ARCHIVED REPORTS_2022 [REV] RCSI W/CHANGES

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2-13 Forward Composting Facility SWT Engineering Report of Composting Site Information - August 2019January 2022 z:\projects\allied waste\forward\resource recovery facility\5 yr permit rvw 2019-2020\rcsi 2019\text\sec 2.doc It should be noted that the generalized subsurface profile described below has bee n greatly simplified for the purpose of this report. Beginning at the existing ground surface (approximately elevation 39 feet MSL), these six layers are as follows: ⧫ A 10- to 15-foot layer of light brown material ranging from lean clay to silt and s ilty clay. This layer is dry and very stiff to hard in consistency, with moisture contents typically at or below the plastic limit. A UU triaxial compression test performed on a sample at a depth of 15 feet in CH2M HILL Boring BH-4, which indicated an undrained shear strength of 5,145 pounds per square foot (psf). SPT blow- counts in this layer are typically greater than 50. ⧫ A 16- to 20-foot layer of material ranging from silty sand to clayey sand. This layer is dry to moist and typically dense to very dens e. Within this layer, a 5-foot zone of very stiff lean clay (UU shear strength of 2,262 psf) was encountered in Boring BH -2 at an approximate depth of 19 feet. This layer was not encountered in other borings and is believed to be discontinuous. ⧫ A 5- to 10-foot layer of very stiff yellowish -brown lean clay. This material is moist, with moisture contents between the plas tic and liquid limit s. A consolidation test indicates that this material is consolidated to approximately three times the effective overburden stress overconsolidation ratio (OCR) of 3. The OCR is the ratio of the maximum effective stress the soil has been subjected to in the past (preconsolidation pressure) to the existing effective stress in the soil. A UU triaxial compression test at an approximate depth of 34 feet in CH2M HILL Boring BH-1 indicated a shear strength of 2,286 psf. ⧫ A 30-foot layer of material consisting of very stiff to hard silt and lean clay. This material is moist to wet, with moisture contents between the plastic and liquid limit. This material has an approximate OCR of 1.7 (CH2M HILL Boring BH -1, approximate depth of 78 feet). ⧫ Very dense poorly graded sand was observed near the bottom of the CH2M HILL Boring BH -1, starting at an approximate depth of 100 feet. Sand was also observed in EMCON's Boring No. 17 to 178 feet, the maximum depth drilled (EMCON, 1972). Soil type and properties of the materials encountered in the geotechnical investigations performed by EMCON and InsituTech were generally consistent with the conditions determined by CH2M HILL described above. In EMCON's subsurface investigation, a Modified California Sampler (with a 140 -pound hammer with a 30 -inch drop) was used to collect samples. Blow counts recorded with the Modified California Sample r typically ranged from 20 to 60 blows per foot. Unconfined compression tests were performed on silty clay samples between depths of 3 and 10 feet. The compressive strengths ranged from 1,310 psf to 5,100 psf for an average of approximately 3,000 psf (No te: the undrained shear strength is defined as one half of the unconfined compressive strength) (EMCON, 1972). Shear strengths based on InsituTech's CPT data (performed to a maximum depth of 50 feet) typically ranged from approximately 4,000 psf to over 10 ,000 psf. Based on these CPT logs, a lower bound shear strength value of 2,000 psf was used in InsituTech's stability evaluation for WMU E (InsituTech, 1987 ). Table 2 summarizes the results of the subsurface soil investigations for the six generalized zo nes mentioned above. 2.2 Monitoring