MECHANISED TUNNELLING | ITA GUIDES


perform in the long operating life of a bored tunnel? The guide discusses ways to inspect and judge


the condition of such segmental lining during the operational life of TBM-driven underground assets. It does so across 35-pages, after the first third of text- heavy narrative, but the illustrations and tables that come in the last-two-thirds are effective. With a focus on the post-construction, longer-


Above:


Rear shield view of a Single Shield Hard Rock TBM. Image credit: Herrenknecht; Source: ITA guide on TBM equipment specifications; also photos on pp28


To address this need, the guide gathers those


“considerations necessary for assessing the resultant forces” for shielded TBMs when operating in open mode in highly fractured or weak rock (i.e., low strength and high deformability). That said, it should be noted that the guide, as any other, is not the last word in any matter. As the report says, “It should not be considered as a definitive or exhaustive guide for the design of TBM equipment, which is also subject to other standards and procedures.” And, of course, the main thrust forces under


Below: ITA guide on segmental lining assessment during operational phase of bored tunnels.


consideration here for the ground conditions are not the only capabilities required of a TBM, for more “capacity is installed in the machine to enable TBM steering, to guarantee proper margins of safety during excavation and to handle unexpected circumstances.” It adds: “The final assessment of suitable thrust capacity depends also on a proper project-specific risk analysis defined by the designers and contractors.” This report is also


discussed further below. In the third of the


ITA reports being discussed in this article, the focus is not on the shield wholly or in part but is a full switch to the segmental lining erected by the TBM, but mostly from the perspective of the post-construction phase of a project: how does it


term condition of the tunnel infrastructure, the guide has looked at bored tunnels across a range of ages, whether it is refurbishment required or repairs to damaged segmental lining. It is noted that the behaviour of a segmental tunnel is subject to change with time, and the operational phase is, by a long way, the majority of the life of the lining - materials and as structural systems - of such an underground asset. The report, it says, keeps “the tunnel owner in mind,


but can be useful to all persons active in the inspection, monitoring and/or maintenance of bored tunnels.” Below, in the box panel, the report is discussed further.


TBM EQUIPMENT SPECIFICATION A highly illustrated guide, dominated by useful tables, the ‘Recommended Guidelines for Tunnels Boring Machine Equipment Specifications’, from WG 3 (Contractual Practices) & WG14 (Mechanised Tunnelling, Task Group 2) is an excellent reference and very accessible. In considering technology and cost in light of risk


and procurement, the guide observes, “Increased efficiency of tunnel excavation depends upon resolving the tension between making effective use of such technological advances and the requirement for contractors to win contracts by offering lower prices, or more advantageous proposals, than their competition.” A challenge comes from costs being lowered


with bid offers that include minimal TBM capability, it adds, and in reaction some parts of industry has seen projects owners and designers make increased use of prescription-based specifications to meet requirements they deem to be necessary. This response, though, “may hamper” the contractors and manufacturers to use new technology. And, partly in consequence of this tension, there have been moves to the use of Alternative Delivery Concepts. Within this approach, it goes on, “A combination of performance and prescriptive elements may offer an attractive solution” but “needs to be carefully implemented” to avoid risks of conflict between them. The guide says the goal is not to be overly


prescriptive by going beyond the fundamentals required and consequently constraining an experienced contractor. Best to add prescriptive elements for project-specific needs or “risks which are deemed critical for the success of the project.”


...FINAL ASSESSMENT OF SUITABLE THRUST


CAPACITY DEPENDS ALSO ON A PROPER PROJECT- SPECIFIC RISK ANALYSIS...


26 | July 2026


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