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OCR for page 53
53 9 0.2 8 0.175 7 log-normal 0.15 distribution 6 mlnx = 0.007 lnx = 0.350 0.125 Number of Pile-Cases Relative Frequency normal distribution 5 0.1 m x = 1.066 4 x = 0.351 0.075 3 0.05 2 1 0.025 0 0 0 0.5 1 1.5 2 2.5 3 KSX = Ratio of Static Load Test Results over the Shaft Capacity Prediction using the R&W Method for Mixed Construction Figure 39. Histogram and frequency distributions for Ksx for 44 cases of drilled shafts in sand + clay. 2. Searching for common factors that can be compared, for average prediction is conservative and hence the mean case example establishing a connection between resistance with an FS = 2.25 relates to an overprediction ratio of 3.1 factors and factors of safety (e.g. see section 3.2.3). (1.368 2.25). In comparison, the use of FS = 2.25 with a specified CAPWAP at the BOR is reasonable and is associ- The following sections deal with various aspects asso- ated with an acceptable probability of failure for a single pile ciated with the recommended factors and means for their application (approximately 1.85%; see Figure 32). The use evaluation. of a large factor of safety for the static analysis appears to be very sensible, as most of the methods overpredict the actual 3.5.2 Working Stress Design capacity. The WSD existing factor (FS = 3.5) is probably based on historical cumulative experience and matches the The traditional factors of safety presented in Table 1 can presented results without being excessive or wasteful. The now be evaluated in light of the available data. For example, data summarized in Figure 45 are used to demonstrate this the COV for the ENR equation and the WEAP analyses are issue. For example, the average static capacity analysis of a 0.910 and 0.724, respectively, which practically means that driven pile in clay results in a mean underprediction ratio of the methods are unsuitable for the purpose of capacity pre- about 0.82 and 0.72 for and methods, respectively. The diction (see Figure 33). The reduction in the factor of safety actual factors of safety in theses cases are 2.87 and 2.52. from 3.50 to 2.75 when adding WEAP analysis to static cal- These factors of safety are in good agreement with the actual culations (as shown in Table 1) is therefore unfounded. Nor factor of safety when using the CAPWAP BOR results con- does the use of unspecified CAPWAP (general case) justify sidering the bias (FS = 2.61; see Figure 45). However, using the reduction of the factor of safety to 2.25, even though the CAPWAP results at the EOD, considering the bias, results in

OCR for page 53
54 6 5.5 0.12 5 log-normal distribution 0.1 4.5 mlnx = 0.101 lnx = 0.494 4 0.08 Number of Pile-Cases normal distribution Relative Frequency 3.5 mx = 1.229 3 0.06 2.5 x = 0.509 2 0.04 1.5 1 0.02 0.5 0 0 0 0.5 1 1.5 2 2.5 3 KSX = Ratio of Static Load Test Results over the Shaft Capacity Prediction using the C&K Method for Mixed Construction Figure 40. Histogram and frequency distributions for Ksx for 46 cases of drilled shafts in rock.