How to solve the alignment problem when a tunnel is being constructed at both ends simultaneously?

Produced by | Science Popularization China

Author | Jing Bo (geological engineer)

Producer | China Science Expo

On August 17, the Jiuwan Dashan No. 4 tunnel of the Guinan High-speed Railway in Huanjiang Maonan Autonomous County, Guangxi, was successfully opened. So far, all 108 tunnels along the Guinan High-speed Railway have been opened. The Guinan High-speed Railway under construction is an important part of the Baotou-Haikou channel, the main channel of my country's "eight vertical and eight horizontal" high-speed railway. It has a designed speed of 350 kilometers per hour and is expected to be put into operation by the end of 2023.

Guinan High-speed Railway Bridge and Tunnel from a drone camera | Image source: Xinhua News Agency

With the construction of transportation projects such as China's high-speed railways and China's expressways, China has become the country with the most and most complex tunnel and bridge projects in the world, with more than 20,000 tunnels and more than 1 million bridges built, including many ultra-long tunnels over 3 kilometers in length. So how do these tunnels, which are often tens of kilometers long, achieve accurate penetration?

Measurement: the key to whether a tunnel can be completed as expected

The final successful completion of a tunnel is the result of the concerted efforts of all departments of the entire tunnel project, including exploration, design, construction, supervision and acceptance. The surveying department is responsible for controlling the position and shape accuracy of the tunnel project. Whether the tunnel can be excavated according to the expected position, measurement is undoubtedly the core. Generally speaking, tunnel measurement is divided into two aspects: plane and elevation to ensure the posture of the tunnel.

The surveying work starts from the entire project preparation stage. The surveying department will provide the road design department with the surrounding terrain conditions of the corresponding route. This will ultimately determine whether to build a tunnel and the location of the tunnel opening. When the tunnel design stage is completed and the construction stage begins, the surveying is carried out before the earthwork is moved.

The construction team will first set up a survey control network in the work area, which plays the role of connecting the national surveying and mapping control network and the surveying and mapping control points of the work area, and determines the accuracy of the measurement results within the work area based on the structural strength of the construction control network. Simply put, before the formal construction, some special points need to be set up around the work area, and their distribution must not only meet the needs of rapid measurement in the work area, but also be affected as little as possible by the construction. These control points are connected to the national benchmark points to measure their corresponding positions, and all subsequent measurement work starts (starts) and ends (checks) based on these special points.

High-precision measurement requires advanced surveying and mapping tools

After the construction control network is set up, the corresponding construction layout is to be carried out. Layout is the same as the marking of carpentry lines or home decoration, which is to guide the construction team to carry out construction. In the process of tunnel excavation, it is necessary to indicate the direction of excavation and mark the center line and waist line of the tunnel, which defines the boundary of tunnel construction, that is, the so-called "pointing where there is a hit". However, due to the special environment of the tunnel, electromagnetic waves are difficult to penetrate the thick rock wall, so the most commonly used modern measurement GPS measurement methods such as RTK (real-time dynamic differential technology) cannot be used. Generally, photoelectric measurement equipment such as total stations are used to complete plane measurement and leveling measurement.

Total station used in tunnels | Image source: Author

The total station is designed based on the principle that light travels in a straight line. In actual work, measurement errors may be caused by dust in the tunnel. At the same time, since the instrument operation must be completed by the staff, various errors cannot be avoided in theory. However, through multiple measurements, adjustment calculations and reasonable layout of the traverse network or triangulation network, coupled with the computing power and highly automated optical equipment of the new total station, adjustment calculations can be made quickly. Modern total stations replace the optical scale with a photoelectric scanning scale, and replace the manual optical micrometer readings with automatic recording and display readings, so that the errors generated are ultimately very small, reaching an error of only a few centimeters or millimeters per kilometer.

The TBM (Tunnel Boring Machine), which is increasingly used in tunnel construction and has a higher efficiency, is equipped with laser target correction to ensure that the engineering equipment can proceed as designed.

