Gerrards Cross tunnel segments are being lifted in by crawler crane at site of collapse.
Tunnel segments on the site of a new Tesco at Gerrards Cross in Buckinghamshire were this week being replaced in an attempt to kick start the project following a partial tunnel collapse in 2005 (NCE 7 July 2005).
A 280t crawler crane had installed about 20% of a total 63 precast units for main contractor Costain along a 65m stretch surrounding the 30m long collapsed section when NCE visited the site last Friday. When all 300mm thick segments are in place, the originally designed tunnel structure will be ready to be used as permanent formwork for the newly designed 600mm thick reinforced concrete arch that will be a continuous structure.
The tunnel is being monitored for movement. “It’s a very complex piece of civil engineering because of how the structure behaves,” said Costain project manager Martin Baughurst. Thermal movement causes 3mm to 4mm of movement but the requirement to remove old infill and replace it with new material above the structure to create a new area for the supermarket means movements of up to 20mm are also expected.
Limits have been set not to exceed 60mm and three monitors installed on alternate units – put in by Network Rail to check the tunnel’s stability after the collapse – will provide readings around the clock.
The original arch comprised precast units which connected in the middle, but will ultimately be structurally redundant. A new design by Scott Wilson will add a significant layer of extra support to this arch that will bear the weight of the store foundations.In addition, some fill material has been supplemented in the new design with lightweight foamed concrete.
The original cost of the project was £20.3M, and although Tesco refuses to reveal the revised cost of the project, in its 2007 annual report it wrote off £35M against the project.
The Health & Safety Executive is still investigating the collapse, but last June said: “The evidence shows that the reason the tunnel collapsed was linked to the way in which the infill material was placed over the concrete arch structure at the part that failed.” Tesco received Form B approval from Network Rail to go ahead with the rebuild of the tunnel vault over the Chiltern railway on 8 January.
Each 23t segment is being lowered into place during nighttime possessions and are expected to be completed by mid-April ready for concrete pouring.