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waste bank is the Mancos Shale Formation, which forms the steep slopes below the Rollins • member. According to geologic hazards maps for the area (USGS, 1978b1, rockfalls and expansive soils are potential hazards at this site. Rockfalls were evident in the vertical Rollins sandstone rock outcrops above the proposed coal mine waste bank, and the colluvial material found on the hillside. Rockfalls are not expected to be a factor in design, but may prove to be a maintenance issue, (i.e., removing fallen rock fragments from the waste bank during the life of the project). No remedial activities need to be conducted to prevent rockfalls in the area. Expansive soils originate from the Mancos shale underlying the site that become wetted from rain or the placing of coal waste with a high moisture content. Effects of expanding soils can be minimized by quickly routing water away from contact with the shale. The planned underdrain system will channel any water within the coal mine waste bank to the toe, and the diversion ditch system will route excess runoff from the waste bank to reduce contact with the Mancos shale. • 2.2 SUBSURFACE CONDITIONS Due to steep terrain and vehicle access problems, two geotechnical boreholes were completed on the hillside of the proposed waste bank. Three additional boreholes were drilled in the area of the toe in the old coal loading area. The location of these boreholes is shown on Figure 2.1. The two hillside holes, WBH-B1 and -82 were drilled to determine the depth of colluvial materials, to obtain material strengths, and to confirm depths to the Mancos Shale Formation. Boreholes WBH-B3, -B4, and -B5 were important in determining strengths of foundation materials at the toe of the proposed waste pile. They also determined depths to the Mancos formation. In Borehole WBH-84, a piezometer was installed to monitor subsurface water conditions. The boreholes were drilled by a local drilling contractor using a Central Mining Equipment (CME) model 55 drill rig. A 4-inch solid or 7.5-inch hollow stem auger was used to drill through the unconsolidated material. Standard Penetration Test (SPT) drives were taken within this unconsolidated material using standard American Society for Testing and Materials (ASTM) methods, including a 2-inch outside diameter (O.D.) split-spoon sampler and 140 • pound (Ib) drop hammer. Drives were usually taken in 5-foot depth intervals to 20 feet, then 96266/1424.RPT Coel Mine Weste Bank Final Design for the Bowie No. 2 Mine June 1998 WESTEC 3 ADCpUACY-O]