A rectangular TBM named "Jiangong" with the meaning of accomplishment-independently designed and manufactured by CREG, completes the factory acceptance test (FAT) in Deyang, Sichuan Province on 26 May 2016. The machine will be applied to a 57-meter tunnel undercrossing the South Renmin Road with an overburden of 5 meters.
The crown of the 6.02m×4.52m machine is 1.1 meters below a φ600mm sewage pipe while the bottom is 3 meters above a subway line. Top-down geology to be crossed is silty clay, fine sand, coarse sand and loose cobble sands to dense ones. Cobble sand makes up more than 70% of the overall ground and the maximum grain size exceeds 20cm.
Cobble sand ground features low stability and shallow overburden, thus making it difficult to keep a steady working face. For that reason, cases of rectangular box jack TBMs applied to this kind of geology remains rare in China up to this day.
"Jiangong" by CREG has adopted a tailored design to cope with the general and potential challenges faced by TBM tunneling in cobble sand ground. In regards to excavation, 10 cutterheads of 4 different specifications are assembled on the same surface for coordinated work which enlarges the agitating area considerably, enables thorough muck agitating and ensures effective control over ground settlement. The structure of 4 biggest cutterheads resembling spokes of a wheel, plus dis cutters significantly increases the rock breaking capacity of the TBM and allows smooth flow of cobble sands by virtue of sufficiently big opening ratio.
In terms of mucking, to deal with big size cobbles in the ground, two large diameter screw conveyors are used to build up a mechanism of discharging supported by breaking. Moreover, the two screw conveyors are designed to meet the requirement of advancing speed and maintain the balance of mucking and cutting amount.
The first two rectangular TBMs (10.12×7.27m and 7.52×5.42m) manufactured by CREG were used to excavate motorways and sidewalks respectively for Zhengzhou Zhongzhou Avenue Undercrossing Tunnel Project. The two machines have fininished the four tunnels on September 2014. The great success of this project won CREG an order from Singapore Land Transport Authority who were then looking for trenchless technologies that could excavate lengths of up to 150m in soft soil at shallow depth for T221 Lot of Thomson East-Coast Line Project. The 7620mm×5645mm rectangular TBM by CREG started to bore on 11 May 2016.
Following the achievement of rectangular TBMs in soft soil, CREG has continued its efforts to further expand the application scope of such machines. That's how the project-specific rectangular TBM for challenging Chengdu Cobble ground comes into being.