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The factors of safety calculated by the Case 3 Spreadsheet and SLOPE/W were similar (< 1% difference), which gave confidence in the use of the SLOPE/W suite to analyse more complex cases in terms of drained analyses. 5.5 Case 4: Two layer soils with phreatic surface Case 4 analysed the same embankment as Case 3 but now with a phreatic surface (the same phreatic surface as Case 2). Elevation (m) 15 10 Embankment 5 Foundation 0 0 5 10 15 20 25 30 35 40 Distance (m) Figure 5.12: Case 4 embankment geometry showing two soils and phreatic surface as entered into SLOPE/W 5.5.1 Stability charts results Table 5.13: Factor of safety from stability charts Analysis Undrained Drained Taylor Bishop & Morgenstern Spencer Janbu 0.90 n/a n/a 0.94 n/a 1.64 1.50 n/a The short-term factors of safety using Taylorâs and Janbuâs stability charts were the same as were calculated for Case 3. These values gave close (< 4% difference) representations of what was calculated in Table 5.14 using Bishopâs simplified method. The factor of safety using Bishop and Morgensternâs stability charts was 1.64 (15% higher than Bishopâs simplified method values in Table 5.15). This factor of safety depended on the depth to the phreatic surface (zw) which varied for each slice. the resultant factor of safety may not necessarily be the most critical since the depth to the phreatic surface (zw) was estimated to the midpoint. Further in situ testing to determine the depth to the phreatic surface could be undertaken, however it would be difficult to determine the phreatic surface position compared 62