Graduate Thesis Or Dissertation
 

Evaluation of ocean wave-induced liquefaction in a large scale wave flume

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  • Ocean waves propagating over cohesionless seabed deposits produce cyclic shear stresses within the deposit. Under certain conditions these stresses may cause a progressive build-up of pore pressure. Pore pressure accumulation can result in liquefaction or a substantial decrease in the effective stress with attendant large deformations of the seabed deposit. In recognition of the need to study ocean wave-induced liquefaction, a series of large scale wave flume tests were conducted. The test program consisted of generating a series of uniform waves over a sand deposit and measuring the pore pressure response. The pore pressure response was studied for a variety of soil and wave conditions. Specific variables addressed in the study included drainage conditions near the surface of the deposit, relative density of the deposit, magnitude of wave loading, and previous wave loading history. Both the cyclic and mean pore pressure responses in the deposit were determined. Cyclic shear stresses within the deposit are generated by wave-induced cyclic pressure fluctuations at the mudline. For a given wave length, the cyclic shear stresses increase with increasing wave height. Wave-induced liquefaction of a fine sand was observed. A decrease in effective stress and liquefaction resulted from an increase in the mean pore pressure during undrained wave loading. The mean pore pressure increased owing to a transfer of intergranular stress to the pore fluid. Pore pressures cycled about the shifting mean pressure at the same frequency as the wave loading. Mudline pressure fluctuations were not affected by pore pressure changes occurring below the mudline. Liquefaction resulted in gross disturbance of the deposit. Pore pressure accumulation was not observed during drained loading. Several drained tests were conducted for a range of initial relative density and wave conditions. During the drained tests, the relative density of the sand layer increased and settlement of the sand was observed. The increased density of the sand layer was accompanied by attenuation of the cyclic pore pressure fluctuations during the initial wave loading cycles of the test. Following attenuation, the cyclic pore pressure fluctuations remained constant to the end of the test. The effect of previous wave loading was evaluated by comparing the response of tests subjected to previous wave loading with those not subjected to previous wave loading. Previous wave loading resulted in increased stability of the sand with respect to liquefaction potential. The increased stability was the result of an increase in relative density as well as changes in the structure of the deposit.Measured mean pore pressure responses for all the tests were compared to the response predicted by a theoretical model. Theoretical predictions agreed with the measured response except for the case where the deposit was subjected to a previous wave loading history. The potential for pore pressure accumulation and liquefaction of a seabed deposit decreases with: 1) increased drainage at the surface of the deposit, 2) increased relative density, 3) decreased magnitude of wave loading, and 4) previous wave loading.
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