The term tunnel boring is a familiar one, that deals with creating a tube-like passage through mountains or underneath the earth’s surface to avail transportation, drainage facility, and as a part of mining. Traditionally, tunnel boring was done manually using explosives and other methodologies to blast rocks. It was a risky and time-consuming process that affected the environment to a greater extent. To overcome the drawbacks of conventional tunnel boring processes, tunnel boring machines(TBM’s) were introduced in the 19th century by Sir Marc Isambard Brunel to minimize the effort on digging and blasting tunnels. In this article, we can discuss some important aspects of the tunnel boring machine that utilizes hydraulic power for operation.

With a tunnel boring machine, it is possible to create a circular cross-section in the desired diameter by digging through soil and rock surfaces. The tunnel boring machines have many benefits over conventional methods. Some advantages are listed below.

• Improves the safety of the worksite
• Eliminate noise and other pollution caused by the blast of explosives.
• Smooth walls are created with TBM and it reduces the need for ventilation.
• Minimize the cost for tunnel lining
• Help to accomplish the tunnel boring in a short timespan
• Can be operated on a minimum support
• Reduces the probability of rock damage

Along with the merits, TBM has some drawbacks like the cost for construction, difficulty in transportation, fixed circular geometry, and limited flexibility. 

IMPORTANT PARTS OF TUNNEL BORING MACHINES

TBM is a large mechanical drilling structure that can be divided into 3 different parts like the cutter- head (front), the tunnel shield (middle), and the trailing gear (rear). The cutter head is located in the front and it has direct contact with the surface that needs to be drilled. The continuously rotating cutter head contains cutting tools to break rocks and to drill the soil. Behind the cutter head, TBM has a tunnel shield that develops tunnel rings to protect both the machine and workers. The trailing gear is the rear part of TBM that can manage the excavated materials and includes a control mechanism for operators. A short description of components in the tunnel boring machine is provided below.

Cutter Head: Cutting blades and cutting rolls are mounted on the cutter heads and it faces the surface that needs to be tunnelled. It rotates 360 degrees and is equipped with nozzles to inject water to smoothen the hard earth surfaces.  

Grippers: Grippers are supporting arms that use the hydraulic press to provide radial thrust to tunnel walls. Every TBM will contain grippers and it helps the cutter head to move forward.

Conveyors: The conveyors in TBM carry away the excavated soil or rock from the cutter head using different mechanical transportation like auger conveyors, conveyor belts, and mine cars. 

Erector: The erector plays its role during tunnel ring creation. This rotating system picks up the curved pre-build concrete segments one by one and assembles them properly to create tunnel rings that protect the workers and TBM.

Press Frames: The hydraulic piston-cylinder arrangement that allows the forward movement of the cutter head is called as press frames. The cylinder extends and retracts using the pressurized hydraulic fluid.

WORKING OF TUNNEL BORING MACHINE

The important functions performed by the tunnel boring machine are tunnelling and ring building. When the machine excavates soil from the tunnel surface in the desired length, it is necessary to set up a tunnel ring for safety. This tunnelling and ring building phase continues alternatively until the end of the tunnel building.

The first process is tunnelling. Here, the cutter head is the front part of Tunnel Boring Machine(TBM) and blades, cutting rolls, nozzles, etc… are attached to these rotating components to break rocks and other materials on the tunnel surface. For the forward movement of the cutter head, the system uses a set of hydraulic cylinders that are supported by the finished part of the tunnel and these cylinders extend to apply a forward force of up to 400bar  on the cutter head. The cutting wheel is driven by a set of hydraulic motors using a gear rim. This excavated soil is carried towards the rear end of TBM with the help of belt conveyors. The screw conveyor that is driven by hydraulic motors takes the material from the excavation chamber to the belt conveyor.  

The next phase is ring building and it occurs in the shield area under atmospheric conditions. The TBM has pre-build concrete curved segments called the lining segments. These lining segments are transported into the tunnel in mine cars and then they are lifted and positioned accordingly using the hydraulically controlled crane arm called erector. To assemble the segments precisely, the hydraulic cylinders retract and develop a gap for erector to place the lining segment next to the previously positioned segment. After placing the new segment, the hydraulic cylinder extends to secure the position and to bolt it into the previously created tunnel ring.

The system also contains hydraulic power units(HPU’s) that are powered by electric motors. The HPU in Tunnel boring machine has a reservoir that stores compatible hydraulic fluid and three hydraulic pumps that serve different components of TBM. The first pump assists the cutter head for its forward movement, the second pump will power the thrust, steering and anti-roll fins, and the third pump powers the conveyors and auxiliary circuit.