Vegetation and slope stability

Transpiration is the vaporisation of liquid water contained in plant tissue and the vapour removal to the air.

However it can be assumed that the chance of slope failure following saturation by storm events or periods of extended rainfall will be lessened as a result of transpiration.

[2][3] It is important to note that desiccation cracks can potentially be extended by vegetation in dry weather promoting the deeper penetration of water to a potential slip plane and increased water pressure into the soil during the wet periods.

[2] Studies in Malaysia[4] have shown that there is a significant relationship between root length density, soil water content and ultimately slope stability.

Shading helps prevent the desiccation of the soils which results in shrinkage and cracking allowing the deep penetration of rain water.

Plants need to have a high leaf to root ratio and have the ability to persist through hot summer months in order to provide effective shading of the soils.

The larger trees should be planted at the toe of the slope with a potential rotational failure as this could increase the factor of safety by 10%.

[2] If larger trees are removed from the toe area of a slope there will be both a reduction in soil strength due to the loss of evapotranspiration effects and a reduction in applied loading which may result in temporary suctions in clay soils which could lead to softening as the available water is drawn in to compensate for the suction forces.

This is similar to the recognised softening of overconsolidated clays due to the relaxation of overburden pressures when placed in the top layers of an embankment from deep cutting.

[5][6][7] When roots grow across the plane of potential failure there is an increase in shear strength by binding particles.

The way that roots interact with the soil is intricate but for engineering purposes the available force contributions may be measured with in situ pull out tests.

Roots which have no branches tend to fail in tension and pull straight out of the ground with minimal resistance.

[2] Roots that do not have branches generally fail in tension and pull straight out of the ground with only minimal resistance.

[5] Vegetation can also control water erosion by limiting surface processes such as sheet wash and overland flow.

The fine root network may have an apparent enhanced cohesion, which is comparable to geosynthetic mesh elements.