A £6m project to install a concrete deck slab over 150m of abandoned coal mine workings has been completed near Edinburgh. Donaldson Associates designed an innovative slab with transition zones of Vibro stone columns and Tensar polymer geogrid reinforced stone platforms for the project.

Subsidence of the undocumented shallow mine workings was first recorded in 1880, and in 1992 crown holes of up to 2m diameter appeared. Attempts to fill the workings were only partially successful, and further subsidence occurred, imposing 60 km/h speed limits on the 200 km/h railway.

The recently installed slab deck is supported on 336 steel and concrete mini-piles between 9·5m and 27m in length, down to solid rock. The alignment was transferred to a 300m temporary slab during a 54h possession, and the track reinstated afterwards in a series of short possessions. To avoid differential settlement of the track in the transition zones onto both the permanent and temporary slabs a 12·5m zone of Vibro stone columns was overlaid with a 35m long 150mm to 550mm deep stone layer reinforced with Tensar geogrid, a high performance geosynthetic grid. Levels were monitored throughout the first two weeks of 200 km/h operation, and no differential movement was found.

Tensar International, Great Britain

Reader Enquiry Number 139