Long-term settlement following SCL-tunnel excavation

Summary

Long-term settlement at the start of the project was considered marginal and was believed to not be significant in comparison to the magnitude of the direct settlement influence from tunnel excavation. However, long-term effects of SCL tunnelling at Crossrail C510 Whitechapel and Liverpool Street Stations were identified at an early stage of the project. Ground movement did not stabilise soon after the tunnel face left the zone of influence as expected. Long term settlement has resulted in significant increase of the magnitude, extent of the settlement trough, total volume loss, slopes and deflections.

This paper provides a thorough investigation of the long-term settlement for various excavation stages and after completion of SCL tunnelling. A definition of long-term settlement is given, with an overview on the geology and the excavation and support method principles of LaserShell.

Two particular areas at Whitechapel Station have been investigated providing up to 2 ½ years of long-term surface settlement observation after the last construction activity. These areas comprise green field settlement at Vallance Gardens and Kempton Court buildings where no settlement mitigation could be implemented. Long term ground movement between construction stages and after construction has nearly doubled the settlement directly induced by tunnelling and increased volume loss by up to ca. 110%.

This investigation gives an overview of the development of settlement rate over time and addresses seasonal effects. Particular attention is given to slopes and deflections which are observed to be adversely affected by long-term settlement in contrast to the opinions in the textbooks.

The long-term settlement is most likely caused by the drainage effects of the permeable tunnel lining on the surrounding ground.