BANK STATION UPGRADE | TECHNICAL


Step 1: Primary SCL Top Heading


Full thickness primary SCL


Theoretical excavation line


Primary SFR sprayed concrete lining


Step 2: Preparation of Joint


Theoretical excavation line


Primary SFR sprayed concrete lining


Step 3: Primary SCL Bench/Invert


Theoretical excavation line


Primary SFR sprayed concrete lining


TH (750 - 1250) 1000


TH


Radial joint to next round


Toe removed and joint cleaned


400 (200 - 400) B/I


Excavate top heading/bench Apply primary SCL to given dimensions


300 50mm B/I Excavate bench/invert Break off toe and form min. 150mm clean radial construction joint DO Step 1: Primary SCL at stepped joint Step 2: Toe break-off Step 3: Primary lining to full thickness B/I


Apply primary SCL to full thickness


Left, figure 3:


SCL radial joint system adopted on BSCU - design (above), construction (below)


Full thickness primary SCL


TH


Defined stepping


clean radial construction joint No cracks, no rebound, no clay


Broken back –


Perfect blend-in


cases are reduced. In a sheet membrane scenario, water can potentially migrate between the linings and find its way all around the tunnel cross section, applying full hydrostatic pressure on the SL. In some tunnels the design did switch to a sheet membrane system with cast SL, to be installed using a travelling shutter that could produce a smooth finish. In locations with consistent geometry, that would be exposed to the public, this was found to be more efficient for cost and programme purposes. The cost savings were in part related to a more efficient use of materials and resulted in a carbon reduction through greater replacement of CEM 1, as well as lower wastage. As Dimmock (2022) rightly stated: zero carbon


tunnelling should start from efficient, state of the art design, even when carbon reduction has not been set as a specific target. Designers should be flexible to differing requirements in different locations, but equally requirements should be easy to keep up-to-date and justifiable as technology evolves. Setting a sensible set of design principles is key if we wish to have efficient linings and reduce tunnelling’s carbon footprint. Obviously, these principles should be tailored to the specific conditions of the project and need to be agreed with the client and the CAT 3 checker. LU tunnelling standards avoid prescribing design items but allow


for efficiencies to be found to suit the circumstances. Principles are set during scheme design; when information is available to make informed choices. Design assumptions for BSCU include:


1. Despite being located outside the waterproofing membrane, the PL is considered to be durable and part of the permanent support. A durable PL was credible due to unaggressive ground /groundwater condition revealed from chemical tests in addition to extensive knowledge of material behaviour in London Clay. A 75mm sacrificial layer outside the primary lining, as applied across the Crossrail route, was not considered to be required.


2. Considering a 5-year design life for the PL during construction allowed us to use a lower load partial factor (1.2) for the temporary loading cases than the permanent load partial factor (1.4 in LU Standard S1055) prior to installation of the SL. This led to a leaner PL thickness without compromising the overall support for a 120-year design life.


3. Non-linear analysis of the fibre reinforced concrete lining was allowed. No crack width criterion was set for the serviceability limit state of the SL as the tunnels were assumed to be fully tanked inside the waterproof membranes and the publicly accessible areas in the station are clad. This led to design of


May 2024 | 25


Varies


Varies


Toe break off


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