Tunnel boring machines (TBMs) and microtunnel boring machines (MTBMs) are used to construct underground tunnels and passages with circular cross-sections. They both use a drilling mechanism, known as a cutting head, to remove material and advance the equipment forward.
However, both these pieces of equipment possess unique characteristics that make them better suited to specific construction requirements.
As their names suggest, the primary and most obvious difference between the two is their size. TBMs are capable of boring tunnels up to 60ft (18m) in diameter. This attribute makes them ideal for constructing vehicular passages, such as underground tunnels and subway systems. Due to their sheer size, TBMs also possess more raw power, allowing them to drill through difficult strata such as hard rock.
MTBMs, on the other hand, are significantly smaller in size in comparison to their TBM counterparts. These machines are typically used to construct microtunnels, i.e., tunnels with diameters ranging from 2ft (0.6m) to 5ft (1.5m). As such, MTBMs are generally usually used for installing underground water and wastewater pipeline systems, as well as utility conduits.
Another major difference between these two types of drilling equipment is their mode of operation. As tunnel construction moves forward, the cutting head of the TBM loosens the surrounding soil, subsequently transporting the cuttings to the entrance pit using a system of screw and belt conveyors.
Once the cutting head has advanced to an appropriate distance, precast concrete liner segments are transported into the tunnel using mine service vehicles and installed individually by the TBMs ring erector system. Upon successful installation of the liner segments, the machine’s hydraulic jack system pushes the cutting head forward, and the lining process is repeated.
During microtunneling, the cutting head grinds the soil into smaller particles and mixes them with a drilling fluid. The slurry mixture is then transported to a separation plant at the surface, which recycles and reinjects the drilling fluid via a system of discharge pumps.
During this process, individual sections of pipe or culvert are inserted into the entrance pit and pushed forward by the machine’s jacking system, while the cutting head simultaneously loosens the surrounding soil.
All of these processes (drill, jacking, etc.) are controlled remotely by an operator positioned in a control room located at the ground surface.