Study On Large-diameter Slurry Shield Tunneling With Half-chamber Air Pressure Method In Clay Strata

Large diameter slurry shield tunneling in some clay strata is prone to the problem of mud-cake formation on the shield cutterhead,and the problem can cause a decrease in the tunneling speed and an increase in the energy consumption of shield tunneling,which may also have an adverse impact on the surface structure.The background of this paper is the shield tunnel construction of Zhengzhou Yuji intercity railway undercrossing the trunk canal of South-North water transfer project.In view of the problem of large diameter slurry shield mud-cake formation on cutterhead,the tunneling mechanism under the conditions of half-chamber air pressure method and full-chamber mud water method is analyzed in this paper.The main contents and results of this paper are as follows:(1)Firstly,this paper expounds the principle of half-chamber air pressure method of slurry shield,and analyzes the setting method of liquid level and pressure value of mud water shield excavation under half-chamber air pressure.Under the half-chamber air pressure method,the upper half of the excavation chamber is compressed air and the lower half is mud,and the excavation chamber is not fully excavated by mud-water.Reasonable setting of the height of the liquid level and the pressure value at the excavation chamber is the key to ensure the smooth tunneling of the shield.The height of the liquid level is set to be about 2/3 of the height of the excavation chamber,and the pressure value is set to be about 3.2bar.During the construction,the shield tunneling machine maintains an efficient,safe and stable tunneling state.(2)Secondly,this paper puts forward the cutterhead"mud bridge"model and velocity model of shield tunneling,and analyzes the mechanism of half-chamber air pressure method to solve the problem of cutterhead mud-cake formation.According to the"mud bridge"model,under the half-chamber pressure method,the mud-water pressure behind the mud bridge can be disappeared,and the mud-cake can be easily squeezed into the chamber.After using the half-chamber air pressure method,the knot rate of mud-cake decreased from 92.8%to 20%.The effective opening rate of the cutterhead increased from 2.6%to 28.9%,and the equivalent diameter of the cutterhead opening increased from 0.30m to 1.08m.According to the tunneling velocity model,the calculated velocity is only 2%different from the actual tunneling velocity,so the opening rate of the cutterhead calculated by the“mud bridge”model can be better applied to the tunneling velocity model.(3)Thirdly,this paper studies the characteristics of shield tunneling parameters under the half-chamber air pressure method,and puts forward the evaluation model of shield tunneling efficiency for evaluating the shield tunneling.After using the half-chamber air pressure method,the tunneling speed of shield tunneling increased from 4.9mm/min to 25.4mm/min,while the thrust decreased from 93069k N to 74473k N,and the torque decreased from 10712k Nm to 6814k Nm.The rotation speed remained around 1.4 rpm.At the same distance of tunneling(30m),the energy consumed by shield propulsion decreased from 6×10~8k J to 7.4×10~7k J,which reduced 7/8approximately.The efficiency index of shield tunneling increased from 3.7to 26.1,and the mean dispersion value of scatter distribution of shield tunneling parameters decreased from 0.1 to 0.03.The tunneling efficiency of shield tunneling is greatly improved under the half-chamber pressure method,and the parameters of shield tunneling are stable.(4)Finally,this paper monitor the surface settlement under the half-chamber air pressure method.Meanwhile,analyzing the stability of excavation surface,and studying the deformation characteristics of surface settlement.The results show that the limit support pressure ratio is 0.6.And the displacement of the center point is-260mm and the surface settlement is-110mm when the excavation surface becomes unstable.Compared with the full-chamber mud water method,the displacement at the center point of the excavation surface is 30%less.Meanwhile the surface settlement is 36%less,and the formation disturbance area is small.The maximum displacement occurs below the center of the excavation surface.With the same support pressure ratio,the displacement and surface settlement of the center point of the excavation surface under the half-chamber pressure method are smaller.The ground settlement is stable and the final settlement is well controlled.The final settlement is-4.0mm.At the same time,the diachronic curve of surface subsidence under the half-chamber air pressure method is strong.The setting of pressure value of excavation chamber and the quality of grouting at the tail of shield will have a great influence on surface deformation.