Frequently Asked Question | E-Berk Tunnel Boring Machine

History of Tunnel Boring Machines?

The first successful tunnelling shield which is commonly regarded as the forerunner of the tunnel boring machine was developed by Sir Marc Isambard Brunel to excavate the Rotherhithe tunnel under the Thames in 1825. However, this was only the invention of the shield concept and did not involve the construction of a complete tunnel boring machine, the digging still having to be accomplished by the then standard excavation methods using miners to dig under the shield and behind them bricklayers built the lining. Although the concept was successful eventually it was not at all an easy project. The tunnel suffered five floods in all. It is also noteworthy that Marc Brunel’s son who was the site engineer went on to become what is generally thought of as Britain’s greatest engineer, Isambard Kingdom Brunel.

Diagram of tunnelling shield used to construct the Thames tunnel Improvements on this concept were used to build all of the early deep railway tunnels under London in the early 20 th century and lead to the name ‘ tube ‘ which is the nickname all Londoners call their metropolitan railway and give tunnels made by this method their characteristic round shape..

In other countries tunnel boring machines were being designed to tunnel through rock. The very first actual boring machine ever reported to have been built is thought to be Henri-Joseph Maus' Mountain Slicer designed in 1845 dig the Fréjus Rail Tunnel between France and Italy through the Alps, Maus had it built in 1846 in an arms factory near Turin. It basically consisted of more than 100 percussion drills mounted in the front of a locomotive-sized machine, mechanically power-driven from the entrance of the tunnel however it was not used, and the tunnel was finally built using conventional methods.

In the United States, the first boring machine to have been built was used in 1853 during the construction of the Hoosac Tunnel. Made of cast iron, it was known as Wilson's Patented Stone-Cutting Machine, after its inventor Charles Wilson. It drilled 10 feet into the rock before breaking down and the tunnel had to be completed many years later, using less ambitious methods.
what is tbm
We need to move on nearly 100 years when James S. Robbins built a machine to dig through what was the most difficult shale to excavate at that time, the Pierre Shale. Robbins built a machine that was able to cut 160 feet in 24 hours in the shale, which was ten times faster than any other digging speed at that time.
The modern breakthrough that made tunnel boring machines efficient and reliable was the invention of the rotating head, conceptually based on the same principle as the percussion drill head of the Mountain Slicer of Henri-Joseph Maus, but improving its efficiency by reducing the number of grinding elements while making them to spin as a whole against the soil front. Initially, Robbins' tunnel boring machine used strong spikes rotating in a circular motion to dig out of the excavation front, but he quickly discovered that these spikes, no matter how strong they were, had to be changed frequently as they broke or tore off. By replacing these grinding spikes with longer lasting cutting wheels this problem was significantly reduced. Since then, all successful modern tunnel boring machines use rotating grinding heads with cutting wheels for boring through rock.
Below is an example of a tunnel boring machines which is equipped with a back hoe Whilst the cutting head has been a breakthrough on soft material the shield with a backhoe is still a cost efficient and well utilised solution even today.

What is Tunnel Boring Machines

A tunnel boring machine (TBM) typically consists of one or two shields (large metal cylinders) and trailing support mechanisms. At the front end of the shield is a rotating cutting wheel. Behind the cutting wheel is a chamber. The chamber may be under pressure (closed machine) of open to the external pressure (open machine) Behind the chamber there is a set of hydraulic jacks supported by the finished part of the tunnel which push the TBM forward. The rear section of the TBM is braced against the tunnel walls and used to push the TBM head forward. At maximum extension the TBM head is then braced against the tunnel walls and the TBM rear is dragged forward.
Behind the shield, inside the finished part of the tunnel, several support mechanisms which are part of the TBM are located: soil/rock removal, slurry pipelines if applicable, control rooms, and rails for transport of the precast segments. The cutting wheel will typically rotate at 1 to 10 r pm (depending on size and stratum), cutting the rock face into chips or excavating soil (usually called muck by tunnelers). Depending on the type of TBM, the muck will fall onto a conveyor belt system or into skips and be carried out of the tunnel, or be mixed with slurry and pumped back to the tunnel entrance. Depending on rock strata and tunnel requirements, the tunnel may be cased, lined, or left unlined. This may be done by bringing in precast concrete sections that are jacked into place as the TBM moves forward, by assembling concrete forms, or in some hard rock strata, leaving the tunnel unlined and relying on the surrounding rock to handle and distribute the load. While the use of a TBM relieves the need for large numbers of workers at increased pressure, if the pressure at the tunnel face is greater than behind the chamber a caisson system is sometimes formed at the cutting head this allows workers to go to the front of the TBM for inspection, maintenance and repair if this needs to be done under pressure the workers need to be medically cleared for work under pressure like divers underwater and to be trained in the operation of the locks.

