Abstract: During the construction of largediameter shield tunnels in waterrich sandy pebble strata, the following difficulties are often encountered: collapsing tunnel faces, water gushing, rock falls, and jamming of screw cutters. Meanwhile, due to the high shield torque, high muck temperatures, and large volumes of slag, soil conditioning becomes essential. Accordingly,  many laboratory experiments are conducted to explore the optimal mixing proportion of bentonite and foaming agent, and according to analysis, proportion and evaluation indices for soil conditioning  are proposed. The following conclusions are drawn: (1) The viscosity of bentonite increases with increasing sodium bentonite ratios. (2) The foaming multiples and halflife period of the foam solution increases with increasing foaming agent concentration. (3) The fluidity of the conditioned soil significantly increases with increasing proportions of bentonite and foaming agent; the permeability of the conditioned soil decreases with increasing proportions of bentonite and increases with increasing foaming agent proportion. Finally, it is recommended that the bentonite viscosity should not be less than 35 s, bentonite volume should be 10%~12% of the soil volume, foaming multiple of the foaming agent should be less than 15, and volume of foam should be limited to approximately 20% of the soil volume. The soil conditioning effect is verified by actual soil sampling and excavation parameter analysis.

Key words: largediameter earth pressure balance (EPB) shield, soil conditioning, waterrich sandy pebble strata, laboratory experiments, bentonite, foaming agent