The direct shear test is favored by many geotechnical engineers because of their simplicity and repeatability. These tests measure the maximum shear strength and shear strength parameters of soils and other particulate materials. In this paper the effects of box size on the strength parameters (the friction angle and cohesion) of soil is studied. The used soil samples are classified as SC according to the unified soil classification system. The soils were well graded with dry density ranges between 1.67 – 1.82 g/cm3. There are different size shear boxes in use today to find the effect of the specimen size on the shear strength parameters used in foundation design. In present study three different size shear boxes were used. The undistributed sample soils with almost same properties were tested in three square shear boxes of varying sizes (60 mm, 100 mm, and 300 mm). The samples were sheared at a constant rate of 1 mm/min. In this study 45 sets of direct shear tests were performed to study the influence of the size shear boxes and density on the strength parameters. The results from this study show that scale effect in direct shear tests. The large and medium scale direct shear (300 mm × 300 mm shear and 100 mm × 100 mm shear) gives a higher cohesion and lower friction angle compared with the results of the small-scale direct shear test (60 mm × 60 mm shear). The tests indicate that the friction angle and cohesion were seen to increase with increasing density in each of the three boxes. These observations suggest that friction angle and cohesion controls with the scaling effect and physical properties.

Keywords