The Thelwall Viaduct (grid reference SJ664883) is a steel composite girder viaduct in Lymm, Warrington, England. It carries the M6 motorway across the Manchester Ship Canal and the River Mersey. Its location on the motorway network is between junctions 20 and 21 of the M6, the former being also known as junction 9 of the M56.
It actually comprises two entirely separate bridges, one of 4,414 feet long carrying the northbound carriageway, which was the longest motorway bridge in England when it was opened in July 1963, and one 4,500 feet long carrying the southbound carriageway which was opened in 1995.The longest single span is the one of 336 feet crossing the ship canal.
In July 2002 a failed roller bearing was discovered and it became necessary to close all but one northbound lane. As the M6 at the time carried an estimated 150,000–160,000 vehicles per day, this led to serious congestion. The viaduct was not completely reopened to daytime traffic until February 2005, and subsequently remained partially closed at night for further remedial work to take place. In all, 148 bearings were replaced, with the repair scheme costing around £52 million.
Given the bridge's height and openness to the elements it has frequently been the subject of speed reductions due to strong gusts of wind that badly affect the stability of high-sided vehicles. On several occasions lane closures have resulted as a consequence of articulated vehicles simply being blown over. However, the open sides of the bridge are a deliberate design feature to reduce the likelihood of snow drifts building on the carriageways.
In April 2011 a massive free party took place under the bridge, with reportedly over 5,000 ravers in attendance