Slope stability of forested slopes considering effect of tree root and steel bar reinforcement

Abstract

A tree root crossing through upper soil layer into bedrock increases shear strength of soil and renders the slope more stable. The effect of root reinforcement on soil strength has been well researched through the mechanistic and empirical studies, but yet few studies have examined how tree roots in combination with steel bar influence slope stability. Further, most of previous research lacked consideration of effect of slope displacement on the reinforcement of tree roots and steel bars.By using actual distribution of tree root, an analysis of slope stability considering tree root reinforcement as a function of groundwater level and slope displacement will be proposed. Larger displacement of slope results in greater reinforcement of tree root. The resisting force and displacemen increase until the slope becomes stable. For some reasons (external load, more rainfall. etc), if the slope became unstable again, the process of landslide repeats until Fs=1. Tree root functions like reinforcement material and improves factor of safety of forested slope, Fs≥1. However, target factor of safety (also called design factor of safety Fds) of a forested slope must be equal to or higher than a standard value (such as Fds= 1.2 in this paper). Magnitude of Fds depends on the importance of slope. Thus, we need extra reinforcement even if the slope has tree root reinforcement. There are many methods to reinforce slope. In this paper, steel bar is chosen for two reasons: 1) steel bar has the same behavior as a root in a landslide; 2) steel bar is one of very few methods that can sustain the vegetation on slope. By applying model of inclined root proposed by Nghiem et al.(2001, 2003), a design method is presented for root fibers reinforcing slope in combination with steel bars.