**7.1. External stability checks**

As with any stabilization structure, soil bioengineering solutions must be checked from a structural point of view to ensure that both external (sliding, overturning, bearing capacity and slope failure; **Figure 1**) and internal stability conditions are satisfactory. These checks must include both decay and living plant effects, in order to reflect the changes during the lifetime of the bioengineering solution. The external stability checks are usually performed in line with existing geotechnical engineering design standards and the stability is expressed in terms of a factor of safety (FoS; e.g. [32]). In this book chapter, both the FoS expressions for bare and vegetated soil [24] and the use of lumped global FoS for the sliding and overturning checks are proposed. The resistance to sliding (FoS<sup>s</sup> ) will be affected by the evolution of the RAR value with time across the sliding plane [33], while the resistance to overturning (FoS<sup>o</sup> ) will be affected by the pull-out force evolution with time due to root growth (**Figure 4**). As shown before, the overall slope stability of a bioengineered slope can be assessed using existing slope stability analysis methods [34] taking into account both long-term (drained) and short-term (undrained) conditions.
