tunnel boring machine

Tunnel boring machines are used to excavate tunnels with a circular cross section through a variety of geologies. They can be used to bore through hard rock or sand or almost anything in between. Tunnel diameters can range from a meter (done with micro-TBMs) to more than 14 meters. Tunnels of less than a meter or so in diameter are more typically done by horizontal directional drilling than TBMs. Tunnel boring machines are used as an alternative to drilling and blasting (D&B) methods. A TBM has the advantages of not disturbing surrounding soil and producing a smooth tunnel wall. This significantly reduces the cost of lining the tunnel, and makes them suitable to use in built-up areas. The key disadvantage is cost. TBMs are expensive to construct, difficult to transport and require significant infrastructure.

What they are

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 a rotating cutting wheel is located. Behind the cutting wheel there is a chamber where, depending on the type of the TBM, the excavated soil is either mixed with slurry (so-called slurry TBM) or left as-is. The choice for a certain type of TBM depends on the soil conditions. Systems for removal of the soil (or the soil mixed with slurry) are also present. Behind the chamber there is a set of hydraulic jacks supported by the finished part of the tunnel which are used to push the TBM forward. The action here is caterpillar-like. The rear section of the TBM is braced against the tunnel walls and used to push the TBM head forward. At maximum extention 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 can be found: dirt removal, slurry pipelines if applicable, control rooms, rails for transport of the precast segments, etc. The cutting wheel will typically rotate at 4 to 10 rpm (depending on size and geology), cutting the rock face into chips or excavating soil (muck). Depending on the type of TBM, the muck will fall onto a conveyor belt system and to carried out of the tunnel, or be mixed with slurry and pumped back to the tunnel entrance. Depending on geology and tunnel requirements, the tunnel may be cased or lined. This may be done by bringing in pre-cast sections of concrete that are jacked into place as the TBM moves forward, or by assembling concrete forms, and pumping concrete in. In hard rock geologies the tunnel may be left unlined, relying on the surrounding rock to handle and distribute the load.

Shields

Modern TBMs typically have an integrated shield. The choice of a single or double shielded TBM depends on the type of geology and the excavation speed required. Double shielded TBMs are normally used in unstable geology, or where a high rate of advancement is required. Single shielded TBMs, which are less expensive, are more suitable to hard rock geology.

Urban tunneling and near surface tunneling

Urban tunneling has the special challenge of requiring that the ground surface be undisturbed. This means that ground subsidence must be avoided. The normal method is doing this is to maintain the soil pressures during and after the tunnel construction. There is some difficulty in doing this, particularly in varied geologies (imagine tunnel boring through a region where the upper portion of the tunnel face is wet sand and the lower portion is hard rock). TBMs with positive face control are used in such situations. There are three common types: Earth pressure balance (EPB), Bentonite slurry (BS), and compressed air (CA). The Compressed air method is the oldest, but is falling out of favour due to the difficult working conditions it imposes. Both types (EPB and BS) are clearly preferred over open face methods in urban environments as they offer far superior ground control.