Abstract
If the free vertical movement of the upper rigid part of the shear box is hindered during shearing, a frictional force is mobilized between the specimen and the vertical walls of the shear box. This causes either unloading (for contractant soils) or additional loading (for dilatant soils) of the specimen during shearing. If no correction of the applied vertical load with respect to the wall friction is taken into account, the resulting shear strength can be either underestimated (for contractant soils) or overestimated (for dilatant soils). For example, in a particular investigation of a normally consolidated soil, the measured friction angle from a direct shear test was almost 8° smaller than the angle from a triaxial test. This paper, therefore, presents a method for direct measurement of the frictional force at the contact between the vertical walls of the box and a fine-grained soil. If the wall friction is taken into account, the friction angle from the shear box coincides well with the angle from triaxial tests. If the wall friction cannot be measured during the test, a sufficiently large vertical gap should be adjusted in case of soft soils, in order to enable non-restrained settlement of the upper part of the box during specimen contraction.
Published Version
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