Study On Ground Conditioning In EPB Shield Tunneling Through Sandy Stratum

Among the shield tunneling methods, up to now, the earth pressure balanced(EPB) shield tunneling is the most universal type. The earth pressure balanced shieldtunneling machine is primarily applicable to the excavation of cohesive soil stratum.Due to the weak soil plasticity flow, the big friction coefficient and the high strength, thethrust of the shield machine increases significantly when tunneling through sandystratum. Thus, the output difficulty of soil, the cutterhead torque and the screw conveyortorque increase. Also, the advance velocity reduces and it is not even enough to go ondriving. The cutterhead heats quickly and wears badly as same as the screw. So it isdifficult for EPB shield tunneling to construct in sand layer. That’s to say, it is notapplicable on this occasion.Based on the non-adaptivity of EPB shield tunneling through sandy stratum, thisresearch on ground conditioning is conducted. With bentonite as soil modifier,laboratory soil tests are made to obtain the optimal concentration and expansion time ofthe bentonite. By comparing the shear strength, permeability and slump before and afterthe add of bentonite slurry relatively in mixing proportions, the appropriate bentoniteconcentration and slurry mixing ratio are obtainedThe results prove that the plasticity and fluidity of soil improve effectively whilethe shear strength and permeability decrease sharply with the use of1:10bentoniteslurry. When the slurry mixing ratio remains20%, the temperature and attrition ratedecrease obviously. At the same time, the output of soil is smooth and the advancespeed increases. The conditioned soil still can’t meet the requirement of the optimalslump. The laboratory soil tests show that it is the best in terms of both modifiedparameters and economy when bentonite slurry to gravelly sand in a ratio of5:1andloess to bentonite in a ratio of2:3 Meantime, the three-dimensional finite element numerical analysis technique is used tocompare and analyze the calculated values of ground settlement before and after groundconditioning, and the calculated values after ground conditioning is compared with themeasured data. The results indicate that3D simulation of shield tunneling machinecrossing sandy stratum with FEM software is feasible. The measured data fit well withthe calculated values. The effect of ground conditioning is also verified. The researchresults are expected to offer reference to the solution of non-adaptability problems, suchas the difficulty of earth pressure balance, the output difficulty of soil, the obviouscutterhead torque and the quick abrasion.