Experimental Investigation Of Backward Erosion Piping In Two-stratum Dike Foundations

Backward erosion piping is a prominent failure mechanism in water-retaining structures.It is a phenomenon in which shallow hollow spaces(pipes)are formed in an upstream direction(backward erosion)at the interface of a sandy aquifer and an impermeable top layer.Erosion at the tip of the pipe causes pipe lengthening.A zone of loosened sand grains forms around the pipe during pipe development,which may have an effect on the equilibrium of forces on the grains at the tip and walls of the pipe.The current researches on the erosion mechanism that causes the pipe to lengthen and the loosened zone around the pipe are not deep.And current prediction models for backward erosion piping are unsuitable for dike safety assessment.This research focuses on experimental investigation of progression of backward erosion piping,movement characteristic of particles at the tip of the pipe,loosened zone around the pipe,the criterion for pipe progression and the prediction model for backward erosion.Experimental investigation of pipe progression.This paper presents configuration experiments at three scales with a range of sand types.The processes of pipe progression and loosened zone formation were observed and recorded.A series of experiments was performed in a configuration with a limited sand sample width(10 mm)in order to visualise the erosion process near the pipe tip from the sidewall,when proper sand types were used and an artificial pipe was created.The video recordings of the erosion process of grains at the pipe tip during pipe development provide valuable information about movement characteristics of particles at the pipe tip,that can serve as a basis for a new criterion for pipe progression.Digital images captured during model tests are used to analyze the displacement of loosened particles around the pipe.Investigation of movement characteristic of particles at the pipe tip.The local area near the tip of the pipe is the control area for the seepage stability of the pipe.The observed contour of the pipe tip was an irregular curve.A scarp at the edge of the pipe tip was formed with an estimated height of several grains.The movement characteristic of particles at the tip of the pipe is concluded on the basis of the observation of the sand transport at the plane and section of the pipe tip in the recorded video.That is,the grains at the toe of the scarp of the pipe tip move upwards into the pipe one by one,ongoing erosion causes the heightening of the scarp,and the local grains in front of the pipe tip to collapse.Investigation of the loosened zone around the pipe.Images captured during pipe development were analysed to obtain displacement fields for particles around the pipe,which are analysed to determine the size and degree of looseness of the loosened zone and estimate the hydraulic conductivities of the loosened soils.The quantitative data of the loosened zone is obtained.Sand column experiments subjected to upward vertical flow were performed and analysed to study the permeability characteristics of loosened soil at free face.Reference is provided for the permeability coefficient of loosened area near the pipe,where the method of image analysis is not applicable.Numerical simulation of piping tests are carried out to analyze the influence of loosened zone on hydraulic conditions around the pipe.The influence of loosened zone on the tip stability is analyzed from the point of redistribution of local stress at the pipe tip caused by the displacement of loosened particles.The results showed that,the loosened zone has a limited effect on the hydraulic conditions near the pipe,but has an important effect on the stability of the pipe tip.A criterion for pipe progression.Based on the mechanical analysis of the erosion process at the tip of the pipe,the vertical equilibrium of single particle at the toe of the pipe tip is adopted as a criterion for pipe progression.The seepage velocity at the toe of the pipe tip is equal to the minimum lifting flow velocity of the control size particle,which is proposed as the general form of the criterion.The relationship between the seepage velocity at the toe of the pipe tip and the local hydraulic head of the tip is established based on the analytical solution of the local seepage flow at the pipe tip.Based on this,the concrete application formula of the criterion is established.Using the criterion for pipe progression to predict the pipe-forming erosion processes.This research presents a new method to establish the prediction model for pipe progression in the three-dimensional seepage field of two-stratum dike foundations.At the mesoscale,the criterion for pipe progression is used to determine the local hydraulic conditions around the pipe tip that cause the pipe to progress.On the macro scale,the relationship between the upstream head and the hydraulic head near the pipe tip is established based on the hydraulic head continuity condition.And the upstream head that cause the pipe to progress is determined.Based on this,a prediction model for backward erosion is developed.In this paper,information about the movement law of particles at the tip of the pipe,the quantitative data of the loosened zone around the pipe,the criteria for pipe progression and the establishment method of the prediction model for pipe progression are obtained.The findings can serve as a basis for novel piping prediction models for two-stratum dike foundations with different boundary conditions.