This is an
update to the story of the construction, collapse and recovery of the
tunnel constructed over the Chiltern line at Gerrards Cross to permit
the building of a Tesco store. Just to remind those who have lost
touch with this
saga, bitterly opposed Planning Permission was obtained followed by
drilling in 1999, work started by contractors Jackson in 2003, the
tunnel structure partially collapsed on June 30 2005. Costain
already on-site erecting the store itself as a sub-contractor and took
over the whole contract in 2006. Quiet maintenance and
investigation occupied 2007 and in to 2008. This page pulls
together some of the activities in 2008 which have been on a larger
scale that many people realise.
along topic lines, rather than chronologically, and starts with the
of the Tesco store framework, beginning in March end ending in May.
topics include site access changes, a few trains and the present main project
to enable a crane to replace the missing segments.
Click on a
thumbnail to see a larger image. You can go the TescoTunnel
home page here.
Removal of the store structure.
Some bits of the framework were removed in 2005 to allow clearance of the fill material over the tunnel crown
but most of it was still present in March 2008. Removal seemed a much more difficult task than the original erection because Network Rail was taking a stringently low-risk position, insisting that any work involving lifting near the exposed tunnel crown could only be done within a Possession which meant nightwork.
The safety netting used in 2005 was still in place but was replaced as being life-expired.
Dismantling began in earnest with the northern part of the frame, which should have been in use as the store entrance for nearly three years by now. In many of these pictures, you will see Costain's obsession with safety, not only of its people but of site structures.
along Packhorse Road is still behind the bridge parapet which should
have long since gone. The middle two verticals here are supported
on temporary steel beams so that a safe access route had to be created
for the dismantling job because of the considerable void under the
framework at this point.
A piece of the
framework is removed carefully. Note the man watching the
operation. I have not seen a piece of machinery operating unless
it is supervised or observed from outside the machine, sometimes by
more than one man. The only exceptions seem to be simple
tranportation devices such as dump trucks.
had high-level work to do wearing a safety harness.
drillings continued while the framework was being dismantled.
Note the line of concrete panels guarding the allowed route for
the drilling machines.
frame is stripped to the simple basic structure and will be gone in a
few days. The temporary steel beams referred to previously are
A truck is
ready to take some of the framework pieces back to Norfolk where they
came from in the first place. The steel framework
in the foreground spanned the space over the tunnel crown where the
material had been removed. The verticals where burned off rather
than being unbolted and will presumably have to be fabricated again.
All the framework verticals were attached to concrete pads sunk in to the fill material. The pads were levered out and stockpiled for removal by trucks.
varied a lot in size and some were cut in half with this handy angle
grinder before being extracted.
Access to the site and within it is
In April a
prefabricated bridge was delivered and soon erected between the north
and south sides of the construction sitte. Until this event, only
very small machinery could be moved over the crown of the tunnel
and as only the north side of the site has access from the road, the
southern side was quite isolated.
A note about orientation. If you take the train through Gerrards Cross to local places like High Wycombe, passengers think of themselves as going to the west, whereas those going to Birmingham and beyond are travelling north. In fact the line goes through GX almost exactly on a SE to NW axis so, to save complicated language, we think of the tunnel being on an east - west axis.
The new bridge
spans the cleared crown of the tunnel, with bases
well back from the tunnel segments.
Here is the
finished bridge from the south side. There are
some other points of interest in these pictures. In the first
the original Costain site office cabins are visible and they will
in this story again later. On the other side of the bridge in this one
can see a road trailer being loaded with dismantled framework pieces -
white bags are big soft cushions spread along both sides ot the trailer
to catch anyone falling off it.
bridge, a piece of equipment like this, working on stabilising the
southern cutting side, had to be delivered by a complex process.
This was in March 2007.
West of the
main road bridge that divides the main eastern part of the site from
the smaller western part, there is space for a large crane. Its
task was to lift machinery over the railway during an overnight
The next morning, this machinery is waiting to move from the west site to east site, but it is not an easy journey.
The fill material beyond the southern arch of the road bridge was cleared just enough to create a very steep access route to the southern side of the main site. Of course, when the work of the machinery completed, the crane had to be brought back and the whole process gone through again in reverse.
Trains are running all the time - here
are a few examples.
A Chiltern Railways 165
runs west, or north if you prefer. The amount of time available
observe works on the site from a passing train is very brief.
is no view of the works before entering the tunnels from either end.
A WSMR set heads towards London.
A Down WSMR service slips through the eastern tunnel.
This is the interior of the eastern tunnel. At this point the two tracks are hard over to the right of the arch, but they slew across to the left to join the alignment of the original double track mainline. There is room for a 4-track railway, with catenary, through the tunnel.
tunnel, work is underway in January. The fill material was
removed in 2005 down to the level of the stitching beam, seen here
along the centre of the picture. Here men are digging out fill
material to allow measuring instruments to be placed each side of the
Later in the
year, some fill material was carefully returned to the tunnel crown to
stabilise its shape and position. By September, more had been
added, but the stitching beam was left exposed in the locations
being excavated in the previous picture.
Project to replace missing tunnel
A project scheduled to take 22 weeks is underway to enable replacement tunnel segments to be positioned by a mobile crane. Preparation work includes reshaping the ground to give stable positions for the crane, creating suitable access routes and piling to support itthe cranes.
In July the
site looked like this. The blue site office cabins in their
original location, quite narrow tracks each side of the gap and steep
access slopes. Much of this scene was soon to change.
The site cabins were removed and erected elsewhere. This allowed the north cutting side to be reshaped, leaving the old cabin foundations standing proud. This made a wider space at the base of the cutting.
cabins housed all the Costain people involved in erecting the store
but managament, professional and technical staff have moved to an
suite in Station Road that overlooks the site.
Shaping the access route with the site cabins in their new location on the right.
Meanwhile on the south side of the site, strong supports have been put in place to make the route along the railway strong enough for the work to come.
Shaping the access route on the south side.
This machine has been on-site for a while, boring large and deep holes for supporting piles. Here it is at the western end of the northern side of the segment gap. An interesting feature is that it is entirely controlled by a hand-held device, including its movements round the site.
machine made a hole here, perhaps more than one, and now
reinforcing wire is being lowered into it. One set is held up
a shaft across the hole while another is lifted and attached, before
assembly is again lowered into the hole.
machine is in the distance where the previous picture placed it.
The next set of wires is being attached to the earlier set. This give some idea of the depth of the bore.
This link will take you to a page created in 2006 about the cranes used to place the original tunnel segments.
developments become apparent I will attempt to incorporate them in the
You can go
TescoTunnel home page here.
Pictures and text © Guy Gorton 2008