Seepage—Failure Experiments on Multi—Layered Sand Columns: —Effects of Flow Conditions and Residual Effective Stress on Seepage-Failure Phenomena—

Abstract

ABSTRACT An apparatus was designed for one-dimensional seepage-failure tests. The apparatus consists of seepage cylinder, constant-head device, and open piezometers. Using the apparatus, a series of seepage-failure experiments on the one- and two- layered sand columns was carried out. It was found that the flow condition : (i) (exerted-pressure type) or (ii) (uniform-flow type) is one of the most important factors in the seepage failure of sand columns, where and k are the coefficients of permeability of the rectifying filter and the sand layer, respectively. The following results were then obtained : (1) The modes of failure of sand columns are able to be predicted theoretically from the residual effective stress diagrams. (2) The one-layered sand column (with no loaded filter) collapses in quite different ways according to the above two flow conditions. (3) In the case of the flow condition of the exerted-pressure type, a multi-layered sand column collapses at the smaller hydraulic head difference than that in the uniform-flow type. (4) When the relative density Dr is small almost all the one-layered sand columns (with no loaded filter) collapse at the smaller hydraulic gradient than the theoretical critical one. (5) As the residual effective stress within a sand column increases, the friction between the vertical cylindrical surface and sand particles becomes large, and thus the resistance to failure of the sand column increases. As a result the sand column collapses at the larger hydraulic head difference than the theoretical critical one. (6) The effect of the cylindrical surface friction on the critical hydraulic head difference of a sand column is able to be estimated from a magnitude of the residual effective stress.