A mechanism of pore pressure accumulation in rapidly sliding submerged porous blocks

The generation and dissipation of pore pressure excess due to the rapid, mutual sliding of two water-saturated blocks which are composed of glued arrays of cylindrical rods is herein theoretically examined. The mathematical model consists of three ordinary differential equations describing the motion of a solid mass coupled with the continuity of a fluid mass. These equations are numerically solved for the following conditions: (a) motion induced by the constant horizontal velocity of the sliding mass; (b) constant drag force acting in a horizontal direction; (c) gliding on the irregular, inclined surface that envelops the rods. Theoretical predictions for case (a) favorably compare with experimental results reported in literature from an investigation carried out on a physical model resembling the treated theoretical system. Primary results show that the pore pressure excess depends upon physical and geometrical parameters, and furthermore, that displacement of the rods and pore pressure excess are strongly coupled. Maximum pore water pressure occurs when “fluidization”, i.e. the complete balancing of submerged block weight by pore pressure excess, is achieved. The latter does not coincide with the rising of the upper block (hydroplaning). A cyclic motion is reached under condition (a), when dynamic pore water pressure increases from a negative to a maximum positive value and then falls again to a negative value. On the other hand, non-cyclical motion and monotonically increasing pore water pressure are obtained under sliding conditions (b) and (c). The response of the model under condition (c), which is of particular interest for run-out analyses of flow slides, is compared to the sliding response of ordinary submerged blocks on a slope, showing significant differences and pointing out the roles of particle diameter, layer extent and medium permeability. The occurrence of different pore pressure states for fluidization and hydroplaning is highlighted. This, hopefully, can open the way to a reliable treatment of the mechanics of sub-marine flow slide propagation.