TECHNICAL | BANK STATION UPGRADE
SCL construction canopy
Northern Line Adit 3
SCL access tunnel cross passage No. 4 Northern Line
New Northern Line southbound platform
Right, figure 10: SCL excavation (green) around existing assets
Operational Northern Line platforms
New SCL tunnels Existing assets
superstructure model created by Robert Bird Group
to assess the building response due to the tunnelling- induced settlements given by the substructure model. Nasekhian et al. (2021) gives a detailed account. In addition to the complex modelling, TfL and the
contractor Dragados approached Cambridge Centre for Smart Infrastructure and Construction (CSIC) to install a highly innovative system to monitor the piles in the building in real time. Understanding the responses of intercepted piles due to tunnelling is crucial, yet very limited case histories have been reported, mainly because of the difficulty associated with deploying conventional instrumentation in existing piles. On BSCU, distributed fibre optic sensors were installed
into two existing large-diameter 25m-long end bearing piles – one intercepted fully and one partially by the 6.5m diameter running tunnel. The sensors were used to monitor behaviour during tunnelling, pile interception, transfer structure installation and post construction. The use of fibre optics to measure strain in the piled foundations was an important aspect in the verification of the design of the pile interceptions. Barker et al. (2020) provides further details. It is believed to be only the second project in the UK to use optical fibre instrumentation to observe the behaviour of existing piles for reuse and the first project for piling interception for a tunnelling project.
UPHILL EXCAVATION OF AN INCLINED BARREL A new triple escalator now travels between the Central Line platform tunnels, improving the connection to the Northern Line and facilitating faster interchange between the different parts of the station. Unusually, the inclined escalator tunnel was excavated uphill rather than downhill due to restricted access (see Figure 9). Uphill escalators had been built on Crossrail, but in this case there was insufficient room for a pilot tunnel to repeat their methodology. The new escalator also intercepted several existing structures that had to be backfilled
30 | May 2024
and then broken out, some just 0.5m from operational platforms. Kumpfmueller et al. (2022) report the design considerations, construction sequence decisions and operational risk management, and discuss the existing assets’ response to uphill tunnelling based on a comprehensive monitoring scheme.
LESSONS LEARNT In addition to the items described above, some of the further lessons learnt during the works on BSCU included: ● Maximising the use of mechanical excavation doesn’t have to increase the risk of damage to existing assets. During the early design stages the basis of using hand-mining techniques to excavate the connections to existing assets was in part based on perceived risk of an excavator damaging the brittle cast iron. The repeated excavations up to, over and around existing assets on BSCU, all while trains and passengers safely continued usage within, were managed using sensible mitigation measures such as toothless buckets, probing/measuring ahead, watchmen inside the platforms, and good communication of the risks to all involved.
● Depending on the scale, you don’t have to commit early to one waterproofing system throughout a project, but should manage the decision-making process on a tunnel by tunnel basis in conjunction with the construction team. While local areas of complexity might suit the adaptability and lining efficiency potential of a sprayed system, relatively long straight tunnels within a complex system might still be optimally constructed using a sheet membrane approach and merit the initial expense of a shutter.
● Opening frame designs can be optimised using high strength steel to reduce the size of members, making them easier to transport to their required location and simplifying lifting plans. Reinforced concrete solutions can have further benefits where the route
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41
