7.2.1 Group A cargoes contain a certain
proportion of small particles and a certain amount of moisture. Group
A cargoes may liquefy during a voyage even when they are cohesive
and trimmed level. Liquefaction can result in cargo shift. This phenomenon
may be described as follows:
-
.1 the volume of the spaces between the particles
reduces as the cargo is compacted owing to the ship's motion, etc.;
-
.2 the reduction in space between cargo particles
causes an increase in water pressure in the space; and
-
.3 the increase in water pressure reduces the
friction between cargo particles, resulting in a reduction in the shear strength
of the cargo.
7.2.2 Liquefaction does not occur when the cargo
consists of large particles or lumps and water passes through the
spaces between the particles and there is no increase in the water
pressure.
7.2.3 A cargo shift caused by liquefaction may occur when
the moisture content exceeds the TML. Some cargoes are susceptible to moisture migration
and may develop a dangerous wet base even if the average moisture content is less than
the TML. Although the cargo surface may appear dry, undetected liquefaction may take
place, resulting in shifting of the cargo. Cargoes with high moisture content are prone
to sliding, particularly when the cargo is shallow and subject to large heel angles.
7.2.4 In the resulting viscous fluid
state cargo may flow to one side of the ship with a roll but not completely
return with a roll the other way. Consequently the ship may progressively
reach a dangerous heel and capsize quite suddenly.