Abstract:

Objective: Rainfall is one of the important causes of slope instability. It is generally believed that plant roots can effectively reinforce soil and increase slope stability. In general,shallow landslides occur during and shortly after precipitation events. The function of soil reinforcement and its change dependent upon the soil volumetric water contents remain poorly studied because of the complex root-soil interactions and difficulty of the underground observations. This study investigated the influence of soil moisture on soil cohesion and internal friction angle,and identified the root failure mode under different soil moisture,which aims to expand our knowledge about the dynamics of slope stability during rainfall. Method: In this study,a common shrub species (Vitex negundo) in north China was selected. In different periods after rainfall,the undisturbed root-soil composite samples and plain soil samples without roots were collected from soil profiles. The shear strength of the undisturbed root-soil composite and plain soil samples were measured for calculating the cohesion (c) and internal friction angle (φ). Here,remolded samples of root-soil composite and plain soil were also tested for comparison. Result: 1) The increase of root pullout strength of V. negundo exhibited a single peak curve with the soil volume water content,and the strength reached the maximum value when the soil volume water content was about 18%. 2) Plant roots were able to significantly reinforce the soil. Compared with root free soil,the cohesion of the undisturbed and remolded root soil composite increased by 15.8 and 7.5 kPa,and the internal friction angle increased by 3.1° and 1.1°,respectively. 3) The cohesion and friction angle of rooted soil were negatively correlated with soil water content,generally. With the increase of soil moisture from 13% to 40%,the additional cohesion of undisturbed and remold samples decreased to 5.9 and 2.6 kPa,respectively. Additional internal friction angle of the undisturbed samples decreased to 0.1°,while that of remold samples increased to 1.8°. Conclusion: Plant roots can play an important role on both cohesion and internal friction angle,however,the reinforcement provided by roots would be decrease significantly with precipitation-driven water content increase. Thus,it is suggested that in the area or season with heavy rainfall,the additional cohesion provided by roots should be evaluated more conservatively. This study provides a theoretical basis for establishing a dynamic root reinforcement model considering soil moisture.

Key words: addition cohesion of root, soil volume water content, pull-out strength, root-soil composite, root area ratio