Analysis of unsaturated soil columns with application to bulk cargo liquefaction in ships

Over the last 40 years, there have been several ship losses carrying granular mineral ores that are believed to have been caused by the sudden shifting of the cargo as a result of liquefaction. It is known that the initial moisture content during loading is important for cargo types which are considered susceptible to liquefaction. To limit the moisture content, the degree of saturation must be below 80% in compaction tests designed to reproduce the energy associated with loading cargo into a ship's hold. During transport, ships can experience storms causing cyclic loading which leads to densification of the cargo and increases in the degree of saturation. There can also be transient increases in pore pressure and associated reductions in the cargo material's resistance, which when the ship heels increase the likelihood of the cargo shifting and in the extreme, the ship capsizing. In this paper, a recently developed fully coupled dynamic finite element analysis and constitutive model for unsaturated soil is used to perform a parametric study to explore the capabilities of the numerical code and the mechanics of the problem. The numerical analyses are shown to be capable of running for up to 2500 cycles and simulating both cyclic loading and simultaneous consolidation and drainage, a computationally very demanding task. Outcomes of the analyses are used to discuss the factors influencing cargo stability and vessel instability.