Recently, the first full-face hard rock tunnel boring machine (TBM) in China used for auxiliary construction in an expressway tunnel project, the Yuecheng Liangshan TBM, successfully bored 12,318 meters. With this, the parallel pilot tunnel of the Daliangshan No. 1 Tunnel, a key control project of the Lexi Expressway and the first expressway TBM construction pilot tunnel in China, has been fully holed through, marking a solid step toward the goal of achieving full completion and opening to traffic of the Leshan-Xichang Expressway (Lexi Expressway) in 2025.

The Lexi Expressway starts within Mabian Yi Autonomous County of Leshan City, passes through Leibo County and Meigu County of Liangshan Prefecture, and ends on the south side of Zhaojue County, connecting with the planned Xichang-Zhaotong Expressway. It is the first expressway to penetrate deep into the heartland of the Greater and Lesser Liangshan Mountains and the Yi ethnic inhabited areas, with a total length of 152 kilometers and an investment of 33.5 billion yuan, and a bridge-tunnel ratio of 81.78%. Once fully completed and opened to traffic, the expressway will end the history of no expressway access in the three counties of Leibo, Zhaojue, and Meigu. At that time, the drive from Zhaojue to Mabian will be shortened from the current 5 hours to about 1.5 hours, and driving from Zhaojue to Chengdu will take only 3.5 hours. It will play an important role in opening up the dual transportation corridors between the Chengdu Plain and the Panxi region, serving the economic and social development of the Greater and Lesser Liangshan Mountains and the Yi ethnic inhabited areas, and achieving common prosperity for all ethnic groups.

High Standards Implemented for Safety and Environmental Protection Requirements, Giving Rise to the TBM

The Daliangshan No. 1 Tunnel of the Lexi Expressway is 15.366 kilometers long and is the longest expressway tunnel in Southwest China. About 8.2 kilometers of the tunnel's entrance section lies within the Mamize Nature Reserve of Sichuan Province. Due to the strict restrictions of the environmental protection area, no ventilation shaft can be set up where the tunnel passes through the Mamize Nature Reserve. In addition, the tunnel passes through two fault fracture zones, limestone, and water-rich syncline sections and other unfavorable geological conditions, with a maximum daily water inflow of 110,000 cubic meters. If traditional inclined shaft auxiliary construction were adopted, there would be risks of water inrush and sudden flooding, as well as uncontrollable investment and schedule. To effectively improve the intrinsic safety level of tunnel construction and strictly implement all environmental protection requirements, the project team, focusing on "replacing people with mechanization and reducing personnel with intelligent systems," transformed the traditional expressway tunnel construction model. Based on detailed analysis of the tunnel's complex geological conditions and special geographical location, the team organized the design of multiple schemes including a 3-kilometer pilot tunnel, an 8-kilometer pilot tunnel, and a 15.3-kilometer pilot tunnel, and conducted comprehensive comparison and selection from the aspects of environmental protection, prevention of water inrush and sudden flooding, operational ventilation, and disaster prevention and rescue. Finally, the cross-section layout of "one tunnel, three bores" was determined, and the combined construction technique of "TBM pilot tunnel + drill-and-blast main tunnel" was adopted for the first time. Through advanced design concepts and advanced construction techniques, the Daliangshan No. 1 Tunnel has minimized its impact on environmentally sensitive areas and reduced construction safety risks.

Innovative Adoption of TBM Pilot Tunnel + Drill-and-Blast Main Tunnel Overcomes Multiple Challenges

The Daliangshan No. 1 Tunnel passes through "3 major structures + 2 large faults" and various strata of different properties, making construction extremely difficult. By using the pilot tunnel for advance construction, the unfavorable geology of the tunnel was identified, reducing the safety risks of main tunnel construction. After the pilot tunnel was holed through, four additional auxiliary working faces for the main tunnel were added, greatly improving construction efficiency. Because the Daliangshan No. 1 Tunnel is located in a remote area, emergency rescue pressure is enormous. After completion and opening to traffic, the pilot tunnel will serve as a ventilation, smoke exhaust, and disaster prevention and rescue channel, shortening the smoke exhaust distance from 5,000 meters to 750 meters, increasing ventilation efficiency by more than 6 times, and improving rescue efficiency by 4 times compared with the traditional mode of mutual rescue between the left and right tunnels, effectively reducing safety risks during operation.

The "King of Hard Rock TBMs" Sets Multiple Records

The "Yuecheng Liangshan" TBM, known as the "King of Hard Rock TBMs," officially began boring in October 2020 after commissioning in the Daliangshan No. 1 Tunnel of the Lexi Expressway. Over 36 months, it set multiple national records, including a first-ever single-head daily advance of 49.12 meters for an expressway TBM in China, a monthly advance of 723.82 meters, continuous boring of 3,680 meters in 6 months, 6,387 meters in 12 months, and 8,825 meters in 18 months. It reached the contract boundary point 6 months ahead of schedule and achieved pilot tunnel holing-through 2 months ahead of schedule. The successful application of TBM construction for the pilot tunnel of the Daliangshan No. 1 Tunnel shortens the construction period by at least 3 years compared with traditional drill-and-blast construction and effectively reduces the number of workers inside the tunnel, which is of great significance to the intelligent construction of expressway tunnels in China. It is reported that the TBM will next be applied and promoted on the Panzhihua-Yanyuan Expressway and the Chuanwen Expressway in Aba Prefecture within Sichuan.

Domestic Open-Type TBM Brought into the Daliangshan Mountains Achieves Multiple Results

As the first expressway tunnel project in China with TBM auxiliary construction, the Daliangshan No. 1 Tunnel of the Lexi Expressway is highly valuable for promotion. It not only provides technical support for TBM construction of expressways in China, but also drives a new round of technological innovation for TBMs in expressway construction. According to the complex geological characteristics, the participating construction units carried out adaptive modifications to the TBM, adding key technologies such as advanced pipe-roof support, immediate steel bar row protection, mechanical emergency shotcrete system, and real-time cutterhead expansion. In response to research topics such as cost quotas for TBM construction in long and large tunnels, pipeline ventilation, non-stop advanced geological forecasting, and selection and control of TBM boring parameters, in-depth cooperation was carried out with domestic professional colleges and research institutions. To date, 22 patents and construction methods have been obtained, technical guidelines have been compiled, and honors such as the Gold Award of the Second National Highway Micro-Innovation Competition held by the China Highway and Transportation Society and the First Prize of the China Highway and Transportation Society Tunnel and Underground Space Engineering Innovation Award have been received.