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IV. SUBSURFACE CONDITIONS 4.01 General The site geology consists of alluvial soils overlying sedimentary bedrock. The alluvial soils consist of a relatively thin layer of silty and clayey over-bank deposits (common referred to as "overburden" in a gravel mining area and referenced herein as such) overlying a deposit of sand, grave], and cobbles {coarse alluvium) that has infrequent, relatively thin clay lenses (fine alluvium) within it. Bedrock appears to be relatively uniform throughout the site and consists of gray mudstone. Fill along dirt roads occasionally overlies the alluvium. The reclaimed backfill slopes consist of clayey or sandy fill. Dirt roads appear to be constructed on top of unmined alluvium, except for areas along the southern edge of the site where road appears to be constructed on top of reclaimed slope backfill. A subsurface profile aligned around the perimeter of the ponds is presented in Figure 4. Information from previously performed borings by Rocky Mountain Consultants is included in the profile, since strata depths and thickness appeared to be consistent with borings performed by Haley & Aldrich. Detailed descriptions of materials encountered are presented in the following sections of the report. Unified Soil Classification System group names and group symbols are used for soil. Apparent density was determined by correlation with blow counts from standard penetration tests. Field tests performed on the bedrock are summarized in Table II and VI. Laboratory tests performed on the soils and rock are summarized in Tables III, IV, and V. Photographs pertaining to the geology are provided in Appendix E. ~ j 4.02 Overburden Overburden was encountered in 28 borings. The borings show that the overburden material is generally 1 to 11 fr thick, but was evident as thick as 16 and 17.5 in borings HA-23 through HA-25. The overburden consists primarily of brown to dark brown fine and coarse alluvium. A relatively thin layer of fill was occasionally observed overlying the alluvium and was incorporated into the overburden category. Overburden material is comprised of gravel, sand, and clay particles and classifies as clayey sand (SC) and sandy lean clays (CL). Relative density of the overburden ranged from loose to very dense with standard penetration values ranging from 4 blows per foot (bpf) to 50 blows for 5 in. 4.03 Reclaimed Slope Backfdl Reclaimed slope backfill material (backfill) was encountered in all ten hand auger borings along the edge of the ponds and three test borings along [he slurry wall alignment. A. Pond Edges Backfill, as revealed by shallow hand auger borings, appears to be variable in composition and density along the east pond. Backfill is comprised of gravel, sand, silt, and clay particles, with soft sandy lean clay (CL) on the south side (HA-SH), medium dense silty sand and clayey sand with gravel (SC) on the east and north sides