At around 11 am on 19 August, a rectangular TBM, supplied by CREG broke through a 57-meter tunnel undercrossing the South Renmin Road at depth of 5 meters in Chengdu, Sichuan. The rectangular machine, also known as “Jiangong”, is the world’s first of its kind applied to cobble sand ground up to this day.
The geology encountered by the machine comprises uneven soft and hard strata, silty clay, fine sand and a section of full-face cobble sand whose maximum grain size ranging up to 400mm. Moreover, the underpass project takes place at a critical zone. Only 3.1m above the underpass is Chengdu Metro line 1. In-between the tunnel and ground lie 15 municipal pipelines among which the nearest one is simply 0.5m away from the underpass. The construction must ensure continuous ground traffic, no damage to municipal pipelines and minimum disturbance to the metro line 1 overhead. All those challenges make the tunneling project one of the most complicated urban construction works in Sichuan Province and even the country.
The rectangular TBM was designed with particular attention to the major concerns aforementioned. To deal with the anticipated potential risk of ground settlement and likeliness of screw conveyor blockage by boulders during the early stage of boring, the R&D team of CREG worked on slurry improvement to achieve appropriate transfusion in soil chamber and balance of compressed air pressure. To minimize the adverse impact of large size cobble sands on cutterhead rotation, they boosted the cutting abilities of cutters.
According to Wang Yang, the person in charge of The Tunneling Subsidiary of Chengdu Jiangong Luqiao Company, the daily advance of the machine exceeds 1.5m per day. This trenchless method not only increases the construction efficiency and shortens the schedule but also saves as much as 10 million yuan in terms of this project. The ground traffic remains consistent throughout the tunneling work. The dust and noise pollution is limited to a minimum as well. It is indeed a green construction method.
The PIC of Chengdu Construction Committee summarized, the underpass project features impressive economic and social benefit. The successful practice shall serve a precedent of trenchless technology applied to underground projects in Sichuan Province. We are looking forward to the widespread application of this technology in the future and similar municipal public works.
Jiangong’s satisfactory performance in cobble sand ground turns out to be vigorous support for expanding the application scope of rectangular TBM, an area in which CREG shows tremendous interest.