Shear strength of the root permeated soil increases due to the mechanical effects of root reinforcement and most of the past studies have been conducted to capture this effect under saturated soil conditions. However, the soil adjacent to the tree roots is usually in an unsaturated condition and this leads to alterations in root-soil interaction mechanisms and associated shear strength of the root permeated soil system. In this paper, the increment in shear strength is studied considering both the effect of suction and root reinforcement patterns. A number of direct shear tests were conducted for different suction levels in root-permeated and unreinforced soil specimens. The results indicate that the shear strength behaviour of the soil-root system is governed by the level of suction and root failure patterns and a new mathematical model incorporating the effect of both parameters is proposed.
Strengthening soft and weak soil by way of root reinforcement is a well-known strategy that is adopted worldwide. In Australia, native gum trees remain evergreen throughout the year and have been utilised to stabilise transportation corridors by way of reinforcement provided by the roots and the suction generated within the root domain as a function of evapotranspiration through the canopy. A mature gum tree can induce a missive total suction pressure exceeding 30MPa through its root water and solute uptake in terms of matric plus osmotic suction. This cumulative effect of matric and osmotic suctions contributes to the overall shear strength of the soil, but the significant osmotic suction is often ignored in classical geotechnical engineering that does not consider the presence of trees. This study is an attempt to demonstrate the important role of osmotic suction, because it is directly proportional to the solute concentration in the soil and the solute uptake mechanisms of the surrounding vegetated ground.
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