Research On Reducing The Torque And Anti-spewing Of Epb Shield Tunnelling In Low Sand Content And Strong Permeability Sandy Pebble Strata Of Lanzhou Subway

Based on the background of Lanzhou Metro Line 1 earth pressure balance(EPB)shield tunnelling project of Century Avenue Station-Olympic Sports Center Station(Airshaft),and the full understanding on grain gradation and permeability of Lanzhou sandy pebble strata,anti-spewing and cutterhead torque reducing performance of conditioned soil were studied by slump test,permeability test,friction test,cutterhead torque test and numerical simulation.The soil conditioning scheme of EPB shield tunnelling in water rich sandy pebble strata was determined,which can provides some guidelines for reducing the torque and screw conveyor anti-spewing during EPB shield tunneling in low sand content and strong permeability sandy pebble strata of Lanzhou subway.The main research contents and results include the following parts:(1)Study on grain gradation and permeability of Lanzhou sandy pebble strataDuring the excavation of the subway stations,the distribution,genesis,grain gradation and permeability characteristics of Lanzhou sandy pebble are studied based on the summary of soil investigation reports,site investigation and sieve analysis test.Results showed Lanzhou sandy pebble is poorly graded and lacks of intermediate particles.Compared with the typical sandy pebble strata in Beijing and Chengdu,the sandy pebble has the characteristics of low sand content and strong permeability.(2)Blowout control during EPB shield tunnelling in sandy pebble stratum with high groundwater pressureSoil conditioning was carried out to the sandy pebble soil involved in the EPB shield tunnelling by bentonite slurry,foam alone or mixed,when the slump was in the range of 15～20cm,the optimal injection ratio was determined.Then,the permeability coefficients,anti-blowing capacity of conditioned soil were tested by the constant water head permeability test and high water pressure breakdown test in a barrel model.Through theoretical calculation,the soil conditioning scheme with good workability and low permeability was determined.Results showed the conditioned soil with injection ratio of 12.5%bentonite slurry,10%foam singly or 7%bentonite slurry and 8%foam mixed can meet the requirement of 15～20cm slump.The permeability coefficient and breakdown water pressure of the conditioned soil can meet the anti-spewing requirement at the outlet of the screw conveyor under 30m water head pressure.The conditioning scheme with injection ratio of 7%bentonite slurry and 8%foam respectively is remarkable.(3)Test of reducing the friction torque of cutterheadThrough the barrel model,slide plate and shear friction test,the friction coefficients of different conditioned sandy pebble were studied,which provides parameters for reducing the torque of the cutterhead by soil conditioning.The results showed:compared with the sandy pebble of 8%water content,the improved sandy pebble can reduce the friction coefficient by 20～50%,and the foam is better than bentonite slurry in reducing friction coefficient.The corresponding friction coefficient between the soil and machine is 0.2～0.3 when the torque of the cutterhead is 50%～80%rated torque and the pressure of the soil chamber is kept at 0.8～1.6 bar through the calculation of the torque of the cutterhead and earth pressure balance calculation of screw conveyor.(4)Test of cutterhead torque in conditioned sandy pebbleThrough the cutterhead model test,the resistance torque of each part of the cutterhead was tested in different soil with different types of cutterhead.The influence rules of soil conditioning on cutterhead torque were studied.Results showed the stirring torque and shear torque account for a large proportion in unconditioned soil.In conditioned soil,friction torque of the cutterhead,the opening shear torque and the stirring torque of the other parts account for 50%～70%,10%～20%and 20%of the total torque respectively.Soil conditioning can significantly reduce the torque of each part of the cutterhead,especially for the torque of the stirring rod and the cutter,which can be reduced by 75%to 90%,the opening shear torque can be reduced by 65%-85%,and for the cutterhead friction torque,it can be reduced by more than 50%.The opening shear torque of the cutterhead is not only related to the total opening area,but also related to the size of the opening.The torque of the cutterhead decreases with the increase of the opening ratio,the opening ratio increases by 20%,and the torque of the cutterhead reduces by 10%.When the injection ratio is same,the effect of reducing the torque by foam is a little better than that by bentonite slurry.(5)Permeability numerical simulation of the conditioned soil in the EPB shield tunnelling.Using the FLAC3D finite element software,three-dimensional numerical simulation of water pressure diminishing in the screw conveyor with different permeability coefficients of conditioned soil was carried out.The effective of the conditioning scheme for anti-spewing in shield tunnelling was proved,which can provide references to anti-spewing for EPB shield tunnelling in water-rich sandy pebble.The results are as follows:the water pressure distribution of the soil in the screw conveyor is linearly decreasing from the inlet to the outlet,and the outlet water pressure of the soil decreases with the decrease of the permeability coefficient.If the water pressure is about 300kPa at the bottom of the machine,when the permeability coefficient of the conditioned soil is 5.61×10-5cm/s,the water pressure at the outlet is near to zero,which is the critical permeability coefficient to ensure no spewing.Compared with the result of high water pressure breakdown test,the safety coefficient of anti-spewing is 1.29,2.08 and 3.79 at the inlet,middle and 1/4 screw conveyor length to the outlet respectively,and has high anti-spewing reliability.