Major progress has been made in the much-watched Lanzhou urban rail transit construction. On the afternoon of June 3, the reporter came to the launch shaft construction site at Shiji Avenue Station of the Lanzhou urban rail transit test section, and witnessed the launch of "Jincheng No. 1", the first shield machine for the Phase I construction of Lanzhou Urban Rail Transit Line 1.

　　At the construction site more than 20 meters underground, the reporter saw the massive "Jincheng No. 1" shield machine crouching in the launch shaft, ready to start. After the shield segment installation was completed and all preparatory work was in place, the "Jincheng No. 1" shield machine began to slowly bore into the ground.

　　"This is the moment most worth recording, marking that Lanzhou metro construction has begun to enter a new stage of interval tunnel engineering construction." Zhang Zhiguo, deputy project manager of the Lanzhou rail transit engineering test section project of China Railway 21st Bureau Group, introduced that "Jincheng No. 1" is the first metro construction shield machine in Lanzhou's urban construction history. It is an earth pressure balance shield machine that integrates optics, mechanics, electronics, hydraulics, sensing and information technologies, and has functions such as excavating and cutting soil, transporting muck, assembling tunnel lining, and measuring, guiding and correcting deviation. It was specially "tailor-made" according to the actual conditions of the water-rich sandy pebble stratum in the test section of the Phase I project of Lanzhou Rail Transit Line 1. The total machine length is 84 meters, the main machine length is 13.2 meters, with 1 equipment bridge and 6 trailers, weighing 508 tons, of which the cutterhead plus front shield weighs about 160 tons. The average tunneling speed can reach 450 m/month, and the performance parameters of the whole machine have reached the domestic leading and international advanced level.

　　It is understood that the working principle of "Jincheng No. 1" is to use the full-face cutting cutterhead installed at the very front of the shield to cut the soil at the face into the retention sealed chamber behind the cutterhead, and to maintain appropriate pressure in the chamber to balance the water and soil pressure at the excavation face, so as to reduce disturbance to the stratum soil caused by shield advancement and thereby control surface settlement. During muck removal, a screw conveyor installed at the lower part of the sealed chamber continuously discharges the muck to the discharge port. The screw conveyor controls the amount of excavated material through rotation speed, and the excavated amount closely matches the cutterhead cutting speed, so that the sealed chamber is always filled with soil without being overly full. This type of shield avoids the main disadvantages of the local compressed-air shield, and also eliminates the relatively costly control system, slurry transport system, slurry treatment and other links and equipment of the slurry pressure shield.

　　According to the construction plan, after "Jincheng No. 1" shield machine starts drilling from Shiji Avenue Station, it will tunnel from west to east along the right line of Line 1 to Yingmentan Station, then be dismantled and lifted out, transported to Shiji Avenue Station, and launched a second time from the launch shaft of the left line, reaching Yingmentan Station for a second time, after which it will be lifted out from underground. The left line of the interval is 652.823 meters long, and the right line is 574.649 meters long, all constructed by the shield method. During the entire interval construction project, shield tunneling needs to overcome the unfavorable geological conditions of "water-rich sandy pebble" strata. By tackling technical difficulties such as ground settlement control, backfill grouting control, interval tunnel measurement control, and universal ring segment assembly control, 2 hoisting operations, 2 dismantling and lifting operations, and 1 short-distance transport are required. The tunneling speed of the shield machine can reach an average of 12 to 15 meters per day. It is expected that by the end of this year, the tunnel shield construction task of the Lanzhou Metro Line 1 test section will be completed.

　　"According to the plan, single-line breakthrough of the test section will be achieved in mid-September, and double-line breakthrough will be achieved by the end of the year." Feng Shichuan, deputy general manager of Lanzhou Rail Transit Co., Ltd., introduced that the start of drilling of the "Jincheng No. 1" shield machine marks that Lanzhou rail transit construction is being comprehensively and rapidly advanced. The shield construction of the metro tunnel in the test section will accumulate experience for the scientific selection of shield machines in the construction of the entire Lanzhou urban rail line, the determination of tunneling control parameters, and the summary and mastery of various risk control technologies.