Determination of the surface-mat effect of grassland slopes as a measure for shallow slope stability

Herbaceous vegetation is an important factor for control of shallow slope failure. While roots of herbaceous vegetation rarely cross potential shear planes, they increase soil cohesion in the top soil and create a surface-mat effect. The surface-mat is a dense layer of laterally interwoven plant fibers, which create considerable horizontal tensile strength of the top soil. Through redistributing impacting forces from unstable towards stable slope areas, the surface-mat can prevent shallow slope failure. We present a method for measuring the horizontal tensile strength of the top soil, which estimates the surfacemat effect. On four montane grassland slopes in Northern Italy, grass sods were cut out from the soil and their tensile strength was measured with a traction machine. The scope of this study is to investigate potential soil physical and biological factors influencing slope stability. Generally, the surface-mat effect decreased with soil depth. Bulk density, soil moisture and aboveground vegetation parameters showed no correlation with the surface-mat effect. Root length density was weakly correlated. Soil water saturation had a marginally significant negative correlation (p < 0.1) when root length density was low, but no correlation when root concentration was high. Vegetation type and management influenced the surface-mat effect. We conclude that the low influence of wet conditions on the surface-mat effect makes it a promising strategy for shallow erosion control. In addition, our findings suggest, that specific vegetation management can increase the surface-mat effect.