Evaluation of chalk rock mass properties in subcrop and outcrop in London; examples from the Tideway and Lee tunnels

The Tideway and Lee tunnels represent the two deepest tunnels constructed in the Chalk in London. The extensive ground investigations for both projects, and the construction records from the Lee Tunnel, provide a unique opportunity to evaluate the rock mass properties of the Chalk in London both in subcrop, below the London Palaeogene deposits, and at outcrop. In particular, variation in the properties of the Chalk are reviewed in relation to depth, overlying geology and the proximity to geological structures (faults, folds) and quaternary features (pingos, scour hollows). Comparison is also made between rock mass properties, engineering classifications (e.g. CIRIA grade) and in-situ testing results to provide general guidance for preliminary ground model conceptualisation and characterisation for similar future tunnelling projects in the Chalk.