Wave-soil tank calibration and geotechnical studies of seafloor instability

Dawe, C. Roy (1984) Wave-soil tank calibration and geotechnical studies of seafloor instability. Masters thesis, Memorial University of Newfoundland.

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Abstract

The general scope of this research is to understand the problem of seafloor instability due to ocean surface waves with particular reference to the offshore activities on the Canadian east coast. An evaluation of the wave effects on seabed stability is necessary in the determination of the long term stability of bottom-founded offshore production platforms and transportation systems for the Grand Banks and Sable Island regions. -- A wave tank was designed and built for the purpose of conducting laboratory tests with sediments similar to those found on the east coast. The tank is 8.5 m long, 0.76 m wide and 1.0 m deep, with a capability of generating regular waves of maximum heights of about 0.12 m. The wave length can be varied from 0.5 m to 4 m. The representative seafloor was a bed of sand about 0.4 m deep, underlying a water column up to 0.5 m deep. The tank can be tilted so that slopes of up to 1:10 may be formed at the water-soil interface to represent seabed slopes. A beach was designed and built to absorb the wave energy and to minimize the effect of reflected waves. Reflection coefficients of less than 10% were obtained for the frequently used wave types. Reflection coefficients as low as 5% can be obtained for some specific sets of wave characteristics. -- Waves with a steepness between 0.04 and 0.1 caused failure of the test bed of fine sand when it was in a loose state. Porewater pressure was measured by embedding pressure transducers inside the soil. Soil failure by liquefaction was confirmed by the measured pressures. However, the sand always stabilized again after about 10 to 20 cycles resulting in a denser soil. The soil failure appears to be the result of transient wave effects rather than porewater pressure buildup. This is more pronounced when the soil is sloped. -- In a parallel theoretical research of the same problem, solutions have been reported using Biot’s equations. Comparison of the measured wave induced porewater pressures with the reported theoretical values show a good correlation in some cases. -- The present studies show that the seabed will be subjected to mass movement under specific wave conditions. This failure depends on the seafloor sediment type and its initial insitu state. In combination with the bottom currents, the effect of the waves on the seabed could be quite severe.

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