Calibration of Multiple Resistance Factors for Drilled Shafts in Multiple Strata

Abstract eng:
Drilled shaft is advantageous for its low noise and vibration during installation. Soil profile with multiple soil layers is often encountered in several metropolises around the world. Analysis and design of drilled shafts in this type of soil profile should consider resistances contributed from different soil types. For reliability based design, the multiple resistance factor design (MRFD) format is the realistic approach, so that different level of safety measure required for clay, sand, gravel, and rock layers can be considered. In this study, the MRFD factors are developed based on a load test database of drilled shafts in multiple strata. The correlations models for unit side resistance are re-examined based on an expanded global database, and the transformation uncertainties are evaluated. The calibration for the MRFD factors considers not only the transformation uncertainties in the correlation models but also the measurement errors in the geotechnical parameters. The resulting MRFD factors are found to be non-uniform, but simple regression equations as given in the form of Eq. (1) can be developed to determine the calibrated MRFD factors. γ=C₀ + C₁δ_su + C₂δ_σ^'v + C₃δ_qu + C₄R_c + C₅R_c² + C₆R_s + C₇R_s² + C₈R_r + C₉R_r² where δ is the c.o.v. of the measurement error for the designated geotechnical parameter; R_x is the ratio of the nominal resistance: R_x = S_x^*/( S_c^*+S_s^*+S_g^*+S_r^*). The regression coefficients c_i, i = 0 to 9, for all MRFD factors corresponding to a target reliability index of 3.0 are shown in Table 1. The maximum errors of the regression equations are typically small.

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