Cuttings and Embankments
|Cuttings and Embankments|
|M3 Twyford Down cutting under construction|
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There are three options for a road to make its way through a hilly part of the country. The first is for it to follow the landscape, whether up and down, or round and back again. The second is to push a hole through the hill in such a way that the material fills the dip on either side (cut and fill), the third is similar, by using tunnels and bridges. This page looks at the middle of these options.
First let's take a look at exactly what we mean by each of these words:
A cutting is simply described as where the land has been excavated to allow the road to pass at a lower level than the original ground level. They are commonplace across the country, and can range from a slight notch cut into the side of a hill to provide a level ledge for the road to pass, to deep gorge like rock cuttings where the sun hardly ever reaches. Depending on the geology of the area, they may either be constructed using large earth-moving plant such as excavators, bulldozers and scrapers, or in rocky areas, drilling and blasting may be required to break up the rock.
An Embankment is where the level of the land has been raised to cross a depression, or area of boggy ground. Embankments which only raise the road level by a little amount are often described as Causeways, but either term is correct. Embankments are perhaps more common on Railway lines, where gradients were more critical, but can also be found on roads across the country.
Embankments may be built from suitable material excavated from elsewhere along the route, or imported from other sites. The weight of the embankment will cause settlement of the ground underneath the embankment over a period of time following construction, the amount depending on the size of the embankment and the make up of the existing ground. If this is not taken into account during construction, the finished road will sink, and may suffer significant damage. Therefore, to speed up settlement a number of techniques may be used including surcharging the embankment (building it higher than eventually required and letting it settle for several months before removing the excess material and building the road) and installing drainage to remove ground water from under the embankment, speeding up settlement.
A different type of embankment is sometimes used, particularly in Urban areas, where the road is raised up on what appears to be a solid concrete box. Railways may have used long arched viaducts, and many roads use a series of piers to create an elevated road, but in some circumstances, two concrete retaining walls filled with rock and tied together by the road deck is used.
False cuttings occur when a road runs at natural ground level, but appears to be in a cutting as a result of embankments being constructed either side of the road. This may be done for a number of reasons, for example to provide environmental screening (noise bund) to reduce the visual or noise impact of a road on an adjoining community, or as a means of using excess material gained from a true cutting elsewhere on the site without having to pay the expense of disposal off site. In some cases, the side of the embankment away from traffic is built with a very shallow slope. This is more costly in terms of temporarily obtaining land for the scheme, but does allow more land to be returned to other uses, for example agriculture, after the scheme is complete.
Cut and Fill
This is a term which is used where the material from a cutting is used to fill in the next hollow, and so provide a smooth, level road. In the past it was a common practice, reducing the amount of material to be moved away from or brought to the site. However, in recent years the practice has become a lot less used. This is largely due to concerns over the quality of the material being used, and its propensity to subside or collapse completely. Previously this could often be accounted for by adjusting the slope angle on either side of the embankment, or by planting. However, the costs of the additional landtake (perhaps including property) has made this less feasible, particularly in areas where the material is least suitable in the first place.
As a result, there have been occasions where lorry loads of material are removed from a site, and a similar number of lorry loads of rock are brought in instead. This has, obviously, led to environmentalists campaigning against the road in question.
In a flat landscape, where road and motorway or rail meet, there is a desire to create a new level to allow them to cross without interrupting either flow. Bridges are the natural answer, and in order to acheive this the minor route, or road, is lifted up on a short embankment, broken by the bridge. In such cases, there is rarely any option but to import the material from off-site. However, there are also examples, particularly common next top railway bridges on the Somerset Levels, of the material being excavate right next to the bridge, leaving small ponds behind.
Grade Separated Junctions also require roads to cross at different levels, using bridges. A variety of embankments, cuttings and elevated roads are used across the country to achieve this. Indeed, some junctions feature all three solutions in the one site.