UNSTABLE SHAFTS in Railtrack's Strood railway tunnel in Kent will be filled and stabilised with lightweight concrete, it was revealed this week.
Collapse of a capped shaft in an orchard on 12 June prompted the immediate closure of the North Kent Line between Higham and Strood. Railtrack has since identified another shaft in Hancock Close, Strood, as potentially unstable and evacuated eight homes as a precaution.
Inspection specialists from Inspectahire were called in last week to survey all six remaining capped construction shafts on the 4km tunnel with a pole mounted video camera. The results showed that both the collapsed shaft and the Hancock Close shaft would require concrete plugs.
Hydrogeologists have also been called in to investigate the cause of the collapse, in which the shaft cap and backfill collapsed into the shaft above the rail line. Initial findings are that changes in groundwater conditions in the chalk hillside caused the problem.
The collapse comes after two other rockfalls in the tunnel in the past seven months. The first, in December, was repaired with shotcrete and steel supports, but the repair caused destabilisation of the chalk, causing another rockfall in May, a day after the tunnel reopened.