Experimental investigation of initiation of backward erosion piping in soils

Experiments were conducted on field and laboratory-mixed soils in a new true-triaxial test apparatus to assess factors that influence initiation of piping in non-cohesive and cohesive soils. Three modes of piping behaviour were recognised: concentrated leak erosion, backward erosion, and suffusion. The amount and type of the fines present in the soils significantly influenced the initiation and mode of piping. The presence of non-plastic fines significantly reduced the required seepage velocity to initiate piping (critical seepage velocity), whereas the presence of plastic fines greatly increased the hydraulic gradient required to initiate piping. Concentrated leak erosion occurred along the soil/apparatus interface in soils with plastic fines at very low seepage velocity, but required very high hydraulic gradients. Backward erosion was the primary mode of failure in non-cohesive soils, and this mode required a seepage velocity of 0·8–1·1 cm/s to initiate piping in uniform sand. The critical seepage velocity was found to depend on the magnitude of the stress state, pore pressure, initial void ratio, and seepage direction. It is found that an increase in maximum principal stress and seepage angle and a decrease in void ratio caused an increase in the critical seepage velocity, whereas the minor and intermediate principal stresses did not have a significant influence on piping initiation. However, the influence of intermediate and minor principal stresses may be important for plastic soils prone to hydraulic fracturing. Suffusion was the primary mode of piping in soils that contain very fine non-plastic fines. Initiation of suffusion required seepage velocity an order of magnitude less than that required to induce backward erosion. Once suffusion initiated, it progressed to backward erosion of the soil skeleton at much lower seepage velocity than required for similar soils without non-plastic fines.