Nonlinear seismic behavior of pile groups in cement-improved soft clay

Centrifuge tests were performed to investigate the effects of ground improvement on the seismic behavior of pile groups in soft clay. The soil profile consisted of four lightly overconsolidated clay layers overlying a dense layer of sand. The pile groups had a symmetrical layout consisting of 2×2 piles spaced at 3.0 pile diameters and were driven into both unimproved soft clay and soft clay improved by a simulated Cement Deep Soil Mixing (CDSM) method. The centrifuge model was subjected to seven different earthquake events with peak accelerations ranging from 0.03 to 0.66g. The foundation level motions of the improved pile groups were different than the surface free-field motion. The foundation level motion for the unimproved pile group was, however, identical to that in the free-field. Higher peak accelerations were observed in the pile cap of the group with smaller CDSM block (GIS) compared to the unimproved pile group (GU) and the group with the largest CDSM block (GIL). Higher pile cap to the soil surface spectral ratios were also obtained for the GIS group in both short and long periods. Cement-Deep-Soil-Mixing was effective in reducing the peak displacements of the GIL pile cap. The peak displacements of the GIS pile cap remained about the same as the GU pile cap. As the size of the ground improvement increased, the fundamental period of the pile groups reduced. The estimated fundamental periods of the GIS and GU pile groups were, however, close to each other. Acceleration and displacement response spectra of the foundation level motions in comparison to the fundamental periods of the pile groups provided insight into the observed acceleration and displacement responses. The adhesion between soft clay and CDSM blocks helped to reduce the soft clay settlement in the vicinity of CDSM blocks compared to the free-field and the vicinity of unimproved pile group. More residual excess pore water pressure was, however, generated in the vicinity of CDSM blocks compared to the free-field and the corresponding location in the unimproved pile group, likely due to vibrations of the CDSM blocks and the piles.