Analyses of convergence and utilisation of SCL-tunnels during excavation

Summary

This paper provides a comparison of predicted and measured displacements and deformations for SCL tunnels at Crossrail C510 Whitechapel Station. It contributes to the current ‘lesson learnt’ discussion on Crossrail projects in terms of design optimisation. The paper shall feed this discussion with thoroughly investigated data as the authors believe in a significant potential of design optimisation without compromising safety and stability.

An overview is given on the geology and the excavation and support method when using the principles of the LaserShell. The method of in-tunnel displacement measurements is described as well as the layout of monitoring arrays.

The investigation starts with the regular time dependent graphs, where a typical range of absolute displacements can be identified. It is also looked at the distance to the disappearing face at which displacements are fading.
Absolute displacements are used to separate whole body movement from deformation and to calculate convergences, vertically and horizontally. A typical range of convergence of the sprayed concrete lining can be identified.

Utilisation, i.e. the ratio of actual stress and permissible stress, is used to calculate the typical as-built stress level in the sprayed concrete lining.

This analysis covers pilot tunnels and enlargements, the influence of openings for cross passages on the ‘mother’ tunnel and the side wall drift excavation for the Westbound Cavern at Whitechapel which is one of the largest SCL-tunnels in London Clay ever built so far.

It is compared how predicted displacements and deformations tie in with the as-built results. It turns out that the predicted deformations are generally larger than the ones measured. An overestimation of the design loads could be a likely reason which has significant impact on the order of the expected stress level in the lining. Adapting the loads to match measured convergence would bear a potential of reducing support measures without compromising the factor of safety.