Experimental study on soil conditioning in cobble layer by use of earth pressure balanced machine

Many difficulties are encountered during EPB tunnelling through the cobble layer because of high soil friction and low flow characteristics. In order to solve these problems, soil conditioning with additives has become an indispensable practice to enhance the properties of the soil and make the soil suit for the machine. However, the researches on soil conditioning are limited to an extent, and the use of conditioning additives is often based on trial and errors at the work site or the workers' experience. The soil conditioning for EPB excavation in the cobble layer of the 2nd-stage project of Beijing Metro No. 10 is studied, a series of laboratory tests are performed to investigate and assess the properties of additives and conditioned soils. The optimum injection ratios of additives (foam injection ratio of 30%, slurry injection ratio of 10%, and the optimum slump value of 150～200 mm), which are obtained by laboratory tests, are applied to the penetration tests. The comparative analysis of excavation parameters before and after penetration tests shows that the optimum injection ratios of additives significantly improve the performance of EPB machine. The results can be used to provide guidelines for the excavation by use of EPB machine under such soil conditions.