Laserfiche WebLink
Design and Operations Plan <br />Western Gravel E & P Waste Disposal Facility <br />2.3.2.6 Slope Stability <br />October 24, 2011 <br />Page 10 of 51 <br />excavation may be conducted in order to construct the cells. A dozer equipped with rippers will conduct <br />bedrock excavating activities. Blasting will not be conducted in order to excavate bedrock. <br />• Since the landfill will be constructed atop bedrock located along the terrace face, potential of significant <br />slope erosion appears low. <br />• The only floodplain relevant to the facility is the White River located along the north and east sides of <br />the site. The White River is located approxim ately 45 feet below the facility. Only ephemeral drainages <br />that flow to the White River floodplain are located along the north faces to the west and east of the <br />landfill area. <br />Because the north side of the landfill is locate d on a terrace above the White River, a potential geologic <br />hazard exists. Therefore, in addition to the geologic hazard assessment discussed in Section 2.3.2.5, a slope <br />stability evaluation was conducted. Slope stability evaluation results are discussed below. <br />Slope stability analyses were conducted using the proposed final waste and soil cover grades of 6 <br />(Horizontal) to 1 (Vertical). Stability of the materials used to construct the cells was evaluated three <br />different ways including the following. <br />• Stability of final cover soils in a saturated condition. <br />• Stability of waste materials placed without toe confinement provided by cell side slopes. <br />• Overall global stability of a waste cell under static conditions and constructed to final design <br />configurations including adjacent terrace slope. <br />• Overall global stability of a waste cell under dynamic conditions (seismic coefficient of 0.128) and <br />constructed to final design configurations including adjacent terrace slope. <br />Stability of recommended final cover s oils was analyzed using an infinite slope stability model under both <br />static and dynamic (seismic) conditions. The proposed cell waste slope and overall global stability were <br />analyzed using the "Simplified Bishop" and "Mohr - Columb" methods in order to determine the factor of <br />safety against failure. Material strengths were estimated based upon available data associated with WG or <br />other projects that accept similar waste and are constructed in areas that exhibit similar soil and bedrock <br />materials properties. In accordance with industry standards, the required minimum factor of safety used for <br />this project is 1.5 under static conditions and 1.2 under dynamic conditions . <br />The calculated factor safety of fmal soil cover materials failing over the waste is 4.3 a nd the calculated factor <br />of safety of failure within waste materials is 1.83. The factor of safety associated with a large scale global <br />slope failure occurring within the completed cell and through the adjacent terrace slope is 4.32 under static <br />conditions and 2.41 under dynamic conditions. Stability analyses conducted under dynamic conditions and <br />with a failure through the underlying bedrock surface exhibited a factor of safety of 2.62. Based upon the <br />above calculated factors of safety, the proposed design and assumed materials properties suggest that the <br />facility will be stable from failures associated with final soil cover, above grade waste materials, and from a <br />large scale failure occurring at the facility under both static and dynamic (seismic) cond itions. A more <br />detailed discussion of the stability analyses methods, specific soil parameters, calculations, and data are <br />provided in Appendix B. <br />NWCC, Inc. <br />