Mechanisms of root-soil reinforcement in bio-embankments of sloping farmland in the purple hilly area, China

Abstract

Bio-embankment is an important soil and water conservation measure in the purple hilly area in China, which can effectively improve the ability of cultivated soil layers to resist rainfall erosion and runoff scour. In contrast, the ecological effect of bioembankment depends on the stability of the earth bank. Taking the natural grass bank as a control (CK), the root distribution, root tensile properties and shear resistance of root-soil composites for 3 typical soil and water conservation bio-embankments, namely, Morus alba Linn (Morus), Zanthoxylum bungeanum Maxim. (Zanthoxylum) and Medicago sativa (Medicago) were analysed. The results included the following: (1) The root system of the bio-embankments generally decreased in extent with the soil depth; fine roots in the 0–10 cm depth were most prevalent and significantly higher than those at the other depths, and coarse roots were mainly distributed in the 0–30 cm layer. (2) The stress-strain curves of the roots of each bio-embankment were single-peak curves without clear strain softening phenomena. The smaller the root diameters were, the smoother the stress-strain curves, and the lower the capability of the earth bank to resist collapse. The larger the root diameters were, the lower the tensile strength. The average root tensile force was highest for Zanthoxylum (73.91 N), followed by Medicago (68.07 N) and Morus (61.88 N), and the average root tensile strength showed the same trend, 16.52 MPa for Zanthoxylum, 16.08 MPa for Medicago and 13.02 MPa for Morus. (3) The bio-embankment measures significantly improved the soil shear resistance, especially under vertical loads of 100 kPa and 200 kPa. The soil internal friction angle showed a significant log-positive correlation with root morphological parameters of root length density (RLD), root surface area density (RSAD) and root weight density (RWD), while the soil cohesion force showed a positive linear correlation with these parameters. The results provide effective parameters supporting for the design of bio-embankments and promoting the use of soil reinforcement with suitable species selection in protective earth banks for stability in the purple hilly area.