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16 <br />3-1 <br />3.0 LOWER TAILINGS DAM MODEL <br />The available and newly obtained information discussed in Chapter 2.0 <br />was combined into a geologic and hydrologic model of the lower tailings <br />dam. The model incorporates the geologic and hydrologic conditions <br />anticipated in the embankment and foundation. The flow paths and <br />hydrostatic pressure within the system as a whole are defined. The <br />purpose of the model is to quantitatively evaluate the stability of the <br />dam against slope failure and against distress due to uncontrolled or <br />excessive hydraulic conditions. <br />The model is based on observations at various points, internal and <br />external to the structure, with extrapolation of conditions between <br />these points. Conditions between measured points may be different than <br />those assessed in the model, and conditions can change with time. For <br />the purpose of structure evaluation and remedial measure assessment, the <br />model is considered appropriate. <br />3.1 DAM EMBANKMENT <br />3.1.1 Materials <br />The central portion of the dam, where borings were advanced through the <br />fill, consists primarily of silty and clayey sands (USCS SM and SC) <br />soils and some sandy silt and sandy clay (USCS ML and CL) layers with <br />low plasticity. Cobbles and rocks are also frequent in the fill of the <br />central portion of the dam. The fill in this region is considered <br />medium dense to dense in consistency and layered, although of generally <br />similar composition. <br />The materials in the outer zones on the upstream and downstream portion <br />of the dam were not investigated by drilling. Consequently, the <br />material characteristics were estimated from available information and <br />data. The original designs specified coarser material in this region. <br />16 <br />