Planning a monitoring campaign for a natural submarine slope prone to static liquefaction is a challenging task due to the sudden nature of flow slides. Therefore, gaining a better insight by monitoring the changes in pore pressure and acceleration of the soil mass, prior to and at the onset of static liquefaction, of submerged model slopes in the laboratory, helps in quantifying the minimum required triggering levels and ultimately the development of effective margins of safety for this specific failure mechanism. This study presents a set of physical model tests of submarine flow slides in the large-scale GeoTank (GT) of Delft University of Technology, in which a tilting mechanism was employed to trigger static liquefaction in loosely packed sand layers. Novel sensors were developed to locally monitor the hydro-mechanical soil responses acting as precursors of the onset of instability. The measurements indicated that soil instability can initiate at overly gentle slope angles (6–10°) and generate significant excess pore water pressures that intensify the deformations to form a flow slide. Moreover, it was observed that the onset of instability and its propagation are highly dependent on the rate of shear stress change and the state of the soil. The obtained data can be used for the future validation of numerical models for submarine flow slides.

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In current modelling of excess pore pressures (EPPs) below marine structures, the irregular nature of cyclic loads and the real storm development are not taken into account. The effect of the irregular cyclic loading in time is investigated in this paper. The wind, wave and turbine loads on a gravity based foundation (GBF) are derived in the frequency domain. The real storm development is based on the CoastDat dataset. The load input is used in a program which takes the generation and dissipation of pore pressures under cyclic loading into account. Also, densification is included. The results show that the first storm in the lifetime of the GBF results in the highest EPPs. The EPP decreases in time, due to significant dissipation and densification during the build-up of a storm. Therefore, not the storms with the largest cyclic loads but the storms with the fastest build-up result in the highest EPPs, since this limits the process of densification. A large scatter is found in the maximum values of EPPs due to the irregular nature of the loads.

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The liquefaction flow slide is an important failure mechanism for under water slopes composed of sand. The failure of the sand body is the result of liquefaction of loosely packed sand, which suddenly looses a large part of its strength and starts behaving as a fluid. Liquefaction flow slides occur unexpectedly and develop at a very high rate, resulting in considerable damage. As a consequence, we have little detailed information on this phenomenon; the actual nature of the failure can only be established afterwards. The lack of detailed information constrains the engineer who needs to assess the slope stability. The available methods are strongly simplified, as a basis for more advanced techniques is lacking. This thesis is aimed at the improvement of the assessment of liquefaction flow slides. The first part contains a theoretical treatment of the underlying physics, which form the basis of an advanced calculation model. In addition, a large scale experiment has been developed, the Liquefaction Tank. We managed to reproduce laboratory liquefaction flow slides by gradually tilting the sand bed. The result is surprising; at a very gentle slope the sand bed suddenly and seemingly spontaneously liquefies. The occurrence of these experimental liquefaction flow slides depends on the density of the sand and the rate of tilting. The measurements provide valuable new insights that can be used for the further development of new models. @en

The liquefaction tank is an experimental facility developed to conduct physical scale model tests of liquefaction flow slides. We developed the liquefaction tank to evaluate the performance of advanced numerical models for submerged slopes composed of sand. For the long-term, the research with the liquefaction tank aims at composing a database with high-quality experimental results of liquefaction flow slides, in which properties related to the soil, degree of saturation, geometry, triggering and mitigating measures will be varied.
This paper addresses the first results obtained with the liquefaction tank. We used a fluidization system to create a uniform, loosely packed sand bed. The liquefaction tank was subsequently tilted uniformly, while measuring the pore pressures at the base of the sand bed. Furthermore, the stability of the slope was monitored using a camera system pointed at the transparent side of the tank. We conducted around 30 tilting tests on a level sand bed while varying consolidation time, density and tilting rate.
We were able to reproduce liquefaction flow slides below a particular threshold density. The moment of failure was noted by an instant, uniform liquefaction of the sand bed, preceded by an abrupt increase of excess pore pressures. The results in terms of failure angle and measured pore pressures were consistent and reproducible. The measured failure angle was much lower than anticipated from results of element tests in literature. Future research aims at relating the results to the response during undrained triaxial tests and the effect of mitigating measures.
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