Full underwater operation
Build a high-speed rail tunnel on the seabed of one hundred meters
1.06 MPa
The design of the Pearl River Mouth Tunnel adopts a combination of mining method and shield method.
The maximum burying depth of the mine section has reached 115 meters, and the maximum water pressure of the shield tunnel is 1.06 MPa, which is more than 10 standard atmospheres, which is equivalent to the water pressure of the famous Turkish Bosphorus highway tunnel.
◎Reporter Jiao Yang
On June 10, news from the Jiangmen Engineering Construction Headquarters of China Railway Guangzhou Bureau Group Corporation, a major transportation project in the Guangdong-Hong Kong-Macao Greater Bay Area, which started for nearly a year, is the new Shenzhen-Jiangmen high-speed rail (hereinafter referred to as the Shenzhen-Jiangmen high-speed rail) important control project. The Shiziyang Pearl River Mouth Tunnel project is progressing smoothly. The inclined shaft enters the tunnel 20 to 30 meters, and the shield part will start in November.
The Shenzhen-Maoming high-speed railway passes through 5 cities including Shenzhen, Guangzhou, Dongguan, Zhongshan and Jiangmen. It is a component of the Shenzhen-Maoming high-speed railway with a total length of 116 kilometers.
The Pearl River estuary is crossed by a tunnel. The project is located at the estuary of the Pearl River between Dongguan and Guangzhou. The total length of the tunnel is 13.69 kilometers, the design speed is 250 kilometers per hour, and the planned construction period is 56 months.
"The Pearl River Mouth Tunnel was designed with a combination of mining method + shield method. The maximum buried depth of the mine method section reached 115 meters. The maximum water pressure of the shield method tunnel was 1.06 MPa, which exceeded 10 standard atmospheres. The water pressure of the Prussian Highway Tunnel is comparable.” said He Weiguo, head of the China Railway Sixth Survey and Design Institute Group Co., Ltd. Tunnel Institute (hereinafter referred to as the China Railway Sixth Institute Tunnel Institute), which undertook the entire design process.
With such a large water pressure, why does the deep river high-speed rail crossing the Pearl River Estuary use the full underwater tunnel method?
How to solve the technical difficulties faced by the design solutions?
Demonstration and selection of bridge and tunnel schemes lasted for 10 years
The deep river high-speed rail is an important part of the national "eight vertical and eight horizontal" high-speed rail main channel coastal passages, and also a major transportation infrastructure in the Guangdong-Hong Kong-Macao Greater Bay Area.
After completion, the Guangdong-Hong Kong-Macao Greater Bay Area will realize a half-hour high-speed rail life circle and economic circle.
Cross rivers, lakes, rivers and seas, or use bridges, tunnels, or bridge tunnels, island bridges and tunnels.
The deep river high-speed rail crosses the Shizi Ocean of the Pearl River Estuary. Why use the full underwater tunnel method?
He Weiguo told the reporter of Science and Technology Daily that the Jiangmen-Maoming section of the Shenzhen-Mao (ming) high-speed rail was opened in 2018. The Jiangmen-Shenzhen section needs to cross the Pearl River Estuary Lion Ocean area. Due to the complex geographic environment factors, various economic and technological factors are considered. Factors, the road-rail dual-use bridge scheme, the single bridge scheme, and the tunnel scheme have their own characteristics, but which one to use has not been finalized.
According to the plan, the Shenmao High-speed Rail Road-Rail Bridge leads from Humen, Dongguan (opposite the Shajiao Fort), and enters Nansha, Guangzhou after crossing the Lion Ocean.
"The demonstration and selection of bridge and tunnel schemes took up to 10 years." He Weiguo said.
Most experts believe that the use of a road-rail bridge in the bridge plan can increase the economic benefits of crossing the river, but it will have a certain impact on urban planning, navigation and flood control.
In addition, during the construction period, the construction of the bridge piers will pollute the marine environment to a certain extent; in the later operation and maintenance stage, the bridge has poor resistance to the marine environment and severe weather, and the operation and maintenance costs are high.
After a long period of repeated scientific comparisons and selections, in August 2017, China Railway Group issued the first "Environmental Impact Assessment of the Shenzhen-Jiangmen Section of the Newly-built Railway Shenzhen-Maoming Railway", which determined the plan for the Shenzhen-Jiangmen High-speed Railway to cross the Pearl River Estuary. Use a bridge to change to a tunnel form.
Combining new and old methods to deal with complex operating environments
The Pearl River Mouth Tunnel is a controlled project of the deep river highway. The sea area is about 11.05 kilometers long. It is constructed by the "shield + mine" combined construction method. It is expected to be completed and opened to traffic in 2025.
“There are currently three sea-crossing railway tunnels under construction in the Greater Bay Area, namely the Zhanjiang Bay Subsea Tunnel of the Guangzhou-Zhanjiang High-speed Railway, the Shantou Bay Subsea Tunnel of the Shantou Railway and the Pearl River Mouth Tunnel of the Shenjiang Railway. The Pearl River Mouth Tunnel has the longest length and the largest buried depth. The technical difficulty is the highest." He Weiguo said that the hydrology and geology of this tunnel project are extremely complex, and the surrounding environment water is more corrosive.
Statistics show that the largest underwater railway shield tunnel built in China is the Shiziyang Tunnel of the Fo-Dongguan Intercity Railway, with a maximum buried depth of 64 meters and a maximum water pressure of 0.78 MPa.
