Volume 29, Issue 6 (November 2006)
Specimen Size and Scale Effects of Direct Shear Box Tests of Sands
The direct shear test has survived over the past 50 years in geotechnical engineering applications because of its simplicity and repeatability. Many laboratories perform direct shear box tests on sands to determine the friction angle φ′, or shear strength of the sand for engineering design purposes. However, there are different size shear boxes in use today and the effect of the varying specimen size on the resulting friction angle used in foundation deign has never before been investigated thoroughly. Five sands with different properties were tested in three square shear boxes of varying sizes (60 mm, 101.6 mm, and 304.8 mm), each at three relative densities (dense, medium, and loose). Results of the direct shear tests show that the friction angle φ′ can be dependent on specimen size and that the influence of specimen size is also a function of sand type and relative density. The tests indicate that for well-graded, angular sands, φ′ decreases as box size increases and that the influence of box size is dependent on relative density. The paper provides a description of the test methods and presents the test results.