Study On Formation Conditions Of Diffuse Failure In Coarse-grained Soils By Laboratory Model Tests

Diffuse failure refers to the phenomenon of overall sudden collapse and flow-like movement constructed saturated loose granular geo-materials.This failure is likely to occur in loess area,loose debris accumulation area,weathered sand area and mining waste accumulation area.Since this type of damage occurs suddenly and there are no obvious signs before the damage,it often has catastrophic consequences.At present,domestic and foreign scholars have carried out a lot of research work on diffuse failure,and have achieved some results,but there are few researches on the formation conditions and mechanism of the diffuse failure,and lack of unified understanding.In this paper,the coarse-grained soil accumulation is taken as the research object.Firstly,the basic physical properties of the test soil are obtained by Laboratory tests.Then,the fine-grain content in the coarse-grained soil,the compactness of the soil and the size of the slope model are taken as the control variables by Laboratory flume test.The formation conditions and mechanism of diffuse failure in coarse-grained soil were determined by test phenomena and various sensor data.Finally,the effects of various control variables on the diffuse failure in coarse-grained soil were studied.The main research results are as follows:(1)The basic physical properties of coarse-grained soil are obtained by laboratory tests.Permeation test,compaction test and sedimentation analysis of coarse-grained soil shows that the permeability coefficient of coarse-grained soil deposit decreases with the increase of fine-grain content and soil compactness;Its maximum dry density first increases and then decreases with the increase of fines content;The sedimentation value increases with the increase of fine particle content in the soil,and decreases with the increase of soil compactness.(2)The formation conditions of diffuse failure in coarse-grained soil are determined.The flume test shows that only when the content of fine particles in the soil is within a certain range(27%~36%),and the soil is in a relatively loose state(D_r?0.3 with 27%fine particle content,D_r?0.4 with 36%fine particle content),diffuse failure will happen.When the content of fine particles in the soil is less than27%,the soil particles on the surface of the slope will roll off;As the compactness of the soil increases,the slope won't slide when the water height reaches 30 cm.When the fine particle content in the soil is between 27%and 36%,and the relative compactness D_r>0.4,the stream erosion will occur in the slope.When the content of fine particles in the soil is 45%,no matter what the conditions of soil compactness,the slope will be partially collapsed and destroyed.(3)The deformation and failure process of diffuse failure in coarse-grained soil is analyzed.The whole process can be described as follows:during the process of water infiltration into the slope,the soil below the permeability surface is gradually saturated,the pore water pressure rises,and when the soil reaches saturation,the pore water pressure Basically stable;Before the slope model is destroyed,although no displacement occurs,the pore water pressure at the leading edge of the slope increases sharply and produces obvious vibration inside;then the slope slides down in a short time,and the vibration signal reaches the strongest.The internal displacement is constantly increasing.After the sliding of the slope,the pore water pressure drops,the vibration signal drops to zero,and the internal displacement of the slope reaches the maximum value.(4)The mechanism of the failure of diffuse failure in coarse-grained soil is analyzed.The surge of pore water pressure at the leading edge of the slope is the cause of the failure.It is found through analysis that there are two reasons for the surge of pore water pressure at the leading edge of the slope:One is due to the saturation zone,part of the fine particles flow out of the slope through the pore channel between the coarse particles,the slope body settles and compresses the pore channel,and the fine particles move to the front edge of the slope to accumulate and block the channel and the sensor signal shows that the pore pressure increases before the vibration signal.The second reason is that some of the fine particles in the saturated zone flow out of the slope through the channel between the coarse particles,as the channel increases,the internal collapse of the front edge of the slope causes the pore water to be strongly compressed,and the sensor signal shows that the vibration signal increases before the pore pressure.(5)The factors of diffuse failure in coarse-grained soil are analyzed.By comparing the start-up time,sliding time,the pore pressure,microseismic acceleration and internal displacement data of the diffuse failure to study the influence of the fine grain content in the soil,the relative compactness of the soil and the size of the slope model.The results show that with the increase of soil compactness,the internal erosion of the soil becomes weaker,the start-up time of the slope failure becomes longer,the sliding time becomes longer,and the increased pore pressure in the leading edge first decreases and then increase,the sliding distance increases,and the vibration in the leading edge increases.With the increase of the fine content in the soil,the start-up time of the slope failure becomes longer,the sliding time becomes longer,the increased pore pressure increases in the leading edge,the sliding distance increases,and the vibration in the leading edge increases weakens;When the width of the slope model is reduced,the start-up time of the slope failure becomes longer,the sliding time does not change much,the sliding distance increases,and the vibration during the damage increases.(6)The influence of the influencing factors of diffuse failure in coarse-grained soil on the critical water height is analyzed.The critical water height ratio Q is used to measure the variation of the critical water height of diffuse failure in coarse-grained soil under the different influencing factors.The results show that with the increase of soil compactness,the critical water level height ratio(Q)of the slope with different fine grain content and model size increases linearly.The fitting results of Q values under 27%fine particle content,36%fine particle content and reduced model size were:y=0.5832x+0.2231,y=0.7301x+00.926 and y=0.7739x+0.2493.Under the same compactness,increasing the fine particle content or reducing the size of the model will increase the excitation water level required for the diffuse failure.