Experimental Study On Suffusion Characteristics Of Granite Residual Soil And Its Solution Effect

While carrying out large-scale infrastructure construction,Shenzhen has also caused a large number of underground engineering problems.Statistics show that land subsidence has become one of the major accidents that threaten the lives and property of citizens in Shenzhen.The phenomenon of soil suffusion caused by the leakage of underground pipelines is the main reason for the occurrence of underground cavities and the decline in soil structure strength.At present,the research on soil suffusion mostly foc uses on cohesionless soil.However,the completely decomposed granite(CDG)soil widely distributed in Shenzhen,and the soil not only has high content of coarse and fine particles,but also has the characters of softening and disintegration.Considering the current lack of research on the suffusion performance of CD G,and the complexity of the urban underground pipeline solution environment,it is necessary to take the solution electrolyte concentration and p H value as factors to study the suffusion performance of the CDG and the effect of solution.In order to study the effects of solution electrolyte concentration and p H value on the suffusion characteristics of CDG,a large-scale triaxial seepage test device(Diameter × High = 150 mm × 300 mm)and a triaxial seepage test acquisition system were developed.The triaxial seepage device is able to simulate the stress of the soil in the field environment,but also to capture the pore water pressure information at each position through local sensors.The newly developed outflow collecting device not only has the ability to collect the information of fine particle mass and the volume of outflow,but also the sample can be saturated through backpressure method by externally supplying air pressure.The effects of solution electrolyt e concentration and p H value on the suffusion performance of CDG in vertical seepage direction were studied b y using the above apparatus.The test results show that the erosion development model of granite residual soil can be divided into three stages for description.During the seepage stabilization stage,the soil permeability coefficient is stable,the deform ation and erosion are small,and the local hydraulic gradient distribution at each location is uniform,reflecting that the soil structure is stable at this time.During the particle initiation and migration phase,the fine particles begin to flow with the leachate,so the deformation of the soil and the quality of the fine particles flowing out suddenly increase.With the further development of fine particle migration,the soil began to become clogged,and the soil structure tended to be stable.In the stage of erosion development,the permeability coefficient,deformation and fine particle loss of the soil significantly increase.Combined with the local permeability coefficient change information of the soil,it was found that particle migration and blockage are the reasons for the difference in seepage performance at each stage.Changing the electrolyte concentration or p H of the solution did not affect the erosion development model of granite residual soil.After adding electrolyte to the leachate,the permeability coefficient,deformation and erosion of the soil decrease,and the characteristic hydraulic gradient increases.When the leachate is more alkaline,the permeability coefficient,deformation and erosion of the soil increase,and the characteristic hydraulic gradient is reduced.Combined with relevant chemical analysis,it can be known that under low electrolyte concentration and alkaline environment,the thickness of the bound water film of the clay particles increases,thereby reducing the resist ance to the relative movement between the particles,so the particles need to be migrated.The seepage force is reduced,and the ability of the soil body to resist seepage erosion is weakened.