Abstract: To examine the construction adaptability of dual-mode shield(earth pressure balance mode and slurry balance mode) tunneling in medium coarse sand-weathered rock composite strata under earth pressure balance mode, a case study is conducted on the tunnel section between Chishajiao station-Luntou station of the Guangzhou metro line 12. Due to the construction risks of shield tunneling in medium coarse sand-weathered rock composite strata, i.e., mud cake on cutterhead and mud gushing of screw conveyor, anti-gushing and anti-mud cake test devices are designed to examine the soil conditioning effect by dispersants and bentonite slurry. Finally, the control points of shield tunneling under various modes are proposed, providing the soil conditioning method of "foam agent and dispersant based, supplemented by bentonite slurry". The research results indicate the following: (1) Dispersants can effectively prevent the cutterhead from mud cake, and 4% dispersion content is the optimal. (2) Bentonite slurry can effectively improve the flowability of sand, with a good improvement effect when the addition ratio is 10% and the viscosity is greater than 80 s. (3) By conducting soil conditioning while keeping a rational soil level in soil chamber can achieve safe and efficient tunneling in medium coarse sandweathered rock composite strata.

Key words: soil conditioning, composite strata, mud cake test, dispersant, anti-gushing test, bentonite, dual-mode shield, earth pressure balance mode