Abstract
Vegetation, particularly roots, serves different functions in relation to increased shear strength under saturated and partially saturated conditions. Quantification of mechanical contribution of roots due to their tensile strength, and relationships of various vegetation parameters and plant-induced suction, as well as shear strength, have been widely studied. Although shear strength is directly related to the volume change characteristics of soil, dilative or contractive behaviour of root-permeated soils has not been of significant interest so far. This study investigates how volume change during shearing is related to the hydrological and mechanical characteristics of vegetated soils relevant to slope stability and shear strength of root-permeated soils under partially saturated conditions. Direct shear tests, on specimens planted with a mixture of species from different plant functional groups, were performed with an Inclinable Large-scale Direct Shear Apparatus (ILDSA). Matric suctions were monitored throughout the test with tensiometers. Vertical and horizontal displacement graphs were plotted to investigate the volume change behaviour. Maximum dilatancy angle was found to be positively correlated with plant-induced suction and net normalised stress, both of which were linked to root biomass and the root:shoot ratio. It was found that maximum dilatancy is controlled by matric suction and net normal stress.
Highlights
Understanding and modelling the mechanical behaviour of any soil requires the evaluation of the volume change characteristics during shearing
Matric suctions were monitored during shearing, and the matric suction at peak shear stress was taken as the average value of the tensiometer readings
Matric suction ranged between 11 kPa to 97 kPa
Summary
Understanding and modelling the mechanical behaviour of any soil requires the evaluation of the volume change characteristics during shearing. Expansion in volume during shearing, has been thoroughly investigated, and is well established for soils under saturated conditions [1]. There are limited number of studies on the volume change behaviour of unsaturated soils. Constant-water-content triaxial compression tests on a poorly graded sand were used to validate the stress – dilatancy theories for unsaturated soils [2]. Models have been developed to quantify and explain the dilatant behaviour of unsaturated soils, such as an extension to the Modified Cam-Clay model [3] or a model defining the peak strength as a function of dilatancy [4]. A limited number of papers can be found in the literature on the dilatancy of unsaturated soils, common observations among them [5-8] are:
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