The Experimental Study Of Vacuum-electroosmosis Drainage For Shield Slurry

In the construction of underground shield tunnels,a large amount of slurry with high water content is produced.Due to the high moisture content of the slurry and the additives such as bentonite and foaming agent,the property of the slurry are high water content,low permeability and strong plastic flow.It is prone to environmental pollution problems such as throwing and dripping during transportation,and it occupies a lot of land resources when the slurry is piled up.Sometimes improper stacking also pollutes the environment and even forms geological disasters.Therefore,it is necessary to reduce the volume of slurry and the cost of transportation and reuseAlthough the traditional mechanical dehydration method can effectively remove the free water of the slurry,but it is subject to the hydraulic conductivity of the soil when the slurry contains lots of clay.Dewatered solid slowly in sludge treatment results into difficult to effectively remove the bound water.In recent years,engineers and technicians have introduced the electroosmosis drainage method of soft foundation reinforcement into slurry hardening.Because it has nothing to do with the size of soil particles,is not limited to the hydraulic permeability coefficient of slurry,and can effectively remove the bound water in the slurry.However,the unclear mechanism of electroosmosis dehydration and the unclear design method of electroosmosis have always restricted the development of electroosmosis technology.Therefore,it is particularly important to study the mechanism of electroosmosis dehydration and design of test methods.(1)The mechanism of open slurry vacuum negative pressure drainage is explained in the paper.It is pointed out that the vacuum negative pressure has two processes for slurry drainage: first,the water in the slurry is sucked out under the action of the pore pressure;the other is that the water in the pore channel moves to the negative pressure boundary under the action of the gas pressure.(2)Based on the diffusion electric double layer model,based on the B-P and the Helmholtz-Smoluchowski electroosmotic model,the expression of the flow velocity of the electroosmotic flow in the micro-channel is derived and the expression is numerically analyzed in conjunction with MATLAB.The results show that the Zata potential indicated by the microchannel and the applied electric field gradient are proportional to the flow rate of electroosmosis.(3)COMSOL is used to solve the drainage consolidation control equation of vacuum electroosmosis,and the time-space distribution of excess pore water pressure in the process of vacuum electroosmosis is plotted.The results show that electroosmosis produces negative ultrastatic pore water pressure in the soil.The water pressure value of ultra-static pore water pressure is controlled by two factors.The external factor is the voltage,and the internal factor is the ratio of the hydraulic permeability coefficient and the electrical permeability coefficient of the soil.The smaller the ratio,the larger the ultra-static pore water pressure.So electroosmosis method is more suitable for the treatment of low-permeability fine-grained soil.(4)The experimental research on the dehydration technology of the shield slurry is carried out,and an electro-osmosis combined with vacuum negative pressure drainage test device is developed,in which vacuum negative pressure,electro-osmosis and vacuum negative pressure incorporated with electro-osmosis technology are used for the slurry that is initially treated by sedimentation to conduct dehydration test.The test results show that the drainage effect of the combined method is better than that of the single method alone.The combined method not only discharges seepage bound water but also discharges free water.The moisture content of the treated slurry is greatly reduced(close to the plastic limit),and the slurry is effectively hardened.