Sand Response to Cyclic Rotation of Principal Stress Directions as Induced by Wave Loads

Abstract

In order to explore the nature of cyclic loading in seabed soil deposits due to travelling waves, an analysis was made to determine the stresses in an elastic half-space subjected to continuously distributed sinusoidal loads on the surface. The result of the analysis showed that the cyclic loading generates an alternate changeover of two components of shear stresses, i. e., the horizontal shear stress and the stress difference between the vertical normal stress and horizontal normal stresses. It was further shown that the alternate changeover in stresses as above takes place in such a way that the principal stress axes continuously rotates while holding the deviator stress (difference between two principal stresses) at a constant level. In an attempt to simulate the actual stress conditions as above, a triaxial torsion shear test apparatus was manufactured, and controlled combinations of torsional and vertical stresses were applied so that the same nature of cyclic loads as described above could be applied to hollow cylindrical sand specimens prepared in the test apparatus. The test results showed that, even when the amplitude of the combined shear stress (deviator stress) is maintained constant, the plastic deformation as represented by the pore water pressure build-up could take place in the sand, if the rotation of the principal stress axes is executed during the cyclic loading test, and consequently the cyclic strength of sand is reduced, as compared to the cyclic strength obtained from the conventional type of cyclic triaxial test apparatus.