Landslide and rockfall prevention - is vegetation a good thing for slope stability?

Soil and rock slope stability – is vegetation coverage a good thing for slope stability?

This week Slope Stability Southwest have been undertaking a stability assessment on sea cliffs at Towan Beach in Newquay, as part of the ongoing redevelopment of the former Trebarwith Hotel site. As is often the case with the killas type geology of Cornwall, the upper parts of the cliffs are commonly weathered, in many cases back to a residual soil, and are often covered in vegetation, ranging from grasses and shrubs, to bushes and even mature trees.

It is fairly well accepted that vegetation coverage can play a key role in maintaining the stability of slopes. Here are some beneficial effects of vegetation on slope stability:

Soil Reinforcement: Vegetation, especially deep-rooted plants and grasses, helps reinforce the soil on slopes by binding soil particles together. Plant roots penetrate the soil and create a network of fibres that increase soil cohesion, reducing the risk of soil erosion and landslides.
Water Absorption and Control: Vegetation intercepts rainfall and reduces the amount of water that directly infiltrates the soil. This reduces the pore water pressure within the slope and helps maintain its stability. Plants also absorb water through their roots, reducing the saturation of the soil and minimizing the potential for slope failure.
Surface Protection: The presence of vegetation provides a protective layer on the slope surface, shielding it from the erosive forces of wind and water. Plants absorb the impact energy of rainfall, preventing it from directly hitting the soil and causing erosion and shallow landslides / landslips. The plant canopy also reduces the velocity of rainwater runoff, allowing it to infiltrate into the soil gradually.
Increased Shear Strength: Vegetation can enhance the shear strength of the soil on slopes. The roots create a mechanical interlocking effect, increasing the resistance of the soil to sliding. Additionally, plant roots reinforce the soil by consuming some of the moisture, promoting soil compaction and increasing its strength.

However – vegetation coverage, when unchecked and uncontrolled can become a big problem!

Where vegetation coverage starts to include bushes and trees, then root intrusion into a rock mass can be severe, resulting in opening of fractures and the loosening of blocks on the slope face. Even relatively small trees and bushes on a slope face can result in penetration and intrusion of the roots into the rock mass of several metres or more. The is particularly prevalent within the killas lithologies of Cornwall, where roots can penetrate deep into the weathered metasediments, mudstones and sandstones.

Movement of the trees by the wind causes significant leverage by the roots within the rock mass, causing further loosening of the rock mass and infiltration by water.

Maintenance of soils and rock slopes is key. Vegetation can be hugely beneficial in the maintenance of slope stability and the avoidance of landslides, landslips and rockfall. The last this we want to do is denude our slopes of appropriate vegetation, including grasses, plants and small shrubs.

However trees and large bushes can be a big problem, particularly on rock slopes, and should be removed as soon as possible to prevent significant damage and destabilisation to your slope face. The removal of trees and vegetation from a significant slope face is risky however, injuries can occur as a result from a fall from height, and injury and damage to property can occur as a result of falling or collapsing vegetation from a slope face. I necessary vegetation and tree removal may need to be undertake by rope access professionals.

If you have concerns about the stability of a soil or rock slope on your property, then call or contact Slope Stability Southwest for a free no obligation stability appraisal.

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