Effects of gravel columns on static and dynamic properties of sand: a laboratory investigation

Vibrated stone columns (composed of gravel in this study) are widely used for ground improvement. However, the composite behavior of sand-gravel composite columns (SGCC) under combined static and dynamic loading is not yet fully understood. This study systematically investigates the static and dynamic properties of SGCC through consolidated drained, consolidated undrained, and cyclic triaxial tests. The results show that gravel columns enhance the strength and deformation characteristics of SGCC through suppression of shear dilatancy. Under drained conditions, the peak strength ratio between SGCC and sand decreases with increasing confining pressure, whereas under undrained conditions, the ratio exhibits a positive correlation with confinement. The failure stress ratio of SGCC is 1.1 times that predicted by the conventional composite modulus method (CMM). The dynamic shear modulus and damping ratio of SGCC follow trends similar to those of sand and gravel, following a hyperbolic distribution. Compared with sand, SGCC exhibits an increased dynamic shear modulus and a reduced damping ratio. The dynamic modulus coefficient of SGCC is 1.2 times, and the damping ratio parameter is 1.5 times, the values estimated by CMM. Furthermore, the dynamic strength of SGCC is 1.1 times that of sand. These findings reveal the limitations of conventional CMM predictions and provide refined parameters, improving seismic safety design for stone-column foundations.