This paper introduces a double shear apparatus for experimenting the shear strength of root-soil composites. The key component of the apparatus consists of three metal shear rings, which have 7.5 cm in length of two, 5 cm of one in length and 10 cm in inner diameter of the all. The rings are installed as a whole cylinder with the short one (5cm) in the middle and the two longer ones (10cm) in the sides. The prepared root-soil specimen is filled into the cylinder before experimenting. In experimenting, the shear rings with specimen are placed horizontally on the experimented board, then fix the rings in two sides and leave the middle one moveable. The normal stress is applied horizontally on one side of the shear ring via weights through a lever and a fixed pulley which can change the force from vertical to horizontal. In shearing, the middle ring is pushing by a Stepper motor, then the specimen has two shear planes between each neighbor two of the three rings. The apparatus was used to measure the shear strength of Medicago sativa root-soil composite specimens under natural moisture content. A group of experiments were successfully completed. The results suggest that the double shear apparatus can effectively measure shear strength of root-soil composites, and that the roots in soil can significantly enhance shear strength. Relative to conventional direct shear apparatuses and triaxial apparatuses, the double shear apparatuses can be usable to larger size specimen with wider shear area, which can reduce the scale effect. In addition, the three rings constitute with the shear ring effectively solves the problem of shear experiment for the composite matter that granular material (soil) contain flexible material (root). The double shear provides an effective approach for experimenting the shear strength of root-soil composites.

double shear apparatus, root-soil composites, shear strength, soil, Medicago sativa