Study Of Soil Flow Mechanisms In Internal-Stable Soil Using Transparent Soil And PIV Technology

Soil flow,a significant form of seepage failure,refers to the simultaneous suspension and movement of soil particles under the influence of upward seepage.Terzaghi’s critical hydraulic gradient,proposed based on the overall force analysis of particle assemblages,has been widely accepted.However,discrepancies between laboratory test outcomes and the predicted critical hydraulic gradient have been observed,which are currently explained predominantly on the basis of ideal model assumptions without directly observing internal soil phenomena.Alongside discrepancies in the critical hydraulic gradient,traditional laboratory tests have noticed distinct soil flow failure occurrences.Some research has suggested a link between these events and particle gradation.However,the focus of current laboratory tests is largely on macroscopic hydraulic phenomena,with a lack of observation on microstructural evolution,making it challenging to thoroughly comprehend the mechanisms underpinning different soil flow failure incidents.Consequently,it becomes crucial to scrutinize the macroscopic hydraulic phenomena and microstructural evolution of various gradation specimens,establish their correlation,and amalgamate theoretical analysis to elucidate the causes for deviations in the critical hydraulic gradient and the occurrence mechanisms of different soil flow failure phenomena.This would provide substantial guidance for the prevention and control of soil flow failure.For this research,a transparent soil seepage failure test and observation apparatus was developed.With this apparatus,transparent soil flow tests were carried out to assess the macroscopic hydraulic phenomena and microstructural evolution of various gradation specimens.Particle Image Velocimetry(PIV)was utilized to quantitatively examine the particle velocity fields.Subsequently,based on theoretical analysis,we introduced the concept of "particle equilibrium hydraulic gradient" to explain the creation mechanisms of different soil flow failure phenomena.A modified critical hydraulic gradient,considering viscous force modification and boundary friction,was proposed.Compared to existing theories,the theoretical critical hydraulic gradient of this study is in closer alignment with experimental and literary data.The research components are as follows:1、The developed apparatus,capable of real-time imaging of specimen profiles during seepage failure tests,was evaluated under both normal and hypergravity conditions.The PIV analysis of these images facilitates the capture of instantaneous particle velocity fields.The apparatus has proven to meet high-precision velocity measurement requirements for high-speed fields,with a systematic error of less than ±0.5%,while being minimally impacted by factors such as vibrations from baskets and electromagnetic interference in centrifuges below 50 g.2、Soil flow tests were conducted on 13 gradations of transparent soils,prepared using fused quartz sand as the solid phase and mixed oil as the liquid phase.These gradations belonged to three categories: uniform particles,clast support,and matrix support.The qualitative analysis revealed the following:a)The macroscopic hydraulic phenomena during the soil flow process vary among different gradation specimens.All specimens display a decrease in hydraulic gradient and an increase in flow velocity during soil flow failure.However,the failure process of the matrix support specimen exhibits noticeable stages.After reaching a stable state post soil flow failure,an increase in the upstream liquid level maintains a relatively stable hydraulic gradient in the uniform particle specimen,while in the clast support and matrix support specimens,the hydraulic gradient increases with the increase in the upstream liquid level until failure occurs at the secondary peak hydraulic gradient.b)Three different failure modes exist within the soil flow range: in the uniform particle specimen,pore expansion and height increase during soil flow failure,remaining fluidized post failure.In the clast support specimen,a horizontal crack appears during soil flow failure,splitting the specimen into upper and lower parts.The upper part lifts as a whole,while the lower part undergoes structural reorganization,culminating in a relatively stable new structure post failure.In the matrix support specimen,initial pore expansion is observed during soil flow failure,followed by a sudden failure that forms seepage channels.After failure,a relatively stable structure is usually formed,however,occasional blockages by fine particles in the seepage channels may occur.c)The observed critical hydraulic gradient is influenced by the soil structure,decreasing with the rise in equivalent particle size,and increasing with the expansion in the difference of coarse and fine particle sizes.It initially increases and then decreases with the enlargement of the fine particle component within the specimen.3、PIV calculations on image sequences of three typical specimens were conducted to obtain particle velocity fields.Analysis indicates that particle velocities at the specimen sides are smaller,suggesting that boundary friction significantly influences particle motion and should be considered when calculating the theoretical value of the critical hydraulic gradient.The cumulative distribution function of particle velocities,obtained from statistical analysis of the velocity fields,can characterize the particle velocity distribution and reflect the motion state of the specimen.4、The fluid forces acting on individual particles within the specimen during the seepage process were analyzed.Based on the assumption that the fluid forces acting on an individual particle are balanced with its weight,the concept of a "particle equilibrium hydraulic gradient" was introduced to preliminarily explain the generation mechanisms of different soil flow failure modes.Then,considering the balance between fluid forces acting on particle assemblages and their weight,as well as boundary friction forces,a modified critical hydraulic gradient that considers viscous forces modification and boundary friction was derived.Analysis of the experimental and literature data reveals that the theoretical critical hydraulic gradient proposed in this study aligns more closely with the measured values.