3-D numerical investigations into the shear strength of the soil–root system of Makino bamboo and its effect on slope stability

Abstract

This study attempts to quantify the reinforcement effect of the Makino bamboo ( Phyllostachys makinoi Hayata) root system on the stability of slopeland through numerical analyses and in situ tests. Based on the field surveys of Makino bamboo root morphology, a three-dimensional (3-D) numerical model of the soil–root system consisting of the reverse T-shape tap root and hair roots was developed and successfully applied to the finite element simulations of in situ pull-out tests. In the simulations, the soil mass was simulated by a soil element with a perfect elastic–plastic (or Mohr–Coulomb) material model whereas the root system was simulated by a ground anchor element with a linear elastic material model. In addition, a mechanical conversion model with simple mathematical form, which enables a direct transformation of the ultimate pull-out resistance into the shear strength increment of soil–root system was proposed. The conversion model offered a convenient way to quantify the reinforcement effect of the Makino bamboo root system required for the 3-D slope stability analyses. The numerical results indicated that the shear strength increment of the Makino bamboo soil–root system ranged from 18.4 to 26.3 kPa and its effect on the slope stability was insignificant when compared with those adverse influence factors such as the steep slope angle (=50–70°), shallow root depth (=0.8–1.0 m) and large growth height (>10 m) of the Makino bamboo forest slopeland. It can be also speculated that the tension cracks widespread over the slope surface due to the wind loading acting on the bamboo stems and the sequential rainwater infiltration is the dominating factor in the collapse failure of slopeland. For a Makino bamboo forest slopeland with medium slope (25° < slope angle β < 40°), the reinforcement effect of the Makino bamboo root system can mobilize its maximum stabilization capacity when compared with those of slopeland with mild ( β < 25°) and steep slopes ( β > 40°). Conclusively, the contribution of the Makino bamboo root system to the stability of slopeland is not as significant as expected.