troubled Gerrards Cross site, three years after an earthworks induced tunnel collapse halted work.
The project has been in limbo since June 2005 when a major rail accident was narrowly avoided after a 30m section of new tunnel caved in onto the live Chiltern railway line (NCE 7 July 2005).
At the time, contractor Jackson was creating a 320m length of tunnel over the railway using precast concrete arch units. The aim was to build up the ground over the tunnel to form a site for a new Tesco supermarket.
After the collapse, Costain replaced Jackson as contractor and Scott Wilson replaced White Young Green as designer.
The two firms have been reviewing the construction method and working on a revised design since March 2006.
Network Rail has now granted Tesco Form A approval in principle for its new construction method. Detailed design is underway and is expected to go to Network Rail for Form B approval next month.
The new construction method involves removing the existing supermarket frame along with all remaining fill material.
The existing precast structural concrete tunnel lining will remain and new units will be added by crane at the collapse point.
These units and existing sections will be used as a permanent formwork for a new insitu reinforced concrete arch.
This has the advantage of having no joints and also offers an extra support that will bear
the weight of the store and its foundations.
Once the tunnel is complete, construction of the supermarket will begin.
This is expected to take 26 weeks. The store’s foundations will be modified but the store will retain the original design.
Blame for the original collapse is still to be established, but Tesco has already lost a court battle with its insurers over compensation it paid out to Chiltern Railways, the operator of trains using the line at the time of the collapse.