9.7.3  Vegetation

Bio-technical stabilization and soil bioengineering stabilization both entail the use of live materials – specifically vegetation. Biotechnical stabilization utilizes mechanical elements (or structures) in combination with biological elements (or plant) to arrest and prevent slope failure and erosions. Biotechnical stabilization can be characterized by the conjunctive use of live vegetation with retaining structures.   Plant material increases soil strength through the transfer of root tensile strength to soil shear strength, buttressing and arching. Bio-engineering systems provide additional support beyond that which can be provided by single plants. As the plants mature they increase in strength and provide increased resistance to natural forces.  Vegetation on slopes provides protection against erosion and shallow sliding. Roots reinforce or bind the soil and provide cohesion that improves stability against shallow sliding. Plant roots reduces pore pressures within slopes by intercepting rainfall (reducing infiltration) and by evapotranspiration (Wu et al., 1994).  The small size of flexible roots mobilise their tensile strength by soil–root friction and increasing shear strength of the soil root-matrix whereas, the large size roots intersect the failure plane and act as individual anchors in the soil. Roots can also prevent the generation or propagation of tensile facture or cracks. The Mechanisms of root reinforcement of grass plants and tree is shown in figure 34.



Figure 34.  Mechanisms of root reinforcement of grass plants and tree



Figure 35 shows the example of combination of active and passive protection techn iques to stabilize the slope.