Investigating the susceptibility of iron ore to liquefaction

Liquefaction of cargo in a bulk carrier can cause the cargo to shift, which in turn can cause a ship to list and eventually can cause the ship to capsize. When liquefaction occurs, a material undergoes a transition from a solid to a liquid state and, in general, it is caused by the pore water pressure increasing above the total stress and overcoming the effects of cohesion (i.e. shear strength becomes negligible). In this paper, a number of iron ore samples, representing loosely the range of iron ore cargos from around the world, are characterised and tested. The grain size distribution and particle density were determined along with the tests which are prescribed by the International Maritime Solid Bulk Cargoes (IMSBC) code to assess the liquefaction potential: the Proctor/Fagerberg test, the penetration test and the flow table test. It is found that the results from these test methods correspond reasonably well; however, they are not applicable for all ore types and the conditions at which the tests are performed do not correspond with the conditions encountered in the ship. Therefore, one cannot quantify the risk of liquefaction based on the results of these tests alone, and further investigations on the liquefaction mechanism and the factors which influence this process are required. Part of these investigations have shown that the liquefaction behaviour is strongly linked to the grain size distribution, both in terms of particle size and grading. It is anticipated that the permeability, compaction and water retention behaviour are also of influence in determining liquefaction potential and are governed, in part, by the grain size distribution. Examples of these properties are presented, although the experimental programme is still ongoing.