Shear Strength Properties of Coarse-Grained Soils Determined Using Large-Size Direct Shear Test

The paper deals with an analysis of the shear strength behaviour of coarse-grained materials using large-sized direct shear apparatus. The shear strength curve of compacted coarse-grained material allows for determining the three typical shear strengths, i.e. the shear strength for a maximum density of the sample tested (shear strength at point A), the peak shear strength (shear strength at point P), and the critical shear strength achieved for a large horizontal movement (shear strength at point C). Professional laboratories usually apply for determining the critical shear strength two different methods. The first method considers the critical shear strength at point A and the second method considers the critical shear strength at point C. The aim of the article is not to critically evaluate both methods, but to analyze the results of shear tests of gravels. The results of a large series of tests showed that these stresses may differ from each other in some cases, especially for poorly graded materials. In the case of sandy soil, the shear strength at point A and at point C is equal. However, in the case of poorly graded gravel, the shear strength at point C is greater than the shear strength at point A. The greatest difference between these shear strengths was measured in the case of poorly graded medium gravels. The results showed that the ratio of shear strength at point C to the shear strength at point A increases with increasing the grain size of the material tested. The peak as well as the critical shear strength parameters of poorly graded gravels is better represented by a multi-linear or a nonlinear failure line.