Formation of water-escape structure during shock-induced fluidization: The role of permeability contrast

Abstract

Water-escape structure is considered to be a product of fluidization. The exact mechanism of formation of this structure has, however, not been adequately addressed. In the present study, laboratory experiments were carried out to observe the effect of cyclic loading on water-saturated sediment in a close system, with an objective to decipher the specific mechanism of formation of water-escape structure. Present study reveals that an optimum permeability contrast between the successive sedimentary layers of a fining upward succession is the prerequisite for formation of water-escape structures. The onset of fluidization comes into existence when at a critical minor principal stress condition during shearing, the underlying coarse-grained layer attains drained contractive state and the overlying finer-grained layer retains the undrained dilative state. In the contractive layer the sediments tend to become more compact through rearrangements of the particles and displacement of the interstitial fluid. The pore fluid is squeezed out and the sand structure suffers volume contraction. As the upwelling fluid is obstructed by the dilative layer, water-filled cavities are formed at the interface. On progressive shaking these water-filled cavities keep on enlarging and finally the expelled water ruptures through the overlying sand layer resulting water-escape structure. This study also depicts that no influx of external fluid is essential in fluidization of sediments, as was believed earlier.