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OCR for page 50
50 9 0.225 8 0.2 7 0.175 6 log-normal 0.15 distribution Number of Pile-Cases mlnx = -0.232 Relative Frequency lnx = 0.480 5 0.125 4 0.1 normal distribution mx = 0.884 3 0.075 x = 0.424 2 0.05 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 FHWA Method for Dry Construction Figure 36. Histogram and frequency distributions for Ksx for 40 cases of drilled shafts in clay. should be determined based on the smaller of the two, the For piles embedded in alternating soil conditions (granu- FHWA criterion or the maximum applied load on the pile. lar and cohesive): The relationship between the number of tests and the resis- tance factor is based on similar piles (geometry and size) For dynamic testing purpose: t75 = 39 × r (39) tested at the same site (see section 3.4.3). The recommended resistance factors should be applied to the mean capacity Where: determined for all tests. t75 = time to reach 75% of maximum capacity in hours r = pile radius (or equivalent) in feet. 3.4.6 Pile Test Scheduling 3.4.7 Design Considerations Static or dynamic tests (restrikes) should be performed no sooner than before the pile has gained 75% of its capacity. Figure 3 outlines the process of deep foundation design and This can be established as follows: construction. The following sequence of comments address For piles embedded completely in clay: several of the steps in that process in relation to the previous sections: For static testing purpose: t75 = 1540 × r2 (37) 1. When analyzing the field and laboratory testing for For dynamic testing purpose: t75 = 85 × r (38) strength and deformation parameters, two additional
OCR for page 51
51 10 0.175 9 8 log-normal 0.15 distribution mlnx = -0.246 7 lnx = 0.477 0.125 Number of Pile-Cases 6 Relative Frequency 0.1 normal distribution 5 m x = 0.872 4 0.075 x = 0.419 3 0.05 2 0.025 1 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 FHWA Method for Mixed Construction Figure 37. Histogram and frequency distributions for Ksx for 53 cases of drilled shafts in clay. factors need to be established (related to the comments load and the resistance factors to be used during the in section 3.4.3) (a) the number of different "sites" rec- construction. For example, if the required design load ognized in the project, and (b) the level of site variabil- is Fd, the site is of medium variability, and two static ity associated with each site. load tests will be performed, Table 30 indicates that 2. When performing static analysis for the designed deep = 0.75. Using equation 33, FS = 1.4167/0.75 = 1.89 and foundations, resistance parameters from Tables 25 and hence the ultimate capacity for the WEAP drivability 26 should be used for driven piles and from Table 29 analysis can be taken as Fu = 1.89 × Fd. If the design for drilled shafts. Resistance parameters from Table 26 load is established via LRFD analysis (i.e., factored should be used for driven piles under tension. Attention design load) than Fu = Fd/. In case of scour and/or should be given to the efficiency factors as a measure downdrag, both components should be added to the of economic scale. The factors should be applied accord- design load, i.e., Fd + net scour + downdrag. It should ing to the redundancy status of the pile cap arrangement. be noted that the results of this analysis should not be Without prebid pile field testing, the testing planned used for pile capacity prediction in the field. Table 27 during construction (e.g., static and/or dynamic) should provides resistance factors that should be used at EOD not affect the resistance factors used in the design stage if WEAP analysis is required as a prediction method for other than as described in the following item (3). pile capacity based on measured blow count. That table 3. For driven piles, a drivability study is carried out dur- also provides resistance factors associated with the ing the design stage in order to assess the pile installa- anticipated testing method that should be used (in the tion. For this purpose alone, the required ultimate pile same manner as described above for static load tests) if capacity can be established through the required design dynamic testing is to be performed.