ABSTRACT

This paper describes the results of experimental tests performed to investigate pore pressure generation in the seabed under a foundation model inducted by rocking-motion excitation. The experiments were conducted in a soil pit located at the bottom of the flume. The foundation was simulated by a rectangular plate placed on the soil surface. Bespoke test setup was used to translate the horizontal motion of the wave generator piston to a sinusoidal rocking motion of the plate. The pore-water pressure inducted by rocking-motion foundation plate was measured by a set of transducers located in the sediment pit. Pore pressure variation was recorded for the three different periods with two amplitudes of the rocking motion of the model. In all tests, the values of pore pressure built up to maximum value, and then fell off due to the dissipation of the accumulated pressure. The correlation between the basic parameters set-up of rocking-motion and susceptibility of subgrade to liquefaction was analyzed.

INTRODUCTION

During the last few decades, the number of engineering structures, such as breakwaters, oil platforms and turbines built in the offshore and coastal zones has increased significantly. As a consequence, the number of problems that must be solved in order to ensure the stability of these structures also increases. There are many types of environmental loading in an offshore area, but the main are ocean waves and possible earthquakes. The influence of both of these factors should be carefully recognized during the design process of engineering off-shore structures. Although ocean waves are a conventional form of loading for all marine structures, and their effect on the stability of marine structures has been widely investigated (e.g. De Groot et al. 2006; Ye et al, 2014, Ulker et al, 2010), sometimes, design errors are still made in this area. Failures of harbour walls at Malaga and Barcelona noted respectively in 2004 and 2007 were examples of such situations (Campo & Negro, 2011). They were associated with an inadequate consideration of the ground conditions in light of the marine environment. Therefore, it can be concluded that the stability of marine structures under environmental loading is still the main concern for ocean engineers. One of the possible destruction mechanisms is as follows: a severe storm may induce the rocking motions of caisson breakwater, which are transferred to the seabed just underneath the caisson foundation. This motion may cause pore pressure buildup, and in extreme conditions, can lead to subsoil liquefaction and the failure of the structure. Liquefaction takes place when loosely packed, saturated granular soil at or near the ground surface loses its strength in response to strong ground shaking (Sawicki & Kazimierowicz-Frankowska, 2015; Sumer et al., 2008). This phenomenon refers to soil instability and can be caused by various factors such as the inflow of water beneath the soil or a sudden shock caused by earthquakes or human activities. If the subsoil of a specific offshore region suffers from liquefaction, it may become unable to support the weight of the engineering structures located on it. Consequently, these structures could suffer severe damage, or even collapse completely into the ground.

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