Experimental study on laws of scale effects of shear strength of coarse-grained soils

The shear strength is one of the important evaluation indices for the stability of soils. The investigation into the influences of the scale effects on the shear strength of coarse-grained soils has important theoretical significance and application for the strength estimation of soils in high earth-rock dam projects. According to the previous studies, the variations of the maximum particle size d_max and gradation structure can be seen as the main reasons resulting in the scale effects, and the gradation structure can be expressed by a characteristic parameter, the gradation area (S). By changing d_max or S, 24 groups of samples with different gradations of coarse-grained soils are designed. The direct shear tests on all the samples are conducted by a large-scale direct shear apparatus to quantitatively study the multiple influences of the maximum particle size and gradation structure on the shear strength of coarse-grained soils. The results show that the cohesion (c) and the internal friction angle (φ) of coarse-grained soils tend to increase with the increasing d_max, and thus a logarithmic equation relationship between c/φ and d_max is proposed. Both c and φ tend to increase with the decrease of S, and then decrease slightly after S reaches a certain level. As a consequence, an empirical relationship between c/φ and s is established based on the corresponding test results. Furthermore, a new prediction model for the shear strength of coarse-grained soils considering the scale effects is proposed, and the test results in the relevant literatures verify that the proposed model can be applied to different types of coarse-grained soils.