Modelling seepage flow near the pipe tip.

Backward erosion piping is an important failure mechanism in both dikes and dams where uniform silts and sands are covered by a cohesive layer. It is an internal erosion mechanism during which shallow pipes are formed at the interface of the aquifer and the cohesive top layer in the direction opposite to the flow. The hydraulic conditions near the pipe tip determine the onset of erosion near the pipe tip, which causes pipe lengthening. The seepage field near the pipe tip was analysed using the analytical solutions. The two-dimensional (2D) analytical solution available for calculating the seepage field at the vertical section of the pipe tip in three-dimensional (3D) groundwater flow has been validated on the basis of experiments with 3D configurations. The motion characteristics of particles at the pipe tip were explained on the basis of the analysis of the effect of the ditch at the toe of the pipe tip on the hydraulic conditions near the pipe tip. The equilibrium of the soil in front of the pipe tip was analysed to determine the pressure distribution on the top of the sand layer in front of the pipe tip. The unified method for calculating the local critical hydraulic gradient at the pipe tip in both 2D and 3D experiments was proposed. The local critical hydraulic gradients at the pipe tip were analysed for the available experiments, indicating that the influences of grain diameter and scale on the local critical hydraulic gradient are limited, a finding that is essential for the development of a new model. [ABSTRACT FROM AUTHOR]