9 Moon 28 morning of the day 11 time 08 The opening ceremony for the expansion project of the Lanhai National Expressway’s Chongqing-Zunyi section (in Guizhou Province) was held at the Loushanguan East Toll Station. The entire route will officially begin trial operations at midnight on September 29. Kan She Shares Chief Engineer Yang Jian led the team to attend the opening ceremony.
Project Overview
The expansion project of the Lanzhou-Haikou National Expressway, specifically the Chongqing-Zunyi section (in Guizhou Province), has a total length of 119 kilometers. It is being constructed to the standard of a six-lane双向 highway with a design speed of 100 kilometers per hour. The CZSJ-1 contract section, designed by Kanshe Co., Ltd., is 62 kilometers long and features a bridge-to-tunnel ratio as high as 82%. The total tunnel length amounts to 35.5 kilometers, including three extra-long tunnels each exceeding 5 kilometers in length. Among these, the Daloushan Tunnel—formerly known as the Tongzi Tunnel—is the pivotal structure of the entire project. It is a landmark large-section mountain tunnel, marking the first highway tunnel exceeding 10,000 meters in length (with a total length of 10.5 kilometers). It is also the longest highway tunnel in Guizhou Province and the first in China—a two-way, six-lane mountain highway tunnel designed for a speed of 100 kilometers per hour and exceeding 10,000 meters in length. On the right side of the Daloushan Tunnel, two inclined shafts are arranged to provide three-stage longitudinal ventilation; on the left side, one inclined shaft is set up to deliver two-stage longitudinal ventilation. This makes it the first highway tunnel in Guizhou equipped with segmented longitudinal ventilation via inclined shafts.
Affected by karst topography, the Dalou Mountain Tunnel traverses three fault fracture zones and crosses 12 different lithological strata. It integrates a variety of challenging geological conditions, including complex fault fracture zones, high geostress, high gas content, mined-out coal seams, karst formations, and underground rivers. The engineering geology is extremely complex, posing extremely high risks to construction safety and schedule. This tunnel represents a typical example of the construction of an ultra-long, large-span highway tunnel under complex geological conditions.
Technical Features
(1) Efficient and Safe Construction of Ultra-Large-Span Tunnels in Complex Geological Conditions
The tunnel is over 10 kilometers in length, features a unidirectional grade of 1.75%, and is a three-lane, large-span tunnel with extremely complex geological conditions. The tunnel is equipped with three trackless transport ventilation adits and one auxiliary construction adit. Tunnel construction faces numerous technical challenges, including water bursts and mudflows, coal and gas outbursts, long-distance pumping against the slope, and ventilation difficulties in long-distance gas tunnels. The design draws upon the Guizhou Province’s major science and technology special research project—“Research and Demonstration on Key Technologies for Efficient Construction and Intelligent Operation & Maintenance of Ultra-Large-Span Highway Tunnels in Complex Geological Conditions”—and has optimized construction methods for large-section tunnels in high-gas coal-bearing strata, soft rock under high geostress, and weak, fractured Class V surrounding rock. Furthermore, it proposes specific construction measures such as localized grouting to seal water in sections with reverse-slope pumping, significantly enhancing both the efficiency and safety of tunnel construction.
(2) Treatment Technologies for Gas Outbursts in Non-Coal Strata under the Influence of High Geostress
The tunnel passes through the Dongshan anticline section, with a maximum burial depth of approximately 639 meters. The geology in this section is predominantly composed of silty limestone and mudstone from the Lower Series of the Silurian Longmaxi Formation, locally interbedded with lens-shaped black carbonaceous mudstone. During drilling of tunnel blast holes, severe gas spouting, bit sticking, and drill jamming occurred, accompanied by instances of gas combustion. To address the risk of sudden gas outbursts in non-coal strata under high in-situ stress conditions in this section, the design specifically adopted a localized borehole pressure relief and gas drainage technique, thereby ensuring construction safety and significantly improving the efficiency of gas control operations.
(3) Long-Distance Detection and Treatment Technologies for Gas Outbursts in Coal-Strata Formations
To detect in advance the coal-bearing strata and gas distribution in the tunnel breakthrough section, the tunnel construction team innovatively adopted, for the first time, a long-distance deep-hole exploration technique from the surface. This approach provides a solid basis for subsequent treatment of gas outbursts in the coal-bearing strata. Moreover, targeted gas-control technical measures were formulated in advance during the design phase, thereby providing critical technical support for the smooth completion of the tunnel.
(4) Circular Drainage Channel Drainage Technology for Concentrated Karst Water Inflow Points in Tunnels
In response to the characteristics of large water volumes and frequent blockages at karst spring points concentrated along the contact zone between limestone and shale, the design incorporates a circular drainage channel positioned between the initial support and the secondary lining. The two outlets at either end of the circular drainage channel are sequentially connected via side-wall collection wells, laterally buried trenches, and a central drainage ditch, thereby successfully addressing the challenge of draining large, concentrated flows of water.
Meaning and Effectiveness
The Dalou Mountain Tunnel represents a new milestone in the construction of long, large-section tunnels in mountainous regions. Once the expansion project of the Lanhai National Expressway’s Chongqing-Zunyi section (in Guizhou Province), to which this tunnel belongs, is completed and opened to traffic, the travel time between Chongqing and Zunyi will be reduced from three hours to two hours. This will significantly alleviate congestion on the existing highway between Chongqing and Guiyang, creating a “dual-channel” expressway system for southwestern China’s access to the sea. The project holds great significance for promoting economic development along its route.
