Research On Soil Conditioning Study For Earth-pressure Balanced Shields Excavation In Beijing Cobble Layers

Earth Pressure Balanced (EPB) are extensively used to meet the demands forexcavation of urban tunnels because of their technical and economical advantages.The gravel layers are typically instable layer for EPB tunneling, many difficulties areencountered during tunnelling through gravel layers, such as high torque of themachine, high cutter wear, inconvenience of spiral unearthed device, instability of thetunnel face and surface, etc. As a result of this, soil conditioning is increasinglyattracted the attention of the construction side, better plastic is the most importantprotection for earth pressure balance shield smooth, efficient construction. Usingfoam and slurry for soil conditioning is the most efficient way to solve thesedifficulties, but the use of foam and slurry is usually base on experience, it is a bigwastage of resources. The thesis studies soil conditioning for gravel layers in tunnelsection from Xiju to fengguanlu in Beijing Metro Phase II project of line10. Firstoptimize the properties of foam and slurry. And then, obtain the optimal ratio of theadditives by slump test, study the additive influence on the cutter-head torque and soilfriction by mixing test. Finally, evaluate the improved results by Site constructionresults.1、The ideal consistency of the foam rages from3%to4%for EPB in the sandand gravel layers，the ideal consistency of the slurry rages from10%to11%for EPBin the sand and gravel layers.2、To this layer, soil slump in the130~190mm, the soil has good physical andmechanical properties, soil takes the shape of the "cake-like”, with a good flow ofplastic.3、Obtained by mixing test and site test, foam injected into the ratio of30%, mudinjected into the ratio of9%, the ratio is suitable for this project, only inject foam ormud improved to be ineffective, the stirring torque is large, the soil friction is strong.To Beijing’s sand and gravel strata, laboratory test obtained a good result. We didsome site test in n Beijing Metro Phase II project of line10, the experimental resultsin job site show the same variations with the laboratory tests. Through soilconditioning, reduce the amount of additives, the plasticity of the soil is improvedbetter, the torque of the cutter and screw was significantly reduced, the surfacestability was improved a lot.