The Shenzhen-Jiangmen Railway passes through five cities: Shenzhen, Dongguan, Guangzhou, Zhongshan, and Jiangmen. It is an important part of China's "Eight Vertical and Eight Horizontal" high-speed rail network coastal corridor and a major transportation infrastructure project serving the "Belt and Road" initiative and supporting the construction of the Guangdong-Hong Kong-Macao Greater Bay Area and the China (Guangdong) Pilot Free Trade Zone. As a key control project of the entire Shenzhen-Jiangmen Railway, the Pearl River Estuary Tunnel connects Dongguan and Nansha, Guangzhou. With a total length of 13.69 kilometers, it is a single-tube double-track high-speed railway with a design speed of 200 km/h, and is currently China's undersea tunnel with the highest water pressure and greatest burial depth.

Since construction began in July 2020, the Pearl River Estuary Tunnel project has lasted for more than three years. How is this new undersea "artery" being built? What technical support is behind it? The reporter visited the Pearl River Estuary Tunnel under construction.

Maximum burial depth of 115 meters undersea, water pressure up to 1.06 MPa, long-distance crossing of multiple complex strata

Construction difficulty is rare worldwide

Dongguan and Guangzhou in Guangdong face each other across the Pearl River Estuary, and this time they are brought closer by the Pearl River Estuary Tunnel. According to the route planning map, the tunnel enters the ground from Dongguan Binhai Bay Station, crosses the Pearl River Estuary, and emerges at Wanqingsha Town in Nansha, Guangzhou. Once completed, the travel time between the two places will be only 4 minutes.

What many people don't know is that behind the 4-minute shortcut is a 10-year process of scheme demonstration and comparison.

There are many ways to cross rivers, lakes, and seas: bridges, bridge-tunnels, island-bridge-tunnels, or tunnels are all feasible. The Shenzhen-Jiangmen Railway chose an all-undersea tunnel to cross the Pearl River Estuary.

According to relevant data, the sea section of the Pearl River Estuary Tunnel is about 11.05 kilometers long, with a maximum burial depth of 115 meters, equivalent to a 40-story building. As the depth increases, the water pressure also increases. The extreme value borne by the shield machine during construction reaches 1.06 MPa, more than 10 standard atmospheric pressures, equivalent to bearing 10.6 kilograms of pressure on an area the size of a fingernail. In addition, the surrounding water is highly corrosive, which can damage the anti-corrosion layer on the surface of steel bars, causing deterioration of the performance of reinforced concrete structures and affecting their service life.

Why choose an all-undersea tunnel when it must withstand such high water pressure and resist seawater corrosion?

"The Pearl River Estuary Tunnel crosses an economically developed, densely populated, and ecologically sensitive area, which places high demands on construction," Chen Xingfei, chief engineer of the Pearl River Estuary Tunnel project of the Shenzhen-Jiangmen Railway under China Railway 14th Bureau Group, told the reporter. First, disturbance to surrounding communities, shipping lanes, and ecosystems must be minimized to maintain environmental stability. Second, geological conditions must be considered to ensure the safety and durability of the tunnel. To meet these requirements, route selection requires comprehensive consideration of traffic demand, ecological protection, and geological features, as well as full consultation with stakeholders to reach the best solution.

In fact, after considering multiple factors, various schemes such as a road-rail bridge, a single bridge, and a tunnel were proposed. Each has its own characteristics. Most experts believed that the main navigation channel through the Pearl River Estuary is a major waterway transport artery connecting the port cluster of the Guangdong-Hong Kong-Macao Greater Bay Area with international shipping lanes and domestic and foreign ports. Considering that a bridge would have certain impacts on navigation, flood control, the environment, and costs during construction and later operation and maintenance, the road-rail bridge scheme, despite its better economic benefits, was ultimately abandoned in favor of an undersea tunnel.

After deciding how to cross the Pearl River Estuary, the next step was route selection. Based on conditions such as construction sites on the east and west banks of the estuary, route corridor alignment, and project rationality, two schemes—south and north lines—were formed. However, no matter how the route was planned, the complex geological environment of the Pearl River Estuary waters could not be avoided.

