The effect of Poaceae roots on the shear strength of soils in the Italian alpine environment

The presence of vegetation contributes to maintaining slope stability and to reduce soil erosion, both by reinforcing soil shear resistance and influencing hydro-geologic conditions of soil. The contribution of plant root on soil stability had been studied in recent years, but its quantification needs more investigation, in particular concerning the effect of grass root. This contribution can be significant only when the slide is superficial; otherwise their action is negligible compared to arboreal species or shrubs and can be complementary. The grass species widespread in the Alpine environment used in this research belong to the Poaceae family: Festuca pratensis, Lolium perenne and Poa pratensis. This study considers in situ shear tests on soil blocks in the presence or absence of roots made in three different locations, using a specific and expeditious model and methodology. Subsequently the roots properties are compared. Results showed that few grass roots gradually fail and the most of them are inclined to pull out of the soil unbroken. Shear-strengths calculated for root-reinforced soil with Poaceae, yielded values between 50% and 325% higher than directly measured shear-strengths in soil with no roots. The shear displacement had an increase included between 93% and 1544%. The shear time was always longer. Once the trials were made, the clod with roots were all the time compact. (C) 2009 Elsevier B.V. All rights reserved.