On March 12, as the cutterhead of the shield machine on the left line between People's Park Station and West Street Station broke through the soil, it marked the full breakthrough of the entire Chengdu Rail Transit Line 17 Phase II project, and the line construction achieved phased results.
Large-diameter shield tunneling with an optimal construction plan
"The shield machine for the Ren-Xi section (from People's Park Station to West Street Station) completed this time is 126 meters long overall, weighs more than 1,200 tons, and has a cutterhead diameter of 8.63 meters," introduced Yao Tianyu, deputy director of Project Management Center I of the Construction Company under Chengdu Rail Group. "The trains for Line 17 Phase II adopt the largest 8A formation in domestic metro systems, with a maximum operating speed of 140 km/h. To meet the design speed requirements, all shield tunneling sections of Line 17 Phase II use large-diameter shield machines of more than 8 meters for excavation, making it the metro line in Chengdu that uses large-diameter shields to cross the main urban area."
It is understood that large-diameter shield tunneling, due to the enlarged excavation face, especially after entering the main urban area with complex municipal pipelines, numerous rivers, and frequent crossings with existing lines, faces enormous challenges in shield tunneling construction. To ensure the smooth and safe advancement of the shield, Chengdu Rail Group, together with all participating units, invited well-known domestic experts in the shield field to conduct on-site surveys multiple times. In response to the special geological conditions and technical requirements of the section, they selected the optimally configured shield machine and a reasonable construction plan, ultimately successfully and safely achieving the goal of full breakthrough of the entire line.
Shield "sharp turn," multiple measures for precise deviation correction
During shield tunneling, Line 17 Phase II faced the challenge of small-radius curve tunneling, meaning the shield machine does not always advance along a straight line but also encounters turning situations. The smaller the turning radius and the sharper the curve, the more difficulties it brings to construction.
In the section between Provincial Orthopedic Hospital Station and Xiaonan Street Station, workers operated an 8.6-meter large-diameter shield machine to turn on a curve with a radius of 400 meters, while also passing alongside the Nan River during tunneling. "This can be considered a 'sharp turn' in shield tunneling construction cases," introduced Yang Liangang, commander of China Railway Construction Kunlun Metro Investment and Construction Management Co., Ltd., the contractor for Line 17 Phase II. "When the shield machine turns on a curve, the smaller the curve radius, the greater the deviation correction amount; long-distance tunneling requires continuous deviation correction, and if one correction is insufficient, multiple corrections are needed, multiplying the construction difficulty."
To ensure safe and smooth shield tunneling, the construction team optimized the plan in advance and simulated the tunneling posture; during the process, they analyzed data ring by ring and adopted a series of measures such as precise grouting at key locations, strengthened measurement of the formed tunnel, control of segment assembly quality, precise segment layout for turning, advance trend correction, and establishment of a posture warning mechanism, successfully completing the underground "sharp turn."
"Giant doing embroidery," five-step grouting to cross operating lines
During the construction of Line 17 Phase II, there were a total of 16 crossings of existing operating lines, among which the section between Hongxing Bridge Station and Jianshe North Road Station needed to pass beneath the existing Line 3 and Line 6, with a vertical distance of only 5 meters. According to the regulations for shield tunneling beneath existing operating lines, the settlement data of shield tunneling beneath existing lines must be controlled at the millimeter level, which for large-diameter shield tunneling construction is tantamount to "a giant doing embroidery."
The construction team fully applied the mature experience from the previous three phases of line construction in passing beneath existing operating lines. During the 8.6-meter large-diameter shield tunneling beneath existing lines in Line 17 Phase II, they adopted the "five-step grouting method": pre-grouting before tunneling, middle-shield grouting, simultaneous grouting, and secondary grouting during the process, and supplementary grouting after passing through. At the same time, they strengthened whole-process construction control and carried out all-weather automated monitoring of the existing lines, ultimately completing shield tunneling safely and smoothly and ensuring the operational safety of the existing lines.
The Chengdu Rail Transit Line 17 Phase II project has a total length of 24.8 kilometers, starting from Jitouqiao Station (exclusive) and ending at Gaohongcun Station, with 18 stations in total and a design speed of 140 km/h. Once completed, the line will improve public transportation for residents along the route, alleviate traffic pressure from southwest to northeast Chengdu, guide the optimization and adjustment of urban spatial functions, and continuously improve Chengdu's rail transit network.
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