Lindsey Street Bridge – A Structural Solution to Settlement Mitigation
Summary
The C435 Farringdon Station Crossrail Project is currently under construction by BAM Ferrovial Kier Joint Venture (BFK). Significant settlements were anticipated, primarily as a result of deep shaft construction, and a combination of Tunnel Boring Machine (TBM) and Sprayed Concrete Lining (SCL) tunnelling. The method of settlement mitigation specified to counteract these settlements was compensation grouting. Lindsey Street Bridge, located immediately adjacent to the London Undergound (LU) Circle Line and the historic Smithfield Market was identified as a high risk structure which required further structure-specific mitigation. The factors leading to this requirement were as follows:
Anticipated settlements in excess of 100 mm;
The nature of the structure: Lindsey Street Bridge is a Grade II* listed composite masonry and steel structure dating from the 1860’s which has undergone a series of strengthening works over the years. On commencement of the project, the resultant load transfer mechanism needed to be determined; and
The shallow depth of cover between the underside of the bridge foundations and the crown of the inclined shaft to be constructed beneath, as little as three metres.
A mitigation scheme was developed by Crossrail to RIBA Stage D based on installing a reinforced concrete raft beneath the piers of the structure by means of sequenced masonry breakout and propping. This scheme was subsequently developed by BFK to detailed design (RIBA Stage F) with significant Value Engineering, including:
Stepping the slab at the railway interface to eliminate contiguous piling works trackside;
Design basis changed from active to passive pressure redistribution from the bridge pier footings into the underpinning slab and ground beneath; and
Introduction of semi-automated hydraulic jacking solution to allow refinement of level control;
Monitoring of the structure for any movement during the course of the works formed a crucial part of the scheme, and included the use of Shape Accel Arrays (SAAs).
This paper will expand on the design issues outlined above as well as on their implementation on site.