BTS | HARDING PRIZE COMPETITION 2024


Above, figure 6:


Arrangement of pilot and enlarged tunnels within the TSC showing exposed geology


PHOTO CREDIT: BALFOUR BEATTY


I had learnt for inclusion within the TSC PW designs.


The primary lessons to aid construction were: suitable construction tolerances should be defined; tunnel specific concrete mixes should be developed; the requirements for preparation and waterproofing of construction joints must be clear; and, TW profiles must be controlled to avoid unforeseen impacts (e.g., cracking, clashes or design over-utilisation). This paper does not include lessons learnt from the


construction of the TSC PW as, at the time of writing this paper, the works are due to start in the first quarter of 2024, upon completion of the SCL TW.


Geotechnical & structural design The ground loads acting upon the PW TSC were determined using PLAXIS 3D modelling and applied into SAP2000 3D structural models along with the required geometry, concrete properties, internal and external water loads, and seismic loadings. Once the resultant stresses within the structures had


been determined, concrete design calculations were undertaken in accordance with the works information, EC211


and other applicable design standards. This


determined the reinforcement required. Due to the geology, the impact of potential rock


wedges was also assessed using UnWedge which is a 3D stability analysis and visualisation software program. Should potential wedges be identified by geologists, during excavation of the TW, they are locally spot- bolted to hold them in place.


Geometry & space proofing The geometry of the TSC is a balance between providing sufficient space for the PW, limiting excavation volumes, reducing overall complexity, and managing risks. Due to the complex geometry, the PW of all TSCs


has been modelled in 3D as shown in Figure 9. These models were produced using AutoCAD for both general arrangement and RC drawings and were beneficial for planned and unplanned activities, including activity scheduling and pricing.


16 | May 2024


When designing a TW SCL tunnel that is to have


a PW lining, an additional space proofing tolerance should be included beyond any extrados cover zone to reinforcement. This should be considered from the outset and included within design calculations for both the TW and PW. A minimum additional extrados tolerance of 100mm is recommended.


Onshore shaft prefabrication In the Royal Portbury Docks, in which I was based for several months to support BB, onshore prefabrication works were undertaken within the vertical section of the shafts prior to them being loaded onto a JUV and installed into the seabed. PW reinforcement was preinstalled within most of


the liner height and the uppermost concrete pour was undertaken to allow the isolation cap to be installed. Through joint site inspections, I helped to resolve construction queries which enabled the structures to be signed off and for offshore installation to begin. The prefabrication allowed works to occur


concurrently in different locations whilst also reducing risks associated with working in underground confined spaces, such as the TSC. Due to the nature and isolation of the TSC, other scope for prefabrication is limited.


Concrete mixes, assurance & quality As the concrete must be batched onshore and transported a significant distance – both vertically and horizontally, underground – a long open life and workability is required, ensuring the mix remains useable and design compliant at the location of the concrete pours. This has been achieved through use of concrete


additives, such as ground granulated blast-furnace slag (GGBS), microsilica, superplasticisers, retarders, and extensive trialling. To ensure the design intent is met, I have developed a close working relationship with the BB materials team to understand its requirements and to provide support.


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