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additional gob capacity. Stability of the anticipated excavation slopes was first evaluated. <br />Overall excavation slopes of 1.SH:1 V, 2H:1 V, 2.25H:1 V, and 2.SH:1 V were analyzed. These <br />excavations were modeled both as benched excavations and uniform slopes. <br />Once adequately stable excavation geometries were determined, various gob geometries were <br />superimposed on the excavation surfaces and evaluated. In general, geometry of the gob for the <br />analyses was as follows: <br />- A slope of 2.SH:1 V for the first 50 vertical feet of the stockpile (beginning at the toe). <br />- A 15-foot wide bench at a vertical height of 50 feet. <br />- The slope above the first 50 vertical feet to the ultimate height of the stockpile was <br />varied. Slopes of 4H:1 V, 3H:1 V, and 2.SH:1 V were evaluated. <br />- An approximate maximum gob pile height of 118 feet was evaluated. <br />Groundwater was modeled as a phreatic surface in the stability model. The depth to groundwater <br />was assumed to be approximately 40 to 70 feet below the existing ground surface. This is <br />considered reasonable, given that no groundwater or indications of groundwater were observed <br />in the boreholes to depths up to 57 feet. <br />5.2 Shear Strength Parameters <br />Shear strength parameters for the coal waste material were determined based on unconsolidated <br />undrained (UU) and consolidated undrained (CU) triaxial shear tests (presented in Sections 4.2 <br />and 4.3) and aze summarized in Table 5. Total stress strength pazameters for the gob were based <br />on 8 UU triaxial shear tests, and effective stress strength parameters for the gob were based on 3 <br />CU triaxial shear tests with pore pressure measurement. For the stability analyses, the mean <br />undrained shear strength of the 8 W triaxial tests (1,578 pst) was used. A conservative estimate <br />of effective strength parameters for the gob was used in the analyses. This estimate is based on <br />three CU triaxial tests. On-going shear strength testing of compacted gob is recommended to <br />assess the variability in shear strength due to variation in gob composition, moisture content, and <br />placement. <br />Shear strength parameters for the foundation soils were estimated based on materials <br />encountered during site investigation and our professional judgment. The cobble and boulder <br />content of the foundation soils made both in-situ strength testing and soil sampling extremely <br />difficult. Similar colluvial material in the area has performed well at steep slopes. Additionally, <br />excavations in similar colluvial soils at Gob Pile No. 2 have stood at slopes of 1H:1 V or steeper <br />for several months. Sedimentary bedrock outcrops can be observed on the slope above the <br />proposed gob pile. These nearly level units provide stability to the slope above the proposed gob <br />pile. <br />The moist unit weight of the coal waste used in the analyses was 112.5 pcf. This is an average <br />moist unit weight of 114 nuclear density tests performed to date in compacted gob at Gob Pile <br />No. 2. The moist unit weight of native soils was assumed to be 125 pcf. <br />Bowie Gob Pile No. 4 stability report <br />Project #OS-126-GEO <br />Page I 1 of 15 <br />