Mechanical properties and generalized Duncan-Chang model for granite residual soils using borehole shear tests

Granite residual soil, as the weathering product of near-surface granite, is widely distributed in the southeast coastal areas of China. In order to study the influences of weathering degree on the in-situ mechanical properties of granite residual soil, the borehole shear tests are carried out for the residual soil along the depth of foundation pit. The in-situ stress-strian curves under different normal stresses and parameters of the Duncan-Chang model are obtained. Then, the regression relationships between the five model parameters and the weathering degree index represented by the gravel content are determined. According to the Duncan-Chang model, the stress-strain curves of residual soil with different weathering degrees are predicted and compared with the measured data. The results show that the grain compositions of residual soil are significantly correlated with the depth of soil layers. The stress-strain curves obtained by the borehole shear tests show the characteristics of strain-hardening deformation. According to $\phi$ the stress-strain relationship of in-situ tests, the parameters of the generalized Duncan-Chang model, including cohesion c, internal friction angle, stress failure ratio R_f and fitting parameters K and n, can be deduced effectively. Through regression analysis, the parameters can be fitted by the corresponding functions for the gravel content. The stress-strain curves calculated by the modified model are in good agreement with the experimental results, which shows that the generalized Duncan-Chang model can reasonably describe the in-situ mechanical behavior of granite residual soil. This study extends the applicability of the Duncan-Chang constitutive model.