The Protection of Existing DLR Viaduct Against Cross-Passage Construction Impact

Summary

The protection of the Docklands Light Railway (DLR) River Lea Viaduct in East London became the subject of extensive scrutiny as temporary groundwater control was undertaken for cross-passage 14 construction on the Crossrail project. Dewatering was required to facilitate the safe construction of the cross-passage which would encounter layers of interbedded sand and clay of the Lambeth Group.

Initial dewatering resulted in ground movements greater than originally predicted, without achieving the groundwater levels required. This consequently caused concern on the effect further dewatering would have on the stiff-post tensioned concrete box girder, located 40 m away from the cross-passage at its closest point. Therefore the dewatering was temporarily stopped.

This paper describes the revised damage assessment undertaken to assess the impact of the movements that had been observed, and for the asset protection of the viaduct from further dewatering. The revised assessment resulted in tight trigger values, additional precise instrumentation and regular visual inspections being specified to protect the structure.

As further dewatering continued, later supplemented by local permeation grouting, the viaduct was closely monitored. The interpretation of the live monitoring data supported redefining trigger values, where appropriate, to allow beneficial additional movements to occur while maintaining the same risk level imposed by the original tight trigger values.

This paper also describes the challenges encountered with using and interpreting the live monitoring data to continue protecting the DLR River Lea Viaduct, including:

– Establishing a relationship between the dewatering activities and the movements observed on the viaduct;
– Overcoming issues encountered with monitoring instrumentation;
– Predicting future dewatering induced movements by back-analysing observed ground movement data using empirical equations from CIRIA reports.