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40 4 0.2 3.5 0.175 3 0.15 Number of Pile-Cases 2.5 Relative Frequency 0.125 2 0.1 log-normal distribution mlnx = 0.250 1.5 lnx = 0.561 0.075 normal distribution 1 0.05 mx = 1.475 0.5 0.025 x = 0.771 0 0 0 0.5 1 1.5 2 2.5 3 KSX = Ratio of Static Load Test Results over the Pile Capacity Prediction using the Nordlund Design Method Figure 22. Histogram and frequency distributions of Ksx for 19 cases of pipe piles in sand. of piles and drilled shafts is performed via static load testing, 3.3.2 Resistance Factors various methods of dynamic (impact) testing, and integrity for Static Pile Load Tests testing. The first two are carried out to determine pile capacity and integrity while the last is utilized for structural Assigning resistance factors to associate with (in situ) pile quality assurance only. Two issues need to be addressed: (or drilled shaft) static load test results requires an estimate of (1) the testing method's performance and associated resis- the corresponding mean bias and COV. By definition, the tance factors, and (2) the number of tests that need to be car- mean bias is 1.0, since load tests directly measure in situ pile ried out. capacity either to failure or to a maximum applied load (proof Section 3.1 addressed the methods of dynamic analysis test). The COV reflects spatial variation from one pile to most commonly used during driving. The case histories in another at the same site, along with whatever variation is intro- the extensive PD/LT2000 database have widely varied sub- duced by the definition of failure criterion. surface conditions; hence, the direct calibration of the differ- Empirical data of sufficient quality to estimate within-site ent analysis method is applicable to all site conditions. The variability is lacking. Therefore, an assumption is made to evaluation of the required number of tests needs to assess a categorize sites as having low, medium, or high variability single site variability and evaluate how many piles are required and to assign coefficient of variations of 0.15, 0.25, and 0.35 to be tested to guarantee a target capacity. A single site vari- to these three cases respectively (Phoon and Kulhawy, 1996; ability, therefore, utilizes judgment and assigns categories Trautmann and Kulhawy, 1996). that cannot be based on firm data. The following sections In addition to the natural variability within a site, the inter- address issues associated with pile testing. pretation of failure criterion itself (i.e., Davisson's criterion