MECHANISED TUNNELLING | ITA GUIDES
it is, however, for both the design of TBMs and segmental lining. The Introduction
to the report states that the primary aim of the work is to evaluate the resultant thrust force that is required for the TBM to overcome those opposing the excavation, and are calculated “for all anticipated geotechnical scenarios along the alignment during the design phase.” Resultant thrust,
in that it is the summation of other forces - boring to penetrate the
Above: ITA guide on open mode TBM thrust forces in weak rock.
Above right:
Example of Man lock. Image credit: CREG
Below: Example of Screw
Conveyor system for hybrid TBM during site assembly. Image credit: Robbiins
ground ahead; friction around the shield; lining- related; and, to pull the backup train. On top of that summation, as noted before there is more thrust to be added, to: enable steering of the TBM; give safety margins during excavation (for expected as well as unexpected circumstances, respectively); and, to fit with prior, project-specific risk analysis that has been undertaken by the designers and contractors. To get to that point of a cumulative thrust
calculation, the guide first sets out an overview of the scenario in play - that of shield tunnelling, in this case of open mode. It discusses single and double shield TBMs, respectively, noting that while each have application in hard soils and rock their
thrust jacks push off different areas, primarily: single shield TBMs push off the edges of the last built ring of segmental lining; and, double shield machines - in combining the functional principles of gripper and single shield TBMs - have their main thrust jacks push against gripper reaction, and it is the auxiliary jacks that thrust against the edges of segmental lining, if such is being used. The double shield arrangement with different
jacks also allows both lining construction and advance of the drive to happen at the same time. Such TBM tunnelling can be almost continuous, in principle, unlike with single shields. Further, while single shields therefore have
segmental lining in place as part of the tunnel construction works, the double shield TBM tunnelling mainly has such lining but might not have any in use in the tunnel project. With jacks operating differently between the
machines, thrust calculations therefore differ for each type of TBM in open mode tunnelling, in general. However, when in weak rock of low quality or geological shear/fault zones then the double shield’s gripper action “becomes ineffective”, resulting in single mode operation - and it is only the auxiliary jacks that are in use, pushing off the last built ring. In this situation are the required thrust force calculation is as for the single shield TBM. From there the guide performs detailed dives
into thrust force calculations of different types to help establish minimums needed to overcome opposition to the advancement and effective control of a shield, while the segmental lining performs as intended. Consideration is given to ground behaviour in deep tunnels and highly deformable rock mass; gravity loading in jointed rock; loading in weak rock; consolidation; creep; swelling; squeezing ground; and, also looking at geometry, TBM operations and standstill, lubrication. It advises, though, that “more in-depth analyses are always recommended for project-specific scenarios.”
The guides are available for download at the ITA-AITES website, at:
https://about.ita-aites.org/publications/ wg-publications
28 | July 2026
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