Effects of soil suction on root reinforcement in unsaturated forest soil

Abstract

In order to evaluate the effects of soil water suction on soil reinforcement by root systems, the authors conducted direct shear tests under suction control (from 0 to 380cmH₂O). Samples of forest soil from the Fukuroyama-sawa watershed in The Tokyo University Forest, in Chiba, were compared with previously reported values for Toyora standard sand. Bamboo sticks (0.27cm in diameter, 10cm long) were inserted in the shear box as simulated root systems.
In the forest soil, the internal friction angle and the apparent cohesion were proportional to the soil water suction. In Toyora standard sand, on the other hand, the internal friction angle was constant and the apparent cohesion had a maximum value at the critical capillary head. Both the apparent cohesion and the internal friction angle increased when roots were inserted. The reinforcement of the apparent cohesion was proportional to the soil water suction, and the reinforcement of the internal friction angle was constant when the soil was saturated.