Covered Way 126 – Safeguarding a Brittle Structure over a Live Railway during Ground Movements
Summary
Covered Way 126 is a Victorian structure enclosing the London Overground railway at Whitechapel. It consists of masonry abutments supporting cast iron roof beams and masonry jack-arches. It is close to Crossrail tunnels and box excavations, and was expected to be affected by ground movements during Crossrail construction. Calculation of the anticipated settlement, and assessment of its implications on the structure was undertaken by the Arup-Atkins JV.
All parts of the Covered Way were assessed. Particular care was required in assessing the girders due to the brittle nature of cast iron. The proportions of the covered way and the geometry of the girders are not ‘typical’. Consequently the assessment of the girders had some unusual aspects.
The assessment concluded that the consequences of the movement would be severe if no action was taken. Preventative measures to minimise differential movements between abutments during tunnelling were largely ruled out. Simple mitigation works were designed and implemented which modified the articulation of the girders to safeguard against brittle failure. Close monitoring was carried out during construction using strain gauges and other accurate instrumentation. The monitoring data was analysed to show the actual effects of ground movements and temperature changes and cross-checked against the assessment.
During construction, significant movements extended to girders where mitigating works were not expected to be necessary. The mitigation works and monitoring were extended to cover these girders, and the strain gauge results obtained from them provided the means to review the assumptions made in the assessment and relax the movement limits for unmitigated girders.
The paper describes:
– The relevant history and details of the structure.
– The assessment methodology.
– The design and implementation of the mitigation and monitoring works.
– The actual behaviour of the structure as analysed from the monitoring results.