Tunnelling in sand in Farringdon Crossrail Station
Summary
The sprayed concrete lined (SCL) tunnels in Crossrail Farringdon Station have been mainly excavated in the Lambeth group, a highly variable formation comprising generally very stiff, over-consolidated clays with interbedded sand lenses. The platform tunnels, approximately 11.4m wide by 10.6m high, were enlarged from the existing TBM tunnels using SCL techniques following extensive probing from the TBM tunnels to investigate the presence of pressurised sand lenses.
In two occasions, extensive, thick sand lenses with water under pressure were encountered that required significant mitigation measures to be deployed.
At first, a 2 to 3m thick sand lens, with an extent of approximately 100m along the tunnel, was encountered during the enlargement of the westbound platform tunnel (PTW) near tunnel axis level. A systematic in-tunnel depressurisation scheme was successfully utilised by the contractor reducing sufficiently the pore-water pressures in the sand.
The second case was of significantly increased difficulty as a sand lens of similar thickness was encountered in a very unfavourable position, that being in and above the crown of the eastbound platform tunnel (PTE) for approximately 25m along the tunnel. Additionally to the dewatering scheme that was prescribed by the temporary works design, the contractor decided to reduce the risk further by injecting chemical grout. These mitigation measures were combined with the application of toolbox items that enhanced the roof stability through the installation of grouted spiles and the face stability through the sub-division of the top heading into smaller pockets.
In both cases, the implementation of the mitigation measures was successful, enabling the safe completion of the platform tunnels in Farringdon.