Challenges and Development Prospects of Ultra-Long and Ultra-Deep Mountain Tunnels

Social development has led to the placement of high standards on ultra-long and ultra-deep mountain tunnels. Disasters may be encountered during the construction and maintenance of such mountain tunnels due to high geostress, high geotemperature, high hydraulic pressure, and special adverse strata, in addition to various other problems caused by engineering activities. To deal with uncertain geological conditions during mountain tunnel construction, comprehensive geological prediction, refined monitoring, and dynamic design and construction methods based on information technology should be adopted. For the operation and maintenance of ultra-long tunnels, the concepts of dynamic evacuation rescue, active protection, energy conservation, and environmental protection should be fully embodied in order to address significant problems related to ventilation, rescue situations, and energy consumption. Moreover, integrated construction and maintenance should be carried out to achieve digital sensing and intelligent maintenance. New ideas and technologies should be adopted to improve the quality and efficiency of the whole process of construction and operation, and to enable the construction of environmentally friendly tunnels, thus achieving the ultimate goals of safety, efficiency, greenness, and intelligence for ultra-long and ultra-deep rock tunnels. With the development of society, the economy, and transportation networks, the construction of ultra-long and ultra-deep tunnels through mountains has become increasingly inevitable. Ultra-long and ultra-deep tunnels are generally defined as tunnels that have a length exceeding 10 km and a depth exceeding 500 m [1]. Mountain tunnels mainly consist of road tunnels, railway tunnels, and hydraulic tunnels. Although an ultra-long and ultra-deep tunnel potentially features the advantages of safety, environmental friendliness, and speed, the cost and difficulty of project establishment, construction, and operation are considerable. Table 1 lists the ultra-long and ultra-deep mountain tunnels that have already been built or are under construction around the world. According to an incomplete survey, there are 56 ultra-long and ultra-deep mountain tunnels in China and 21 abroad. Among these tunnels, the 57.1 km Gotthard Basis Tunnel is the longest and deepest in the world, the 32.7 km New Guanjiao Tunnel is the world’s longest tunnel above 3000 m, the 18 km Highway Tunnel of Qinling Zhongnan Mountain is the world’s longest double-line highway tunnel, and the 16 km Qinling Tianhuashan Tunnel is Asia’s longest single-hole two-lane high-speed rail tunnel. With the increasing demand for and ongoing progress in tunnel construction technologies, the construction of ultra-long and ultra-deep mountain tunnels will usher in new development opportunities. Due to the high geostress, high geotemperature, and ultra-long construction and operation, these complex tunnel projects must handle unprecedented challenges in terms of design, construction, operation, and maintenance, which demand new ideas and engineering measures.