ABSTRACT
London Power Tunnels (LPT) Phase 2 is a new purpose-built cable tunnel system which will replace the existing buried cable network in the south of London. This £1 billion National Grid (NG) scheme comprises 32.5km of 3m internal diameter deep underground tunnels and 8 shafts which will help keep Londoners connected to safe and reliable electricity supplies. The tunnels extend from Wimbledon in the West to Crayford in the East, reaching depths of up to 60m below ground level. This paper presents the design of the permanent steel fibre reinforced sprayed concrete launch and reception chambers at three of the deep shafts on the project. The paper provides an introduction to the project, the geometrical and geological information for the shafts and details of sprayed concrete lining (SCL) techniques for the chambers. It also presents the detailed design process for the SCL and combined construction techniques using numerical modelling packages PLAXIS and LUSAS. Sensitivity analyses were conducted, using time-dependent SCL stiffness properties, to study the effect of soil parameters, circular and ‘squircle’ chamber shapes, advance rate and excavation sequence. Design of temporary domed SCL headwalls at the end of the chambers was also carried out, for which a range of ground permeability values were investigated to provide a realistic estimate of the applied loading resulting from consolidation of the London Clay. The results of the analyses conclude that these deep chambers can be constructed using SCL techniques for both permanent and temporary cases in these challenging ground conditions.
