Completion of the Tesco Tunnel
Gerrards Cross
2009
A part of Guy Gorton's website


There has been a lot going on at the Costain/Tesco site in 2009, most of it largely unnoticed by the local population.  One important activity has been the replacement of the tunnel segments destroyed during or immediately after the collapse on June 30, 2005.  Even this is not very obvious unless you look over the parapet of Marsham Lane bridge although the sight of the blue framework of the crane visible over the  public car park fence could hardly be ignored.

This page is an account of the final part of this replacement task, made possible by a lot of preliminary ground work and some research and experiment into the design of the segments.  63 segments were placed, 2 of them half-segments to the original design to square off the ends of the original tunnels, 2 half-segments to start and end the sequence of new-design segments, and 59 full size new segments.  The new-design segments are not obviously different from the original ones but they do have a slightly different profile and the mating of the north and south segments at the crown is different.

I have used the word "completion"  but that only means that there is now one tunnel instead of the two we have had since 2005.  There is a brief account of "what next" at the end of the page.

Click on a thumbnail to see a larger image.   You can go the TescoTunnel home page here.

Installing the replacement tunnel segments.

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A 250t crawler crane was brought to and assembled at the Costain site.  The segments weigh around 22 tonnes each, but they need to be lifted at a considerable radius.  Date February 14.

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A very large platform was built for the crawler crane, together with a wide access route.  Preparations included stabilising the ground underneath,  boring some deep piles and safety fences round everything.  The crane placement and preparation work are quite different from those employed by the original contractor.  You can see both the earlier cranes on other pages of this website.  Date February 20.

tseg40The first task undertaken by the crane was to assist the building of this temporary gantry.  All this work was, of course, done at night during a Possession.  When complete, as shown in the next picture, the gantry was used to support the first segment being placed as there was no opposing segment for it to be supported by.  Once segments were in place, the crane could no longer reach the gantry which had to be dismantled from track level and taken out through the tunnel.
The train, 165018, is heading west into the western part of the tunnel.  The eastern, and shorter, part is in the foreground.
In a few more days, trains will be invisible as they were in 2005.  Date February 23.

tseg50With the additional structure on the top, the gantry is ready for use.  Date February 27.

tseg060The first new segments have been placed.  The temporary supporting gantry is now redundant.  This is where your scribe confesses that he fully intended to get out of bed in the early hours to take some nighttime action photos but regrettably his willpower proved inadequate to the task.  Date March 2.


tseg070The base and the channel for the segments survived the 2005 collapse and are in quite good order.  After all the rubble, fill material and other covering was removed only a small amount of remedial work was needed to make them fit for use now.   The foot of each segment is retained in position by wooden blocks and wedges which are later replaced by grouting the gap.
At this stage you can see a half-width segment and three normal size segments in place, but you can also detect that there is a gap between old and new which is where the replacement half-segment will be inserted later.  Date March 2


tseg080Another segment is waiting beside the crane for lifting into place at the next Possession.  Note that it is resting on its side, not on its ends - it is said that the segments are so flexible that if they were to stand for any length of time on their ends, they would become distorted and would take up an incorrect profile.  Date March 4.

tseg090The detail of how the north-side segments fit into the south-side segments at the tunnel crown is slightly different in the new design.  Date March 4.

tseg100Segments queue for placement.
Date March 4.





tseg110North side segments are firmly anchored into their channel.  On the right is the lower part of the platform being used by the crane.  Two things are worth noting here.  Along the right-hand edge of the tunnel bases a rail system will be fixed for a gantry that will straddle the tunnel segments for the work to be done over the top and down the sides of these segments.
The other thing to note is that the entire tunnel length from here to Packhorse Road bridge to the west, on both south and north sides, will have to be excavated to the level you see here.  Whilst a lot of preparatory work has been done, it is not very photo-worthy, but I hope to record some of the action later.  Date March 5.

tseg120The temporary gantry is no longer in use but has not yet been removed.  The space for the old-type half segment shows daylight.  Date March 5

tseg130Here is the space from a different angle.  The stainless steel face to the opposing segment shows clearly from here.  Date March 5.



The next three pictures are of a delivery of yet another segment.  Date March 5.

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tseg170The streak of sunshine on to the track through the tunnel gap will be extinguished permanently in a few days.  I hope the passengers in Chiltern 168002 enjoyed it.  Date March 6.

tseg180Approaching the halfway mark in segment placement.  Date March 11.

tseg190More segments in place and many grouted. 
Date March 13





tseg200Just a small gap left to fill.  Date March 19

tseg210The view along the north side.  Also seen is the crane platform at the right, built up in layers on earlier layers.  Date March 20.

tseg220Meanwhile the team on the south side is mixing and placing grout.  Date March 20.





tseg230Segment placement is almost complete, there being just one half segment to go in at the other end on the left on this picture - the gap is protected by safety fencing.  In the foreground, looking west, it is possible to see the slight difference in profile where the new segments meet the old.  The stitching beams on the new segments do not stand so high so that the top of the concrete is at the same level as on the old segments.  Date April 3.

tseg240The slight profile change shows here too.  On the right is the base of the crane platform.  The ground it stands on is at approximately the level created during the recovery work after the collapse, but hugely reinforced in a number of ways.  Date April 3.

tseg250Looking east towards Marsham Lane Bridge, the almost completed new work completes the link from Packhorse Road bridge to just short of Marsham Lane bridge.  Date April 8.




tseg260Overview from the north side of the linked tunnels.  Date April 8.

tseg270The north side complete.  Date April 8.

tseg280Just the last half-segment waits to be lifted and fitted in the gap, which we have seen in earlier pictures,  between the old western tunnel and the new segments.
Date April 8.



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Finally the last segment is in place so the south side is also complete.  Date Apr 17.




What next?  The old and new tunnel segments seen in this and other pages will be used as formers for the placement of a reinforced concrete outer casing.  The new casing will be the load bearing structure, not the precast concrete segments.
You can skip the page about the west portal and go straight to the continuation of the completion of the main structure here..



You can go the TescoTunnel home page here.

Pictures and text Guy Gorton 2009
Updated 8th June 2009

Link tyo final page added October 2014