BTS | HARDING PRIZE COMPETITION 2024


Right, figure 10: Membrane installed onto SCL


PHOTO CREDIT: BALFOUR BEATTY


In the onshore tunnels, the SCL debonding


membrane was shown to be an extremely effective means of eliminating non-compliant cracking and achieving right first time construction with relatively low reinforcement densities; thus, reducing repairs required and the cost and schedule impacts associated with them. It was therefore taken forward for optional use within the TSC.


Joint preparation & waterproofing In the onshore tunnels, due to access constraints and pour methodologies, it was sometimes difficult and time consuming to form a rough key on horizontal construction joints. Therefore, for the TSC, the design requirements have been reviewed and less stringent criteria considered, including options to coat the intrados of joints where mechanical roughening of the full joint is not possible. When installing the construction joint waterproofing


measures, there could be wet or dry conditions depending on the ground conditions. Therefore, to ensure products function as intended, options for each case were developed in close collaboration with material manufacturers. To meet the designated leakage class requirements,


water bars have been prescribed at vertical construction and movement joints. Due to the high water pressures to be resisted and seismic movements, I have previously helped to substantiate that specific products were suitable within the MW tunnels.


Optimisation & change management Following preliminary trialling of the most complex reinforcement arrangements in the TSC, optimisation of reinforcement provision has been undertaken to improve buildability. Further lessons learnt will be captured as the works progress. As the original PW designs required detailed design


Above: David Maddison 20 | May 2024


substantiation, for post acceptance design changes to be implemented there must be suitably detailed justification developed, and accepted by NNB.


In my previous design coordinator role and now in


my role as design manager, I have led the development of PW ‘field change requests’ (FCRs). In a controlled manner, this enables the accepted designs to be gradually improved upon to capture lessons learnt or to overcome adverse or unexpected conditions encountered in the field.


CONCLUSIONS On the MW, I facilitated collaboration between the designer, contractor, and client to enable complex design and construction issues to be overcome whilst ensuring the project’s overriding nuclear safety and culture requirements are met. As outlined by this paper, technological advancements


have already and will continue to provide enhancements to space proofing, de-clashing, sequencing, remote excavation, and other elements of tunnel design and construction. To be successfully implemented, these technologies require an experienced team with a clear quality vision for the project. From the different aspects of the MW that I have been


involved in, to ensure the successful delivery of complex infrastructure, the critical requirement for success remains early engagement, open communication and collaboration within the team delivering it. Without this, even the most technically advanced project will struggle to succeed. On design and build projects like the MW, this is


best achieved through continuity of the construction team from design into construction with the support of knowledgeable design representatives embedded within the team. The regular sharing of knowledge and implementation of lessons learnt within and between projects should also be facilitated.


ACKNOWLEDGMENTS At the near culmination of my involvement on the HPC MW, I am extremely grateful to all my past and present project colleagues who have given me the opportunity to learn from them over the last decade to enable me to become the collaborative engineer I am today.


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