The Pearl River Delta is a composite delta formed by alluvial deposits from the Pearl River system, with complex water conditions and variable terrain. Survey results show that the geomorphic units of both routes are the Pearl River Delta Plain, and the composition of the bottom overburden in this water area is basically the same, "soft on top and hard on the bottom." The upper part is mainly soft soil and sand deposits, while the lower part consists of Upper Pleistocene cohesive soil and gravel layers, with slightly weathered granite bedrock at the bottom.

"The seabed is uneven and soft and hard are unevenly distributed. The full-face hard rock section is 2.75 kilometers long, the fault zone section is 490 meters long, and the maximum saturated compressive strength of the rock reaches 124 MPa," Chen Xingfei roughly calculated. Under such conditions, the tunnel crossing the Pearl River Estuary must pass through 17 fault fracture zones, 13 types of strata, and 5 types of composite geology. "Its construction difficulty is rare worldwide."

"Lion Dance" frolics in the sea, "Flood Dragon" dives to the bottom, tailor-made domestic shield machines

Using national heavy equipment to solve world-class engineering challenges

In the 15-square-meter cab, machines roar, and the flashing indicator lights on the console seem to tell of the busy work here. Shield machine operator Chen Xingge stares intently at the nine screens in front of him, turning knobs from time to time to adjust parameters. Outside the cab, the huge cutterhead of the shield machine slowly rotates and begins to move forward.

The shield machine is currently the most advanced tunnel excavation equipment. In complex and variable hard rock strata, although it can only advance 6 to 8 meters per day, it is a "capable general" of tunnel engineering. The Pearl River Estuary Tunnel is no exception.

The Pearl River Estuary Tunnel uses a combined "mining + shield" construction method. The shield method means using large shield machines to excavate simultaneously from both ends. The reporter learned that the shield section of the tunnel is 6,520 meters long in total. The "Shenjiang No. 1" shield machine excavates 3,590 meters from the Dongguan Humen direction, and the "Shenjiang No. 2" shield machine excavates 2,930 meters from the Wanqingsha direction in Nansha, Guangzhou. The mining method is used to build tunnels by excavating underground galleries, creating space for the smooth disassembly and recovery of the shield machines and ensuring the safety and order of the entire construction process.

"Unlike ordinary mechanical equipment, shield machines are customized products. Each machine needs to be tailor-made according to the actual project. Different geological conditions have different requirements," Chen Xingfei said. The ultra-high water pressure of the Pearl River Estuary Tunnel has no equivalent engineering experience, shield equipment, or design standards in China for reference. Therefore, the two domestic shield machines designed for the key and difficult points of this tunnel construction are destined to be different.

—A contrasting charm beneath a heavy-industry exterior. The two shield machines are like "steel pangolins," cutting through rock and breaking walls, but their huge "bodies" have an endearing appearance. "Shenjiang No. 1" is clad in "lion dance" livery, majestic and full of Lingnan characteristics; "Shenjiang No. 2" is painted with a "flood dragon" design, symbolizing this "underground flood dragon" riding the wind and waves and being invincible.

—Equipment upgrades enhance shield adaptability. On the "Lion Dance" and "Flood Dragon," many advanced technologies are adopted to ensure safe and efficient excavation. For example, "Shenjiang No. 1" is equipped with "normal pressure + pressurized" disc cutters, a retractable main drive, and integrates a series of intelligent systems such as efficient heavy-duty material transport, automatic pressurization of ultra-high water pressure main drive seals, and shield tail clearance measurement. The R&D team of "Shenjiang No. 2" specifically equipped it with innovative technologies such as a small-spacing normal-pressure composite cutterhead, a retractable main drive, ultra-high pressure automatic compensation, four-circuit pressure maintenance, and an air-cushion direct discharge excavation system, striving for excellence and strict quality control.

—Double-shell design ensures "cicada shedding its shell." How to receive the shield machine after completing its task is a problem the design team must consider. Traditional shield machines are disassembled and transported with the assistance of large lifting equipment, but this is obviously impossible for the Pearl River Estuary Tunnel, and undersea disassembly involves high risks. "We designed a 'double-shell' mode for 'Shenjiang No. 1.' After the tunnel is completed, the outer shell remains in the strata as support, which can ensure the stability of the tunnel face and also provide lifting support for disassembling the shield machine. In this way, the shield machine can retreat from the seabed intact like a 'cicada shedding its shell,'" Chen Xingfei explained.

