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ARCADIS <br />Analysis <br />Slope stability analysis was conducted using Spencer's Method (method of slices with circular failure). <br />The global slope stability of the sector gate section was evaluated using GeoStudio's SLOPE/W® <br />software (v 7.17 built 4921), which is capable of performing Spencer's Method. Spencer's method <br />considers both shear and normal interstice forces, satisfies both moment and force equilibrium, and <br />assumes a constant interslice force function. The analysis was conducted using a piezometric surface <br />based on the normal pool elevation (as depicted on the supplied embankment sections, varies between <br />elevation 5682.5 and 5693.3) and assuming steady state seepage through the embankments, automatic <br />search for a tension crack, and using optimization. Tension crack development was assumed in the <br />model; however, given the granular nature of the material, it is unlikely that a tension crack would develop. <br />Assuming that a tension crack will develop forces the model to search for, and calculate the most <br />reasonable location and also reduces interslice forces to eliminate negative interslice forces. <br />A total of six different stability analysis cases were reviewed and analyzed. Some of the cases that were <br />analyzed where the same as those conducted by Greg Lewicki in January 2000 (Revised March 2006). <br />Some modifications to the cases were made based on subsurface geotechnical conditions that appear to <br />differ from those outlined in the previous analyses. The attached tables (Table 1A, 1B, and 1C) discuss <br />the cases and the results of the stability analyses. Case 6 is the "most likely" scenario and for all stability <br />sections the FOS exceeds the required FOS of 1.3. <br />For purposes of the stability analysis, the use of cutoff trenches or key trenches was not assumed. This <br />leads to more conservative stability model results. Stability analysis results for Pond NHN -002 indicate <br />that the calculated FOS for Cases 1 and 5 were less than 1.3. For Pond NHN -001, Case 1 and Pond <br />NHN -003, Case 5, the calculated FOS was less than 1.3. This is due to factors including: 1) the soils <br />strengths used were the lowest values, 2) the slope of the foundation base, and 3) the lack of a key trench <br />to bind the embankment to the foundation base. As required by CDRMS regulation "the foundation base <br />areas shall be sloped no steeper than 1 H:1 V, and the entire foundation base shall be scarified. Cutoff <br />trenches shall be installed if necessary to ensure stability." Case 6 is adequately stable without the use of <br />a key trench (cutoff trench); however, the use of a key trench is recommended. <br />Attached to this memorandum please find the stability analysis result graphical output files that depict the <br />results for Case 6. These drawings depict the geometry of the proposed embankments, soil properties <br />used in the analysis and the resulting FOS. <br />Based on the laboratory results contained in the Lambert and Associates report, swell pressures of <br />between 350 and 700 pounds per square foot (psf) were measured. This is an important consideration as <br />the weight of the embankment near the toes of each end of the slope could experience differential swell <br />especially when exposed to impounded water and seepage through the new embankments. The potential <br />for swell is primarily in areas where the overlying surcharge load is less than about 700 psf. This would <br />Page: <br />wlsmhrrgk o001537 .0007_sedcad\derrverablesb raft _repa4slopeslabihtyanaysislslope stabikty anaysis and recd mamdaWn me= 11-17-11 docx 3/4 <br />