Abstract:  The earth pressure balance shield encounters many problems, such as multiple machine malfunction and difficult soil conditioning, due to high pebble content, large particle size, and high strength in sandycobble strata. To address these problems, the sandy cobbles are first divided into fine and coarse particles with a grain size of less than 2 mm and greater than 2 mm, respectively. A twophase physical model is established with porosities formed by coarse gravel residue filling and wrapping by fine particles, and the critical fine particle content is deduced. The sandy cobbles are divided into three types, i.e., very difficult to condition, difficult to condition, and easy to condition, according to the critical content ratio and sandycobble strata conditioning laboratory testing. Furthermore, the corresponding conditioning measures are given as follows: single mixing mode of stable foam agent, compound blending mixing mode of foam+bentonite slurry, and compound blending mixing mode of foam+bentonite slurry+polymer. Finally, the recommended measures are applied in a shielded section of the Chengdu metro line 17. Moreover, the soil states and shield tunneling parameters before and after soil conditioning indicate the rationality of the sandycobble strata conditioning and the established classification mechanism.

Key words: earth pressure balance shield, sandy-cobble strata, workability, critical content of fine particle, soil conditioning