Slope Stabilisation by Perennial “Gramineae” in Southern Italy: Plant Growth and Temporal Performance

Abstract

In the territory of Altomonte, a village located in Calabria, in the Southern part of Italy, a new thermoelectrical station is under construction. This work involved major earthworks which regarded new excavated slopes. In order to protect soil from erosion due to rainfall and runoff and also in order to prevent superficial soil instability, it was decided to plant four different species of perennial “gramineae” plants (Eragrass, Elygrass, Pangrass and Vetiver) characterised by deep roots. Works began in November 2002 and ended in May 2003, a period marked by very different climate and meteorological conditions, varying from exceptionally rainy and cold winter to warm and dry spring months. The paper describes the different stages of the project and the monitoring programme for the following months. The extension of the work and the use of four different kinds of vegetation made periodic inspections of the entire site appropriate. Two in situ investigations, respectively performed in August 2003 and in November 2003, are outlined. The aim of these surveys was to confirm the success of the work by verifying the growth of the plants and roots. The principal monitored parameters were the percentage of sprouted plants, the height of the foliage and the depth of roots. The investigations showed good results, keeping in mind the very difficult climatic conditions and the extreme poor fertility of the topsoil laid down upon the clay layer: in particular, high survival rate were measured over the entire area of the works and the root systems have developed sufficiently to grow through the upper topsoil layer (0.2–0.3 m) into the underlying clay layer. In March 2004, a sampling programme was undertaken on the same site. Direct shear tests were carried out in the laboratory in order to evaluate the increase in shear strength of the rooted soil mass. The research involved the recovery of three undisturbed samples of soil with roots for each of the four types of “gramineae” plants and three undisturbed samples constituted only of soil, from the surface to a depth of 1.0 m. The tests were performed in a large direct shear apparatus on 200 mm diameter samples. The test results allowed to evaluate the roots’ contribution of the different gramineous species and to underline the direct correlation between the increase in soil shear strength and the root tensile strengths. In particular, an increase in cohesion ranging between 2 kPa and 15 kPa was recorded, according to the different species: the maximum values of increase in shear strength were reached by Vetiver roots, which are also characterised by the highest tensile strength.