Laboratory Modeling Of Opening And Burst Exit Of Backward Erosion Piping Near Seepage Exit In Two-stratum Dike Foundations

Two-stratum dike foundations with a low-permeability blanket layer overlying a sandy layer of higher permeability is widely being seen on dams and levees foundations.This foundation is lifted by the confined water during flood,and there are often within dangerous conditions such as flowing soil and piping propagate behind the embankment.Backward erosion piping(BEP)is responsible for a large percentage of the seepage failures in this type of configuration.BEP occurs within the blanket layer at downstream of the levee when individual non-plastic soil particles are removed at a seepage exit point,then progresses “backward” against the seepage flow forming a tunnel or “pipe” leading toward the seepage source.BEP is still one of the least understood mechanisms of internal erosion even though its assessment of mechanisms has been conducted over years but mostly have provided insight into the behavior of pipe progressing rather than BEP initiation.However,initiation stage relates directly to the level of pipe progression under different hydraulic conditions,as a means of assessing erosion potential.This paper will discuss the characteristics of soil movement near the seepage exit when BEP occurs in terms of different levels of the blanket layer weakness.1.Laboratory modeling of initiation stage of BEP.According to the characteristics of soils movement corresponding to the weakness of the blanket layer,that can be separated into two types: one is easily to be eroded(opening exit)and the other ones is difficult(burst exit).A wide variety of exit conditions were tested,including one-dimensional cylinder,threedimensional cylinder and flume test.Observation and recording had been performed throughout initiation of soil loosening and expansion of the loosened zone around the exit area,and the process of particle movement from the initiation around the exit to the formation of the channel in these two types.2.Measuring characteristics of soil loosening around the seepage exit(opening exit).As one-dimensional cylinder laboratory modeling program has been performed with real-time data collecting and video recording throughout each test,the loosened void ratio and hydraulic conductivity was measured.Simple FEM models were developed for each stage using an inverse analysis procedure to investigate the results of soil loosening and found the effective stress along the interface increases as the extent of the loosened zone,lead to establishment of an exponential relationship between the critical gradient needed to transition from the original density of the soil to the loosened zone and effective overburden pressure of the loosened soil.Consider the forces in the flow regime acting on a grain at the exit face,the exponential behavior has been confirmed.3.Measuring characteristics of seepage field around the seepage exit(opening exit).Based on the three-dimensional cylinder laboratory modeling program results,three-dimensional finite element analyses were conducted to indicate the process of BEP progression between soil loosening and pipe development.The pipe-tip equations had been applied to verifies the rationality of its application to the law of the channel progression around the exit.A mathematical method for the key parameters to control the initiation and progression of the pipe tip had been proposed with consideration of the effects of the narrow ditch in front of the tip,and come to specific criterion for the initiation of critical conditions.4.Mechanism analysis of seepage field and predicted critical affected area within failure moment around the seepage exit(burst exit).Regarding the seepage field analysis,flume test had been performed and the characteristic of the pore pressure changes at the moment of pressure relief was considered,the simplified strip load model had been applied to assess the seepage field at the initial stage of BEP.Characteristics of BEP failure within burst exit had been confirmed through comparison of the seepage field between opening exit and burst exit.The forces in the flow regime acting on a grain around the seepage exit had been used to assess the critical affected area.In the equilibrium analysis,the intergranular forces between the particles and the related concepts of river sediment dynamics are considered,then the range of particles that can reach the critical conditions can be assessed,which provides insight into predicting the degree of BEP failure.