In order to evaluate the influence of a root system on soil reinforcement, we performed direct shear tests with soil water suction control. The suction in the tests ranged from 0 to 50cm H
2O, and the normal stress ranged from 42 to 140gf/cm
2. Toyoura standard sand was used for the tests. Two kinds of material simulating the root system were inserted into a shear box. The first was a bamboo stick (0.27cm in diameter, 10cm long); the second was a nylon ner (0.05cm thick, 10cm wide, 15cm long). The former simulates a thick root system; the latter, a fine root system.
The experiment results are as follows. With respect to the relation between the suction and the increase of shear strength for the same normal stress, we found peak points of shear strength at 30cm H
2O of suction for the bamboo stick and at 40cm H
2O for the nylon net. The effect of root systems on soil reinforcement was observed in both apparent cohesion and internal friction angle. Suction values greatly affected the increase of apparent cohesion, contrary to the internal friction angle which increased independently with suction. These results suggest that the soil reinforcement depends not only on the resistance force of the root system itself, but also on the interaction between the resistance of the root system and the shear strength which is affected by both normal stress and soil moisture conditions.
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