| Soil should be looked at in three parts: |
| 1) Top soil - Uppermost layer comprising of disintegrated rock particles with an a mixture of organic remains and materials. Generally loose, friable particles with little or no cohesive properties. Will sustain plant growth. |
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| 2) Sub soil - Secondary layer of disintegrated rock particles with little or no organic matter. Generally but not always firm cohesive material with some capability of sustaining vegetative growth. |
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| 3) Base soil - Comprising of sedimented, compacted or large rock masses, with no real ability to sustain plant growth. |
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| Erosion Control deals with preventing the wearing away of the upper soil layer. Soil is eroded by the actions of an external influence such as rain, water run off, wind or foot and animal traffic. |
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| Soil Stabilisation deals with reinforcing the subsoil and topsoil layer where these layers become detached from the base soil layer. Instability can be caused by water ingress between the layers, steepening the embankments where gravity pulls the layers down and newly soiled embankments where lack of cohesion reduces the frictional resistance between layers. |
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| Soil Reinforcement is where the whole soil mass needs restraining and in particular where the base soil is to be steeper than its natural angle of repose. A retaining wall may be needed to hold a slope in place to allow the construction of buildings or roads and railways. |
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