2018-10-24_PERMIT FILE - M2018055 (6)

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E Slope Stability Analysis June 29,2018 Page 3 silty sand above and gypsum below. D&A Test Hole 4(TH-4)showed a sandy gravel thickness of 55 feet, while TH-5 showed a sandy gravel thickness of 50 feet. The sandy gravel material was relatively dense, allowing a standard California sampler to advance an average of 3 inches after 50 blows of a standard 140- pound hammer. Gypsum was encountered in three out of seven boreholes.Approximately 8, 10,and 9 feet of gypsum in TH-4, TH-5, and TH-7, respectively, were drilled before these borings were terminated. Groundwater was encountered at approximately 27 feet below the ground surface (bgs) in TH-7 during drilling. 5.3 ME Test Pit Investigation and Drilling Program On May 25, 2018, WJE personnel visited the site to perform a test pit investigation. Six test pits were dug by an operator from Site Resource Management (SRM) using a track-mounted CX 240B Case excavator. The locations of the test pits were focused to the west and middle portions of the site per(Figure A.3). The test pits were generally 15 feet wide and were excavated to varying depths ranging from 15 to 25 feet, depending on the tendency of any one test pit to cave in due to a lack of lateral support at depth. Four general units were identified during this investigation including 1)silty or silty sand soil,dry,orange- brown; 2) sand, relatively clean to silty, medium-to coarse-grained, poorly graded, moist, light brown; 3) clay, somewhat plastic,highly laminated,moist-wet,grey, orange,red, and brown; 4) alluvium-gravelly cobbles with a clean sand to silty,clayey sand matrix.The sand was present as an approximate 2 foot-thick layer within the alluvium in TP-4 while the clay was observed in TP-3 as a discrete 6 inch layer within the alluvium. In general,the gravel, cobbles,and boulders within the alluvium unit were rounded to well-rounded,elon- gated, and somewhat imbricated. Visual estimates of grain size percentage distributions by volume were performed in the field. We have categorized clasts based on the Unified Soil Classification System for boulders, cobbles, gravel, sand, and fine diameter sizes. Typically, there were 0%to 5% boulders, 60% - 65% cobbles, 15% - 20% gravel, and 15% to 20% matrix material, although there were some instances where the majority clast size was between 2 and 4 inches,making the distinction between cobble and gravel grain size percentage distributions difficult to estimate, since in the Unified Soil Classification System, particles smaller than 3 inches are considered gravel, while particles larger than 3 inches are considered cobbles. For example, in Test Pit (TP) 6, the majority clast size within a portion of the alluvium unit was approximately 3 inches. It is possible that gravel was the main alluvium constituent by volume in some instances, but this was not commonly observed. The rock types within the alluvium unit mostly consisted of crystalline igneous and metamorphic rocks including quartzite,granite,granodiorite, and basalt.A weak,red sandstone was also observed, although it accounted for less than ten percent of the alluvial rock type where we observed it. WJE personnel returned to the site on June 4-5, 2018,to perform borehole logging. Three boreholes were drilled by an operator from Dakota Drilling with a truck-mounted rig using a 5-inch diameter ODEX head. The locations of these boreholes are displayed in Figure A.3. Approximately 6 to 7 inches of silty overbur- den was present above a gravel and cobble alluvium material in either a clean sand or silty sand matrix in all three boreholes.A 2 to 3 inch-thick clay layer was observed at 34 feet bgs in TH-1. Silty sand layers up to 2 feet thick were observed near 60 feet bgs and at 27.5 feet bgs in TH-2 and TH-3, respectively.Neither gypsum nor groundwater was encountered during this drilling program. The total depths of the boreholes ranged from 62 feet to 69.5 feet. Appendices B and C are test pit and borehole logs,respectively.Appendix D includes photos of the test pit investigation and drilling program.