Type of TBM Machines

Slurry Machine,

This is used for soils usually of varying hardness. The excavated soil is mixed with slurry to create positive face pressure required to sustain the excavation. This is known as a closed machine. The system for the removal of the soil involves pumping the soil mixed with slurry to plant located outside the tunnel that separates the slurry from the muck allowing its recirculation. See sketch below.

Earth pressure Balance machine,

This is a closed machine and is used usually for softer fairly cohesive soils. In this case the positive face pressure is created by the excavated ground that is kept under pressure in the chamber by controlled removal through the rotation of the screw conveyor. The muck is thereafter removed by a conveyor belt and/or skips.

Rock Machine,

This is used for excavating rock. The rock is crushed by the cutters (often discs) and removed on conveyors and/or skips. Cutters are specifically designed to resist hard abrasive material.

Maximum Diameter for Slurry and EPB

The biggest shields ever built to date are the 2 Shanghai slurry TBMs of 15.43m diameter. They were designed for a road tunnel. The biggest EPB shield is one that bored the M30 road tunnel of Madrid. Its diameter was 15.20m. It was fitted with a double cutter-head aligned on the same axis. The inner one was of 7m diameter. These cutter-heads could operate independently of each other

a) The cutter-head. Indeed the bigger the size the stronger structure of the head must be. As a result there is less room for openings and therefore more difficulties to let the bored materials located closed to the axis of the head,
b) Movement from the cutting chamber. This is why the double head system was helpful to solve this problem. The rheology of the muck in the cutting chamber in an EPB system. If the muck is too dry it is difficult to remove it from the cutting chamber. If it is too wet the pressure at the entrance of the screw conveyor (hydrostatic behaviour but with a high density) is too high and the ability to keep a proper pressure gradient along the screw not achievable. This means there is no pressure drop through the screw conveyor therefore a very high risk of collapse of the front face.

Maximum Water Head

Beyond 3.5 bars (35m water head) at the crown of the shield the pressure is an issue in respect of the maintenance in the cutting chamber. Indeed with a pressure above 4.5 bars the working hyperbaric conditions are not easy with air: short working period, narcotic effect of the nitrogen, toxicity of the oxygen. Saturation diving is possible (Westerschelde tunnels) by breathing heliox (mix of helium and oxygen) or trimix (mix of nitrogen, oxygen and helium) but it needs special equipment and professional divers.

Use of TBM in squeezing conditions

This is a main issue. In that case the cutter-head should be equipped in its periphery with an over-cutting tool to let the body of the shield to move. This one has not a perfect cylinder shape but a cut conic shape to be able to escape from the squeezing effect behind the cutter-head.

Maximum speed of excavation reached

In a closed mode (under pressure) the speed is lower than in an open mode (no pressure). With an EPB the contractor can afford to bore under open mode subject to his assessment of the stability of the front face. Underwriters should check that contractor’s assessments of the stability of the front face are consistent with the stability assessments of the designer team and their geologists The speed of the shield depends on how it is driven (thrust, cutter-head rpm, ...) but also by the geotechnical parameters of the soil or rock (compressive strength, degree of fracturing,..), its abrasivity and how its ability to keep stable the pressure in the cutting chamber.
In a closed mode the speed is in the range of 0 to 8cm/min