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HEPWORTH-Pll.WLAI< GEOTECHNICAL
May 5, 2014
Derek Gangroth
475 Panoramic Drive
Silt, Colorado 81652
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Job No. 114 041A
Subject: Observation of Excavation, Proposed Addition to Residence,
475 Panoramic Drive, Near Silt, Garfield County, Colorado
Dear Derck:
As requested, a representative of Hepworth-Pawlak Geotechnical observed the
excavation at the subject site on May 1, 2014 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
February 28, 2014, Job No. 114 041A.
The proposed addition is similar to that discussed in our previous report. Spread
footings bearing on the natural soils designed for an allowable bearing pressure of 1,500
psf were recommended for foundation support of the addition, with some risk of
movement.
At the time of our site visit, the foundation excavation, which was essentially complete,
had been cut in two levels from about 5 to 9 feet below the adjacent ground surface.
The soils exposed in the bottom of the excavation consisted of stiff, very sandy silty
clay in the shallower cut areas and medium hard and highly weathered claystone shale
in the deeper cut areas. The excavation subgrade had been compacted. ResuJts of
swell-consolidation testing performed on samples of the highly weathered claystone
taken from the site, shown on Figure l, indicate the material is slightly to moderately
compressible under conditions of loading and wetting, with a nil to low swell potential
when wetted under a constant light surcharge. No free water was encountered in the
excavation and the materials were slightly moist.
The conditions exposed in the excavation are somewhat different than those previously
encountered . It should be feasible to found the addition on spread footings bearing on
the exposed materials designed for the recommended allowable bearing pressure of
1,500 psf with some increased risk of foundation movement . The risk of movement is
due primarily to the variable bearing conditions, and especially if the bearing materials
Derek Gangroth
May 5, 2014
Page 2
become wetted, and precautions should be taken to prevent wetting. The movement
may be differential with respect to the existing structure which should be considered in
the design.
Prior to the footing construction, loose disturbed materials should be removed 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 in this letter are based on our observation of the
materials 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
reconunendations contained in this letter
If you have any questions or need further assistance, please call our office.
Sincerely,
attachment
Job No. 114 041A
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Moisture Content = 13.8 percent
Dry Density = 118 pcf
Sample of: Highly Weathered Claystone
From: Bottom of Northwest Portion of Excavation
0 -r--*-...........
c 1
:-....... ...... 0
·~ .... ,
.... ~ ~ IO a.
~ 2
I ~ '\ 15 ~ ' ~ 3
E Expansion ~I> 0 upon
{) wetting 4
0 .1 1.0 10 100
APPLIED PRESSURE -ksf
Moistl.J'e Content = 8.9 percent
Dry Density = 133 pct
Sample of: Highly Weathered Claystone
From: Bottom of East-Central Portion of
ti. 1 Excavation c: .Q
Cl> :a (~ ~ 0
I --~~ ~ ~ E """ '""""' h ~ ... ·c;; 1 ~ Expansion I'. 0. " E upon ~ 0 wetting (.) 2
0.1 1.0 10 100
APPLIED PRESSURE -ksf
114 041A ~ Heoworth-Pawlak GeatechnbJI
SWELL-CONSOLIDATION TEST RESULTS Figure 1