Comparative geotechnical analysis of slope stabilization through conventional, soil and water bioengineering, and combined solutions

The sustainable mitigation of hydrogeological hazard through the geotechnical stabilization of natural and artificial slopes is an ethical and technical goal of increasing global relevance. In this context, “gray” geotechnical stabilization solutions involving the use of inert materials, injections of cement mixtures and steel elements, have been prevalently used in the past decades and have thus come to define the present “conventional” approach. These solutions may meet engineering performance criteria but are unable to attain desirable sustainability standards. The practice of Soil and Water BioEngineering (SWBE) draws from ancient empirical experience and is rapidly gaining new momentum due to the increased focus on environmental protection and requalification. SWBE and can be effectively conducted through the design and implementation of nature-based solutions (NBS) by using living plants, alone or in combination with locally available materials, to improve the engineering performance of ecosystems while fostering an increase in their biodiversity and environmental value. The domain of applicability of NBS is limited to quasi-surficial instability phenomena, since the root systems which provide resistance to destabilizing forces are found mainly at shallow depths from ground surface. Moreover, biological and physical processes intervening in NBS result in the temporal variation of their mechanical resistance and engineering performance. “Combined” solutions involving the presence of – and synergy between - gray and green solutions may ensure the simultaneous attainment of safety and sustainability. This paper describes the conceptual standpoints and operational framework used for the comparative assessment of the engineering design performance of conventional, NBS, and combined solutions for a slope stabilization intervention on a site located near Florence, Italy. Stability is assessed quantitatively through limit equilibrium methods for multiple scenarios defined in terms of technological solutions, temporal stage, and level of engineering conservatism in design parameters. Temporal trends of the factors of safety against sliding are defined statistically and assessed qualitatively and quantitatively. The comparative analysis suggests that the combined solution provides the best option at the Montisoni site as it ensures sufficient short-terms, post-stabilization stability as well as increased stability overtime due to the improvement in the mechanical contribution of NBS components. The paper brings innovative contributions with respect to the equivalent geomechanical modeling of NBS and combined solutions in limit-equilibrium analyses and to the discussion of criteria to be considered in the assignment of design values in stability analyses.