Abstract:Based on the analysis of the forces acting on the fine grains in the skeleton pores,a capillary-tube model is introduced to characterize the development of piping in noncohesive soils. With the model,the relationship among the washout of fines,the permeability of soils and the hydraulic condition of incipient motion of grains is investigated,and a set of formulae of the critical hydraulic gradient for the incipient motion of movable grains and the permeability coefficient considering the washout of fines are obtained. The application of the formulae to some examples of piping tests gives reasonable results in comparison with the experimental observations. The theoretical results illustrate that the washout of the fine grains has an important influence on the permeability of noncohesive soils. In addition,it is proved that piping failure takes place more easily in the piping-typed soils with a bimodal grain-size-distribution than in the soils with a unimodal grain-size-distribution after some fine grains are washed out.