Abstract: When earth pressure balance (EPB) shield tunneling in the sandy cobble strata of Phase 1 project of the Chengdu rail transit line 17, it encounters difficulties, such as unstable pressure on the excavation face caused by water and mud gushing from screw conveyor and the cobbles accumulating at the bottom of the pressure chamber. As a result, the bentonite slurry and foaming agents are used in a laboratory test to condition the sandy cobble strata. The results are as follows: （1）The optimal modifier mixing proportions of bentonite is observed with a mass ratio of 5% and foam with a volume ratio of 10%~30%. On the basis of this, the soil′s fluidity, workability, and impermeability can be maintained in an optimal state. （2）With the objective of using the minimum quantity of bentonite using a 150~200 mm soil slump with no segregation, a threedimensional curved surface is established for the interaction between soil slump, bentonite amount, and water volume. Moreover, a mass ratio of bentonite to water of 1∶6 is determined to provide the most economic benefit based on the boundary between segregation and antisegregation areas. Furthermore, these results have been applied to the test results for the Phase 1 project of Chengdu rail transit line 17, resulting in a safe and highly efficient shield tunnel.

Key words: sandy cobble strata, earth pressure balance shield, soil conditioning, slump test, permeability test