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9 Landslide Enhancement and Analysis <br />This section of the report discusses the stability analysis performed on the landslide that comprises <br />the left abutment of the dam. The objective is to determine the stability of the landslide under <br />existing and proposed conditions and the impact that groundwater has on the overall system. Further, <br />the impact of seismic loading on the landslide was included in the analysis. <br />9.1 Landslide Stability Analysis <br />The main objective of this portion of the slope stability analysis was to evaluate the stability of the <br />landslide under existing, proposed, and seismic loading conditions. The landslide stability analysis <br />was performed on Cross-Section A, which provides a section through the left abutment and through <br />two installed inclinometers and a.number of piezometers. Having across-section through the <br />instrumentation provides information about the location of the failure surface and porewater pressure <br />data. Furthermore, correlations between slope movement and porewater pressures have been made <br />(Norfleet, 1999). These correlations provide important information about soil strength, especially for <br />the back-analysis which is described below. <br />~ 9.1.1 Geometry <br />Figures 12 and 13 show the existing and proposed geometry modeled at Cross-Section A, <br />respectively. The geometry consists almost entirely of colluvium, except near the toe of the landslide <br />where bedrock is found. The colluvium is separated into three layers: Upper colluvium, Failure <br />Surface Material, and Lower colluvium. The contact between the lower colluvium and the bedrock <br />beneath the spillway and access road is unknown because no soil borings reached the contact. The <br />location of this contact is not relevant for modeling purposes, however, because the failure surface <br />for stability calculations is being forced through the observed failure surface and never enters into the <br />lower colluvium. This is described in greater detail below. <br />~ 9.1.2 Groundwater <br />' As mentioned previously, groundwater plays a major role in the stability of slopes. The likelihood of <br />slope movement increases when groundwater levels rise. Fortunately, the correlation between <br />groundwater levels and slope movement on the landslide is well documented (Norfleet, 1999). In <br />' this research, piezometer readings that corresponded to the initiation of slope movement were <br />determined and these water levels were called the "threshold" values. Table 12 shows the threshold <br />' values for the previously installed piezometers. <br />P:\Mp]s\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 39 <br />1 <br />