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HEPWORTH-PAWLAK GEOTECHNICAL
February 27, 2015
Sunsense Solar
Attn: Mark Item
1629 Delores Way
Carbondale, Colorado 81623
mark @sunsensesolar.com
Hepworth-Pawlak Geotechnlcal, Inc.
5020 County Road 154
Glenwood Springs, Colorado 81601
Phone: 970-945-7988
Fax: 970-945-8454
Email: hpgeo@hpgeotech.com
Job No. 114 473B
Subject: Observation of Excavation, Proposed Equipment Pad, Sunnyside Ranch
Community Solar Project, 4543 County Road I 00, Garfield County,
Colorado
Gentlemen:
As requested, a representative of Hepworth-Pawlak Geotechnical, Inc. observed the
excavation at the subject site on February 24 and 27, 2015 to evaluate the soils exposed
for foundation support. The findings of our observations are presented in this report. We
previously conducted a subsoil study for design of pile foundations for support of the
solar panels at the site and presented our findings in a report dated November 19, 2014,
Job No. 114 473A. The current observation was conducted according to our agreement
for services with Sunsense Solar dated February 24, 2015.
The proposed equipment pad will be constructed as slab-on-grade with perimeter turned
down edges 24 inches deep and be about 13 feet by 35 feet in plan dimension. The slab
will be reinforced to support equipment load bearing pressures up to about 300 psf and
shade roof post and beam type structure load bearing pressures up to about 600 psf along
the slab perimeter.
At the time of our visits to the site, the excavation had been completed and steel
reinforcement bar was being placed within the slab and turned down perimeter edge. The
excavation for the perimeter edge appeared to be cut up to about I Y2 feet below the
adjacent ground surface on the west end and near ground surface at the east end. The
soils exposed in the bottom of the excavation consisted of road base apparently overlying
native silt and clay soils which had been removed and replaced to a shallow depth to
install electric line conduits. The backfill soils were reportedly compacted. The interior
slab area was covered with crushed rock reportedly about 18 inches deep. Results of
swell-consolidation testing performed on a sample of the onsite soil taken from below the
road base, shown on Figure I, indicate the soils are highly compressible under conditions
of loading and wetting. The soils exposed along the perimeter trench bottom were probed
by hand on February 27th with push depths of 6 to J 6 inches. The shallow push depths
apparently refused in the road base and the deep push depths in the underlying native
Sunsense Solar
February 27, 2015
Page2
soils. No free water was observed in the excavation and the soils were moist to very
moist.
The soil conditions exposed in the excavation are variable and we cannot confirm their
suitability to support the proposed structure because we were not present when the work
was performed. The natural silt and clay soils previously encountered on the property are
moisture sensitive and tend to settle when wetted and keeping the bearing soils dry will
be critical to satisfactory perfonnance of the equipment pad. Considering the light
loading of the equipment and shade structure, the slab foundation can be used for support
with the risk of differential settlement mainly if the bearing soils are wetted. Backfill
placed around the structure and against the slab perimeter should consist of relatively
impervious soil (not crushed rock as shown on the plan elevation details) compacted to at
least 95% of standard Proctor density near optimum moisture content. The perimeter
slope should be graded at a minimum 5% for 10 feet away from the structure and a
minimum 2% in paved walkway areas. A surface swale with minimum 3% slope should
be provided along the uphill side to direct runoff away from the structure.
The recommendations submitted in this Jetter are based on our limited observation of the
soils exposed within the perimeter foundation excavation and the previous limited
subsurface exploration at the site. Variations in the subsurface conditions below the pad
structure could increase the risk of slab 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 -PAWLAK GEOTECHNICAL, INC .
Rev. by: DEH
SLP/ksw
Attachment: Figure I -Swell-Consolidation Test Results
Joh No. I 14 4738
Moisture Content = 17.1 percent
Dry Density = 93 pcf
Sample of : Silty Sandy Clay (Fill)
From : Bottom of Perimeter of Excavation
0
~ ~ 1 ......._
~ '-, 2
I'-. Compression
" ~t> D
upon
~ "' wetting '*-3 -----~I--
c: ~ (_ /
v'
.Q
(/)
(/)
4 ~ a.
E C)
0 \ (.)
5
6 \
7
8 \
9 \
10 \
11 \
I)
12
0.1 1.0 10 100
APPLIED PRESSURE -kst
114 4738 ~ SWELL-CONSOLIDATION TEST RESULTS Figure 1
HEl'WORTll-PAWLAK GEanCHNICAI.