On October 9, at the construction site on Tinghong Road in Jiangnan District, Nanning City, Guangxi, as the massive cutterhead of the "Zhuangmei" shield machine broke through, Guangxi's first high-speed railway shield tunnel was successfully completed, marking major progress in the construction of an important project of the New Western Land-Sea Corridor—the newly built Nanning-Chongzuo Railway (hereinafter referred to as the "Nan-Chong Railway").

Since the start of construction of the Liucun Tunnel, the contractor has applied a visual intelligent construction management platform, precisely controlled the tunneling parameters of the shield machine, comprehensively strengthened settlement data monitoring, successfully completed the underpassing of six existing railway lines, achieved "zero settlement," solved a number of industry construction challenges, and obtained a batch of technical achievements such as "rapid tunneling technology in mudstone strata." After 20 months of continuous tunneling, the tunnel was successfully completed as planned, clearing the last obstacle for track laying along the entire line.

Contractor Overcomes Numerous Technical Challenges

The tunnel has a total length of 5,725 meters and is located in Jiangnan District and the Economic and Technological Development Zone of Nanning City. Due to the dense above-ground buildings and the crisscrossing of railways such as the Yunnan-Guangxi High-Speed Railway, municipal roads, and metro lines, building the railway on elevated structures above ground is extremely difficult.

To realize the goal of bringing high-speed rail into the city and comprehensively utilize underground space, the contractor carried out technical research and designed the use of a large-diameter shield tunnel to pass underground, of which 4,006 meters were constructed using a slurry balance shield machine with an excavation diameter of 12.86 meters. The tunnel is designed as a single-tube double-track tunnel, where high-speed trains meet inside the tunnel, minimizing the environmental impact on the city.

During this period, the contractor overcame numerous technical challenges.

First, the shield diameter is large, and the long-distance crossing of complex geology makes construction highly difficult. The Liucun Tunnel segment has an outer diameter of 12.4 meters, an inner diameter of 11.3 meters, a ring width of 2 meters, and a wall thickness of 0.55 meters. A slurry balance shield machine with a diameter of 12.86 meters was used for single-head continuous tunneling of 4,006 meters, making efficient construction organization inside the tunnel highly difficult; tunneling through large-section, long-distance sandstone strata causes severe tool wear and high tool replacement frequency. Under tight schedules, the shield machine must reduce failures and maintain a high availability rate; at the same time, targeted design was needed for the mudstone strata in the section to prevent mud cakes on the shield cutterhead and improve the shield machine's adaptability to complex strata.

Second, the engineering environment is complex, and construction risks are high. The shield tunnel is located in the urban area and passes under many structures, including 6 existing railways, 6 municipal roads, 1 metro line, 12 building clusters, and 27 various underground pipelines, with a cumulative underpass length of 3,120 meters, accounting for 77.8% of the total shield section length. The complex engineering environment, high requirements for subsidence and settlement control, and high construction risks meant that during the continuous underpassing of 6 existing railways, the subsidence was controlled within 2 millimeters.

Third, the proportion of fine particles is large, and the amount of waste slurry generated after screening is large. According to the geological particle analysis report, the content of particles below 25 microns reaches 72%, accounting for a large proportion. The slurry cannot be effectively screened and treated, and the amount of waste slurry generated per ring is large. Under high environmental protection pressure, abundant rainfall, and strict management of spoil disposal sites, the drying treatment and disposal of waste slurry are costly and difficult to organize, and slurry treatment directly determines tunneling efficiency.

Application of Rapid Tunneling Technology in Mudstone Strata Doubles Construction Efficiency

During the construction of the Liucun Tunnel, a batch of rapid tunneling technologies for mudstone strata were put into practice and summarized.

Mudstone strata have the characteristics of high viscosity and swelling-shrinkage, sticking to the cutterhead like rice cakes sticks to teeth, causing mud cakes on the shield cutterhead and seriously affecting construction efficiency. The project department fully leveraged its rich experience and core technical advantages in the large shield field, carried out technical research according to the stratum characteristics of this area, carried out multiple upgrades and transformations of the tunneling equipment and slurry circulation treatment system, optimized construction organization, planned 8 tool replacement locations in advance, used improved tools and rearranged tool types, increased tool service life, reduced the number of hyperbaric chamber entries, and fundamentally solved the phenomenon of suction port blockage through pipeline modification. Construction efficiency doubled, with an average daily advance of 12 meters during stable production. After 20 months of continuous tunneling, the tunnel was successfully completed according to plan.

In addition, the Liucun Tunnel was the first railway shield tunnel in China to adopt the method of fixed-point embedded channels in segments. It is understood that domestic railway shield tunnels previously used post-anchoring for catenary channels, which led to maintenance difficulties after railway operation, falling anchor bolts and nuts, and even dangerous situations such as catenary falling, endangering traffic safety. The Liucun Tunnel was the first in China to adopt the fixed-point embedded channel method in segments, which is simple in process, convenient to install, stable in quality, and long in service life. It avoids the disadvantages of post-anchoring and is of great significance for ensuring traffic safety.

To ensure the smooth progress of the entire line, a 24-hour shift system was implemented to accelerate the construction of auxiliary tunnel structures and ballastless track, striving to complete all construction tasks within 4 months, reach track-laying conditions, and prepare for track laying.

The Entire Nan-Chong Railway Has Entered the Track-Laying Stage and Is Expected to Open to Traffic in 2022

The Nan-Chong Railway is the first high-speed railway fully invested and built independently by Guangxi. It starts from Nanning Station in the east and passes through Wuxu Airport and Fusui County to Chongzuo City. The main line has a total length of 119.29 kilometers, a design speed of 250 kilometers per hour, and an estimated total investment of 18.322 billion yuan.

It is reported that the planning and construction of the project are of great significance for implementing the strategic deployment of building a country with strong transportation, accelerating the construction of the "five vertical and five horizontal" trunk railway network, helping build a magnificent Guangxi in the new era of socialism with Chinese characteristics, and promoting the construction of the New Western Land-Sea Corridor and the "Belt and Road."

The project is expected to be completed and open to traffic in 2022. By then, Nanning Station, Nanning East Station, and Nanning Airport will be directly connected by bullet trains, enabling seamless transfers between aircraft and high-speed rail. It will take only 12 minutes by bullet train from Nanning Station to Wuxu Airport, and travel time from Nanning to Chongzuo, a border port city between China and Vietnam, will be shortened from the original 2 hours to 30 minutes.

At present, the entire Nan-Chong Railway project has entered the track-laying stage. The total track-laying mileage (including station tracks) is about 249 kilometers, of which 49 kilometers (19.68%) has been completed. For post-station works, the main structures of Chongzuo South Station and Fusui South Station have been topped out and have entered the construction stage of roof steel structures and decoration.