Study On The Slurry Performance And Face Stability Of SPB Shield Tunnel In Cobble-rich Soil

With the rapid development of China's economy,the process of urbanization is unprecedentedly accelerated,and the scope of urbanization is expanding.The problems of urban land tension,traffic congestion,and environmental degradation have become increasingly prominent.In urban environments with dense buildings,rail transit and intercity railways have to adopt a large number of tunnel solutions.At the same time,due to the sensitivity of the urban environment and the continuous advancement of shield technology,urban tunnels are increasingly being built using shield methods.In Beijing,Chengdu,Lanzhou and other cities,cobble-rich soil are widely distributed.Compared with general soft soil layers,cobble-rich soil has loose structures,large porosity,and low cohesion.There will be many difficulties in the process of shield tunneling.This study is based on the Beijing-Zhangjiakou High-speed Railway Tsinghuayuan Tunnel Project.The tunnel is driven by a slurry balanced shield tunnel,which mainly crosses the cobble-rich soil.When the slurry balance shield is constructed in this type of stratum,problems such as difficulty in forming slurry membrane,difficulty in slag generation by the shield,and poor stability of the driving face are easy to occur.In view of this,this article comprehensively uses theoretical analysis,numerical simulation,indoor tests and field measurements to focus on the membrane formation mechanism of the slurry balance shield tunneling face of the cobble-rich the matching relationship between the slurry ratio and the cobble-rich soil,and the cobble-rich soil.In terms of slurry transport performance and soil stability of the large-diameter slurry balance shield tunneling face,the research results can provide direct reference and technical support for the Beijing-Zhangjing High-speed Railway Tsinghuayuan Tunnel Project,and can also provide slurry slurry for shields under similar stratum conditions.Selection of configuration and tunneling parameters provides reference,which has very important theoretical significance and engineering application value.The main research results of the paper are as follows:(1)A self-developed slurry permeation test device was used to carry out slurry graded loading permeation membrane formation tests in consideration of formation changes.The effects of formation permeability coefficient on slurry permeation process,membrane formation morphology,and percolation loss in cobble-rich soil were studied.The matching relationship between slurry and stratum discussed the division criteria of slurry membrane type.The self-developed dynamic slurry permeation test device was used to study the cyclic process of slurry membrane failure-formation under the condition of cutter disk rotation failure,and revealed the change law of slurry percolation and filtration loss in the sand and cobble formation with the cutter disk rotation failure.Several groups of slurrys with different specific gravity,viscosity,and sand content were prepared by the control variable method,and slurry permeation membrane formation tests were conducted in similar formations of the Tsinghuayuan Tunnel to determine the key indicators that affect the slurry membrane formation in cobble-rich soil,and Furthermore,the best slurry optimization scheme was obtained,and the problem of difficult membrane formation in the high-permeability Cobble-rich soil was solved.(2)Based on the calculation method of particle flow,a scale model of coupled slurry permeation membrane formation based on FLUENT-EDEM was established.The permeation,blockage,and migration processes of slurry particle groups in the formation pores were studied,and the slurry permeation membrane formation was revealed microscopically.The mechanism of permeation was obtained by changing the related parameters.The permeation trajectory and membrane formation state of the slurry under different particle sizes,densities and pressures were obtained.The influence of relative particle size,slurry density and slurry pressure on membrane formation was clarified.Classification method of the size(D/d)of the slurry membrane.(3)The free-settling test device was used to study the relative starting flow velocity of large-diameter pebble in the vertical tube of the circulation system and the similarity of the movement law of the pebble in different solutions,which verified the feasibility of using brine mixed solution instead of slurry for the circulation system test.Using the circulation system test device,the starting flow velocity and migration morphology of large-sized particles of pebble at different positions in the circulation pipeline were studied.The starting flow velocity and passing capacity of the pebble at different positions in the circulation system were obtained.The influence rule of the flow velocity is proposed to improve the pipe wear of the slurry circulation system.(4)The FLAC3D finite difference software was used to establish a three-dimensional space model.The effects of slurry parameters(slurry pressure and permeability coefficient)on formation pore water pressure and excavation surface stability during slurry balance shield construction were studied.The slurry parameters and formation pores were analyzed.The change law of water pressure and formation displacement are internally related.The slurry pressure and slurry membrane permeability coefficient of the Tsinghuayuan Tunnel slurry balance shield to maintain the stability of the excavation face were determined,the stability of the heading face was controlled,and the surface settlement and displacement in the ground were effectively controlled.