The west portal of the Tesco Tunnel
Gerrards Cross
A part of Guy Gorton's website

The west portal of the Tesco tunnel will be the only part of the structure in full public view, and will be a prominent feature as seen from the station platforms and footbridge.

Much preliminary work was done in this area in 2004 and 2005 until it was brought to an abrupt halt by the tunnel collapse on June 30 2005.  In fact, some important work was done between the collapse and the re-opening of the tunnel and railway in August that year.  An account of that work has been on-line here previously but it is now available again by clicking on the picture left below which is how the site looked in 2005/6.


The temporary structure which enabled all the work under the bridge to be done was removed in late 2006.   Work was done to stabilise the cutting slope in March 2007 by equipment, seen below, which was lifted over the railway from the north to the south side by a large crane.

This new section takes up the story again, after some three years of only sporadic construction activity.  To see a larger image, click on the thumbnail.


Early in 2009 the west portal looked like this.  But the peace was about to be disturbed as contractor Costain moved into action.

By early April this busy scene shows that the north-side cutting slope has also been stabilised.  A wall made of concrete components has been erected to separate the working railway from the construction area, and has a safety fence attached to its top.  Ground material has been moved around and steel for temporary structures is being delivered and positioned.  The shadow on the right is from the existing bridge.


The view on the north side illustrates nicely that the 2-track railway only occupies half the tunnel, although if it was ever enhanced to 4 tracks with electric catenary and continental loading gauge stock, the tunnel could handle it.  The ground has been prepared for the placement of a steel structure to support the additional tunnel arch segments.

The steel frame is now in place ready for the first tunnel segment to be placed.  On the right the massive reinforced concrete structure supports tunnel segments but it goes right through the arch of the bridge and has taken over the structural support of the road bridge itself.  The roadway over the bridge will be widened on this, the west side.  Under the bridge arch the tunnel segments are in one piece but will be extended by two-piece segments.

One of the tunnel segments waits to be placed on the structure seen in the previous picture.  The placement will take place at night during a possession so that the safety of passing trains is not put at any risk.
This part of the tunnel was always planned and clearly the segments had already been made when the collapse occured and are now being brought from store.  This one looks unchanged, but some have been modified.


The first segment is now in position, supported by the steel frame.  The stainless steel hinge plate awaits its meeting with another segment placed on the south side.


The first segment on the south side is now in place.  It is resting on foundations created in 2004/5 but has itself been modified, probably to provide more steel loops for interlacing with the ground foundations.  We are now in early May 2009.

All the pairs of segments are now in place, including one obviously new one and a half-width one to complete this section of the tunnel.

This view through the old road bridge arch shows the newly placed tunnel segments being stitched together to ensure that they do not move relative to each other.  A similar stitching beam will be created on the south set but the two are not connected to each other.
In the station is a Chiltern 165 bound for High Wycombe and beyond.  For people who are interested in such minutiae, I think it is unit 165023 which is just discernable on the original image!

The next stage was to create the tunnel portal itself with its impressive curvature.  But first another supporting steel frame is needed and this colourful framework appeared.  It had to cover the entire railway to support the concrete pieces and provide access for the men working on it.

t11The steel frame now spans the railway and is ready for the shaped pieces to be placed on it.  In the meantime on the crown of the tunnel to the right, temporary red steel beams are being secured to the concrete segments.  With the addition of more beams at angles, the steelwork will ensure that the north and south sides of the tunnel do not move relative to each other.  This was not done in the original construction but it has been for the full length of the tunnel during reconstruction.

The first of the shaped pieces is in place together with its unseen equivalent on the north side.

Most of the shaped pieces have been placed.  Unlike the tunnel segments they cannot be self-supporting so they are strongly anchored at ground level and will finally be held by the reinforced curved beam which we will see later.


The three dimensional curvature of the portal is well displayed here.
At this stage, debonding material has been fixed to the outside of the tunnel segments and a base created  in preparation for the building of the new tunnel structure over the top of the original segments.

This picture has rather a lot going on!  The tunnel segments on the right now  are encased in curved reinforcing steel and ready for concrete to be poured into the curved shuttering.  Exactly the same applies to the north side.
On the left, access scaffolding, safety fencing and access platforms are in place.  In an earlier picture we can see the ground level anchorage, but now it is all encased in concrete.  The crane has changed too, now being lighter but with a very long reach.  The date is mid-September 2009.

This gracefully curved reinforcing steel will soon disappear into the concrete ring beam that will form the visible part of the tunnel portal.

The ring beam is now complete, although still surrounded by steel supports and woodwork, but it is entirely self supporting.  It is now early November 2009.

t18Late November and the west portal stands clear of most of its clutter.
It was not long before more safety fences and other new clutter was put in place, for there is still much to do before the space between the portal and the bridge behind it is filled and shaped ground created to the north and south in  the present cutting.

You can go the TescoTunnel home page here.


Pictures and text © Guy Gorton 2010

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