Prediction of Three-Dimensional Dynamic Soil-Pile Group Interaction in Layered Soil by Boundary Element Analysis and Seismic Cone Penetration Tests

A series of full-scale dynamic vibration tests and cyclic lateral tests are to be performed on a single pipe pile and a 2x2 pile group as part of an ongoing research project. The parallelized boundary element method (BEM) code BEASSI was modified and used to predict the three-dimensional vibration response of the pile group with account of the layered soil profile including material damping and radiation damping in the viscoelastic system. The concept of the disturbed-zone model for pile groups is employed to account for pile installation effects, soil inhomogeneity, and stress-and strain-dependent modulus and damping in the near-field, while simultaneously capturing three-dimensional wave propagation and rigorously accounting for radiation damping in the far-field. To acquire accurate soil profiles as input to the analyses, a comprehensive site investigation was recently conducted, including seismic cone penetration testing with pore pressure measurement (SCPTu), standard penetration testing (SPT), and Shelby tube sampling. In the present paper, the acceleration sensor responses from SCPT tests at various depths are analyzed by the cross-correlation method to obtain shear wave velocity profiles, and the results are compared to those by the arrival time and cross-over methods, as well as empirical correlations to CPT tip resistance and sleeve friction. Material damping in the soil is also estimated from the SCPT data by the spectral ratio slope (SRS) method, which aims to minimize effects of radiation damping. The BEM program is then used with the resulting shear wave velocity profiles to calculate impedance functions in the frequency domain, which explicitly quantify the three-dimensional interaction between all the piles in the group. A general formulation based on the sub-structuring method is employed to analyze the dynamic response of pile groups using the impedance functions to obtain accelerance functions of the pile cap in vertical and coupled lateral-rocking vibration modes for validation. The findings demonstrate the accuracy and stability of the cross-correlation method for estimating shear-wave velocity from SCPT data, and an average minimum material damping ratio of 2.6% by the SRS method for the clay soils encountered. The impedance functions of the pile group indicate strong frequency-dependent dynamic soil-pile-soil interaction for the case examined. Results of this study will aid development of the proposed disturbed-zone continuum models, and provide insights into future physical pile group tests.