Direct shear test analysis of root–soil complex considering the change of shear surface area

Direct shear test is one of the important test methods to obtain shear strength index. With the increase of shear displacement, the area of shear plane decreases gradually. According to the normal stress and shear stress, the shear strength line was obtained and the index of shear strength was calculated. However, the phenomenon of shear area decreasing was neglected in the calculation, and the influence of shear area variation was not considered. Based on the change of shear area in the process of shearing, a mathematical model was established, and the formulas for calculating the normal stress and shear stress considering the change of shear area were derived, based on the direct shear test of root–soil complex, the indexes of shear strength were compared and analyzed. The results show that the improved calculation method has a greater effect on the cohesion, and the cohesion obtained was higher than the original calculation method, but it has little effect on the angle of internal friction. The results of this study can be used to evaluate the effect of plant roots on soil strength and to prevent soil erosion and shallow landslide effectively.