Design and temporal issues in Soil Bioengineering structures for the stabilisation of shallow soil movements

In the last decades, soil bioengineering has gained considerable popularity worldwide, especially in hillslope, riverbank, and earth embankment stabilisation works. The use of plants as a building material transfers the plant multifunctionality within engineering structures and meets the demand rising from society for more environmental-friendly approaches to structure design. In addition, soil bioengineering approach complies with public policies, such as EU strategies concerning the green infrastructures, the circular economy and the green deal, as well as the global framework defined by the Global Goals for Sustainable Development. Despite the widespread use of soil bioengineering techniques and their adherence to the new directions of public policies, however, quantitative and temporal aspects of soil bioengineering approach are not fully considered. Construction criteria typical of civil engineering approaches, such as safety, serviceability and duration, need to be evaluated and fulfilled by bioengineering measures in an analog way as for technical construction, with the additional effort to consider spatial and temporal variability of their properties. In particular, it must be considered at the same time the dynamic temporal development of vegetation (especially the dynamic of root reinforcement) and the decrease in strength of some additive construction ma-terial (e.g. wood decay). A comprehensive design scheme, comparable with the one for conventional engineering structures, is still lacking. In this paper, knowledge and tools aiming to address the above issues, are reviewed, with particular reference to points associated with the structural stability of soil bioengineering structures for shallow landslide stabili-sation. The paper: - provides the background to the traditional engineering design for slope stabilisation, and the normative state of the art of geotechnical and construction design, as well as the residual risk of slope stabilisation; - considers the temporal issues associated with the vegetation and the effect of soil bioengineering structures on the stability of natural, man-made, and anthropogenically influenced slopes; - develops a decision framework for soil bioengineering applications that explicitly considers quantitative and temporal aspects of soil bioengineering approach, highlighting critical parameters governing the design with vegetation against shallow slope instability; - critically debates the issues of the quantifiability of vegetated slopes and the use of permanent and temporary inert material.