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4 <br />Mr. Timm Comer <br />Manager, Environmental Resources <br />ADR Water Line <br />January 3, 2010 <br />amec?y <br />The stability of the PSSA embankment resulting from potential failure of the water pipelines, <br />was assessed using a limit equilibrium approach, using the program Slide 5.0 (RocScience, <br />2008). The embankment stability was evaluated according to the Spencer's Method of Analysis <br />(Spencer's Method). Spencer's Method considers potential failure masses as rigid bodies <br />divided into adjacent regions or "slices", separated by vertical boundary planes and is based on <br />the principle of limit equilibrium, i.e., the method calculates the shear strengths which would be <br />required to just maintain equilibrium, and then calculates a "FOS" by dividing the available shear <br />strength by the required shear strength. Consequently, the FOS calculated by Spencer's <br />Method indicates the percentage by which the available shear strength exceeds, or falls short <br />of, that required to maintain equilibrium. Therefore, a FOS in excess of 1.0 indicate stability and <br />those less than 1.0 indicate instability. The greater the mathematical difference between the <br />FOS and 1.0, the larger the "margin of safety" (for a FOS in excess of 1.0), or the more extreme <br />the likelihood of failure (for a FOS less than 1.0). <br />For the stability analyses, the strength properties for the structural fill and foundation were the <br />same values as those used for Amendment No. 9 and previous amendments. The base-case <br />for the PSSA embankment stability represents dry conditions within the fill. The FOS for the <br />base case is 1.8, as shown in Figure 4. <br /> <br />Material Properties <br />Material: Foundation <br />Strength Type: Mohr-Coulomb <br />Unit Weight: 1501b1113 <br />Cohesion: l pat <br />Friction Angle: 40 degrees <br />Water Surface: None <br />Material: Structural Fill <br />Strength Type: Mohr-Coulomb <br />Unit Weight: 1201W113 <br />Cohesion: 1 pat <br />Friction Angle: 88 degrees <br />Water Surface: None <br />g <br />.150 .flee .50 .0 5D 100 150 200 250 eW 350 00 <br />Figure 4 Base-Case (Dry) PSSA Embankment Stability <br />MACINTOSH HD:USERS:JLUPO:D000MENTS:CCSV:WATER LINE letterdoc 4