DIFFICULTIES WITH the highly complex repairs needed for a weakened section of Birmingham's M6 Spaghetti Junction have further delayed its reopening by 10 months, the Highways Agency admitted this week.
Over pounds 1.5M worth of strengthening work on the elevated sliproad, partially closed in March last year, finally gets under way on Monday - at triple the expected cost. But the heavily trafficked route will not be open until next August - 19 months late.
Understrength concrete beams supporting the slip road over an electrified rail line have defied repeated design scrutiny from all but the most advanced three dimensional finite element analysis. Even when repairs start, contractor Taylor Woodrow Construction has been ordered to stop work immediately if drilling tolerances of stressing bars exceed a few millimetres.
'Repairs are so sensitive we would have to re-analyse the entire solution if bars go off line,' said HA project manager David Cropper. 'Numerous analysis attempts suggested the beams were so weak that they could not even support their own weight. But clearly this was incorrect as they show no distress even under heavy traffic loading.'
The 70m long V-shaped viaduct carries two converging sliproads. One is totally closed but the other is a main feeder route between the M6 and Birmingham city centre. Its 20,000 vehicles/day flow is currently channelled across the centre of the deck, creating extensive traffic delays (NCE 19 February).
The problem centres on four 2m deep concrete crossbeams each up to 42 m long and located between insitu and precast deck sections. They were built integral with insitu side span slabs but also contain a lower downstand shelf supporting the central railspan's precast deck beams.
Repairs involve strengthening lower beam sections by inserting horizontally over 350, up to 2.5m long stressing bars at numerous high-stress hot spots across each beam.
'The conceptual solution was worked out last winter and repairs were expected to start then,' said Cropper. 'But adding prestress to an insitu beam can create additional stress problems elsewhere and the design did not work out as expected. We needed a second row of bars to relieve tensile forces and must monitor repairs.'