Discovery of two bodies in a car buried under a landslip close to a tunnel portal in Dorset has raised questions about the maintenance of slopes above the tunnel.
The landslip at the northern end of the Beaminster Tunnel occurred just after 10pm on 7 July after prolonged heavy rainfall in the area. It damaged the tunnel portal and left several hundred tonnes of debris on the road.
The bodies were found after Dorset Fire Service was asked to investigate the landslip debris on 16 July. Local consultant PCRM Consultancy’s geotechnical specialist Peter Chapman said landslips had occurred around the tunnel portals before.
“The tunnel passes through the Upper Greensand so lies above the [geological] unconformity and is unaffected by the periglacial landslips that are common in other parts of West Dorset.
“Normally the natural slopes in the Upper Greensand are relatively stable but around the tunnel, the slopes are oversteepened,” he said.
“The weathering of the cut slopes can cause shallow instability and slumping, particularly when waterlogged.
“The exceptionally heavy rain on 7 July is likely to have caused shallow mudslides that may have resulted in material building up behind the masonry parapet, which is not designed to be a retaining structure, and causing its failure.”
Chapman said gabion baskets have been installed around the southern portal to add support.
The slopes above both portals used to be heavily wooded but much of this was cleared about six years ago.
“It is debatable about whether the vegetation is a good or bad thing,” said Chapman. “Roots can break down the surface layers of the soil but the roots can also help bind the top layers.”
Dorset Police closed the road after the incident and Dorset County Council engineers were waiting until the slope had dried out to inspect the area, in case there were further landslips.
But questions about why the landslide debris was not checked earlier have been raised.
Dorset Police has voluntarily referred the investigation to the Independent Police Complaints Commission.
The county council said a small landslip also occurred at the southern portal but that the
interior of tunnel, built in 1832, was intact.
It said the tunnel was inspected every two years with a principal inspection every six years and the most recent inspection was carried out in October 2011.
The council also supplied details of recent work on the structure but the maintenance regime seemed to focus on the tunnel itself.
The council said: “The county council does not own the land above or either side of the tunnel, although occasionally does work there to make sure the public using the road and tunnel is safe.”