It is a process whereby moisture in the cargo migrates to the surface due to compaction and vibration resulting in the development of flow state.
In a fine grained, moisture laden cargo the spaces between cargo grains are filled with both air and water. Whilst at sea the cargo is subject to forces due to the vibration and rolling of the vessel. These forces cause the inter-grain spaces to contract. The water in the spaces between grains is subject to a compressive force but as it is a liquid it cannot be compressed. This has the effect of reducing the inter-grain frictional force that holds the cargo in a solid state. Where enough moisture is present the reduction in inter-grain friction due to the ship’s motion and vibration can be sufficient to cause the cargo flow like a liquid i.e. to liquefy.