Crossrail Tunnelling In The London Clay Formation

The Crossrail project in London (UK) spent seven years excavating and lining tunnels and shafts within the London Basin. Construction activities encountered, and dealt with, a range of geological strata, including recent deposits of Made Ground, Alluvium and River Terrace Gravels, over-consolidated London Clay, variable mixed sediments of sands gravels and clays, uniform and fine-grained sands, and the underlying sedimentary Limestone (Chalk). Approximately 42km of segmentally lined bored tunnel were completed using pressurised Tunnel Boring Machines (TBMs), and over 12km of tunnel were supported utilising Sprayed Concrete Linings (SCL) with spans of up to 17m, along with sprayed concrete, diaphragm wall and piled shafts and underground structures.

This paper provides an outline of the project, concentrating on the central tunnelled section. The broad geological/hydrogeological setting of the central tunnelled area is described, with an examination in more detail of the perception and reality of tunnelling in the over-consolidated London Clay. This material has often been described as the ideal tunnelling material, but it is variable and not risk free, and offers challenges for both design and construction. The London Clay has been compared with the similar Keswick and Hindmarsh Clays that underlie much of the Adelaide city area. Tunnel construction utilising TBM and traditional excavation coupled with the use of SCL for support through the London Clay will be discussed. The paper will consider in particular the influence of historical experience of tunnelling in London Clay on modern approaches and risk perceptions.