The platform and pedestrian tunnels for the Crossrail Stations at Whitechapel and Liverpool Street have been constructed using Sprayed Concrete Lining techniques more than 30 m below the congested urban streets of London. Pre-construction ground investigations indicated that the tunnels would be formed predominantly in London Clay with invert close to or within the under-lying Lambeth Group soils. There was evidence of intermittent water-bearing channel sands in the Lambeth Group which could adversely impact stability during tunnel construction.
A depressurisation strategy was prepared prior to commencement of the works. The aim of the strategy was to identify and depressurise any water bearing granular horizon in the vicinity to ensure the safety and security of the unsupported headings during the SCL works. Initially a surface depressurisation scheme had been proposed. This option was not pursued due to the very limited surface access available for well installation combined with the intermittent nature of the channel sand horizons. Instead the strategy adopted for the majority of the works relied on in-tunnel investigation with pumped wellpoints targeting the water bearing channel sands. The platform tunnels were formed with an initial pilot tunnel, which often had sufficient clay cover below invert such that depressurisation was not required. The pilot tunnel could then provide access, allowing investigation and depressurisation, prior to enlargement.
This paper describes the in-tunnel depressurisation strategy used at Whitechapel and Liverpool Street Stations. The in-tunnel ground investigations were undertaken by probe drilling with drill patterns based on the anticipated geological profile. Depressurisation of target horizons was then achieved with vacuum wellpoints with unpumped wellpoints used as piezometers for monitoring. A vital part of the strategy was the development and continual updating of the ground model using both face and probe drilling logs. This proved essential for identifying target water bearing horizons and in understanding their response to pumping.
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