Underground holes as big as king-sized beds forced the sudden closure of the seven-mile stretch of the A6 south-east of Market Harborough, contractors have revealed.
The holes have formed on both sides of a bridge south-west of Desborough on the A-road, as embankments leading to the bridge settled over time. The bridge structure itself is sound.
It was the discovery of these unstable embankments in a routine inspection on January 18 which forced Northamptonshire Highways to immediately close the road.
Ground penetration radar and dynamic probing confirmed that, not far beneath the busy road, the ground on both sides of the bridge now contained large underground holes.
The seven-mile stretch of road between the A14 and the B576 junction to Desborough – which only opened in 2003 – will remain closed while repair work continues.
Weather permitting, the road is due to re-open by the end of February.
Head of operational delivery for Northamptonshire Highways Leon Jolly said: “There has been gradual settlement on both sides of the bridge, in the embankments leading up to the bridge structure, resulting in holes beneath the surface.
“How big are the holes? Maybe two metres by two metres by one metre.”
“We have to deal with this quickly, so it doesn’t deteriorate further.
“It’s very early to say how this occurred, but to see it happen to this extent is quite rare.
“I haven’t come across one like this in 15 years.”
About 40 people have been involved in the repair project so far, with about 10 on site at the moment.
When the Mail visited yesterday (Thursday), workmen had dug away the entire embankment on the southern side of the bridge and were starting to infill it again with compacted material.
Work on the embankment on the northern side of the bridge has not started yet.
Northamptonshire Highways has not said how much the unexpected repairs will cost, saying it will not fully know until workers dig up the northern embankment.