A New Shear Strength Law for Weak Water-Saturated Clay Soils

The conventional method of determining the strength parameters of weak, water-saturated clay soils involves applying vertical compressive loads, which cause significant water to be expelled from the specimen through the slot in the shear apparatus. At the same time, the weakest part of the clay sample begins to extrude plastically in a paste-like form through the same slot. Under these conditions, it is impossible to accurately test weak clay soils using the unconsolidated-undrained shear method. This is because the resulting strength parameters are overestimated due to the unintended compaction of the soil under compressive loads. The author therefore proposes applying vertical tensile loads to the soil sample that are lower than the load corresponding to the elastic limit of the sample under tension. In this case, the shear strength of weak, water-saturated clay soils is determined using a formula that differs from the one proposed by Coulomb. The maximum tensile load on a sample of water-saturated clay must be less than the tensile force corresponding to the elastic limit of the clay under tension. Applying tensile loads to samples of weak, water-saturated clay soil within the range of its elastic deformation causes a circular meniscus to form in a thin layer along the shear plane of the shear apparatus. This minimally reduces the original diameter of the sample by no more than 1 mm and does not affect the results of shear strength tests on weak, water-saturated clay soil samples. One of the main conditions of the proposed method is that the tensile forces applied to samples of weak, water-saturated clay soil must not reach the stage of plastic flow, as this would violate the initially defined equilibrium conditions. The applied tensile loads must not exceed the elastic limit under tension. The adhesive strength of the tested clay soil is determined after three to four tests by extending the shear strength graph until it intersects the negative part of the x-axis. The cohesion value is determined by extending the linear portion of the shear strength graph until it intersects the y-axis. The angle formed between the shear strength line and the horizontal x-axis represents the angle of internal friction of the clay soil. A dependence graph showing the relationship between vertical tensile loads and shear strength, obtained using the proposed method, provides more accurate strength parameters than a dependence graph showing the relationship between vertical compressive loads and shear strength, obtained using Coulomb’s law. The latter overestimates the strength parameters of weak, water-saturated clay soil.