Feb 222009
 

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I left you with the question regarding this structure, its relevance to the underlying geology and how it works.

This structure takes the River Wheelock beneath the Crewe to Manchester railway line near to Sandbach, Cheshire, England. This gentle river would normally need a simple bridge or large culvert if it was not for the underlying geology.

The bedrock here is, or was, Triassic salt. Since the 1930s salt was extracted here by brine pumping. This involves pumping water into the salt which dissolves it, pumping the resulting brine back to the surface and then evaporating it to produce table salt. The problem was that there was little control where the pumped water was going to, or indeed coming from. Dissolution of the underlying salt causes the land surface, and here the railway embankment, to subside. It also creates local lakes called flashes and Elton Flashes can be seen in this wider Google Earth image below with the River Wheelock flowing beneath the railway just left of centre in the picture.

flashbridge

So, as the land surface subsides, the River Wheelock will flow through successively higher tubes underneath the railway.

As the subsidence also causes the rail line to dip downwards, the track bed has to be built up with ballast to keep it level. However, this narrows the height between the rails and the overhead electrical wires so the gantries are designed so that they can be jacked up. These can be seen in the upper picture, and the one below shows the transition from normal gantries on stable ground to the jackable ones in the subsidence area.

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This salt related subsidence has caused engineering problems elsewhere. Nearby,the Trent and Mersey Canal passes beneath a bridge. Because of subsidence the banks of the canal have had to be built up otherwise the water would spill over the edges. However, this brings the canal level so close to the underside of the bridge that canal boats couldn’t pass underneath so the bridge had to be raised. From the brickwork under the bridge in the image below it can be seen that this has happened several times, and the current bridge is also jackable.

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