Effect of Specimen Size on Undrained and Drained Shear Strength of Sand

Failure of granular soils can be defined by a friction angle that can be obtained from direct shear tests or triaxial tests. The results of triaxial tests can be influenced by various experimental conditions such as confining pressure, shearing rate, end constraint, and specimen size. In this study, the effect of specimen size on the undrained and drained shearing behavior of Nakdong River sand was investigated using loose (Dr = 40%) and dense (Dr = 80%) specimens 5 and 10 cm in diameter. Friction angles such as the undrained total stress friction angle, undrained effective stress friction angle, and drained friction angle obtained from Mohr's stress circle decreased slightly as the diameter of the specimen increased, regardless of relative densities. The difference between friction angles caused by different specimen sizes was at most 2.6° for an undrained total stress friction angle of dense sand. In most cases, there was little difference between the friction angles of large and small specimens (less than 1°). The difference between an effective friction angle from undrained tests and a drained friction angle from drained tests was up to 7° for loose sand, but was negligible for dense sand.