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  Information - NH3.14 The role of vegetation in slope stability

Event Information
To analyse the contribution of vegetations in erosion control and slope stability, one need to think of its hydrological, biological and mechanical role. The mechanical contributions arise from the physical interactions of either the foliage or the root system of the plant with the slope. The hydrological mechanisms are those processes of water use and movement in the slope when living plants exist in the soil. In addition, the existence of plants on the ground affects the biological process in the soil underneath and also in the surrounding, which may have both positive and negative roles in slope stability and erosion control. The stability of any slope depends on the strength of the soil material comprising of slope and slope geometry. Less can be done to change the slope geometry while we consider the stability of natural slopes. However, more can be achieved if appropriate soil bioengineering techniques are used. When properly installed and maintained, vegetation can protect slopes by reducing erosion, strengthening soil, and inhibiting landslides which increase general slope stability. The use of vegetation to manage erosion and protect slopes is relatively inexpensive. It does not require heavy machinery on the slope, establishes wildlife habitat, and can improve the aesthetic of the surroundings. The net effect of vegetation is usually beneficial to slope stability. As mentioned earlier, the existence of plants affects the strength characteristics of soil material and also hydrological characteristics of the slope. Furthermore, it will enhance the biological activities in the soil, which also affects the physical and chemical characteristics of soil and natural drainage system of the slope. Therefore it is required to study these three phenomena separately. In its mechanical contribution, different types of root systems that plants can provide strengthen the stability of a slope by both fibre reinforcement of the near surface soil and binding soil structure together into a larger unit through tap or lateral root networks. Roots reinforce the soil, increasing soil shear strength. Tree roots may anchor into firm strata, providing support to the upslope soil mainly through buttressing and arching. Roots also bind soil particles at the ground surface, reducing their susceptibility to erosion. On the other hand, plants (especially of tall hard wood trees) may have negative effects also. Trees exposed to the wind which transmits forces into the slope may cause slope failure by increasing the lateral loads on the slope. Weight of trees surcharges the slope, increasing normal and downhill force components. Tree weight in some situations is beneficial to slope stability (in cases of "unweight" slopes). In some cases plant roots may cause the direct physical breakdown of rock by the pressures exerted during growth. Small root hair cells enter bedrock cracks and increase in length and width, gradually prising the cracks apart. Pressures have been measured as high as 1.45 MPa sufficient to break up rock. While analysing the role of plants on slope stability all these aspects should be carefully considered. The hydrological cycle describes the ecological processes involved after a drop of water has entered into the ground as rainfall, dew, snow or in any other form. The vegetation plays a vital role in hydrological cycle and surrounding ecosystem. Foliage intercepts rainfall, causing absorptive and evaporative losses that reduce rainfall available for infiltration or making splash on the surface. Roots and stems increase the roughness of the ground surface and the permeability of the soil, leading to increased infiltration capacity, which may have positive or negative role in slope stability depending upon the soil type. Depletion of soil moisture may accentuate desiccation cracking in the soil resulting in higher infiltration capacity. Furthermore, the plant roots increase the soil suction reducing pore water pressures, which again significantly increases the cohesion and also the friction angle to some extent. In an experimental investigation carried out in a highway embankment in Germany, an increase of effective cohesion from 1.1 KN/mē to 6.3 KN/mē and friction angle from 33.1° to 34.7° were observed. Vegetations play significant role in the ecosystem. It will enhance the biological activities in the soil which affects the physical and chemical characteristics of soil and finally the strength characteristics of soil. It also influences the movement of water in the soil, which again affects the stability of slope. Therefore while talking about the role of plants in slope stability, its biological effects (which is an indirect effect) should also be taken into consideration. Although the protection of the slope against shallow seated landsliding and erosion control are key benefits of soil bioengineering techniques, it can be applied anywhere as a single system and also in combination with other civil engineering structures. In its direct influences, there are five main functions that the plants may perform within a soil bioengineering system (in a vegetative structures): support, anchor, drain, reinforce and armour. Depending upon the type of plants used in soil bioengineering, the nature of slope and soil characteristics, the plant may perform one or more of its functions and plays a vital role in the slope stability. Moreover, plants' influence on processes of erosion and drainage is both direct and indirect. Indirect effects arise due to complex links with hydrology, slope stability, soil cohesion and associations with fluvial systems. The indirect erosion and slope stability importance of vegetation is perhaps best illustrated when it is altered or removed, promoting overland movement of water as a function of reduced infiltration capacity and rapid, concentrated and highly erosive flow due to a lack of surface resistance. Considering the complex nature of influences of plants on slope stability, more field oriented experimental research works on different soil bioengineering system are required to quantify the role of different plants in slope stability. In this session, we wish to bring together researchers from various scientific communities (geologists, geotechnical and hydraulic engineers, soil scientists, etc.) to create a broad interdisciplinary forum for discussion on soil bioengineering techniques and their effects on slope instability. Contributions that address the interplay between vegetation and soil properties are particularly welcomed, as well as case-histories where the focus is on the monitoring of the remedial works.

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