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Culverts

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Culverts are a very common feature on the road network, and come in a surprisingly large variety of sizes. Essentially, they are just small bridges, but with modern construction methods, very large culverts are sometimes used in place of bridges!

The simplest form of Culvert in use today is a basic ridged plastic pipe, laid in the stream bed and bedded in with soil before the road structure is built above. The ridges prevent the material above from crushing the pipe, and the design can be adapted to fit a variety of water courses. Wide streams can see a row of small pipes, deep channels can be spanned with a larger pipe, sometimes partially buried below the stream bed. Above a certain size, the plastic pipe is generally replaced by a concrete pipe, or at least sectional pipe, which provides more rigidity under the material above.

In the past, before plastic pipe became commonplace, it was common to use corrugated galvanized steel pipes, however as can be expected these were prone to rusting, and so collapse. Few remain on the network today, compared to those that have existed. However, corrugated steel pipes have made a partial come back, and can now be found in use as form work for large concrete structures. These can extend to significant sizes, enough to carry a river and access road to one side large enough for a Land Rover, as is seen at Edendon Bridge on the A9. The primary differentiation between these structures and bridges, despite their significant size, is that they are still essentially pipes with soil between the top of the structure and the road itself.

As culverts generally cross small water courses, with some notable exceptions, they tend to be more common in hilly regions, where many small streams cross the landscape. This often leads to a small waterfall out at the downhill end, as the pipe is generally laid horizontally. Whilst this isn't often a problem in small upland streams, in other locations it can conflict with wildlife conservation. In such instances, pipes may be laid deeper and with a larger circumference than is necessary to allow the passage of wildlife. The bottom half of the pipe is then infilled with material to produce a simulated river bed.

The other common location to find Culverts is in low lying areas, such as the Somerset Levels, where they are used to span ditches, drains and rhynes. In many cases, the water is slow moving and as such the pipe may be located completely below normal water level, as there was perceived to be no need to provide for flood water or changes in flow. Whilst this may have had an impact on some of the recently reported flood events, it is unlikely to be a significant factor. One of the reasons why the pipes are set below water level is to minimise the hump on the road often found in these low lying areas where a road crosses a bridge, and with the generally unstable nature of the land leading to subsidence of the road structure, the elimination of such extra humps is desirable.

Box Culverts

One of the oldest types of Culvert is the Box Culvert, where parallel walls were built on either side of the water channel, and then large slabs of stone laid across the top to form a simple pipe. This obviously only works up to a certain size, but can be used for great lengths. It was a type often used in the Scottish Highlands by General Wade on the military roads, by his successor Caulfeild and also by Thomas Telford on his Highland roads. Many of these culverts can still be seen on abandoned routes, and occasionally on roads still in use. It was also doubtlessly used extensively across the rest of the British Isles in the same era, but such structures are rare survivors.





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