Group A cargoes are those which pose a risk from either liquefaction or dynamic separation. The IMSBC Code defines cargoes which may liquefy as:
“Cargoes which contain a certain proportion of fine particles and a certain amount of moisture. They may liquefy if shipped with a moisture content in excess of their transportable moisture limit.”
In such a cargo composed of fine particles and containing moisture, the spaces between the particles are filled with both air and water. Whilst at sea the cargo is subject to forces due to the vibration and motions of the vessel. These forces cause the particles to move closer together and reduce the size of the spaces (voids) between the particles, which is known as densification.
The reduction in void volume forces water and air out of the cargo. Where water cannot flow quickly enough in response to densification (due to some physical condition of the cargo) and saturation increases, the water in the spaces between the particles cannot be easily compressed (because it is a liquid) and therefore becomes pressurised. Increased pore water pressure causes risk of liquefaction or dynamic separation.
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