Journal of Waterway, Port, Coastal, and Ocean Engineering

Understanding of the physical backgrounds is essential for good engineering practice with respect to liquefaction of sandy soils around and beneath marine structures. Several types of liquefaction can be distinguished. The corresponding physical phenomena are briefly described. Among them are: compressibility of soil skeleton, dilation, contraction, elastic versus plastic deformation, interaction between pore water and soil skeleton, compressibility of pore water, monotonic versus cyclic response, instantaneous versus residual liquefaction, drainage and pre-shearing. In each particular case just a limited number of these phenomena is relevant and needs to be modeled. A survey of typical cases is presented and the relevant phenomena are discussed. Most will be elaborated on in other papers appearing in this issue. This paper may help to discover the relationship between the different papers.

The conventional approach to pipeline stability design is reviewed and compared with experiments. A failure mechanism of pipelines on liquefied seabeds is analyzed. An analytical model to predict the pipelines sinking depth is proposed, and the comparison with measurements shows that the analysis is consistent with observed behavior.

This paper summarizes the results of an experimental and theoretical investigation of: (1) pipeline flotation in a soil (liquefied under waves); and (2) density of the liquefied soil. In the experiments, the soil was silt with $d_{50}=0.078\mathrm{mm}$ . Pipeline models of $2\mathrm{cm}$ diameter were used. They were buried in the soil at different depths in the range $3–15.5\mathrm{cm}$ . The total depth of the silt layer was $17.5\mathrm{cm}$ . Waves (with $17\mathrm{cm}$ wave height and $1.6\mathrm{s}$ wave period, the water depth being $42\mathrm{cm}$ ) were used to liquefy the soil. The pipes with specific gravity smaller than 1.85–2.0 floated when the soil was liquefied, the critical specific gravity for pipe flotation. The lower bound of the above range corresponds to the initial pipe position near the surface of the bed, and the upper bound to that near the impermeable base. Furthermore, the pipe floated (or sank) to a depth where the pipe specific gravity was equal to the previously mentioned critical specific gravity for flotation. The density of liquefied soil was determined in two different ways in the experiments: (1) from the aforementioned flotation tests where the pipe acted as a “hydrometer”, the instrument to measure fluid density; and (2) from the force balance equation for the pipe, in the vertical direction, corresponding to the critical condition for the pipe flotation. The results from these two methods agree well. The density of liquefied soil varied with depth; it was 1.85 near the surface of the bed and 2.0 near the impermeable base. A hydrodynamic model has been developed to predict the density of liquefied soil. The model is based on the force balance (in the vertical direction) for a soil grain of the liquefied soil. The model indicates that the density of liquefied soil is influenced by the soil category; the soil “class”; the initial soil specific gravity; the specific gravity of soil grains; and the coefficient of lateral earth pressure.

This paper presents the results of an experimental study on liquefaction around a pipeline buried in a soil exposed to a progressive wave. The soil used in the experiments was silt with $d_{50}=0.045\mathrm{mm}$ . The pore-water pressure was measured in the far field and on the pipe simultaneously. The tests indicate that the buildup of pore pressure and the resulting liquefaction in the soil are influenced by the presence of the pipe. The pore pressure builds up much more rapidly at the bottom of the pipe than in the far field (at the same level as the pipe bottom). By contrast, the buildup of pore pressure at the top of the pipe is not influenced radically by the presence of the pipe. The tests further indicate that as the liquefaction initially occurs in the very top layer and develops downwards, this picture changes in the vicinity of the pipe; in the latter case, the liquefaction initially occurs at the bottom of the pipe, and develops along the perimeter of the pipe upwards. The influence of the “no-slip” condition at the pipe surface on the end results has been investigated and found very significant. The influence of the wave height, the influence of the pipe diameter, the influence of the no-liquefaction-regime conditions, and the influence of a sinking pipe have also been investigated.

This paper presents the results of a numerical modeling study of wave-induced liquefaction around offshore structures. The finite-element model was first validated against both analytical solutions and laboratory data and was shown to be able to reproduce the observed behavior of the pore pressure variations. The model includes a constitutive model that can predict both residual and momentary liquefaction. The model was used to study liquefaction around a buried pipeline under wave action. In the analysis, it was assumed that the soil in the trench was loose and therefore susceptible to residual liquefaction. The numerical modeling focused on some of the questions that were raised in the physical modeling studies of Sumer et al. in 2006 and Teh et al. in 2003. In particular the influence of pipe diameter, boundary conditions around the pipeline, trench depth, etc., have been assessed and the results are in general agreement with the experimental findings.

Coastal or offshore structures such as pipelines installed on the seabed are submitted to cyclic horizontal loads either by the direct hydrodynamic action of waves or through the cyclic movement of risers or flow lines transmitted by floating structures. In fine sandy or silty soils these cyclic loads can induce a liquefaction of the surrounding bed, which can play an important part in the processes of erosion, trenching, or self-burial of the pipes. As part of the LIMAS program, a full-scale physical model was built to study the fluid-soil-structure interaction with special emphasis on the conditions leading to liquefaction around a pipe instrumented with pore pressure sensors. The experiments indicate a development of excess pore pressure at the pipe-soil interface much higher than the effective overburden stress, and a lateral visualization provided evidence of the liquefaction of a soil band in the vicinity of the pipe. The penetration of the structure can be related to the phenomenon of liquefaction.

Based on previously available results and the recent research within the Liquefaction around Marine Structures program this paper formulates practical guidelines for design of pipeline on-bottom stability on noncohesive seabeds. Existing methods for calculating the probability of seabed liquefaction are recommended and the requirements for site investigations are discussed. A new formula is presented for the embedment of pipelines in liquefied soils. The implications of the method presented here are somehow similar to the density guidelines used for pipeline stability in the 1960s and 1970s but this work provides a clearer scientific justification for the methods.

Results of large-scale model experiments in a wave flume are discussed. These experiments are concerned with the study of the generation of transient or instantaneous and residual pore pressure in a seabed beneath a caisson breakwater subjected to both pulsating and breaking wave loads. The simulated seabed and drainage conditions correspond to those encountered in a loose sand bed with thin clay or silt layers. Even under such unfavorable conditions total liquefaction due to residual pore pressures could not occur during the experiments. It is shown that the residual pore pressure is essentially generated by the caisson motions due to breaking wave loads and that they are closely related to residual soil deformations, which may lead to the failure of the breakwater.

The foundation of a vertical breakwater or offshore gravity platform may fail due to wave loading. The question is discussed whether liquefaction phenomena in a sandy subsoil, such as the generation of residual excess pore pressures and cyclic mobility, may significantly contribute to such failure. Theory, descriptions of prototype failure cases, and several scale tests are analyzed. It is concluded that the most spectacular failure type “liquefaction flow failure” is only possible in the case of a subsoil of very loose sand or silt combined with a low drainage potential, e.g., by the presence of a clay layer or large structure dimensions. Three other potential failure types are more likely in other cases: stepwise liquefaction failure, stepwise failure, and wobble failure. Their relevance increases with decreasing relative density and decreasing drainage potential. Brief recommendations for assessment of each failure type are presented.