Performance of reliability-based design code formats for foundations in layered soils

Simplified reliability-based design (RBD) formats in the form of Load and Resistance Factor Design (LRFD) and Multiple Load and Resistance Factor Design (MRFD) are applied to 42 drilled shafts installed in layered soil profiles underlying the city of Taipei. Clay, sand, gravel and rock layers or some partial combination thereof are encountered within the depth of penetration. It is reasonable to require a foundation reliability-based design code to achieve a reasonably consistent target reliability index over this range of design scenarios, as they are representative of the ground conditions underlying Taipei. The simplified RBD formats are calibrated using two approaches. They are the design value method (DVM) and the more recently proposed quantile value method (QVM). In general, for layered soil profiles, conventional formats containing resistance and load factors are unable to achieve the prescribed target reliability index with the same consistency as that reported for homogeneous soil profiles, regardless of the reliability calibration approach. For the drilled shaft examples considered in this study, it was found that the direct application of quantiles in the simplified RBD format is more reasonable for layered soil profiles.