TECHNICAL | BANK STATION UPGRADE
the PL/SL without rebar, except L-bars at tunnel
junctions to assure connectivity between the child/ parent tunnel. Relaxing crack width requirements in the industry is becoming more common (Norrish 2023).
4. A minimum thickness of 150mm was set for the fibre reinforced SL to prevent penetration of the tunnel fixings to the waterproofing membrane (max 100mm embedment for fixings considered).
5. The minimum period of fire resistance of the tunnel lining is 90 minutes. According to the tabular method of concrete design against fire in EC2 the lining thickness must be minimum 140mm, which is less than the 150mm minimum thickness chosen for the SL. The fire design strategy was to assume the SL a sacrificial layer.
Below, figure 4: Squareworks (in red) connecting the SCL tunnels (in green) to the existing Northern Line platform (in grey) at Concept Design
By applying the above principles, the design of SCL tunnels on BSCU used a leaner lining system in comparison with Crossrail, with a similar SFRC lining specification. Table 1 compares lining thicknesses of a platform size tunnel at Bank versus Crossrail Liverpool Station. It is observed that a 35% reduction in the overall lining thickness and up to 50% reduction in the secondary lining has been achieved. No rebar in the lining reduces the carbon footprint even further. At T-junctions, thickenings were localised to the opening side with a simple step in the tunnel lining at crown and invert (see Figure 2). A small change, but every little saving in the design helps. For further details refer to
Interchange tunnel Cross-passage No.1 Cross-passage No.2
Cross-passage No.3 Temporary access tunnel mid
New sprayed concrete lining Northern Line platform tunnel
Cross-passage No.4 Adit 1
Staircase stub access tunnel
Adit 2
Existing Northern Line platform tunnels
Escalator Adit 3
Docklands Light Railway (below) Lift connection adit
Temporary access tunnel south 26 | May 2024
Nasekhian & Feiersinger (2019), which highlights the design considerations for different components of the combined lining.
RADIAL JOINTS One of the areas of SCL tunnelling that Crossrail had worked hard to improve was management of exclusion zones and safety of the site team, particularly when working with open ground and freshly applied concrete (Anthony 2020). Both the design and construction team on BSCU wanted to build on this; with an area to address being the need for the tunnelling team to enter the face to install radial bars to connect top heading to bench excavations, or benches to inverts. The resulting radial joint system requires care and supervision, but can be fully mechanically excavated, meaning that no one needs to enter the tunnel face at any point in a standard tunnelling cycle. This removes the waiting times that have to be managed before Kwika-strip installation etc, and so not only made exclusion zone management simpler (the face is out of bounds all of the time), but also sped up the excavation process, so was considered a win-win. The team of tunnel inspectors were also happy
with this approach and it has been applied on other LU projects since, with similar success. One of the other benefits of removing person entry into the tunnel face is that the thickness of material applied to stabilise it does not have to be dictated, but can be sufficient to prevent the ground ravelling or drying out before the next advance, when it will be removed. A thicker layer of material on the face is only needed for extended shut-downs.
ASSESSMENT OF IMPACT TO LU EXISTING ASSETS In addition to design of the SCL tunnels, DSP carried out impact assessment of tunnelling works on the existing LU Civil assets. Combining both tasks under one team ensured an integrated design solution that considered asset protection and efficient design together. It also resulted in significant time and cost saving. Adopting the staged approach from the LU standard
S1050 and working closely with LU to benefit from their knowledge of the behaviour of their assets, the assessment process was as efficient as possible; allowing focus to be directed to the assets where movements will have the greatest impact. Only the most relevant tunnels and openings were incorporated into the 3D Finite Element Analysis (FEA) models that were created to design the new structures; maximising the benefits of this computational effort and creating a more complete and interrelated model than has been used previously. Using this combined model LU, DSP and Dragados
had more confidence in the predictions and could update models during construction based on measured movements in a way that can be clearly traced and interrogated. In conjunction with focussed assessment,
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