Comparison of 2-D and 3-D slope stability analyses for unsaturated soil slopes

Lulu Zhang, Murray D. Fredlund, Delwyn G. Fredlund, Haihua Lu

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Unsaturated Soils

ABSTRACT: Most past comparative studies between 2-D and 3-D stability analyses have ignored the effect of negative pore-water pressures (i.e., matric suctions) in the soil zone above the groundwater table. In this paper, a comparison is made between 2-D and 3-D slope stability analyses on soil slopes where a portion of the soil profile has matric suctions. The factors of safety on simple geometry slopes, complex geometry slopes which have two intersecting slope surfaces, are investigated under various shear strength parameters and groundwater conditions. For simple slopes with a low slope angle, the difference in factor of safety between a 2-D and 3-D slope stability analysis, (i.e., DFs/Fs2-D), generally ranges from 9% to 16% when fb is equal to 15 degree. The value of DFs/Fs2-D for a steep, simple slope is generally larger than that of a low angle, simple slope. The difference between a 2-D and 3-D stability analysis was most pronounced for concave geometries where a portion of the soil profile contained unsaturated soils. The values of DFs/Fs2-D for concave slopes with a corner angle between 180 to 270 degrees can be as large as 20 to 59% when fb is equal to 15 degree. The case history of the high-wall stability failure at the Poplar River coal mine was back-calculated to illustrate the effect of unsaturated zone on back-analyzed shear strength parameters. RÉSUMÉ La plupart des études comparatives entre les deux dernières 2-D et de la stabilité 3-D analyses ont ignoré l'effet de pressions interstitielles négatives (succions matricielles) dans la zone de sol au-dessus de la nappe phréatique . Dans cet article, une comparaison est faite entre 2-D et 3-D stabilité de la pente des analyses sur les pentes du sol où une partie du profil de sol a succions matricielles. Les facteurs de sécurité sur la géométrie des pentes simples, la géométrie des pentes complexes qui ont deux surfaces de pente qui se croisent, sont étudiées sous différents paramètres de résistance au cisaillement et les eaux souterraines. Pour les pentes simples avec un angle d'inclinaison faible, la différence de coefficient de sécurité entre une analyse 2-D et 3-D stabilité des talus, (DFs/Fs2-D), est généralement comprise entre 9% et 16% lorsque fb est égal à 15 degrés. La valeur de DFs/Fs2-D pour, une pente raide simple est généralement plus grande que celle d'un faible angle, pente simple. La différence entre un 2-D et 3-D analyse de stabilité a été plus prononcée pour les géométries concaves où une partie du profil de sol contenait des sols non saturés. Les valeurs de DFs/Fs2-D pour pentes concaves avec un angle de coin entre 180 270 degrés peuvent être aussi grand que 20 à 59% quand fb est égal à 15 degrés. L'histoire de l'échec de la stabilité à haute paroi de la mine de charbon de la rivière Poplar cas était de retour-calculé pour illustrer l'effet de la zone non saturée sur les paramètres de sauvegarde analysé la résistance au cisaillement . 1 INTRODUCTION Two-dimensional (2-D) limit equilibrium methods (LEM) of slope stability analysis remain the most common method of analysis in slope engineering practice. However, most engineering problems have three-dimensional characteristics that cannot be taken into account by conventional two-dimensional plane strain representations. Most natural landslides are three-dimensional in character and the geometries are too complex to be accurately modeled using a two-dimensional representation of the geometry. Other engineering problems which are inherently three-dimensional include mining pit, tailings and mine rock piles, deep excavation with corners, earth and rock fill dam or levees, earth storage, and municipal solid waste slope (Fredlund et al. 2012b; Yu et al. 2005). A three-dimensional (3-D) analysis can accommodate variations in geometry, pore-water pressures, and material properties across a site. Three-dimensional methods of slope stability analysis are usually extensions of conventional two-dimensional approaches (Hovland 1977; Chen and Chameau 1982; Leshchinsky and Baker 1986; Zhang 1988, Hungr et al. 1987; Lam and Fredlund 1993; Chen et al. 2001; Cheng and Yip 2007; Zheng 2012). Kalatehjari and Ali (2013) undertook an extensive review of 3-D analyses. It is generally assumed that a 2-D slope stability analysis provides a more conservative estimate of a 3-D slope

RÉSUMÉ: rison of 2-D and 3-D slope stability analyses for unsaturated soil slopes

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Cite this article:
Lulu Zhang; Murray D. Fredlund; Delwyn G. Fredlund; Haihua Lu (2014) Comparison of 2-D and 3-D slope stability analyses for unsaturated soil slopes in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper379,author = Lulu Zhang; Murray D. Fredlund; Delwyn G. Fredlund; Haihua Lu,title = Comparison of 2-D and 3-D slope stability analyses for unsaturated soil slopes ,year = 2014}