Bioengineering techniques to prevent soil erosion

The erosion is the most important component of soil degradation. It is a physical process responsible for the continuous remodeling of the terrestrial surface, which causes the removal of the solid material from the soil surface and its deposition somewhere else. In not man-altered ecosystems, erosion is a natural phenomenon that human activities accelerates and causes a progressive degradation of the fertility and therefore the potential productivity of soil. The slope instability is not the result of a single process of erosion, but is much more complex, and there

® C. Prison, 2016

are several arrangements to provide. In order to solve at least in part the problems of instability, we can perform specific arrangement techniques that use biodegradable materials (more or less durable) and vegetation that allow a consolidation of sliding surface (increase in shear strength, reduction of water in the soil and the neutral pressures), without polluting the environment and the natural land balance.

The arrangement interventions with biotechniques determine a reduced impact on the land, improving the aesthetic aspect. They can replace only under certain conditions, physical and environmental, those classics. For example, in cases of modest spaces of climatic extremes or of civil protection, we follow the classical engineering diagrams. The bioengineering is clearly distinguished from the conventional engineering as it uses live plants to stabilize the steeps. While in traditional works the steep is consolidated by supporting walls, in bio-works we try to create an harmonious environment, combining inert materials and plants, obtaining a permeable and draining work of consolidation.

The techniques for the periodic maintenance of existing vegetation and for the reconstitution of the vegetative surface cover play a fundamental role in the context of the operations of maintenance, protection and consolidation of the slopes and the escarpments. The aim is to protect by the vegetation the soil surface from the rain drop impact, ensuring the correct absorption of meteoric water, avoiding the surface water run-off. This goal can be achieved through the correct vegetation management or using techniques that foresee the use of vegetal natural material, exclusively or in combination with other inert materials. To have successful bioengineering interventions, the proper use of the vegetable component is particularly significant either for the technical aspect or for the ecosystemic one. We must therefore use only autochthonous species, typical of the local vegetation of the area of operation.

In bioengineering works, during the delicate period between the end of the work and the development of the vegetation cover (resulting by the work of re-naturalisation: planting process, etc.); the geosynthetic materials, used to protect escarpments, constitute the only defense of the land from the surface rain erosion. They play also an effective action of distribution of the loads and the traction stresses on large surfaces, like other geosynthetics (geonets, geotextiles, geogrids), although with inferior mechanical characteristics. The effectiveness of their contribution lapses in the arc of a few seasons with the fiber degradation, but it is gradually replaced by the development of the herbaceous vegetation. Its natural fiber thus represent the optimal support to the development of the vegetation, increasing the fertility of the soil (biodegradable products), keeping the finer particles and reducing the water evaporation.

To integrate “grassing method” and geosynthetic materials, we can perform drainage interventions whose purpose is to remove and collect the surface water and groundwater in correspondence with unstable slopes or foundation soils, in order to decrease the interstitial pressures and consequently the soil thrusts.

When the cited above interventions are inadequate because the slope is not only subject to surface runoff, but it is also affected by ground instability in the first meters of soil; you can use surface stabilization techniques that have the function to stabilize the more superficial soil layers, and therefore to prevent a hypothetical slope instability in the first meters of depth. The realization of these works follows the bioengineering guidelines by using natural materials such as wood and the installation of herbaceous and arboreal species suitable to the environment and to the subject to intervention area.

In the table 1, I am going to illustrate a summary of the main intervention techniques, describing for each of them the advantages, disadvantages, the applicative limits and the constructional errors. As already mentioned, these techniques can be mutually integrated and matched for optimal prevention, arrangement, recovery and in general for the resolution of environmental destruction issues.

Table 1.

Overview of the main Bioengineering techniques: advantages, disadvantages, applicative limits and constructional errors. [1]