China Railway Engineering Equipment Group Co., Ltd. (CREG) held a launch ceremony June 18 for a water jet hard rock TBM. The ceremony for the TBM, named “Longyan,” was attended by more than 100 Chinese experts and scholars in the field of TBM engineering technology at Zhengzhou, China, on June 18, 2019.
The TBM is for the Wan’anxi Water Diversion Project in Longyan City in Fujian Province. It has a diameter of 3.8 m and a length of about 300 m. It was jointly accomplished by CREG and Yellow River Engineering Consulting Co. Ltd.
The anticipated geology in this project consists of mainly biotite granite and also quartz conglomerate, quartz sandstone, siltstone, argillaceous siltstone, glutenite and conglomerate in some parts. The tunnel alignment is basically rock, and the rock near the entrance is found weakly weathered. The tunnel wall is moist and a small section is present with dripping and linear flow.
The rock type is: 11,258.6 m of class II rock mass, accounting for 80.4%; 1,440.3 m is class III rock mass, accounting for 10.3%; and the rest 1,302.6 m is class IV rock mass, accounting for 9.3%. The rock mass joint is not developed. Therefore the overall rock mass conditions are suitable for TBM tunneling.
The hydraulic pressure rock-breaking system once used in breaking rocks will be expected to reduce the wear of cutting tools and the thrust of TBM by 30% to 40%, increase excavation speed, extend TBM service life, reduce construction cost, shorten construction period, loosen geological constraints in tunnel construction and provide a wider range of options for tunnel location planning.
Mechanism of water flow rock-breaking
The rock-breaking mechanism of water jet mainly bases on the theory of water wedge rock-breaking. When jet flow impacts crack-free rock wall, apart from compressive stress in rock body, tensile stress is generated around the boundary of the impact zone as well. And when this tensile stress exceeds the tensile strength of the rock, the rock wall is pulled apart to form a crack. After the rock wall initially forms a crack, under the action of the jet flow impact force, the water immerses into the crack space, causing the tensile stress concentration zone at the tip of the crack to expand the crack and finally break the rock. This mechanism, similar to wedging a steel wedge into a rock and causing the rock to break apart, is therefore called water wedge effect. The water wedge effect is continuous. The closer the jet flow comes to the rock mass, the greater the water pressure and water wedge effect is. The water wedge effect is more obvious when the jet flow strikes fractured rock stratum.