The "Lion Dance" and "Flood Dragon" have pressed the "fast-forward" button for the construction of the Pearl River Estuary Tunnel. The reporter learned from the contractor that as of now, the "Shenjiang No. 1" shield machine has excavated 2,646 meters, and the "Shenjiang No. 2" shield machine has excavated 2,380 meters, and the project progress bar has loaded more than half.

Tail water turned from "waste" to treasure, intelligent segment assembly

Innovation safeguards green and efficient construction

"Attention all, start advancing!" On the morning of November 17, with the operation of oil pumps, mud pumps, and the cutterhead, "Shenjiang No. 1" began a new round of work 73 meters below the surface of the Pearl River Estuary.

The cutterhead of the shield machine "eats" soil in the working tunnel, but the construction site is clean and tidy, with no muck or muddy water. This benefits from the research results of the "Large Shield Craftsman Studio" and the "Party Member Innovation Studio."

To solve technical problems in the shield tunneling process, party members from the construction, contracting, design, and supervision units established these innovation studios. "Direct discharge of construction tail water will cause serious harm to the environment, so effective treatment of tail water during construction is necessary," Zhang Wei, party branch secretary of the Shenzhen-Jiangmen Railway project of China Railway 14th Bureau Group, introduced to the reporter how to turn "waste" into treasure.

First, the muck is separated from the mud. Zhang Wei said that the mud mixed with muck is transported through the slurry discharge pipeline to the mud treatment plant, where the mud and muck are separated by mud-water separation equipment. After further treatment, the muck is transported to a designated muck yard and can then be used for farming, planting trees, and roadbed construction, thereby achieving green emission reduction and resource reuse.

The gravel-bearing sandstone strata crossed by the Pearl River Estuary Tunnel are rich in iron ions and suspended solids. The initially separated tail water is turbid and brownish-red. To solve this problem, the two studios carried out research.

Zhang Wei further explained that a centrifuge is first used to treat the waste slurry to reduce the specific gravity of the mud; then a mud pump is used to send the treated tail water to the primary sedimentation tank, where composite chemicals are added to accelerate the flocculation and sedimentation of suspended solids in the water; the supernatant after sedimentation is then introduced into the secondary sedimentation tank. After multiple treatments, the tail water looks colorless and meets environmental emission standards and recycling requirements. It can be used for slurry preparation and car washing in the washing bay, thereby protecting the environment and saving water resources.

The working conditions of the Pearl River Estuary Tunnel are complex, and many challenges are faced during construction. With a tight schedule and heavy tasks, the designers and builders insist on using innovation to safeguard green and efficient construction.

"Segments are key to ensuring the integrity of the tunnel structure. The special nature of the Pearl River Estuary Tunnel places higher demands on segment performance and specificity," said Li Bing, project manager of the Shenzhen-Jiangmen Railway project of China Railway 14th Bureau Group. For example, the shield section excavated by "Shenjiang No. 1" is 3,590 meters long and is assembled from more than 16,000 segments ring by ring. Each segment weighs 13 tons, and assembling one segment takes an average of 6 minutes. Both production and assembly are time-consuming and labor-intensive projects.

To further improve construction efficiency, the production and assembly of segments have been equipped with intelligent "wings." Li Bing introduced that by introducing more than 10 types of intelligent equipment such as mold cleaning and oiling robots, intelligent surface finishing robots, and unmanned transport vehicles at the prefabrication base, as well as intelligent curing and unmanned weighing systems, the entire production line requires only 11 workers to produce high-quality segments.

Intelligent equipment is also indispensable in the assembly stage. Unmanned transport vehicles deliver the poured segments to designated positions, and workers only need to operate the remote control in their hands to "command" the segment crane installed on the upper part of the tunnel to start working. With three simple actions—hooking, lifting, and turning—the segment moves along the conveyor belt to the front of the shield machine, waiting to be assembled and embedded into the tunnel wall. "Intelligent equipment has further liberated labor," the construction workers said with emotion.