Contribution of Vegetation to Slope Stability: an Overview of Experimental Studies Carried Out on Different Types of Plants

Abstract

Vegetation has a great importance in erosion control and slope stabilisation, protecting and restraining the soil (at the surface) and increasing the strength and competence of the soil mass (at depth). To evaluate the effect of plant roots on soil shear strength, it is necessary to quantify the bio -technical and mechanical properties of the roots. This paper presents an overview of experimental and analytical studies developed by CESI S.p.A. in Italy and particularly focuses on the evaluation of root tensile strength related to four kinds of plants, commonly used for slope stabilisation in different Italian regions, a nd five kinds of perennial “gramineae” plants, imported from abroad. To evaluate the mechanical properties of the soil and then estimate the contribution of root tensile strength to slope stabilisation, some geotechnical tests were executed; the results were the inputs for a theoretical model that calculated the increase in shear strength (and therefore the increase in the factor of safety) due to plant roots. 1 Foreword The use of vegetation in civil and landscape engineering as a method for erosion control and slope stabilisation has grown in importance during the last 30 years and is now widespread. Vegetation plays an active role both at the surface, protecting and restraining soil, and at depth, reducing the pore water pressure (hydrological effect) and increasing the shear strength of soil (mechanical effect). In particular, this paper is mainly related to the study of the mechanical effect of different types of vegetation and specifically to the contribution of their roots to the surrounding soil shear strength. For this reason, adequate investigations related to the properties of plants and particularly root mechanical properties, such as tensile strength, together with the determination of geotechnical properties of the surrounding soil, are presented . It should be also noted that vegetation could actually have both beneficial and detrimental impacts on the slope stability. Possible ways in which vegetation might negatively affect the balance of forces are slope surcharge from the weight of trees, win d leverage, increase in infiltration capacity due to root penetration and root wedging. In particular, the surcharge causes shear forces on the slopes, but this effect is negligible if compared to the increase in shear strength. Root penetration increase infiltration capacity and could be a detrimental effect, but, also in this case, this fact is accompanied by interception and transpiration as modes of water loss each contribute to the reduction of soil moisture. Wind leverage or wind throw can be a serious problem caused by the overturning moment of wind on trees, or arising from