Experimental investigation of the backward erosion mechanism near the pipe tip.

Backward erosion piping is a prominent failure mechanism in water-retaining structures. It is a phenomenon in which shallow pipes are formed in an upstream direction at the interface of a sandy aquifer and an impermeable top layer. Experiments were conducted at two different scales to investigate the erosion mechanism near the pipe tip that causes the pipe to lengthen at the grain scale. The erosion process of grains at the tip of the pipe was filmed from the top and the side. The observations and video recordings allowed for a better understanding of the features of the pipe tip. A zone of loosened sand grains was formed around the pipe during the pipe development. The extent of the loosened zone was determined by analysing the images captured during the lengthening of the pipe. The motion characteristics of particles at the pipe tip and the erosion mechanism near the pipe tip were obtained by analysing the erosion process observed. The findings can serve as a basis for the establishment of a criterion for pipe progression and the development of a novel piping prediction model. [ABSTRACT FROM AUTHOR]