Journal of Waterway, Port, Coastal, and Ocean Engineering

The problem of wave-induced liquefaction is an important feature in coastal and marine engineering, and is one of the main research topics in the European Union project Liquefaction around Marine Structures (LIMAS). In this context, the gas content of a soil is one topic of interest, as the presence of gas influences the pore pressure response and the gradient distribution in the seabed during wave action. The main objective of the work described herein has been to develop a new sampler that enables in situ measurement of gas content in the seabed, and at the same time retrieves a representative sample of the soil. The sampler should be able to maintain the natural soil structure and amount of gas in the pores to a high degree, and should primarily be used in sandy and silty materials. The major challenge in the project was to achieve perfect sealing of the sample container after sample shearing. This was finally obtained by use of a ball valve and backpressurized ball-valve housing in the lower end of the sample container, and a piston in the upper end. The main problem with the ball-valve concept was wear and clogging from sand particles inside the ball valve, and various sampler designs were tested and evaluated before the final, robust design was chosen. The gas content in the soil is determined in situ, using backpressure for compression of the gas in the sample container, enabling deaired water to be injected in the sample. This water volume is recorded on a burette and is used to determine the total amount of gas in the sample. To obtain the degree of saturation $S_r$ in the soil, the weight and volume of the soil grains is determined after dismantling of the sampler. Laboratory calibration tests show that a known gas volume of $100\mathrm{mL}$ may be measured with $\pm 0.75\mathrm{mL}$ accuracy with the new sampler. The new sampler was used successfully at the LIMAS research site in Capbreton, France, where gas measurements were carried out under various wave and tidal conditions in a sandy seabed. The obtained results showed that the gas content could vary significantly in the upper $0.3–0.6\mathrm{m}$ top layer of the soil. The smallest gas contents, representing 0.2–0.65% of the sample volume, were measured under falling tide conditions after repeated cycles of strong waves. The tests showed that the new sampler can be used successfully under field conditions, provided that great care is taken during mounting and preparation of the equipment.

This paper summarizes the results of an experimental investigation on wave scour around a circular pile in three kinds of soil, namely in dense silt (with a relative density of $D_r=0.74$ ), in medium dense silt (with $D_r=0.38$ ), and in sand (with $D_r=0.23$ ). The dense silt was achieved by wave liquefaction. Upon the completion of the liquefaction/compaction process, the pile was subjected to wave induced scour. The range of the Keulegan-Carpenter number $\left(\mathsf{KC}\right)$ , tested in the experiments is from practically 0 to approximately 20. The Shields parameter in the experiments was such that the bed was live. The scour depth was increased by a factor of 1.6–2 when the bed soil was changed from medium dense silt or sand to dense silt. This is partly due to an increase in the angle of friction. The time scale of scour also was influenced by the density of the soil. The time scale was largest for the dense silt case, and smallest for the sand case in the experiments.

This paper presents the results of field experiments carried out around a bunker from the Second World War situated on the beach. The structure was surrounded by water at high tide and subject to intense wave forcing. By positioning pressure sensors at different levels inside the soil, simultaneous time series of pore pressure measurements were acquired. They demonstrated that the sediment layer on top of the bed does not remain continuously in static equilibrium. Momentary liquefaction was observed as the occurrence of an upward pressure gradient, which could overcome the effective weight of the sediment layer. Using a Fourier analysis, a comparison was made with the theory put forward by Sakai et al. in 1992 whereby the damping of pore pressure variations inside the soil can be quantified. This indicated the existence of a significant amount of gas inside the soil, which was confirmed in the field by geoendoscopic videos.

It is demonstrated that reactions for impulsive loads in saturated sand soil are speed dependent and can be far higher than the reactions for the corresponding slow loads. The reason is that soils rupture and pore water flow interacts, both for contractive sand and for dilatant sand and create additional pore water pressures and additional effective stresses. The traditional geotechnical methods, the extreme method, and the equilibrium methods have been extended to also include the interaction between ruptures and pore water flow. It is shown that zone ruptures generally will be dilatant except possibly for the loosest layering. This means that the pore water will be sucked into the rupture zone when it develops. This may not be the case with line ruptures, where the soil may be mainly contractive and squeeze out pore water during the development. Analytical methods have been developed and compared with laboratory results for falling cones penetrating into saturated sand. Both tests and analysis compared well and showed an increase from 2 to 70 times in average resistance compared with dry sand depending on the relative density of the sand and the ratio between speed of penetration and permeability.

In this paper the capability of a simple mathematical model of compaction/liquefaction applied for prediction of the seabed behavior subjected to the 1999 Kocaeli Earthquake is presented. The compaction/liquefaction model has been calibrated in laboratory on samples taken from the region analyzed and then applied to the conditions corresponding to the seabed in the Karamursel Eregli Fishing Port. Based on numerical simulations the influence of various model parameters on earthquake induced liquefaction phenomenon is discussed.

This paper gives a state-of-the-art review of seismic-induced liquefaction with special reference to marine structures. The paper is organized in seven sections: (1) introduction; (2) seismic-induced liquefaction in which a general account of soil liquefaction is presented; (3) existing codes/guidelines regarding seismic-induced liquefaction and its implications for marine structures; (4) review of the Japanese experience, giving a brief history of earthquakes and design codes, describing the current design code/standard for port and harbor facilities including counter measures and remediation; (5) review of the liquefaction damage inflicted on marine structures in the 1999 Turkey Kocaeli Earthquake, including recommendations which draw on the lessons learned; (6) assessment of postliquefaction ground deformation (more specifically of lateral ground spreading); and (7) tsunamis and their impact. The present paper and the existing guidelines (CEN, ASCE, and PIANC) form a complementary source of information on earthquake-induced liquefaction with special reference to its impact on marine structures.

Wave induced pore pressures in bottom soils have an influence on the geotechnical stability of structures and may also have an effect on the transport of sand. Wave induced pore pressures in a seabed of sand or clay has been an item for extensive research. One of the major difficulties in analyzing and interpreting both laboratory and field data has been related to the question of air/gas content in the pores of fine soil. Only a small fraction of air/gas significantly changes the bulk modulus of the water/gas mixture and also the attenuation of the wave induced pressures in the soil with depth. The reason for the difficulty of analyzing data has been the lack of good instrumentation for measuring the gas content in the seabed soil.