Effects of drilling disturbance and unloading lag on in-situ mechanical characteristics of granite residual soil

The granite residual soil is a kind of special structural soil with both engineering and fabric characteristics of clay and coarse grained soil. Its engineering performance is generally susceptible to disturbance damage and excavation unloading, which may lead to engineering disasters. Through the comparison tests of SBPT and PMT in different predrilling unloading lag time in a typical layer of granite residual soil, the response characteristics of in-situ stiffness attenuation characteristics, strength indexes and bearing capacity of granite residual soil are analyzed. The results show that the weakening effect of drilling disturbance on the strength, bearing capacity and stiffness of granite residual soil is obvious, and the weakening degree increases with the increase of stress release time during unloading period. The order of influence degree of drilling disturbance and unloading delay on each parameter is evaluated by disturbance factor R(u): the effect on stiffness parameter G_s is the largest, followed by undrained shear strength C_u and bearing capacity f_ak. The in-situ G-γ curves of SBPT and PMT tests are obtained by nonlinear stiffness analysis and effectively fitted by the equation proposed by Ken Stokoe. The research results may provide the reference for engineering design and construction in the granite residual soil area.