The railway line between Oxford and Didcot could have remained open if a key process in the repairs of Nuneham Viaduct railway bridge went as intended.

The bridge, which carries the main line over the River Thames east of Abingdon, has been closed since April 3 after significant movement was detected in the viaduct's structure.

It has caused major disruption with rail replacement services forced to run between Didcot and Oxford.

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The line is not expected to be operational again until June 10 while engineers completely rebuild the south abutment where a huge crack has formed.

However, the man tasked to oversee the project said the disruption could have been avoided if initial repairs went as planned.

Stuart Calvert, capital delivery director for Network Rail, said the movement of the south abutment had been monitored for years.

Repairs were set to go ahead, but when the movement accelerated in February and March this year, the engineers were forced to respond quickly.

It was believed the problem was the unstable ground underneath the south abutment.

To amend this, polymer grout was injected into the ground to allow repairs to the abutment. However, when this took place, the movement accelerated further.

Just a month or so later, the bridge reached the maximum limit of movement and was closed.

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Mr Calvert, 59, said: “What we found with the grout was that rather than stopping any movement, the movement accelerated.

“What we don’t know is did that actually cause the movement to accelerate, did it actually slow it down from what it would have done, or did it have no impact at all.

“We genuinely don’t know. It’s a standard way of dealing with this ground stability but it didn’t work. At this stage we just don’t know why.”

He added: “Had the polymer system worked, which is a fairly standard system, the railway would not have shut. We would have had a speed restriction, done the repairs, and then we’d have raised the line speed again.”

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Mr Calvert said Network Rail’s focus was currently on getting the service restored, but there would be a detailed review into how the repairs were managed afterwards.

He said: “There’s been a lot of people saying, ‘how on earth did people let this get there?’, and I get that, but the reality is we were managing this incredibly professionally with a really controlled plan.”

He added: “We’ve got nothing to hide. We’re trying to explain what’s happened, exactly what we’re doing, and how we’re doing.”