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Probability distribution curves 9 8 @RISK Frequency (%) 7 SLOPE/W 6 5 4 3 2 1 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Factor of safety Figure 8.17: Probability distribution curves for undrained conditions Table 8.12: Output data from @RISK and SLOPE/W Outputs @RISK SLOPE/W Minimum factor of safety 0.113 0.334 Maximum factor of safety 3.941 1.467 Mean factor of safety 0.746 0.829 Standard deviation 0.304 0.215 Reliability index Î² - -0.794 82.7* 77.3 Probability of failure (%) *value only given to one decimal place The probabilities of failure were found to vary by approximately 7% in this analysis. The same range of variables was used in both programs to limit any potential epistemic errors. Even with this, @RISK calculated a much greater maximum factor of safety. This difference is assumed to be due to default functions in the programs on how they treat lognormal distributions. The mean factors of safety were approximately 10% different with @RISK having a lower mean factor of safety, and therefore a greater probability of failure. The failure distribution curves are lognormal since the input variable (cohesion) was lognormal. This does not affect the probabilities of failure; the distribution can be easily turned into a normal distribution by taking the log of x-axis values. If lognormal distribution 111