Sodium-based Bentonite Slurry Improvement to Sandy Cobble Soil During Shield Excavation

Sandy cobble soil has characteristics including a large internal friction angle, high permeability coefficient, and poor fluidity, which can result in serious tool wear, groundwater spilling, and transportation difficulties during shield construction. A subway in Luoyang is presented as an example case of shield construction. The optimal soil-water ratio of the bentonite slurry is 1:10 based on a sand and cobble stratum in the shield construction, and the optimal expansion duration is 18 h. Large-scale shear, slump degree, and permeability tests were conducted on a sodium-based bentonite slurry mixed with the sandy cobble soil. It was found that, with increasing slurry injection rate, the viscosity of the sandy cobble soil increased; the sand and stone separation phenomenon gradually vanished; the shear strength, internal friction angle, and permeability coefficient non-linearly decreased; and the slump degree non-linearly increased. The slurry injection rate necessary to meet the friction requirement of sandy cobble soil in the shield construction was 17%; the slurry injection rate necessary to meet the permeability requirement was 14%; and the slurry injection rate necessary to meet the fluidity requirement was 13% - 17%. The total thrust and cutter torque of the shield were found to be reduced by approximately 1/3, and the earth pressure balance maintained relatively well after the sandy cobble soil was mixed with the bentonite slurry, these points being remarkably significant to the high-speed advance of shield construction. This study is important with respect to solving issues concerning sandy cobble layers in subway shields.