Hydro-mechanical reinforcement of contrasting woody species: a full-scale investigation of a field slope

Abstract

Soil bioengineering using vegetation is an environmentally friendly technique for slope stabilisation. Plants stabilise slopes by way of mechanical reinforcement (through root anchorage) and hydrological reinforcement (through transpiration-induced matric suction). However, little is known about the effects of a plant's functional group on slope hydrology and stabilisation. This makes it difficult for engineers to select appropriate species for soil bioengineering. In this study, full-scale field monitoring of a 20 m long vegetated embankment was conducted, with the aim to quantify and compare the hydro-mechanical reinforcement provided by three contrasting woody species (deciduous Corylus avellana, evergreen Ilex aquifolium and evergreen Ulex europaeus) native to a European temperate climate. The rainfall interception, matric suction and in situ soil strength were measured over two growing seasons. Evergreen species differed greatly in their water uptake, and sometimes exceeded the deciduous species. The evergreen U. europaeus induced the greatest suction (> 70 kPa) and soil shear strength (e.g. up to 136 kPa) because of its better developed above-ground shoot architecture for rainfall interception (up to 50% of incoming rainfall) and greater root length density for water uptake. This suggests that careful choice of species could greatly enhance slope stabilisation by increasing the soil shear strength.