The effect of water content on the shear strength characteristics of granitic soils in South China

Shear strength is crucial for slope stability and soil erosion measurement, and shearing deformation is one of the most harmful processes for land and environment degradation. The heterogeneity and anisotropy of shear behavior for granitic soils have been scarcely investigated in previous studies. The pedogenic differentiation of an intact granite weathering profile was characterized by soil properties from surface (0 ∼ 45 cm), lateritic (45 ∼ 110 cm and 110 ∼ 170 cm), sandy (170 ∼ 430 cm) and detritus layers (> 430 cm). Shear strength with response to water content variation (7% to saturation)　for undisturbed soils from these layers was determined by direct shear test and its anisotropic characteristics were explored by shearing samples selected from the vertical and horizontal directions of the profile. Soils in the clay loam layer (depth <170 cm) exhibited a softening tendency at low water content (7 ∼ 14%), and at this water content level, the shear strength generally decreased with increasing profile depth. Water content made significant contributions to shear properties (p<0.01), especially to cohesion strength with regard to the total stress (F=125.46, p<0.01), showing a quadratic polynomial response to water content variation (R²>0.75, RMSE< 5.6, p<0.05). Shear properties with less significant variation along the profile under near-saturation condition (CV<0.25) obtained the peaks at water content in a range of 14 ∼ 21%. The internal friction strength under the total stress condition varied slightly along the profile (CV=0.21, p>0.05) and showed a less apparent anisotropy. The unsaturated shear strength of granitic soils determined by the cement materials and the porosity of granitic soils could be predicted by the constitutive model considering the variation of matric suction contribution to shear strength (Adj-R²>0.47; RMSE<26.13; AICc<56.65). The predominant variation (75%) of the effective cohesion strength was resulted from liquid limit (p<0.01). Silt content contributed to 50% variance of the effective internal friction strength (Adj-R²=0.52, Sig.=0.050). The anisotropic shear strength of clay loam soils was more significant than that of sandy soils, however, its quantified response to water content needs a further investigation. The results will guide decisions on stability analysis and facilitate the understating of unsaturated shear strength mechanism of granitic soils.