UNEXPECTEDLY RAPID mining subsidence forced the Highways Agency to extend emergency speed limits on the M6 in the Midlands last month.
Compression humps on the motorway south of Birmingham have defied expert predictions by suddenly worsening, the agency said.
A side-effect of measures to minimise mining-induced settlement, the humps were expected to have stopped reforming by the end of last year.
They are ripples in the original flexible carriageway at each end of a 750m stretch of 445mm thick reinforced concrete overlay.
This was installed between junctions 3 and 4 to allow UK Coal to tunnel 600m beneath the M6 and exploit a 5m thick coal seam.
As the overlay settles, ripples form in the road surface.
Repairs are done when these humps reach 50mm high. A year ago this was being repeated about every four weeks. Since 2001 the Highways Agency has been planing off the irregularities and frequently resurfacing up to 1km of carriageway.
An 80kmh speed limit has been in force on the rippled section since October 2001.
'Although settlement is occurring less frequently than last year, there has been another big and quite rapid movement recently, and we're still not sure when it will finally stop, ' said Highways Agency area manager Stephen Edwards.
'With the M6 Toll road opening nearby in the not too distant future we will have to make sure the existing M6 is up to standard.' UK Coal has already paid more than £4M for the original overlay and associated works, and will be liable for resurfacing the affected 1.2km when settlement finally stops.
But Edwards confirmed it will be the agency that will pick up the tab if a major interim repair operation is needed before the M6 Toll opens.