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I <br />~ • was used because it is clear that groundwater is present in this area, however it is not clear exactly <br />where the groundwater surface is located. When the estimated slide surface was modeled using <br />the total stress pazameters the resulting factor of safety was 1.05. This result is very close to a <br />FOS of 1, which correlates to the slope being at the point of incipient motion just prior to the <br />slide. Since the results of the total stress analysis closely modeled the condition of the slope prior <br />to the slide, it was decided to check the re-graded post-slide stability of the portion of the slope <br />that experienced movement by utilizing a total stress analysis. The current configuration with an <br />~ estimated failure surface was analyzed to determine the FOS. Utilizing the above mentioned soils <br />data, the current post slide re-graded topography, and the estimated failure surface; the FOS for <br />the stability of the lower portion of the slope was calculated to be 1.35. <br />in addition to the total stress analyses used to determine the stability FOS of the lower half of <br />the slope, an effective stress analysis of the stability of the upper portion of the slope above the <br />head scalp was performed. The effective stress analysis was utilized to model the upper portion of <br />the slope because observations in the field indicate that the water table is significantly below the <br />surface in the upper portion of the hill. The field observations that support a deep water ta61e <br />include the absence of any surface seeps and the lac[ that there was no standing water evident to a <br />depth of approximately twenty feet in test pit P12-1 located in the body of the slide (See <br />Drawings). The failure mode of concern in the upper portion of the slope is a failure surface that <br />would intersect the haul road. The indication of the slope stability analyses is that the Cailure <br />mode that intersects the haul road has a factor of safety of 1.33. In addition to the computer <br />generated quantitative slope analyses, qualitative analyses aze discussed in the results portion of <br />this report. The SLOPE/W input and output printouts aze included as Attachment 1. <br />• 4. Results <br />To address the hillside stability, the failure surfaces were modeled in bosh the pre-slide and <br />re-graded post slide topography. Through modeling it was determined that the re-graded post <br />slide topography has a factor of safety of I.3 for a failure of the material in the lower portion of <br />the slope and for a failure that would effect the haul road. The SLOPFIW stability analyses are <br />attached. <br />In addition to the quantitative analyses, field observations since the slide event indicate that <br />the upper portion of the hillside has been stable for the ten months that have passed since the slide <br />except for a small amount of surface sloughing from the head-scarp prior to re-grading. <br />Furthermore, during the field investigation of the materials that makeup the head-scarp and upper <br />portion of the hillside it was evident that the material is unsaturated. As a result the unsaturated <br />materials that compose [he hillside are likely stronger than the saturated material utilized in the <br />lab to determine the shear strength of the soil and modeled in the SLOPFJW stability analyses <br />that utilized saturated soil mechanics. <br />6. Conclusions and Recommendations <br />Utilizing the available information to model the hillside resulted in a factor of safety of 1.3 <br />for the two major modes of concern. These analyses were based on saturated soil mechanics and <br />are conservatively model the unsaturated condition of the upper portion of the slope. Based on the <br />field observation and the quantitative slope stability analysis it appears that the slope has been <br />displaced in to a relatively stable configuration. The additional re-grading work completed by <br />• Seneca further stabilized the area by reducing the localized steep slopes in the vicinity of the <br />