BANK STATION UPGRADE | TECHNICAL
should ideally be chosen such that the parent/child ratio is greater than one. Two of the adits between the Northern Line platforms,
and the five connections to the DLR, located below the Northern Line, were similarly transferred from a squareworks to an SCL/hybrid SCL methodology. Due to the complexity of connections to the existing assets, which are composed of a variety of materials including pre-1900 cast iron segments, precast concrete segments from the 1990s, brick arched passages, and mass concrete collars, 3D analysis was employed to deliver the assessment of these connections. Pamsl & Nasekhian (2023) and Nasekhian et al. (2023) presented numerical modelling predictions versus the existing assets responses due to SCL connections. Unlike the Northern Line cross passages, for these connections access was from above. It was found that early stabilisation of the assets using a frame or concrete collar minimised movement of the segmental linings.
PILE INTERCEPTION The BSCU scheme necessitated a new section of Northern Line running tunnel that needed to tie into the existing line at each end. At the early stage of the design, nine buildings along the potential route were identified as having a deep foundation, with a risk of either being intercepted or passing close enough to the new tunnel to transmit noise to the building above. It was sadly impossible to avoid all deep foundations without compromising the scheme. Therefore, the track alignment was designed considering the requirements of train speed and track maintenance while also trying to minimise pile interceptions. The final alignment intercepted 26 piles (friction and end bearing), either fully or partially. The most critical case was an unavoidable pile
interception of a nine-storey building with two basements (concrete structure built in the 1970s) at the Northern tie-in. The tunnel route constraints were such that even after optimisation the tunnel fully intercepted four out of 25 end bearing unreinforced concrete piles (shaft diameters ranging from 1.5m to 1.8m) and partially intercepted seven others. A major requirement for the project was to minimise the impact of tunnelling on the piles and retain the capacity required for an additional two floors to be added to the building if it underwent future development. The assessment undertaken to understand the impacts of tunnelling on the building was one of the most challenging parts of the project. The building owner’s technical representative
needed to be sufficiently convinced that the proposed construction sequence (cutting the piles first and then supporting them on a load transfer structure) would not cause the building to exceed the damage criteria agreed. To demonstrate this, a comprehensive soil-structure interaction was undertaken using 3D FEA (see Figure 7). The ground model developed by DSP included the pile-raft foundation, building basement and all tunnel excavation sequences. It was coupled with a
Above: Bank station capacity upgrade project - (top) tunnelling works underway, and (middle, bottom) completed tunnels
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