NUMERICAL ANALYSIS OF A STABILIZED NATURAL SLOPE

Xinli Wu, Hany El Naggar, A.J. Valsangkar

In the proceedings of: GeoQuébec 2015: 68th Canadian Geotechnical Conference & 7th Canadian Permafrost Conference

Session: Landslides and Slope Stability I / Mouvements de terrain et stabilité des pentes I

ABSTRACT: Marginally stable slopes coupled with heavy rainfall represent a common challenge to rapid urban development in hilly terrain. These conditions lead to landslides that can be disastrous causing significant damage to nearby properties and loss of lives. In this paper, the stabilization of a 60 m high natural slope is analyzed using finite element and limit equilibrium methods. The contribution of the post-tensioned ground anchors and face plates used for slope stabilization is considered in the analysis. The strength reduction finite element analysis was able to capture the behavior of the slope before the stabilization and its performance after stabilization. The factors of safety obtained with two different approaches are compared and discussed in this paper. Keywords: Slope Stability, Landslides, Limit Equilibrium, Finite Element Analysis, Ground Anchors.

RÉSUMÉ: Les pentes marginalement stables couplées à des fortes précipitations représentent un défi pour un développement urbain rapide en terrain vallonné . Ces conditions conduisent à des glissements de terrain qui peuvent être désastreux, causant des dommages importants aux propriétés et beaucoup de pertes en vies humaines. Dans cet article, la stabilisation d'une pente naturelle de 60m de haut est analysée en utilisant des éléments finis et les méthodes d'équilibre limite. L™analyse considère la contribution des ancrages au sol par post-tension et des plaques frontales utilisées pour la stabilisation de la pente. L™analyse par éléments finis a permis d™examiner le comportement de la pente avant et après la stabilisation. Les facteurs de sécurité obtenus avec les deux approches sont comparés et discutés dans le présent document . Mots-clés: stabilité des pentes , glissements de terrain , équilibre limite , analyse par éléments finis , ancrage au sol. 1. INTRODUCTION Marginally stable high slopes can lead to disastrous damage to nearby properties or even loss of lives. To mitigate failures associated with marginal slopes, flattening of the slope, construction of retaining walls at the toe, stabilization with ground anchors, and the combination of these approaches is commonly implemented. It is well known that the use of ground anchors is an effective method to stabilize marginally stable high slopes, especially when solutions such as flattening of slopes or construction of retaining walls are not feasible due to site constraints. The stability of a slope can be assessed either by limit equilibrium approach (LEM) or finite element method (FEM). In the LEM, to simplify the problem, the slope is divided into a number of slices. There are a number of methods proposed in LEM based on the different assumptions made regarding inter-slice forces and the way in which overall force and moment equilibrium equations are satisfied. On the other hand, the finite element method (FEM) is a general-purpose method that has many desirable characteristics, which are not accounted for in the traditional limit equilibrium method. FEM is capable of modeling: (1) stresses, movements, and pore pressures in embankments and slopes; (2) the conditions during construction and the construction sequence; (3) the soils response under such conditions as nonlinear stress-strain behavior and non-homogeneous conditions. The strength reduction technique of the finite element method is a popular and powerful tool which has been used for analyzing slopes reinforced by soil nails, e.g. ground anchors. It has been demonstrated by Griffiths and Lane (1999) that the FEM is a more powerful alternative to LEM when assessing stability in their study of unreinforced slopes and embankments. Cheuk et al. (2013) studied the influence of soil nail orientations on the behavior of the ground nail-facing system. The present study also has carried out similar analysis, but these results are not presented in this paper. Cai and Ugai (2000 and 2003) used three-dimensional elasto-plastic shear strength reduction FEM to evaluate the stability of homogeneous slope reinforced with piles or ground anchors. Their research shows that it is the soil-nail interaction that affects the stability of slopes, and that the factor of safety obtained from FEM approach is close to that obtained by the Bishop's simplified LEM. In the research works cited in previous paragraph, either the slopes involved are homogeneous in nature, or the height of slope is less than 30 meters. In addition, the research work cited above mostly pertains to idealized cases to carry out a parametric study. When compared to

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