Simulating the stabilization effect of soil bioengineering interventionsin Mediterranean environments using limit equilibrium stabilitymodels and combinations of plant species

Abstract

One of the critical problems in projecting soil bioengineering interventions is the difficulty in developing precise calculations of the evolution of the structures and the type and combination of vegetation plantations that are planned for the particular site and problem. This is mainly due to the uncertainties in the plant development in each type of soil, the long and short term weather patterns and other edapho-climatic factors that determine limited calculation possibilities causing considerable degrees of uncertainty. This leads to a reduced level of reliability of bioengineering projects in terms of pre-determined factors of safety. In Mediterranean conditions this problem is even stronger due to the high seasonal and year-to-year climatic variability, and high degree of uncertainty in the vegetation instalment and growth patterns. To evaluate the comparative short and long term (up to 20 years) efficacy of the most used slope stabilization techniques, common calculation models were used to assess the factor of safety of those techniques. SLIP4EX was used to evaluate the factor of safety, while the normal Coulomb model was used to assess the resistance to sliding and overturning. Considering the average Mediterranean climatic conditions, the considered environmental conditions were the least favourable, in order to assess the reliability of the “worst case scenario”. Two common tree species in the western Mediterranean region (Pinus halepensis and Quercus faginea) were selected for the simulation and three common soil types were considered (stony clay, loose sandstone and sandy marl). The results of the simulations for different types of substrates, plant combinations and designs, show that, according to the modelling approaches, the use of soil bioengineering techniques, present a reliable effectiveness, confirmed by the experience gathered in interventions in Mediterranean sites with similar edapho-climatic conditions. Additionally, because bioengineering is about the factor of safety of the target vegetation after the decay of the combined structure, the efficiency as a slope support structure of a vegetated log cribwall was compared with the expected developed vegetation. It was concluded that an adequate development of the vegetation ensures all safety requirements even in the worst considered scenarios. Our results also illustrate that there is a generalized lack of data on the geotechnical characteristics of the Mediterranean vegetation, which is particularly critical in a geo-ecological context where bioengineering techniques face important limitations (very few species able of vegetative development of roots and shoots from live cuttings).