TECHNICAL | BANK STATION UPGRADE
Table 1: Comparison of lining thickness - Crossrail vs BSCU Project Crossrail Crossrail BSCU BSCU BSCU
Location Tunnel Platform – Regular Note: * Figure 2 shows design at opening
Liverpool Street Station Westbound Platform – Regular Liverpool Street Station Westbound Platform – Thickened 34.9 Northern Line Northern Line Northern Line
Platform – Thickened closed side* Platform – Thickened opening side*
29.4 29.4 29.4
Depth Int. Dia. (mm) SCL Primary (mm) SCL Secondary (mm) SCL Total (mm) 34.9
8820 8820 8920 8920 8920
400 800 300 450 600
300 300 150 250 250
700
1100 450 700 850
The BSCU project is one of the major underground
projects in London that was developed after the design, and much of the construction, of Crossrail. Crossrail undoubtedly progressed expertise in sprayed concrete lining (SCL) tunnelling in the UK, and also generated and shared lots of lessons for future projects to learn from. We have therefore used it as a baseline a number of times in the following review. The DSP team on BSCU were initially responsible
for design of the SCL tunnels within a wider multi- disciplinary design team lead by URS. As the project developed beyond scheme design, Dragados became the principal designer, leading a group of specialist smaller design companies, including Robert Bird Group, GCG, OtB and DSP looking at the underground structures. The Dragados Construction and Engineering teams worked directly with these specialists to innovate and identify efficiencies on cost, carbon, programme and to improve safety during construction. Now, a year after project completion, we look back on what was achieved, where lessons can be applied elsewhere and what could be done better next time.
Tunnel concrete lining (min 450mm)
Primary lining regulatory layer and waterproofing system (40mm)
Secondary SFR cast-in-place concrete lining (min 250mm)
Assumed tunnel extrados Theoretical excavation line Primary SFR sprayed
SCL – LINING SYSTEM On BSCU, similarly to Crossrail, both primary (PL) and secondary (SL) linings carry the permanent loads; assuming a combined interaction between the lining components rather than composite action. However, unlike Crossrail, the proportion of the long-term ground load taken by the primary lining was not prescribed in standards prepared before design began. Instead, the load share between the primary and secondary linings was the outcome of a three-dimensional numerical analysis, resulting in significant reductions in thickness. Initially, a double-bonded spray-applied waterproofing membrane sandwiched between the linings was adopted for the whole project. Each waterproofing option has its own pros and
cons with respect to constructability. However, from a structural designer’s point of view the main difference is that when using a sprayed membrane, groundwater migration between the linings is very unlikely and it can be assumed that both linings move together under the long-term load condition. This scenario can save on overall lining thickness as the secondary lining load
Primary SFR sprayed concrete lining (min 600mm)
NL cross passages
Right, figure 2:
BSCU project - Northern Line platform tunnel at opening
Tunnel axis +84.900
Northern Line - platform tunnel (Thickened)
24 | May 2024
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