Integrated engineering for quality segmental linings 27 Feb 2020

Concrete segmental lining practice has undergone major development over the last 40 years and should be considered as significant as the advancement of TBM design to the success of the tunnelling industry in the same period. Experience of the essential engineering demands of both product and process, however, is not always carried forward and the results are the repeating of faults, defects, delays and adding to project costs.

Accurate ring build in as-built segmental lining is possible

The tunnelling process is now applied in increasingly challenging ground conditions. Clients expect a low-maintenance asset but projects can fail to deliver in the period between TBM launch and celebratory breakthroughs. TBM engineering is now focused in a few, highly specialized manufacturers developing increasingly capable machines. The same is not observed in the manufacture of precast concrete tunnel linings. The task of manufacturing concrete segments appears easy to the inexperienced and the process can become centrally managed by commercial spreadsheets with low technical input. The result is frequent repetition and re-learning of common difficulties with failure to deliver a quality tunnel product at reliable cost. Projects are getting larger and demand right-first-time to control commercial risk. Project management needs technical appreciation of history, research, advancements and past mistakes to avoid repeated failure to deliver the expectation of defect free segmentally lined tunnels.

The exponential global industry growth over the past decade, of excavating more complex geology and larger projects, has increased risk by all measures. Today, segment manufacture requires aerospace accuracy, delivering a prefabricated lining solution on the limits of physical processes. Without detailed knowledge, procurement decisions are made on simplified commercial costs, without regard for design and production details that will determine the success of the final tunnel asset long into the future.

Specialist concrete is essential to justify segment manufacturing outputs at the limits of process quality and speed. Reliable outputs are vital for cost control and top-quality mixes are required for low permeability, high durability concrete, capable of vibration, finishing and segment production in 10 hours or less. High performance production of thousands of units requires process engineering timed in seconds, through repeated activity. Without segment production knowledge, the costs, the schedule and delayed availability of the lining on site, can increase segment supply contracts by 50-100% over procurement assumptions. Reduced output and high defect rates contribute to direct and indirect costs to the overall tunnel project.

Ring build and TBM operation must integrate precisely to avoid lining damage and defects

Segmental construction requires concrete units with exceptional dimensional precision cast from highly specialized moulds to manage the contact loads exerted from the TBM during tunnel build and prevent cracking and leakage associated with long term maintenance. It is not acceptable to regularly present our clients with an asset incorporating defects either incurred during fabrication or created in the immediate period of construction behind the TBM. When reinforcement is included, damage increases the risk of long-term corrosion and the additional costs of attempted repair and extended program time. Segment damage and repair is commonly accepted as ‘normal’ but causes of defects can be eliminated with application of concentrated experience. Risks must be addressed from project concept, as during construction there are few options to change strategy and, unless addressed, the next project will likely repeat the same errors.

The elimination of steel rebar reinforcement cages should be a determined objective wherever possible to de-risk lifetime maintenance issues in subsurface conditions. Decades of development, principally by Bekaert as the leader, have engineered a range of steel fibres with accurate methods of production, dosing and distribution, with reliable performance verified by sample and full-scale laboratory testing.

Despite these proven specialist materials and the different codes developed for design methodology, the performance of SFRC (steel fibre reinforced concrete) is more likely than ever to be compromised by adoption of fibre options that do not provide reliable distribution or long-term creep properties. Specifications continue to require minimal SFRC performance and do nothing to encourage best in class and further segment design advancement.

TBM build conditions present both possible and predictable deviations placing demands on precast unit joints between interconnected rings with sub-millimeter accuracy of segment body dimensions, connection components and seals. All elements must perform to an expected range of build tolerance without compromise in the macro world of the TBM. Component detailing, including bolts, dowels and gaskets, must respect segment size and shape, and construction must respect the component design limitations. Improved solutions cannot be based solely on component cost or demands on progress rates without fully exploring the higher cost impacts of poor tunnel build.

In addition, the speed of tunnel advance must not compromise annulus grouting without which segmental linings will have irregular support and will almost certainly compromise build shape and tolerance.

A single ring does not make a tunnel - process accuracy and reliable unit production must be repeated many thousands of times to reduce segment fabrication defects to below 0.5%

Integrated component design, must address, for example, gasket displacements limited by the designed jointing system to a much lower degree than standard specifications. Otherwise the design anticipates breaches of ring geometry caused by poor build quality and in some cases the failure to correctly install connection systems when ring build gets out of alignment.

Correctional packing should not be required with accurate segment tolerances and rolling ring design. Reliance on packing and its effect on dowels and gaskets becomes ignored at the tunnel face during the ring build. Packing used only to decrease contact stress and corner damage from solid gasket corners also just introduces more costs rather than addressing the latent problem.

EPDM cast-in gaskets development, with compliant compressible corners, as patented by VIP Polymers of the UK, provide uniform stress and seal contact through the gasket without overstress to the concrete. Gasket selection should not be made based on historical design and unit price alone. The cost savings during segment manufacture and reduced damage in ring build should be taken into account. The risk of hydrocarbon attack in natural or contaminated ground, can require gaskets manufactured with a harder Nitrile material and specialist design review is essential before substitution of EPDM, to fully assess cost and build implications. Small percentage procurement savings between gasket types may result in large additional costs due to repair of cracked segments that far exceed the entire cost of the gasket supply contract value. Inexperienced segment manufacturers and contractors facing rising costs become drawn into arguments with the client or specifier requiring experience related to the spectrum of solutions.

Project tensions - commercial and sometimes legal - do not enhance reputations for quality and delivery. Early project management strategy must prioritise engineering best practice in tunnel lining design, manufacture and installation. Experience and continuous improvement is required to reduce the failures in production processes, delivery logistics, tunnel build quality and construction cost. Unless lessons are learned and applied, these improvement efforts will otherwise go unrecognized.

We celebrate each tunnel completion as an arduous achievement, especially if it is our first, but it will take a more integrated approach and a higher expectation to properly manage tunnel lining defect and delay risks to create the achievable objective of defect free tunnel construction in the coming decade. Defect free is possible and can be achieved.