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HomeMy WebLinkAboutObservation of Excavation~tech 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.