THREE years after the disastrous collapse of a Chiltern line tunnel at Gerrards Cross over which a supermarket was being built, Network Rail has granted Tesco approval in principle for building work to restart using a new construction method.

A major rail accident was narrowly avoided when a 30-metre section of the new tunnel caved in onto the line on 7 July 2005, seriously disrupting Chiltern Railways services for weeks. Contractor Jackson was creating a 320-metre long tunnel using pre-cast concrete arch units to span the railway over which ground was to be built up for the Tesco site.

Following the collapse, Costain replaced Jackson as contractor and Scott Wilson took over from White Young Green as designer and the two companies have been working on a revised design for the past two-and-a-half years.

The planned new design to be submitted shortly to Network Rail for final approval involves removal of the existing supermarket frame and all remaining fill material. New pre-cast concrete lining units will then be craned into the collapse point to provide jointless permanent  formwork for a new insitu reinforced concrete arch which will together support the new
supermarket.

Once the tunnel is complete, construction of the store to its original design but with modified foundations will begin. Completion is expected to take 26 weeks.

Tesco spokesman Michael Kissman said: We are pleased that Network Rail has agreed in principle to the new design concept developed by our team, and we have worked closely with them to agree the detailed design of the tunnel, and they will maintain a presence on site when the development is underway.”