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Subsidence mystery on new DBFO road
00:00 18 Aug 1999
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by Graham Ridout
An investigation is underway to determine the cause of the sudden subsidence that created traffic chaos on a section of the recently completed reconstruction of the M40 motorway, which was one of the first design, build, finance and operate road contracts.
Until the cause of the subsidence is established, it is not known whether DBFO contractor UK Highways will be financially penalised. Four instances of subsidence, each approximately 2mx2m and about 70mm deep, were discovered on 10 August after a period of heavy rainfall and prompted the closure of three of the four westbound lanes between junctions 2 and 3. Within two days, UK Highways (a consortium of contractors Laing and Tarmac with consulting engineer Hyder) had repaired and fully reopened the motorway.
UK Highways' general manager Mike Mercer-Deadman said the cause of the subsidence is "a puzzle and does not appear to be a design fault." The section of the motorway that failed was an existing three-lane flexible construction carriageway that had been widened to four lanes and strengthened by the addition of an 100mm overlay comprising a 50mm asphalt base course with a porous asphalt wearing course.
One suggestion being investigated was whether the subsidence was caused by a "swallow hole" in the underlying chalk substrata which suddenly collapsed as a result of the heavy rain.
UK Highways completed the 24-month reconstruction programme in January 1999. Under the terms of the contract, UK Highways can be "fined" for excessive or unscheduled lane closures. The amount depends on the time of day of the closures and the amount of traffic using the road.
Mercer-Deadman said that it was not yet known whether any penalty would be levied, adding that UK Highways and client the Highways Agency were "still considering the issues."
An investigation is underway to determine the cause of the sudden subsidence that created traffic chaos on a section of the recently completed reconstruction of the M40 motorway, which was one of the first design, build, finance and operate road contracts.
Until the cause of the subsidence is established, it is not known whether DBFO contractor UK Highways will be financially penalised. Four instances of subsidence, each approximately 2mx2m and about 70mm deep, were discovered on 10 August after a period of heavy rainfall and prompted the closure of three of the four westbound lanes between junctions 2 and 3. Within two days, UK Highways (a consortium of contractors Laing and Tarmac with consulting engineer Hyder) had repaired and fully reopened the motorway.
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UK Highways' general manager Mike Mercer-Deadman said the cause of the subsidence is "a puzzle and does not appear to be a design fault." The section of the motorway that failed was an existing three-lane flexible construction carriageway that had been widened to four lanes and strengthened by the addition of an 100mm overlay comprising a 50mm asphalt base course with a porous asphalt wearing course.
One suggestion being investigated was whether the subsidence was caused by a "swallow hole" in the underlying chalk substrata which suddenly collapsed as a result of the heavy rain.
UK Highways completed the 24-month reconstruction programme in January 1999. Under the terms of the contract, UK Highways can be "fined" for excessive or unscheduled lane closures. The amount depends on the time of day of the closures and the amount of traffic using the road.
Mercer-Deadman said that it was not yet known whether any penalty would be levied, adding that UK Highways and client the Highways Agency were "still considering the issues."
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