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

This is the final page in the story of the creation of a platform over the railway to enable a Tesco store to be built.

It continues the story from where we left it in the page about the completion of the tunnel itself in April 2009 and will start from the same vantage point in the south east corner of the site.

The remaining work consisted of three main features - 1. build a new reinforced concrete tunnel over the original structure,  2. bring the site up to the required level by a number of means,  3. build the store and finish the ground level works.

You can go the TescoTunnel home page here.

Constructing a new reinforced concrete tunnel.

June 2009.  Construction of the new tunnel started over the new segments that had been placed in the collapse zone.  This had the advantage that the original fill material had already been removed down to the base of the segments.  Here a solid base has been constructed, spacing material placed over part of the new segments and the north and south segments anchored together at the crown of the arch.  The stitching beam along the crown connected segments on the same side to each other but did not connect north and south segments.
At this stage, the large crane at the right had to be set down to the ground during the day to ensure that nothing was lifted that could penetrate the tunnel structure.  The crane could only be used during overnight possessions of the railway beneath.

June 2009.  This is just a more detailed view of the base of the new tunnel structure.  At the lowest level is the base of the original tunnel segments.  Along its left hand, or outer, edge can be seen a length of steel rail which will soon be used by two Goliath cranes.


July 2009.   The two Goliath cranes, specially built for this project, have been delivered, assembled and commissioned.  They are independently powered, each with its diesel generator and other equipment.  Items can now be moved from one side to the other of the tunnel without the need for a rail possession and thereforeduring normal working hours, although stringent conditions applied to minimise potentiak damage to the tunnel structure..
In the background, on the left, are two machines that are creating borings for reinforced concrete piles - we will see them more clearly later.  At this time, only the area around the collapse was excavated down to the foundations.

July 2009.   We have switched to the north east corner because I do not have a suitable picture that illustrates the process well enough.  Here the reinforcing steel is in place and the first sections of concrete have been poured.  Exactly the same process is taking place on the south side.  The Goliath crane tracks are in a very tight space on this north side but there is no equipment on the crane legs other than the wheels.

August 2009.   The new tunnel structure is clearly visible, as is the southern crane track.  The new structure has a slight wedge shape near the top which will lead to a flat top to the completed work.

September 2009.   This section of new tunnel is now complete.
In the distance, directly under the crane capacity information, is a row of the deep piles, now uncovered as removal of fill material down to base level continues.  There is another invisible row of similar piles along the northern edge of the site which are linked to the visible ones.  Better pictures follow.

October 2009.   The cranes are now moving west to enable the construction of the new tunnel to continue.  On this south side, pre-2005 fill material was not deep, so that smaller piles are sufficient to hold the old cutting side back, but the deep piles to restrain the deep fill material further west are clearly visible.  The huge wall of deep piles to restrain the north-side fill material, together with all the equipment and structures that are on it, is clearly visible.

October 2009.   Just a different angle which illustrates the concrete piles more clearly and shows the horizontal waler beam quite clearly.

Bringing the site up to level. This phase started from the east while tunnel creation work continued.

November 2009.  We have stepped further into the south east corner of the site.  Material from the west end of the site has been transferred to provide the fill between the original cutting side and the lower parts of the new tunnel segment.  The nearest part is being dealt with somewhat differently in order to create a massive structure round the tunnel's east portal.

December 2009.   Progress continues on fitting the tunnel outer casing, but a route is now available over the tunnel roof to allow fill material to be moved from north to south.

December 2009.   The tunnel at the east portal has mass concrete set round it, providing extra protection from any derailed train.

January 2010.   Ponds were created to hold foamed concrete as fill material.  Two of the ponds are visible here, but exactly similar ones are on the north side.  This material is very light at less than 0.4 specific gravity and was used in several layers in all places that are directly over any part of the tunnel.  Whilst loads on the tunnel were reduced by ths method, the main benefit was to minimse the increased load on the original piles designed in the first place to carry the original tunnel, not, of course, the new overbuild structure.  This was probably the largest scale use of the material anywhere in the world.  The ponds were shallow at less than a metre deep, partly because at this very low density the poured concrete has a tendency to collapse into itself, and partly because anyone falling in would immediately sink
The temporary bridge over the site has now gone as material can cross between north and south over the tunnel crown.

February 2010.   Some of the concrete layers are visible here.

March 2010.   Here the foamed concrete is up to the finished level and ready for the final material to be placed.

April 2010.  We have switched to the north west corner of the site because progress was made in parallel with the work progressing from the east.  Several things of interest here.  Immediately on our right are foamed concrete ponds in layers and on the left, the wall and waler beam retaining the original fill material to enable excavation down to the base of the segments.  The Goliath cranes did not progress to the western end of the tunnel where it was possible to move material between north and south sides over the tunnel crown..  They have almost finished their work and would soon be dismantled.

May 2010.   We are still at the north west corner of the site.  It is not obvious from the photo, but there were steep ramps from the lower level up to the level we are standing on.  On the south side, the horizontal waler beam is clear.

June 2010.  Foamed concrete now extends from north to south of the area between the retaining walls.  Heavy vehicles can now cross the tunnel crown..

Ground level works and building the store.

July 2010.  We are still at the north west corner.  The foamed concrete has been covered with fill material and is being carefully levelled. On the right, behind the fencing panels, square holes have been dug in the fill material for the foundations for the steelwork of the store itself which was built by another contractor, Dudley's..

August 2010.   Much of the main steelwork of the store is now in place.  The floor has also been prepared for tiling.

September 2010.  Roofing taking place.  Progress was rapid on several fronts.

October 2010.  The  building really takes shape.  The interior is tiled, many fittings are in place, Tesco is in occupation.

29th November 2010.  We are still at the north west corner of the site.  The shop opened this morning.

29th November 2010.  Final view from the south east corner of the site on opening morning.

You can go the TescoTunnel home page here.

Pictures and text © Guy Gorton 2011