Triple Tunnel Troubles (More Tunnel 1)

In 2005 the near complete “Tesco Tunnel” at Gerrards Cross began to collapse. Fortunately nearby trains were approaching the station in order to make a stop or were at the platform and were able to come to a stand well short of the rubble which fell.

From a distance the hole looks bad, but not atrociously bad.

But the closer you get …

… the worse it looks.

And fom track level …

It was definitely not good news.

At this point it is worth understanding how the tunnel was built.

Load bearing walls were built beside the tracks and arch segments were craned into place …

… which formed the shell of the tunnel.

The first job after the collapse was to remove the debris …

… and then demolish and/or remove the arch segments where it was considered that there might be a risk of further collapse.

This activity was prompted by the obvious fact that the collapsed bit was where the spoil infill was being tipped on to the arches to make a nice base for Mr Tesco’s shiny new store and car park. So all the segments and their incomplete infill work were removed; leaving a shiny new tunnel with a large gap!

It remained so until 2007 when new contractors were appointed to rebuild the missing bit and finish the job.

What was strange about the whole kerfuffle was that the report on the cause of the collapse was, apparently. not to be published! It was only through the efforts of a local resident, the freedom of informational legislation and dogged persistence that the report was – eventually – released into the public domain.

So what did go wrong?

One thought was instigated by a late change in the plans. Originally it was to be a two-track tunnel; but later it was changed to allow a return to the older four-track layout …

… at some future date. Was this change of plan bodged? Apparently not – all revised stresses were correctly calculated for the wider arch.

The report is lengthy and technical, very technical, as you would expect. To understand it completely, you need to stand on a tray of eggs!

So was the arch not strong enough for the tonnes of crud to be heaped upon it?

As far as fbb can tell it was all OK. But the strength of an arch relies on downward pressure transmitted from the top of the arch round the curves to the side supports. In theory, at least, you would need to apply the load evenly from the peak of the arch and then out to the edges.

But if the infill was filled in from the sides of the arch first …

… the egg could crack and collapse. And it did. You would find it much more difficult (and messy!) to stand on a tray of eggs if they were laid sideways rather than on end.

The new contractor actually won some sort of award for sorting it out.