A tunnel boring machine (TBM), also known as a "mole", is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They may also be used for microtunneling. They can bore through anything from hard rock to sand. Tunnel diameters can range from a metre (done with micro-TBMs) to 19.25 metres to date. Tunnels of less than a metre or so in diameter are typically done using trenchless construction methods or horizontal directional drilling rather than TBMs.
Hard Rock Tunnel Boring Machine
In hard rock, either shielded or open-type TBMs can be used. All types of hard rock TBMs excavate rock using disc cutters mounted in the cutter head. The disc cutters create compressive stress fractures in the rock, causing it to chip away from the rock in front of the machine, called the tunnel face. The excavated rock, known as muck, is transferred through openings in the cutter head to a belt conveyor, where it runs through the machine to a system of conveyors or muck cars for removal from the tunnel.
Soft Ground Tunnel Boring Machine
In soft ground, there are three main types of TBMs: Earth Pressure Balance Machines (EPB), Slurry Shield (SS) and open-face type. Both types of closed machines operate like Single Shield TBMs, using thrust cylinders to advance forward by pushing off against concrete segments. Earth Pressure Balance Machines are used in soft ground with less than 7 bar of pressure. The cutter head does not use disc cutters only, but instead a combination of tungsten carbide cutting bits, carbide disc cutters, drag picks and/or hard rock disc cutters. The EPB gets its name because it is uses the excavated material to balance the pressure at the tunnel face. Pressure is maintained in the cutterhead by controlling the rate of extraction of spoil through the Archimedes screw and the advance rate. Additives such as bentonite, polymers and foam can be injected ahead of the face to increase the stability of the ground. Additives can also be injected in the cutterhead/extraction screw to ensure that the spoil remains sufficiently cohesive to form a plug in the Archimedes screw to maintain pressure in the cutterhead and restrict water flowing through.
Source : Wikepedia
Hard Rock Tunnel Boring Machine
In hard rock, either shielded or open-type TBMs can be used. All types of hard rock TBMs excavate rock using disc cutters mounted in the cutter head. The disc cutters create compressive stress fractures in the rock, causing it to chip away from the rock in front of the machine, called the tunnel face. The excavated rock, known as muck, is transferred through openings in the cutter head to a belt conveyor, where it runs through the machine to a system of conveyors or muck cars for removal from the tunnel.
Soft Ground Tunnel Boring Machine
In soft ground, there are three main types of TBMs: Earth Pressure Balance Machines (EPB), Slurry Shield (SS) and open-face type. Both types of closed machines operate like Single Shield TBMs, using thrust cylinders to advance forward by pushing off against concrete segments. Earth Pressure Balance Machines are used in soft ground with less than 7 bar of pressure. The cutter head does not use disc cutters only, but instead a combination of tungsten carbide cutting bits, carbide disc cutters, drag picks and/or hard rock disc cutters. The EPB gets its name because it is uses the excavated material to balance the pressure at the tunnel face. Pressure is maintained in the cutterhead by controlling the rate of extraction of spoil through the Archimedes screw and the advance rate. Additives such as bentonite, polymers and foam can be injected ahead of the face to increase the stability of the ground. Additives can also be injected in the cutterhead/extraction screw to ensure that the spoil remains sufficiently cohesive to form a plug in the Archimedes screw to maintain pressure in the cutterhead and restrict water flowing through.
Source : Wikepedia
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