CGJ7/162A Clifton Bridge

CGJ7/162A is a three-span continuous post-tensioned segmental twin cell concrete underbridge on the West Coast Mainline between Oxenholme and Penrith. The central span of the structure passes over 6 lanes of the M6 Motorway with a span of 57.9m. The two back spans, referred to as the Lancaster Span and Penrith Span, have span lengths of 30.5m.

The structure carries two 25kV electrified lines, the Up Main with a linespeed of 95mph and Down Main with a linespeed of 100mph and 105mph EPS (note error in CRT with regards to existing linespeeds).

The deck is supported by bankseat abutments, and two intermediate reinforced concrete piers situated either side of the M6 carriageways. The structure was constructed in 1967 after the deck was slid into position during a possession.

The deck is simply supported at the abutments but is supported continuously over the two intermediate piers. The free movement end is located at the Penrith bankseat. The two piers are 8.1m high and both are founded on reinforced concrete spread footings.

Investigations completed by Ramboll in 2019 (ref. 1620000224/CGJ7/162A/AR/001), concluded that the current deck has a load-carrying capacity of RA0 at 100mph but that if traffic could be controlled so that only one track is loaded at a time, the bridge cold sustain 15BSUs at 100mph. These conclusions are predominantly due to the unbonded post-tensioning system.

The most recent available detailed exam completed in June 2022, identified several significant structural defects:

• The bearings are heavily corroded but do not appear to be deteriorating.
• Hairline fractures to the high mileage abutment. This is a possible sign that the bearings may have seized.
• Numerous areas of spalled concrete and exposed secondary reinforcement throughout the superstructure and with similar developing to the soffit of central span over motorway.
• Widespread cracking to post-tensioned box girder both internally and externally.