Wave-induced Liquefaction in Seabed Deposits of Sand

Abstract

To explore the nature of cyclic stress alteration in seabed deposits of sand due to travelling waves, two-dimensional stress analysis was carried out on a homogeneous elastic half-space subjected to a series of harmonic loads moving on its surface. The analysis indicated that changes in shear stress occur in such a way that, while its amplitude is maintained constant, the directions of the principal stresses rotate continuously. With a view to simulating such stress changes in the laboratory test, a series of cyclic triaxial torsion shear tests was conducted on loose specimens of sand. The test results indicated that the conventionally defined cyclic stress ratio is reduced by about 30 per cent if the rotation of the principal stress directions is involved in the cyclic loading. It is known that the magnitude of wave-induced pressure at the seabed changes as ocean waves move in towards the shore. In unison with the changes in the pressure wave, the cyclic stress ratio induced in the seabed also changes. However, owing to the wave breaking, there exists an upper limit in the magnitude of the wave pressure and, hence, in the induced cyclic stress ratio. With these characteristics in mind, a set of charts was provided to facilitate computation of the induced cyclic stress ratio in the seabed deposit of water of any depth, on the assumption that the seabed consists of a homogeneous isotropic elastic half-space. The charts are organized so that the induced cyclic stress ratio can be obtained for known wave parameters that are specified by design storms at an offshore location. The cyclic stress ratio thus obtained was compared with the corresponding cyclic stress ratio causing failure in the sand, which was determined in the laboratory with a cyclic triaxial torsion shear test equipment in which continuous rotation of the principal stress direction was executed in simulation of the wave-induced stress conditions occurring in the seabed deposit.