What to do about accumulated errors in super-long tunnels?

So will the construction of super-long tunnels lead to the accumulation of errors as the number of kilometers increases? In theory, this situation will indeed occur, but when building super-long tunnels, as the length increases, the engineering difficulty and construction cost increase much faster than the rate of controlling the accumulation of measurement errors. Therefore, the method of long tunnels and short tunnels is generally chosen for construction - that is, the method of building horizontal tunnels, inclined shafts and vertical shafts in the tunnel to break down the long tunnel into sections of short tunnels. Although such a construction plan will increase the construction cost, it also has the advantages of shortening the tunnel construction period and improving ventilation and drainage conditions during construction. The vertical shafts and inclined shafts excavated during the construction process can also serve as ventilation shafts, fire passages, maintenance and emergency passages after the tunnel is completed, thereby improving the safety factor of the entire tunnel.

For example, the world's longest plateau tunnel that has been completed in my country, the Qinghai-Tibet Railway Xining to Golmud section and Guanjiao Tunnel, is 32.690 kilometers long (the tunnel entrance elevation is 3380.75 meters, and the exit elevation is 3324.05 meters). Due to the harsh construction conditions, in order to adapt to the plateau climate, the inclined shaft split air duct technology was adopted, that is, 10 inclined shafts assisted the main tunnel construction, which greatly increased the air supply and met the needs of high-altitude, long-distance and multi-working face construction.

Schematic diagram of Guanjiao Tunnel line construction | Image source: Author

Moreover, during the tunnel excavation process, the completed sections are also subject to supervision and measurement verification. Once a deviation from the designed route is found, it will be remedied immediately, so that the tunnel is finally completed as designed, with a much higher accuracy than the index fingers of both hands together. Therefore, in the modern tunnel construction process, there is no news about tunnel deviation.

In the past, tunnel construction would not match the two ends.

Of course, it would be a different story if there were no corresponding measurement engineering. As an applied mathematical technology, measurement technology has developed very rapidly with the progress of computational science and optical and electromagnetic technology, and the technology is updated almost every 10 years.

At the beginning of the last century, humans were still using surveying tools such as theodolites that had been developed for centuries, and the surveying efficiency and accuracy were quite low. At that time, in addition to the immaturity of tunneling technology, it was also important for the control of measurement accuracy to meet the requirements. Therefore, in the early line design, it would rather take a detour than design a tunnel. Therefore, in the first half of the last century, the development of railway lines and winding mountain roads were extremely active. Moreover, since these two forms of mountain crossing are mostly built in steep mountains and deep valleys, the surrounding areas are beautiful and majestic, and often become an important landscape. However, with the development of bridge and tunnel technology, this form of transportation is becoming less and less, and is gradually disappearing.

Scenery of the Lewu section of the Chengdu-Kunming Railway | Image source: Author

In addition, when tunnel construction is unavoidable, open cut and covered cut methods, or simply opening a skylight, etc., can also be used. Among them, the most famous is the wall-mounted highway. Although the name contains highway, it is actually a kind of tunnel. Although these methods will use more soil and stone and increase the construction cost, their correction is relatively simple and easy to operate, and very simple and basic measurement tools can meet the measurement requirements.

Although choosing not to build a tunnel can both achieve direct surface observation and reduce the difficulty of construction to a certain extent, it has great limitations on the project itself. In addition, designing tunnels using only straight lines will also reduce the difficulty of penetration without good measurement capabilities, and the error tolerance rate is relatively large. In the past, single-side drilling was often used during tunnel construction, which extended the construction period but did not solve the problem of tunnel docking. The above methods have played an important role in early road construction.

The cliff-hanging road in Jidi was opened by hand, one knife and one axe at a time | Image source: author

China has challenged a series of impossible things in infrastructure construction. Chinese engineers have been researching and breaking through to solve various difficult problems, which fully demonstrates China's strong infrastructure strength and perseverance.

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