"Such a large burial depth and higher water pressure have brought huge challenges to the project, but also contain higher risks. Once there is any deviation, the consequences will be unimaginable." He Weiguo said.
The Pearl River Mouth Tunnel needs to traverse a variety of composite strata such as silt, soft sand, and extremely hard bedrock bumps.
The mine section needs to cross multiple faults, and there is the possibility of water head penetration. Under the action of ultra-high water pressure, pre-reinforcement and support measures are also facing severe tests.
Not only that, the huge water pressure and complex geological environment also put forward higher requirements for shield machines and other equipment.
More than 3 years before the start of the project, the Tunnel Institute of China Railway Sixth Institute Group set up a special scientific research team to design a "mine + shield" combined construction method for passing through the tunnel.
The mining method, that is, the construction method of building tunnels by excavating underground tunnels, is a traditional construction method; while the shield method is a modern method of construction using large shield machines.
According to He Weiguo, the design plans have been analyzed and special plans designed for all the important and difficult points and key nodes in the project.
After many internal and external reviews, the design of the Pearl River Mouth Tunnel was considered to be “under pressure in the construction of extra-long submarine tunnels, shield tunnels in ultra-high water pressure sea areas, and joint waterproofing, and large-diameter shields in deep silt formations. A number of breakthroughs have been made in the technology of large-diameter shield reception and overall dismantling in tunnels of the mining and mining method".
The anti-water pressure and anti-corrosion design ensures the safety of tunnel driving
The on-site construction of the Pearl River Mouth Tunnel is divided into three areas: Humen, Nansha and Wanqingsha.
On July 2, 2020, the construction of the three work areas was in full swing.
The bases of the open-cut sections of the Humen and Wanqingsha work areas are located in a large area of deep silt layer, with high compressibility and low bearing capacity, and it is difficult to control the deformation of the foundation pit during the construction process.
"In order to ensure the safety of later high-speed rail operations, the design plan focused on settlement control and formulated high-precision index requirements." He Weiguo said.
According to the design plan, after the shield launch shaft is completed, two large-diameter shield machines will start from Humen, Dongguan and Wanqingsha, Guangzhou, respectively. The outer diameter of the shield segments is 12.9 meters, and the longest single-head tunneling of the shield is the longest. 3590 meters.
“This means that the shield machine must be replaced with worn tools in the poor geological section under high water pressure. This is tantamount to opening a window in the water, which poses a great safety risk.” He Weiguo said that this design requires Adopt more reliable normal pressure cutter head + air cushion mode tool change technology.
At the same time, for areas with extremely high risk of excavation, the shield machine will also be equipped with corresponding advanced geological detection methods to predict and analyze the front stratum to achieve one machine with multiple functions.
In addition to the underwater tool change problem, there is another bone that is more difficult to gnaw, which is how the shield machine will be received and dismantled in the sea in the future.
He Weiguo said that after a long-distance tunneling of the shield machine, the tunnel must be accurately expanded in the sea to achieve docking. This is like threading a needle in the sea. Not only should sufficient reserve be considered in the design, but also the dynamic control requirements during the tunneling process of the shield machine. It is also extremely demanding.
At the same time, the requirement of zero water seepage must be ensured during shield receiving.
"The design plan adopted the linkage measurement inside and outside the tunnel, so that the shield machine can find the direction for the tunneling construction; through geological survey and advanced geological forecast, the front geological conditions can be found out, and the tunneling parameters and attitude of the shield machine can be adjusted in time to ensure the shield machine Digging in the right direction; set up a large receiving cavern, and reserve a certain fault-tolerant receiving portal and water stop system at the receiving end to ensure that the shield machine can directly enter." He Weiguo said.
In addition to strict checks in the design plan, the use of new techniques, new materials, and new technologies has also become a highlight of the Pearl River Estuary Tunnel.
The ocean is the main source of chloride ions, and chloride ions will penetrate into the concrete to destroy the passive film on the surface of the steel bars, and eventually lead to corrosion of the steel bars, resulting in deterioration of the performance of the reinforced concrete structure, and affecting the service life of the structure.
In order to solve the corrosion problem caused by the ocean, the design adopts the immersion type silane impregnation material to strengthen the anti-corrosion effect of the concrete segment.
For ordinary sea-crossing shield tunnel projects, due to the low water pressure environment, the main consideration is to improve the concrete strength and impermeability standards to resist the erosion of chloride ions.
"The Pearl River Mouth Tunnel uses the small molecular structure of silane materials to firmly attach organosilicon molecules to the surface of the tube segment concrete and the capillary pores to form a dense protective layer." He Weiguo said, this also avoided. Conventional external anti-corrosion materials suffer from damage caused by the shield tail brush when the pipe segments are assembled.
The anti-corrosion measures of the structure itself and the anti-corrosion design of the structural joints are also very important. At the joints of the pipe segments, the design scheme uses two EPDM rubber elastic gaskets to enhance the anti-water pressure effect to ensure Security of the tunnel.
Experts in the industry believe that the Pearl River Estuary Tunnel project has "the "mine + shield" combined construction method is technically difficult, the shield tunneling in extreme formations is long, the construction risk is high in the silt environment, the disaster prevention evacuation organization is difficult to rescue, and the world-class high water pressure has no case." After the completion of the project, it will become a world-class super project, providing unique samples and valuable experience for the world's undersea tunnel engineering technology.