Study On The Formation Process And Effect Of Filter Cake During Slurry Shield Construction In Water-rich Sandy Stratum

In recent years,the construction of urban underground railway and pipeline has developed rapidly.Shield method,as a mature and effective construction technology,is widely used in underground construction.Slurry shield,as a form of shield,has broad application prospects because of its wide stratum applicability.However,when the slurry shield passes through the high permeability stratum,it is prone to a large amount of slurry filtration and difficult to form the filter cake,which eventually leads to the instability of the excavation surface.At present,the waste slurry commonly excavated is difficult to degrade,difficult to deal with and easy to cause water and soil pollution.Therefore,the experimental research on the film-forming of slurry in the high permeability stratum is of great significance.In this paper,the shield construction in water rich sand soil layer is taken as the research object,and a series of studies from slurry preparation to final application effect are carried out by using indoor slurry preparation and property determination test,slurry permeability film-forming test and shield numerical simulation.The natural additives suitable for making slurry and the equivalent filter cake permeability coefficient in the process of slurry permeability are obtained,The role of filter cake in the shield process is evaluated.The main contents are as follows:The influence factors of pulping materials on the basic properties of slurry were studied.Natural and easily degradable agar,guar gum and traditional sodium carboxymethyl cellulose(CMC)were selected as additives.By changing the content of each component of slurry,the influence law of each component on the engineering application properties of slurry such as specific gravity,funnel viscosity and gum ratio was studied,and guar gum,guar gum and CMC were found Agar can reduce the API filtration of slurry,increase the viscosity of funnel and increase the colloid rate.It can be used as a new green additive instead of CMC to prepare for slurry penetration test.The slurry penetration film-forming test under the conditions of different types of slurry additives,grouting pressure and formation particle gradation was carried out.The self-developed slurry penetration device was used for the slurry penetration filmforming test in the formation.The device is transparent and has the ability to adjust the grouting pressure and measure the pore water pressure and formation filtration in real time.The test results show that compared with the traditional additive CMC,agar gum and guar gum have a better effect on reducing formation filtration and promoting the formation of high-quality filter cake.According to the change of pore water pressure,the change law of effective stress conversion rate of slurry in the process of slurry penetration is analyzed.At the same time,according to the calculation of formation filtration,under different slurry ratio and concentration,grouting pressure Variation law of equivalent filter cake permeability coefficient under the condition of formation particle gradation.Using numerical simulation,the model adopts the equivalent filter cake permeability coefficient and formation parameters measured in the laboratory test,and carries out the dynamic numerical simulation of the shield process for the water rich sand soil layer,so as to explore the influence of the slurry film in the shield process to reduce the grouting pressure on the formation pore pressure.The test results show that the effective grouting pressure and stratum gradation affect the distribution of effective grouting pressure in the filter cake and stratum.The effective grouting pressure is mainly distributed on the filter cake(up to 80%).The greater the effective grouting pressure,the higher the proportion distributed on the filter cake and the smaller the proportion distributed in the stratum.