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<br />Memo to Gary Curtiss -2- October 18. 1996 <br />are removed and the pit is allowed to flood. This case <br />was designed to simulate a likely worst case scenario. <br />Case B: The same circumstances as case A except that the flooded <br />pit is once again pumped dry. In such a situation, the <br />stabilizing effect of the water pressure is removed from <br />the slope, but the pore pressures in the soil, which tend <br />to de-stabilize the slope, remain high for a certain <br />period of time until the soils can drain. This effect is <br />termed "rapid draw down", and is considered an extreme <br />worst case for slope stability. <br />Attached to this memorandum are cross-sections depicting three separate <br />failure surfaces that were analyzed for both Case A and Case B. Analysis <br />of failure surface no. 1 indicates that the vertical cut would eventually <br />fail, with factors of safety (FS) of .82 for Case A and .41 for Case B, <br />but the slope failure would not transgress the pipeline easement. <br />Analysis of failure surface no. 2 indicates that slope failure within the <br />easement is possible (FS=1.24) but not likely for Case A, and that slope <br />failure within the easement is likely for Case B (FS=.79). Analysis of <br />failure surface no. 3 indicates that failure to the center of the easement <br />is unlikely for Case A (FS=1.94), and possible (FS=1.25) but not likely <br />for Case B. These analyses indicate that a 55 foot set-back from the <br />pipeline (i.e., one half of the 50 ft. easement is 25 ft., plus the 30 ft. <br />proposed set-back from the easement boundary equals 55 ft.) is probably <br />adequate to protect the pipeline. However, the applicant must provide <br />their own engineering evaluation and recommendations in accordance with <br />the following direction: <br />1. The applicant should have their engineering consultant analyze <br />potential worst case scenarios including cut slopes, saturated soil <br />conditions, and rapid draw down. <br />2. Whatever set-back is proposed by the applicant based on the worst <br />case analysis should be a set-back distance from the actual pipeline <br />alignment, rather than a set-back from the easement boundary that is <br />currently proposed. This is because the pipeline is probably not exactly <br />centered within the easement at all points. <br />3. It must be recognized by the applicant that whatever set-back is <br />approved will define a zero excavation zone where all excavation, <br />including cut-and-fill to create finished slopes, will be prohibited. <br />4. The applicant must commit to carefully marking the set-back line on <br />the ground so that equipment operators do not inadvertently excavate <br />within the set-back. <br />C:\WP51\OWENS.MEM <br />