Back-analysis of bi-directional pile load tests of drilled shaft socketed in weak rock

Back-analyses using the finite-element method were undertaken to evaluate the performance of three bi-directional pile load-tested (BPLT) drilled shafts socketed into weak rock (sandstone) for a cable-stayed bridge structure. The hardening soil (HS) model was employed to define the stress–strain relationships of the soil, rock strata and the bored pile–soil or bored pile–rock interface, using the shear modulus of the interface element $Ginter$⁠, strength reduction factor $Rinter$ and power coefficient $m$⁠. Certain parameters relating to the weak rock could not be determined from the available rock quality designation and the uniaxial compressive strength. A back-analysis procedure was therefore proposed to calibrate the parameters for the corresponding rock strata by comparing the results of the finite-element analysis with those of the BPLTs. In this study, it was demonstrated that the parameters of the HS model and zero-thickness interface element could effectively capture the behaviour of soil–bored pile interaction in weak rock. The proposed parameters will subsequently be applied to the analysis of the full-scale foundation beneath the pylon of the bridge.