Shield machine： ＂iron earthworm＂ for digging subway tunnel

Writing/Photo courtesy of Liu Zhirong

You can’t always stay at home, but you have to travel, or commute to work, or go on business trips, sightseeing, and travel can’t be separated from subways, railways and highways. When you take subways, trains and highways, have you ever wondered: How are those tunnels dug through them?

Early tunnel excavation was manual plus blasting.

In the past, the way of tunnel excavation was manual excavation and blasting. For soil layer and gravel geology, manual excavation was used, and tools such as pickaxe and shovel were used to excavate the tunnel face. As far as rock geology is concerned, pickaxes and shovels can’t be dug, so it is necessary to use explosives to blast rocks at the tunnel face, and then transport the gravel out of the tunnel.

"Face" is a civil construction term, which refers to the excavation face that keeps advancing during tunnel construction.

As far as subway tunnels are concerned, in the past, the "open excavation and landfill method" technology was mostly adopted, that is, the street was destroyed first, and a ditch was dug by uncovering the cover, and then the tunnel wall was poured in the ditch. After the tunnel was built, it was buried and compacted with earthwork, and the street was repaired again. In the early days of Chinese mainland, the subway tunnel was excavated by "open excavation and landfill method", and later the "shield machine excavation method" was adopted. Shield machine was used to dig tunnels in the construction of Guangzhou’s first subway line (Line 1).

In the past hundred years, shield machines have gradually become popular in tunnel excavation. At present, shield machines are basically used in long tunnel excavation.

Britain used a shield machine to dig tunnels in 1825.

"Shield machine" is a name in China, generally known as "tunnel boring machine" (TBM) internationally, and "mole" in civil engineering jargon, because it digs holes underground like a mole.

In 1825, Britain used a shield machine when digging the Thames Tunnel. This shield machine was invented by Sir Mark issam Bald brunell, a French-born British engineer, and he applied for a patent for the shield machine. The Thames Tunnel is 396 meters long and passes through the Thames River in London.

In 1845, the international railway from France to Italy was opened. This railway ran across the Alps, and it was necessary to dig the Freres Tunnel at an altitude of 1,123 meters. This tunnel was 14 kilometers long, which was the longest tunnel in the world at that time. The tunnel ran through the Freres Peak at an altitude of 2,932 meters, that is to say, the maximum thickness of the rock above the tunnel was 1,809 meters.

In order to build this tunnel, Henry Joseph Mauss invented the "mountain cutting machine", which was manufactured in an arsenal near Turin, Italy in 1846. The "mountain cutter" looks like a locomotive, with more than 100 vibrating drill bits in front, which are used to smash the rocks on the face. "Mountain cutting machine" already has some functions of modern shield machine.

In 1853, ebenezer Talbot, an American, applied for the patent of shield machine, and the patent name was "Tunnel Boring Machine".

In 1870, john D bruton, an Englishman, developed and manufactured a shield machine with cutter head, which is very close to the shield machine widely used now.

In 1875, the French National Assembly passed a bill to build the English Channel Tunnel, and the British shield machine was selected for excavation. From June 1882 to March 1883, the shield machine excavated 1882 meters on the British side; Alexander Lavalle, a French contractor who built the Suez Canal, used a shield machine to dig 1,669 meters on the French side. Practice has proved that the efficiency of tunneling with shield machine is very high. However, the project was terminated in 1883 because France was later worried that Britain would use the English Channel Tunnel to attack France.

The success of shield machine in the English Channel Tunnel has inspired many civil engineers. In 1883, Britain used a shield machine to dig the tunnel under the Mersey River from Biegenheit to Liverpool, with a diameter of 2.1 meters and a length of 2 kilometers.

In the 20th century, people constantly improved the shield machine. Nowadays, all kinds of shield machines are constantly running in tunnel projects all over the world every day.

How does the shield machine dig the tunnel?

It is easier to understand the working principle of shield machine from its international common name "tunnel drilling rig": it is a big drilling rig that drills a hole in the ground or mountain.

Shield machine can be regarded as the largest construction machinery in civil engineering, with a large diameter of 178 meters and a length of more than 100 meters. The front of the shield machine is a huge cutter head with cutter head array. When driving, the cutter head rotates to cut the earth and stone on the tunnel face, and the muck flows into the muck bin from the gap of the cutter head, which is conveyed to the belt conveyor by the screw machine, and then transferred to another longer belt conveyor to transport it to the muck disposal site outside the tunnel, or transported to the tunnel by the track muck truck.

Readers with a little common sense should understand that if there is no thrust behind the cutter head, it is impossible to cut the earth and stone on the face. So, how to give the cutter head thrust?

This thrust comes from the hydraulic jack of the shield machine. If the tunnel is excavated on hard rock, two huge hydraulic jacks on the shield machine lean backward against the rock walls on both sides of the excavated tunnel and push the cutter head forward to cut the tunnel face.

If the tunnel is dug in the soft geological layer, the above method cannot be used, because the hydraulic jack will sink into the soft tunnel wall and cannot provide thrust. At this time, it is necessary to use another method to make several small hydraulic jacks push against the lining segment installed at the back at the same time to provide thrust for the cutter head horizontally forward, which is also a common shield method in subway tunnels at present.

