TECHNICAL | MECHANISED TUNNELLING
Above, figure 5: Exemplary anti-roll units: gripper version (left) and disc cutter version (right)
quantify the wear of rock on steel (e.g., Cerchar, L. A.,
or LCPC abrasivity tests). Several attempts were made to convert the resulting values into cutter lifetime. The NTNU Trondheim, in Norway, developed an
alternative approach based on the brittleness, surface hardness, and wear capacity of a rock. The cutter life index (CLI) is calculated by use of surface hardness and the wear capacity on cutter ring steel quality. However, this value is rarely determined on a global scale, involves complicated testing, and is uncommon outside Scandinavia, especially for small-diameter projects. Therefore, semi-theoretical wear calculation for
small-diameter tunnelling projects is based on the Cerchar abrasivity index (CAI), which gives a good indication of abrasivity. A first order estimation of the wear caused by a
measured lab and can be confirmed or corrected by the wear calculation based on the CSM model. For small MTBMs, corrections need made for the smaller diameter of the cutter wheel, and maybe also made for different material and the number of rings. Smaller cutters also result in much less wear volume. Cutting wheel design for a small diameter MTBM
is, therefore, substantially different to that for a large TBM cutterhead – such as with the share of centre and caliber areas on the cutterhead surface increasing as machine diameter reduces. So, wear can be very irregular and precise prediction by cutter type is not possible. Therefore, average values for cutting tool sets are recommended. With this uncertainty, investment in high quality, wear-resistant cutters is advisable, and – if applicable
– to reduce the intervals for cutterhead intervention, especially early on a project.
Main bearing and main drive The design of the main bearing is considered, for many reasons, to be crucial to the success of a tunnelling operation. The main bearing is the critical link between the cutting wheel and the jacking system, including the steering cylinders. It transfers jacking loads via the cutting wheel to the tunnel face, doing so while handling torque generated by the main drive and the rotation speed of the cutting wheel to chip the rock. Further, it seals off the excavation chamber against atmospheric pressure in the tunnel. To minimise risk of failure, especially on projects with
hard rock geology and where used equipment can be allowed, given the importance of the main bearing it is advisable to consider refurbishment or replacement prior to commencing the drive.
Steering cylinders The steering cylinders transfer the thrust force to overcome the ground water pressure and the contact force of the cutters to chip the rock. Their stroke needs to be long enough to facilitate curved drives.
Anti-roll solutions Good lubrication reduces friction between the shield and the rock, although in doing so there tends to be machine roll and which, in small diameters especially, can result in roll at low torque. But the MTBM needs all torque available as well as high revolution and high thrust
Main jacking station Intermediate jacking station
Telescopic station
Jacking pipes
Above, figure 6: Pipejacking key equipment overview 26 | January 2024
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