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 Railways 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.
Work is now set to resume after client Tesco finally received Form B approval from Network Rail for the permanent works required to complete construction of the tunnel.
Contractor Costain can now begin work, rebuilding the tunnel and supermarket to a Scott Wilson detailed design.
Stage one, to begin early next month, will involve keeping the existing tunnel framework and using a crane to add in replacement units where the collapse occurred.
These units will then be used as permanent formwork for a new reinforced concrete arch – a significant design change. The original arch was made from precast units which connected in the middle. The
reinforced concrete arch will be a continuous structure and will add a significant layer of extra support that will bear the weight of the store foundations.
The process of pouring concrete will begin in April and be completed in stages with the reinforced arch structure then waterproofed. This structure will in turn support lightweight foamed concrete and fill material which will create a suitable building platform.
The tunnel is expected to be complete by June 2010 so that the 21 week store construction programme can begin.
The supermarket’s foundations will be modified but the main store structure will be the same as originally designed and approved.
The cause of the original collapse is still to be determined.
After the collapse, Costain replaced Jackson as contractor and Scott Wilson replaced White Young Green as designer. But Tesco has already lost a court battle with its insurers over compensation it paid out to train operator Chiltern Railways for loss of fare revenue.