Aerial view of the Xiaojin end of the Jiajin Mountain Tunnel on National Highway 351

　　On March 13, with the sound of a blast, the main tunnel excavation of the Jiajin Mountain Tunnel on National Highway 351 reached 4,689 meters, completing more than half of the tunneling and taking a big step toward the goal of opening to traffic next year.

　　According to reports, all procedures and all links are required to be seamlessly connected, which can effectively shorten the construction time. Tunnel construction uses a 24-hour "three-shift" cyclic operation, with the excavation of a single working face advancing up to 5 meters per day on average.

　　The Jiajin Mountain Tunnel on National Highway 351 is a single-tube, two-lane tunnel, but at the construction site there are two parallel tunnels——one is the main tunnel, and the other, with a slightly smaller portal, is a parallel pilot tunnel, whose role is to assist construction. According to reports, to speed up tunnel construction progress, more working faces need to be opened. Since the two ends of the tunnel are located in Baoxing County of Ya'an City and Xiaojin County of Aba Prefecture respectively, where the mountains are high and the terrain is steep, opening additional working faces from the side or top would require building auxiliary passages several kilometers long, which is neither economical nor environmentally friendly. After assessment, a construction plan was adopted using a parallel pilot tunnel to assist construction and using a TBM (full-face tunnel boring machine) to excavate the parallel pilot tunnel, which can shorten the construction period by about 1.5 years. This is also the first use of TBM construction in the construction of ordinary national and provincial trunk highways in China. Under efficient excavation, the parallel pilot tunnel was holed through in September last year, one year ahead of the planned construction period.

　　Inside the parallel pilot tunnel, two cross passages connecting to the main tunnel have been completed, and construction workers are excavating the working face of the main tunnel. Together with the two ends at the inlet and outlet, four working faces have been formed. With multiple approaches working together, the construction of the Jiajin Mountain Tunnel on National Highway 351 will be further accelerated. This year, the Jiajin Mountain Tunnel on National Highway 351 plans to complete 2.5 kilometers of main tunnel excavation, with cumulative excavation planned to reach 77%.

　　After construction at multiple working faces begins, the number of construction vehicles entering the site will double. An intelligent auxiliary construction system——a traffic organization and transportation system for construction vehicles——is being installed and is expected to be put into use by the end of this month. By installing positioning systems on vehicles, the system automatically calculates and provides prompts for moving or stopping through the back end, ensuring that construction vehicles can safely pass each other in the parallel pilot tunnel.

　　The Jiajin Mountain Tunnel Project on National Highway 351 is a construction project in the Ministry of Transport's "14th Five-Year Plan" development plan and a major livelihood project of the Provincial Party Committee and Provincial Government under "I Do Practical Things for the People." The project has a total length of about 10 kilometers, of which the main tunnel is 9.35 kilometers long and the parallel pilot tunnel is 9.36 kilometers long. It started construction on December 26, 2021, with a planned construction period of 48 months.

　　After the project is completed, the traffic bottleneck on the mountain-crossing section of Jiajin Mountain will be completely eliminated, vehicles will no longer be troubled by winter ice and snow conditions, and all-weather passage will be possible year-round. Crossing Jiajin Mountain will be shortened from the current 1 hour to 10 minutes.