Wildlife & Traffic
A European Handbook for Identifying Conflicts and Designing Solutions
6.3 Earthworks: cuttings and embankments
Where lateral views of the infrastructure are intrusive, a cutting is one of the best means of hiding it. However, cuttings may themselves become intrusive, fragmenting habitats, creating a notch in the skyline or taking the edge of one side of a hill leaving an ugly scar. A curved alignment over high ground and the careful siting of bridges can help reduce the impacts of a cutting on the skyline. Bridges can be adapted to provide a green connection for fauna and flora.
6.3.2 Varying gradients
Cuttings are usually constructed to a uniform gradient of 1:2 and contrast with natural gradients which are more varied and irregular. Good design can provide better integration with natural landforms and provide opportunities for a variety of habitats to be created.
- Different rock types give rise to different natural slopes and cuttings should reflect these where possible.
- Minor cutting faces especially in upland areas can produce rock exposures with potential nature conservation value.
- It is desirable to reveal the natural bedding plane of the rock to provide a stable slope that will not require for reasons of safety the use of catch fences, wire mesh or other unnatural engineering elements.
- In areas of woodland and rough pasture, an irregular cutting surface finish will provide better integration with the adjacent areas. They can provide niche habitats for certain plants and invertebrates.
- There are benefits in rounding off the tops of cuttings to a gentle profile to create a smooth transition to the natural landform.
Terracing can be used to break up the sides of deep cuttings to overcome their visual dominance. It may benefit structural stability and facilitate the establishment of vegetation. It will also provide opportunity to create microclimatic niches for a range of fauna and flora. Terracing needs to work with the natural bedding planes of the parent rock.
6.3.4 Rock outcrops
Rock outcrops can be created in such a way as to provide a sense of place, passenger interest and nature conservation benefits. They are often preferable to attempts to establish vegetation on very steep slopes.
- A varied profile is needed for visual character and to allow vegetation to establish. Natural regeneration of plants is preferable.
- A safe distance must be left between the outcrop and the carriageway.
- Scree, soil and vegetation must be periodically monitored and stabilised.
6.3.5 False cuttings
False cutting is a means of screening the road from receptors (human and animal) in the surrounding landscape (a depth of 2 m is sufficient to hide cars). It is particularly appropriate in gently undulating ground where a natural cutting cannot be achieved. The best effect is obtained where the backslope is returned to the adjacent landuse. Such slopes can be used to screen nature conservation sites from the effects of infrastructure use, such as de-icing salt spray plumes and drift, lighting and noise.
6.3.6 Grading out cuttings and embankments
Grading out of earthworks can provide integration with the surrounding landscape, whilst ensuring the most efficient use of material. Prominent artificial features can be softened and restored to an appropriate landuse. Grading out of cuttings to slopes shallower than 1:2 helps avoid compaction of soils and makes it easier to establish vegetation.
It is often inappropriate to grade out major cuttings and embankments, either because the scale of the earthworks required makes it impractical or because the grading out would extend into areas of conservation interest. In these cases more attention should be given to the details of the top of cuttings and foot of embankments, variations in gradient and finished surfaces.