Experimental and Numerical Study of the Response of Granular Layer in the Trap-door Problem

Deriving from the traditional problem of behavior of granular materials in silos, the trap-door test is a very basic experimental test that reproduces solicitation met in a wide range of technological applications (soil reinforcement, granular material storage...). In spite of the fact that many analytical models exist for the description of this test, a large part of the behavior of granular layers submitted to a localized basal relative displacement remains obscure: influence of the displacement value, value of the friction angle of the granular matter... Carried out in quasi-static motion and involving several granular materials such as gravels and sands, the experimental study brought to light that trap-door tests systematically break down into three different successive phases depending on the amplitude of the trap-door. Pressure applied on the trap-door by the granular material decreased suddenly for very low displacement values. Then a progressive increase was observed coinciding with a progressive expansion of a subsiding zone from the bottom of the layer to its top. At last the pressure stabilized for the greater displacements of the trap-door. This last phase corresponded with the classical failure pattern used in analytical solutions: a vertical slipping plane at each edge of the trap-door. Because of very different response were obtained with sand and gravel, particularly in the transitional phase, a numerical study were carried out by means of Discrete Element Method. Involving simple spheres and complex shaped particles as clumps, a wide range of materials presenting various friction angles were tested. A neat influence of the peak friction angle on the maximal load transfer phase was observed whereas the last phase was associated with the residual friction angle. In addition, a micromechanical analysis, giving the localization of shear strains underlined the effect of the friction angle on the pattern of arching observed in the material.