b) It might be helpful to the CG if Japan first review briefly the available knowledge of bulk cargoes liquefaction phenomena under dynamic sea loading conditions and the factors which influence it (earthquake soil loading conditions are similar to the dynamic loading conditions at sea) and;
c) A definition of Liquefaction (spontaneous liquefaction) is also required.
Coordinator's comments (1) - Regarding comments by Canada:
Bulk cargo liquefaction phenomena under dynamic sea loading conditions
As pointed out in your comments on paragraph 3.1, many factors have influences on liquefaction of solid bulk cargoes under dynamic sea loading conditions. These factors can be categorized into two types. One is the property of the cargo including moisture content and the other one is condition of stresses applied to the cargo under dynamic sea loading conditions. It should be noted that when acceptance for loading of the cargo is judged prior to shipment, the property of the cargo is evaluated but the conditions of stresses are not considered in general. It means that liquefaction can be prevented by means of limiting the moisture content of the cargo by introducing the safety margin, regardless of the condition of stresses. Furthermore, based on the experience for developing the penetration test procedure, it can be said that the effect of the amplitude of acceleration on the flow moisture point is not significant.
Factors which influence the liquefaction characteristics of solid bulk materials
Basic formula of resisting tangential stress
Liquefaction is that the shear strength of the wet granular material decreases by increase of pore pressure due to cyclic loading. Shear strength of wet granular materials is primarily represented by the resisting tangential stress, microscopically. The basic formula of resisting tangential stress of wet granular materials is as following:
TR = c + tan F*(SN - u)
where, TR : resisting tangential stress
c : cohesion
tan F : internal friction coefficient
SN : normal stress
u : pore pressure
Liquefaction Phenomenon
The resisting tangential stress of the materials is low when the pore pressure of a materiel is high. In such condition, the material flows freely like water. Increase of pore pressure is, basically, caused by the tendency of reduction of volume of void of the material resulted from the movement of the particles due to cyclic loading, in the case that the void of the material is filled mainly with water.
Factors which influence the liquefaction
Increase of pore pressure is the key of liquefaction phenomena. The characteristics of pore pressure increase are primarily related to the permeability and the degree of saturation of the material. Therefore, it is rational to limit the moisture content of cargoes which may liquefy, because liquefaction is not liable to occur when degree of saturation is low, even if the permeability of the material is low. This is the basic idea of the requirement for the materials which may liquefy that requires to limit the moisture content of the cargo at shipment less than its TML.
Permeability is related to two factors, primarily. Most significant factor is the grain size, which can be represented by the effective size, D10, in terms of permeability, and the other factor is void ratio which is related to the grain size distribution represented by uniformity coefficient and the condition of consolidation.
Factors considered in new procedure for evaluating liquefaction potential of solid bulk materials
It should be emphasized that the purpose of the procedure proposed by Japan is not to determine the TML of the cargo. In order to judge whether it is necessary to apply the requirement for the cargoes which may liquefy or not, possibility of increase of pore pressure of the material should be considered under various degree of saturation at shipment. In view of this, factors related to the permeability should be primarily considered and permeability of the material is evaluated implicitly, in the Liquefaction Potential Test.
In the procedure for evaluating liquefaction potential of solid bulk cargoes proposed by Japan, liquefaction potential of the material is judged based on the effective size. In the judgment, safety margin is introduced because the factors other than effective size are not evaluated.
Definition of liquefaction
Liquefaction is the phenomena that wet granular materials lose their shear strength resulted from increase of pore pressure due to cyclic loading.
