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Programme of Repairs
M6 Thelwall Viaduct, Cheshire
Programme of Major repairs
Works Complete
Works were completed in February 2005 and Thelwall Viaduct was fully reopened.
Background
The northbound viaduct was partially closed to traffic in July 2002, after it was discovered a roller bearing, which was installed when the viaduct was refurbished in 1996, had split in two.
Bridge bearings are positioned between the bridge deck and its supporting columns. They allow thermal expansion and contraction of the viaduct between summer and winter as well as flexing due to traffic.
Initially it was hoped repair work could be carried out quickly but an extensive study of all the support bearings revealed serious problems. After it was realised that all the roller bearings would fail in the short to medium term the decision was taken to replace all 148 of them. This decision was made in the interest of public safety and to prevent further damage to the viaduct.
In order to gain access to the bearings scaffolding was erected around each of the 35 piers and 2 abutments that support the viaduct. In total 320 kilometres of scaffolding poles was used, which if laid out end to end would stretch from London to Carlisle.
When replacing a bearing it was necessary to jack up the viaduct to allow the old bearing to be removed and a new one installed. This was a complex operation which first required the strengthening of the beam above the bearing (step 1 in the diagram below). Temporary jacking brackets could then be bolted to the beam (step 2), and the bridge jacked up using four 300 ton jacks (step 3). Once the failed roller bearing had been replaced with a new slider bearing the bridge could be lowered and the jacks and brackets removed and used elsewhere on the bridge.
By simultaneously working at several locations the replacements were carried out as speedily as possible. However, because the viaduct had to be jacked up 148 times - once for each bearing this was not a quick operation.
Ongoing Investigations
The fact that the roller bearings failed after only six years is of considerable concern to the Highways Agency and the cause of failures and the responsibility for them have been the subject of extensive investigations. A number of parties were involved in the design, manufacture and installation of the roller bearings, against some of whom the Highways Agency has commenced legal proceedings, which are ongoing.
Any queries can be forwarded to Thelwall_Viaduct@highways.GSI.gov.uk