The lining segment is the reinforced concrete precast member on the tunnel surface that we can see with our naked eyes in the tunnel. These segments are made in the ground reinforced concrete precast yard. After the segment is transported into the tunnel mouth, there is no need for manual intervention, and the transportation to installation is completely automatic.

The width of the lining segment should match the distance that the hydraulic jack advances each time, that is to say, the width of the segment is as many centimeters as the hydraulic jack advances each time. When the hydraulic jack is pushed to the end, the cutterhead stops rotating to cut the tunnel face, and segments are assembled around the tunnel in the shield section of the shield machine. The segment assembling machine on the shield machine is as dexterous as a human hand, movable, rotatable and retractable, which can assemble segments in place steadily and safely with accuracy of millimeter level.

When assembling a segment, several hydraulic jacks responsible for jacking the segment will contract, leaving a certain space for assembling the segment. After the segments are assembled in place, the hydraulic jack will stretch out and gently hold it, and so on until all segments in this circle are assembled in place. At this point, all the hydraulic jacks began to pressurize evenly, and together with the newly assembled segment, the cutter head was pushed forward by the reaction force to continue cutting the working face and move forward.

During the construction of the shield machine, the cutterhead cuts the tunnel face and assembles segments alternately, and the whole shield machine moves forward step by step under the push of the hydraulic jack until the tunnel is dug through. If the geology is soft, tens of meters can be excavated every day; If you encounter hard rocks, you can only dig dozens of centimeters every day.

The gap between the lining segment and the tunnel wall should be filled with mortar to reinforce the segment, maintain the stability of the whole lining and prevent water from leaking into the tunnel.

The series of complicated processes mentioned above are all completed automatically by the shield machine. There is a central control room in the shield machine, and all key parts of the shield machine have sensors, which transmit the collected data to the central control room. Operators can know the working situation of the shield machine through various data displayed on the display and intervene when necessary.

The cutter head on the cutter head should be maintained and replaced frequently. There is also a sensor on the cutter head. When the temperature of the cutter head rises to a certain value, it will send an alarm signal to the central control room.

How does the shield machine turn in the tunnel?

After reading this, some readers may ask, can the shield machine turn in the tunnel?

Of course you can. Most urban subway tunnels are excavated under the streets, and the tunnels are designed along the streets, with constant turns and "jumping up and down" according to the underground conditions. Highway and railway tunnels should also be selected according to landform, geology and hydrology, which also requires the shield machine to turn 360 degrees.

The turning of shield machine is realized by changing the cutting direction of cutter head with lining segment. Where a turn is needed, the width of each ring of lining segment is different. If turning to the left, the width of the right segment is designed to be wider than that of the left segment according to the turning radius. In this way, the hydraulic jack on the right will tilt the cutter head to the left to realize the left turning. If turning right, the width of the left segment is designed to be wider than that of the right segment according to the turning radius, so that the hydraulic jack on the left will tilt the cutter head to the right and realize the right turn. The steering principle of "jumping up and down" is the same as above.

At present, the largest shield machine in the world has a diameter of 17.5 meters.　　

On September 15th, 2006, the Niagara Tunnel, which was used for water diversion irrigation in Canada, was drilled. The tunnel was 10.2km long and passed 140m underground in Niagara Falls, Ontario, Canada. The shield machine used was "Big Becky" made by Robbins Company of the United States, with a diameter of 14.4m. The cutter head was driven by 15 motors with a power of 4.7 million watts (6,375 horsepower), which was the largest shield machine in the world at that time. The reason why tunnels should be dug in such a deep underground is to prevent shield construction from affecting ground buildings.

In 2011, the 5030-meter-long Spavo Tunnel near Florence, Italy was drilled. The rock shield machine used was Martina produced by Herrick Company of Germany. The shield has a diameter of 15.62 meters, a length of 130 meters, a weight of 4,500 tons, a cross-section of 192 square meters, a total power of 18 megawatts and a thrust of 40,000 tons. Herrick also designed a shield machine with a diameter of 19.25 meters, but it has not yet been built.

Bertha, a shield machine made by Hitachi Shipbuilding Co., Ltd. in Japan in 2012, has a diameter of 17.5 meters, a length of 110 meters and a weight of 7,000 tons. The cutter head has 260 cutter heads. This shield machine was ordered by the Washington State Transportation Bureau of the United States and used to replace the tunnel excavation of Alaska Viaduct on Highway 99 (the upper and lower lanes of the tunnel), which is by far the largest shield machine in the world.

Advantages and disadvantages of shield tunneling

There are many advantages in tunneling with shield machine. It is not affected by ground factors and can be constructed all day; It also does not destroy the ground buildings, cultural relics and natural environment, and belongs to ecological and environmental protection construction; Because the excavation, muck discharge and lining segment are all automated, a lot of labor is saved and the accident risk is reduced.

The main disadvantage of tunneling with shield machine is that the initial investment cost is too high. Most shield machines are customized according to tunnel design drawings, with long construction preparation period, high maintenance cost, and low economic benefits of tunneling short tunnels. Generally speaking, the tunnel length is more than 1 km, which has economic benefits.

Finally, the shield machine can not only dig circular tunnels, but also be designed to dig tunnels with arbitrary cross sections, such as rectangular tunnels and elliptical tunnels. People also use shield machines to dig water pipelines, sewage and drainage pipelines, and even power pipelines.