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II. SUBSURFACE CONDITIONS Haley & Aldrich completed 32 test borings along or near the slurry wall alignment (Borings HA-1 through HA-32) and performed observations in nearby active gravel mining operations to characterize subsurface conditions. The boring locations are shown in plan and profile views on Drawing 103. The test borings involved hollow-stem auger drilling, bedrock coring, bedrock packer testing, and drive sampling of the soil and bedrock. Test borings revealed soil overlying sedimentary bedrock along the alignment. The depth to the top of bedrock ranged from 31.5 to 61.0 ft below ground surface. The soil is alluvium consisting primarily of coarse alluvium (sand and gravel) with occasional lenses of fine alluvium (clay seams) overlain by a relatively thin layer of overbank deposits (commonly referred to as "overburden") primarily consisting of clay and sand with some gravel. Clay seams were up to 8 ft thick. Overburden ranged from 0 to 16.5 ft thick. Observations made at an adjacent site suggest that cobble sized particles up to 5 inches commonly exist in the coarse alluvium and infrequent boulders derived from the underlying bedrock can exist near the bedrock surface. Reclamation backfill from mining activities, consisting of clayey sands, was encountered in three of the borings along the south side, suggesting that the current dirt roadway in this location has been constructed on top of reclaimed portions of the gravel mine. ' The sedimentary bedrock is a mudstone of the Laramie Formation, consisting of interbedded claystone, shale, siltstone, and fine-grained sandstone. Occasional lenticular lenses of limestone were observed at the bedrock surface at an adjacent site. Core samples indicate the bedrock is very thinly bedded and unjointed. Packer test results show average hydraulic conductivity of the bedrock along the slurry wall alignment ranging from 3.6 x 10-b to 1.5 x 10' centimeters per second (cm/sec). Drive sampling indicated that the bedrock typically has a zone of very soft, weathered rock beginning at the top of bedrock and typically extending 0.5 to 4 ftinto the bedrock. Weathered bedrock extending 11.5 ft thick was encountered at one location along the slurry wall alignment. Groundwater in the alluvial aquifer was encountered at approximately 10 ft below ground surface (bgs) during the investigation. The Geotechnical Data Report presents a description of subsurface conditions and the results of surface explorations.