Tunnel Boring Machine
The tunnel boring machine is used to bore tunnels with a circular cross-section through a variety of rock strata and soil and also be designed to excavate non-circular tunnels.
The tunnel boring machine is a good choice for urban tunnels or high cover and best option for rock tunnels with a length over 1.5 km and these machines are widely used around the world. The tunnel boring machine will progressively line each tunnel with curved concrete segments to construct the tunnels.
To bore through anything from hard rock to sand they can be designed and also be designed to excavate non-circular tunnels such as horseshoe, u-shaped, rectangular or square tunnel.
To drilling and blasting methods in rock and conventional (hand mining) in soil, tunnel boring machines are used and have the advantages of producing a smooth tunnel wall and limiting the disturbance to the surrounding ground.
Parts of Tunnel Boring Machine:
There are the following components in the tunnel boring machine as given below;
- Cutter head with cutting discs and tools.
- Muck buckets to carry and dispose of excavated muck.
- Power supply Systems, cutter head rotation, and thrust.
- Bracing system for the tunnel boring machine during mining.
- Equipment for ground support installation.
- Shielding to protect workers and steeling system.
Working of Tunnel Boring Machine:
This machine contains dozens of steel blades that chip away at the soil ahead as it rotates and the cutter-head sits at the very front of the TBM (Tunnel Boring Machine) and uses disc cutters. To allow the machine to move forward, these head-cutters do most of the hard work by spinning and digging away at the earth, and to keep the TBM progressing at a constant rate, the blades (disc cutters) are replaced as needed.
Now the tunnel shield (roof shield) and concrete panels at the mid-section comes and as the outer layer of the tunnel, the concrete panels get installed right behind the shield. The panels are picked up and set in place by a rotating vacuum powered lift as the cutter-head makes progress.
The machines trailing gear comprised in some cases of over 300 feet of gear that supports the TBM and trailing gear includes a conveyor belt that removes all the soil excavated by the cutter-head from the tunnel. As the operating crew needs to keep the machine moving forward so, the trailing gear also holds supplies.
Types of Tunnel Boring Machine:
There are the following types of tunnel boring machine as given below;
- Hard Rock TBMs
- Soft Ground TBMs
- Micro-Tunnel Shield Method
Hard Rock TBMs:
Hard rock TBMs excavate rock with disc cutters mounted in the cutter head and in the rock, the disc cutters create compressive stress fractures. As the disc cutter compresses stress fractures the rocks (muck) get chipped away.
Through the cutter head openings, the excavated rocks are transferred to the belt conveyor, and then to get them removed from the tunnel, these rocks are run through multiple conveyors.
Shielded hard rock TBMs can be used in the fractured rock which erects concrete segments to support unstable tunnel walls and Double Shield TBMs have two modes such as they grip the tunnel walls to advance in the stable ground and the thrust is shifted to thrust cylinders that push against the tunnel segments behind the machine in the unstable, fractured ground.
Soft Ground TBMs:
There are three types of soft ground TBMs as given below;
- Slurry Shield
- Earth Pressure Balance
- Open Face Type
When the ground conditions are granular with very high water pressure slurry shield TBMs are used and by filling the cutter head with pressurized slurry, hydrostatic pressure is applied to the excavation.
By mixing with the excavated material, the slurry also acts as a transport medium before being pumped out of the cutter-head back to a slurry separation plant. Slurry separation plants remove particles of soil from the slurry and are multi-stage filtration systems.
For silts and clays, slurry TBMs are not suitable because the particle sizes of the spoil are less than that of the bentonite clay from which the slurry is made so in this case, the slurry is separated into water and a clay cake which may be polluted is pressed from the water and recycled.
Earth Pressure Balance:
In soft ground with less than 7 bar of pressure, earth pressure balance machines are used and the cutter head is also a combination of tungsten carbide cutting bits, carbide disc cutters, and drag picks.
To balance the pressure at the tunnel face, it uses the excavated material and by controlling the rate of extraction of spoil, the pressure is maintained in the cutter head. To increase the stability of the ground, additives like bentonite, polymers, and foam can be injected ahead of the face and additives can also be injected in the cutter head/extraction screw to ensure that the soil remains sufficiently cohesive to form a plug.
Open Face Type:
For use in rock types with a strength of up to 10MPa and with low water inflows, this type is suitable because it relies on the fact that the ground being excavated will stand up with no support for a brief period of time.
Within the 150mm of the edge of the shield, the cutter head excavates the face and in sync to the circular shape, the shield is put forward with the cutter getting rid of the remaining ground.
To provide the ground support, precast concrete is used and until it gets tight against the circular cut of the leftover ground, the final segment expands the ring.
Micro-Tunnel Shield Method:
To construct small tunnels and diminish in size of the general tunneling shield, the micro tunnel shield method is a digging technique that is used and the micro-tunnel boring machine is quite similar to the general tunneling shield.
Advantages of Tunnel Boring Machine:
There are the following advantages of this machine such as;
- It has a high excavation rate which is calculated on the basis of shift time and the total distance mined or has high advance rates.
- It can work continuously without any significant delay.
- As compared to the other drilling machines that decrease the over break value, TBM causes less rock damage.
- To operate, TBM doesn’t need huge support and on minimal system requirements, it can operate.
- TBM causes less damage to the site as well as to the people working on-site and is safer than other machines.