayey silty sand and gravel with cobbles and small boulders. The excavation subgrade
had been moistened and compacted. In general the samples were to rocky to obtain
undisturbed samples for sell -consolidation testing. Results of swell -consolidation testing
performed on a sample taken from the site, shown on Figure 1, indicate the soils are
slightly to moderately compressible under conditions of loading and wetting. The sample
tested showed a low swell potential which may have been due to a shale rock in the
sample and is not considered representative of the soils. No free water was encountered
in the excavation and the soils were slightly moist to moist.
The soil conditions exposed in the excavation are generally consistent with those
previously encountered on the site and suitable for support of spread footings designed
for the recommended allowable bearing pressure of 1,500 psf. Subexcavation and
replacement of a depth of the natural soil below the footing areas does not appear needed
h.ir-kcr• 303-'41-7119 • Lolohido Spring, 719-633-5562 • Silvert hs me 970-468-19"i9
Jeff Nal ley
April 17, 2015
Page 2
provided some risk of foundation movement is acceptable. Prior to the footing
construction, any loose disturbed soils should be removed to expose the undisturbed
natural soils and the subgrade re -compacted as needed. The bearing soils should be
protected from frost and concrete should not be placed on frozen soils. Other
recommendations presented in our previous report which are applicable should also be
observed.
The recommendations submitted m 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.
If you have any questions or need further assistance, please call our office.
Sincerely,
HEPWORTH Pad.'LAK a
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David A. Young, P.E. % •
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Job No. 114 527A
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Figure 1, Swell -Consolidation Test Results
'-'Ptech
Compression - Expansion %
3
2
1
0
1
Moisture Content = 12.0 percent
Dry Density 102 pcf
Sample of: Sandy Silty Clay with Gravel
From: Bottom of Northeast Portion of Excavation
Expansion
upon
wetting
r
0.1
114 527A
H
1.0
Hepworth—Pawlak Geotechnical
10
APPLIED PRESSURE - kst
100
SWELL -CONSOLIDATION TEST RESULTS I Figure 1J