Abstract：Although the shield tunnel method has been widely used in the underground construction, the technology system must be revised to adapt to different strata. Therefore special design should be implemented to special strata. The excavation in large size cobble and boulder strata with shield method is still the restricted zone around the world. So far, there are no applicable mature technology systems for such strata and no typical successful cases have been reported. According to the mechanical characteristics and space distribution of the large size cobble and boulder strata, the theoretical analysis and the in-situ measurements are combined to systematically study the interaction mechanism between the heavy-duty cutters and the large size cobbles and boulders, and the energy transmission principle between cutter and strata during the shield tunnel excavation process. Based on the energy principle, the rock breaking mechanism with heavy-duty cutters is proposed. The results obtained are as follows: (1) the breaking mechanism of shield tunnelling in large size cobble and boulder strata, i.e., hammering splitting off the rock under damping environment, is proposed on the basis of the stress wave propagation theory and the Rittinger's new surface theory; (2) the formula for calculating the minimum torque required to break the rock is derived, and the design method for shield cutter equipment and rock breakage excavation parameters is built; (3) the breaking mechanism has been verified in the Beijing subway Line 9 Bid 06, the "Military Museum Station—East Diaoyutai station" shield section, in which is the shield tunnel method is successfully applied in the large size cobble and boulder strata. The results can provide reference for the selection of shield type, the shield tunnel construction as well as the feedback design.
