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51 A promising technique for improving base load- recommendations given by Allen (2005). Rock mass proper- displacement response of drilled shafts involves post-grouting ties used with LRFD resistance factors should be based on at the base (base grouting). The technique involves casting average values, not minimum values. drilled shafts with a grout delivery system incorporated into the reinforcing cage capable of placing high pressure grout Three methods are cited for predicting ultimate unit side at the base of the shaft, after the shaft concrete has cured. The resistance in rock. The first is identified as Horvath and Ken- effect is to compress debris left by the drilling process, thus ney (1979). However, the equation given in the AASHTO facilitating mobilization of base resistance within service or Specifications is actually the original Horvath and Kenney displacement limits. According to Mullins et al. (2006), base recommendation (Eq. 25), but with unit side resistance mod- grouting is used widely internationally, but its use in North ified to account for RQD. A reduction factor, , is applied, America has been limited. An additional potential advantage as determined by Table 13 and Table 18 of this report. This is that the grouting procedure allows a proof test to be con- approach was recommended by O'Neill and Reese (1999). ducted on the shaft. Base grouting warrants further consider- The second method is identified as Carter and Kulhawy ation as both a quality construction technique and a testing (1988). The draft 2006 Interim AASHTO LRFD Bridge De- tool for rock-socketed shafts. sign Specifications does not state explicitly the equation to be used in connection with the Carter and Kulhawy method. For evaluation of service limit states, both side and base However, in the calibration study by Paikowsky et al. (2004a) resistances should be included in the analysis. Analytical the expression used for all evaluations attributed to Carter methods that can provide reasonable predictions of axial and Kulhawy is load-deformation response, for example the Carter and Kul- hawy method described in this chapter or similar methods fsu = 0.15 qu (96) given by O'Neill and Reese (1999), provide practical tools for this type of analysis. All of these methods require evalu- in which qu = uniaxial compressive strength of rock. In their ation of rock mass modulus. original work, Carter and Kulhawy (1988) proposed the use of 0.15 qu as a design check, whereas the AASHTO Specifi- CURRENT AASHTO PRACTICE cations treat it as a design recommendation. This unintended usage is inappropriate and does not adequately represent the The draft 2006 Interim AASHTO LRFD Bridge Design most up-to-date research based on regression analysis of the Specifications recommends specific methods and associated available data on socket-side resistance. The third method resistance factors for evaluating side and base resistance of given by AASHTO is O'Neill and Reese (1999). It is not rock-socketed shafts under axial load. These are summarized clear how this differs from the Horvath and Kenney (1979) in Table 20. The resistance factors are based on a calibration method because the equations given by AASHTO are all study conducted by Paikowsky et al. (2004a) and additional taken directly from O'Neill and Reese (1999). The equations TABLE 20 SUMMARY OF CURRENT AASHTO METHODS AND RESISTANCE FACTORS Resistance Method/Condition Factor Nominal Axial Compressive Side resistance in 1. Horvath and Kenney (1979) 0.55 Resistance of Single-Drilled rock 2. Carter and Kulhawy (1988) 0.50 Shafts 3. ON eill and Reese (1999) 0.55 Tip resistance in rock 1. Canadian Geotechnical Society 0.50 (1985) 2. PMT Method (Canadian 0.50 Geotechnical Society 1985) 3. O'Neill and Reese (1999) 0.50 Side resistance, IGMs 1. O'Neill and Reese (1999) 0.60 Tip resistance, IGMs 1. O'Neill and Reese (1999) 0.55 Static load test <0.70* Compression, all materials Nominal Uplift Resistance of Rock Horvath and Kenney (1979) 0.40 Single-Drilled Shaft Carter and Kulhawy (1988) 0.40 Load Test 0.60 * Depends on the number of load tests and site variability. AASHTO LRFD Bridge Design Specifications, 2006 Interim.