TBMs generally have the following functions:
Excavating the ground and removing excavated material
Supporting the tunnelling face
Assembling the tunnel lining
This previous blog explained TBM selection mainly in urban environment, and under soft ground conditions. For tunnelling operations in self-standing ground conditions (usually rock), without significant groundwater in-flows, Function 2 is not needed systematically. Three arch-types of TBMs can be used: open (Function 1 only), single shield and double shield (Function 1 and 3).
Open TBM
Open TBMs, or alternatively called ‘hard rock TBMs’, or ‘gripper TBMs’, perform Function 1 only (excavating and removing spoil). Ground can be seen from behind the cutterhead, hence the origin of the name. To use this type of TBM, the rock mass must be strong enough to be self-supporting in the long term, with indefinite stand-up time, with no segmental lining is required, other than some shotcrete and rock bolts to better utilise the rock’s own structural capacity (Function 3 is therefore not needed). Excavated ground falls to the bottom of the face and gets collected by perimeter buckets, then loaded onto the conveyor, and does not go through the cutterhead. For this reason the cutterhead has no openings and is almost a solid piece. To move forward, they rely on grippers rather than thrusting against the built lining, and therefore no thrust cylinders can be found on these TBMs. The only stoppage of the TBM would be to move forward the grippers, rather than for ring-building.
The significant disadvantage of open TBMs is their inability to deal with unstable ground and ground water inflow. Therefore, modern TBMs are normally equipped with the function of probe drilling and umbrella grouting from the face. Another disadvantage that limits the use of open TBM is the huge thrust pressure on the ground from the grippers. Only competent rock masses can take it.
Apart from these, obviously open TBMs do not have Function 3 (assembling tunnel lining), so where a structural lining is required, this has to be done by either sprayed or formed concrete. Where this needs to be done systematically, another type is needed – see below.
Shielded TBM
Where the ground is less stable, liable to rock fall, or needs to be dry, tunnel lining needs to be assembled within the TBM (Function 3). To do this, a shield is required, providing protection to a rotating segment erector that assembles the tunnel lining, usually made of precast concrete. The shield and the gripper are mutually exclusive (i.e. where one exists, the other can not). So where the shield exists, the propulsion of the TBM has to be provided by the thrust cylinders pushing against the built tunnel lining.
Shielded TBMs have larger range of operability, comprising both hard and soft rock conditions. They are more versatile, so why we use open TBMs sometimes?
A huge win by open TBMs compared to shielded TBMs is that there is significantly less stoppage time. For shielded TBMs, while the ring is being assembled, the TBM will lose the base to thrust against, and will have to wait for the build to complete. This cycle time is usually 15-30 mins, about 30-40% of the whole excavate-assemble cycle. Also, where precast lining needs to be assembled within the shield, an annulus gap has to be provided, filling it costs materials. Furthermore, effective support to the ground can only be achieved after the shield length, so the ground is not necessarily strong enough to wait that long (or far). So where ground conditions permit, always prioritise open TBMs over shielded TBMs.
However, is it possible to have a shielded TBM without the draw-back of the stoppages? This type of TBM below gives an answer.
Double-shield TBM
As the name suggests, double-shield TBMs have two shields, a front shield and a back shield. The back shield has a gripper built in, and have thrust cylinders that act mainly to help segment assembly. When the rings are being assembled, the back shield stays stable, using the built-in grippers, serving as a base for the front shield to move telescopically forward. Once the ring-building is complete, the front shield drags the back shield forward, closing the telescope (‘re-gripping’). Effectively, the stoppage time is the same as the gripper TBM.
Doesn’t double-shield TBM sound too perfect to be true? It certainly has a lot of limitations. Apart from higher costs, double-shield TBMs are a lot more complex than single-shield TBMs mechanically and therefore more liable to break-downs. Debris fallen into the telescopic part between the two shields may jam it causing regripping much harder. Double-shield is also significantly longer, and the excavated ground has to be self-stable over a longer distance before gaining support from the tunnel lining. The fact that a shield is used means that the ground is not sufficiently self-stable, and this means that the grippers do not always find good ground to grip on; when the ground is not good enough, the ‘Plan B’ is to use the thrust cylinders in a conventional way to push against built rings.
Unless the tunnelling length is very significant, the break-down and cost hardly justify the use of double-shield TBMs, hence why they are relatively rarer in use.
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