Observation and Simulation of Root Reinforcement on Abandoned Mediterranean Slopes |
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Authors: | L P H van Beek J Wint L H Cammeraat J P Edwards |
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Institution: | (1) Department Of Physical Geography, Utrecht Centre of Geosciences, Utrecht University, P.O. Box 80115, NL 3508 TC Utrecht, The Netherlands;(2) Department of Civil Engineering, Nottingham trent University, UK;(3) IBED-Physical Geography, University of Amsterdam, The Netherlands;(4) Scott Wilson Pavement Engineering Ltd., Nottingham, UK |
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Abstract: | The mechanics of root reinforcement have been described satisfactorily for a single root or several roots passing a potential
slip plane and verified by field experiments. Yet, precious little attempts have been made to apply these models to the hillslope
scale pertinent to landsliding at which variations in soil and vegetation become important. On natural slopes positive pore
pressures occur often at the weathering depth of the soil profile. At this critical depth root reinforcement is crucial to
avert slope instability. This is particularly relevant for the abandoned slopes in the European part of the Mediterranean
basin where root development has to balance the increasing infiltration capacity during re-vegetation. Detailed investigations
related to root reinforcement were made at two abandoned slopes susceptible to landsliding located in the Alcoy basin (SE
Spain). On these slopes semi-natural vegetation, consisting of a patchy herbaceous cover and dispersed Aleppo pine trees,
has established itself. Soil and vegetation conditions were mapped in detail and large-scale, in-situ direct shear tests on the topsoil and pull-out tests performed in order to quantify root reinforcement under different vegetation
conditions. These tests showed that root reinforcement was present but limited. Under herbaceous cover, the typical reinforcement
was in the order of 0.6 kPa while values up to 18 kPa were observed under dense pine cover. The tests indicate that fine root
content and vegetation conditions are important factors that explain the root reinforcement of the topsoil. These findings
were confirmed by the simulation of the direct shear tests by means of an advanced root reinforcement model developed in FLAC
2D. Inclusion of the root distribution for the observed vegetation cover mimics root failure realistically but returns over-optimistic
estimates of the root reinforcement. When the root reinforcement is applied with this information at the hillslope scale under
fully saturated and critical hydrological conditions, root pull-out becomes the dominant root failure mechanism and the slip
plane is located at the weathering depth of the soil profile where root reinforcement is negligible. The safety factors increase
only slightly when roots are present but the changes in the surface velocity at failure are more substantial. Root reinforcement
on these natural slopes therefore appears to be limited to a small range of critical hydrological conditions and its mitigating
effect occurs mainly after failure. |
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Keywords: | FLAC 2D in-situ direct shear tests root pull-outs root reinforcement slope stability modelling vegetation |
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