On the mechanics of seepage induced cohesionless soil slope instability

Charlie Harrison

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

Session: Landslides and Geohazards

ABSTRACT: hanics of seepage induced cohesionless soil slope instability Charlie Harrison, M.Eng, P.Eng. Golder Associates Ltd., Squamish, BC, Canada ABSTRACT Observations of seepage induced soil slope instability in cohesionless soils have been made by numerous geotechnical engineering practitioners and researchers. However, the mechanics of the mode of failure have not been quantified, or likely even fully understood. Although there is a significant amount of literature devoted to this topic, very little slope stability related literature discusses the failure mechanisms. Researchers studying the stability of zoned earth fill dams have published numerous papers, which discuss the stability of soil subjected to seepage. Applying this work to slopes provides insight into the mechanics of seepage induced slope instability. The author provides further discussion regarding the susceptibility of natural soils to seepage induced slope instability by comparing natural soils to laboratory testing. A possible solution to seepage induced slope instability utilizing Critical State Soil Mechanics is summarized. RÉSUMÉ Observations de l'infiltration induite sol instabilité de la pente dans les sols cohésifs ont été faites par de nombreux praticiens et des chercheurs d'ingénierie géotechnique. Cependant, la mécanique du mode de rupture n'ont pas été quantifiés, ou probablement même bien compris. Bien qu'il existe une quantité importante de la littérature consacrée à ce sujet, très peu de stabilité de la pente littérature liée examine les mécanismes de défaillance. Les chercheurs qui étudient la stabilité de zonage barrages en remblai de terre ont publié de nombreux articles, qui traitent de la stabilité des sols soumis à l'infiltration. L'application de ce travail des pistes permet de mieux comprendre les mécanismes de l'infiltration induit l'instabilité des pentes. L'auteur propose en outre la discussion au sujet de la sensibilité des sols naturels à l'infiltration instabilité des pentes induite en comparant les sols naturels à des tests de laboratoire. Une solution possible à l'infiltration induite instabilité de la pente en utilisant la mécanique des sols de l'état critique est résumée 1 INTRODUCTION Many published papers have provided a good, qualitative analysis of seepage-induced slope instability (Williams 1966; Eisbacher & Clague 1981; Hungr & Smith 1985; Crosta & di Prisco 1999; Evans & Savigny 1994; Cavers 2003). However, none touch upon the quantitative analysis required to sufficiently describe the failure mechanism(s). Terzaghi & Peck (1948) touched on the subject in regards to piping failure in dams, which they described as follows: [Piping] may be due to scour or subsurface erosion that starts at springs near the downstream toe and proceeds upstream along the base of the structure or some bedding plane. Failure occurs as soon as the upstream or intake end of the eroded hole approaches the bottom of the reservoir. The mechanics of this type of piping defy theoretical approach. Although Terzaghi and Peck did not consider natural slopes when discussing the mechanics of seepage-induced erosion, the mechanics of seepage-induced instability in a slope and dam are one and the same, with the latter being a man-made structure with a known groundwater source. The intent of this literature review was to bring together concepts from several areas soil mechanics (soil properties, strength criteria, and filter design), groundwater hydrology, geomorphology, and others and determine a framework for modelling seepage-induced slope instability. Many researchers and practitioners have considered rainfall-induced slope instability; however, rainfall intensities are a trigger, not a failure mechanism. Many studies have been conducted to correlate rainfall and slope failures (Yoshida et al. 1991; Au 1998; Collins & Znidarcic 2004; Zhang et al. 2005; Zhang et al. 2014; and many others), but due to the complex nature of the geology and hydrological properties of the system (Au 1998), correlations fail to capture all slope failures. Most often, studies focused on rainfall induced slope failure are concerned with shallow, translational slides. The inability to find a suitable correlation between rainfall and slope failure is likely due to the lack of knowledge regarding the actual failure mechanism present that is being triggered by the rainfall. This review is meant to form a basis for providing a framework to clear up any misconceptions regarding rainfall induced slope failures. 2 CASE HISTORIES Many case studies have been presented in engineering literature over the years. The following case histories, as described by Harrison (2014), provide examples of seepage induced soil slope instability in cohesionless soils: - 1979 rainstorm in Vancouver, BC (Eisbacher & Clague 1981)

RÉSUMÉ: e mechanics of seepage induced cohesionless soil slope instability Charlie Harrison, M.Eng, P.Eng. Golder Associates Ltd., Squamish, BC, Canada ABSTRACT Observations of seepage induced soil slope instability in cohesionless soils have been made by numerous geotechnical engineering practitioners and researchers. However, the mechanics of the mode of failure have not been quantified, or likely even fully understood. Although there is a significant amount of literature devoted to this topic, very little slope stability related literature discusses the failure mechanisms. Researchers studying the stability of zoned earth fill dams have published numerous papers, which discuss the stability of soil subjected to seepage. Applying this work to slopes provides insight into the mechanics of seepage induced slope instability. The author provides further discussion regarding the susceptibility of natural soils to seepage induced slope instability by comparing natural soils to laboratory testing. A possible solution to seepage induced slope instability utilizing Critical State Soil Mechanics is summarized. RÉSUMÉ Observations de l'infiltration induite sol instabilité de la pente dans les sols cohésifs ont été faites par de nombreux praticiens et des chercheurs d'ingénierie géotechnique. Cependant, la mécanique du mode de rupture n'ont pas été quantifiés, ou probablement même bien compris. Bien qu'il existe une quantité importante de la littérature consacrée à ce sujet, très peu de stabilité de la pente littérature liée examine les mécanismes de défaillance. Les chercheurs qui étudient la stabilité de zonage barrages en remblai de terre ont publié de nombreux articles, qui traitent de la stabilité des sols soumis à l'infiltration. L'application de ce travail des pistes permet de mieux comprendre les mécanismes de l'infiltration induit l'instabilité des pentes. L'auteur propose en outre la discussion au sujet de la sensibilité des sols naturels à l'infiltration instabilité des pentes induite en comparant les sols naturels à des tests de laboratoire. Une solution possible à l'infiltration induite instabilité de la pente en utilisant la mécanique des sols de l'état critique est résumée 1 INTRODUCTION Many published papers have provided a good, qualitative analysis of seepage-induced slope instability (Williams 1966; Eisbacher & Clague 1981; Hungr & Smith 1985; Crosta & di Prisco 1999; Evans & Savigny 1994; Cavers 2003). However, none touch upon the quantitative analysis required to sufficiently describe the failure mechanism(s). Terzaghi & Peck (1948) touched on the subject in regards to piping failure in dams, which they described as follows: [Piping] may be due to scour or subsurface erosion that starts at springs near the downstream toe and proceeds upstream along the base of the structure or some bedding plane. Failure occurs as soon as the upstream or intake end of the eroded hole approaches the bottom of the reservoir. The mechanics of this type of piping defy theoretical approach. Although Terzaghi and Peck did not consider natural slopes when discussing the mechanics of seepage-induced erosion, the mechanics of seepage-induced instability in a slope and dam are one and the same, with the latter being a man-made structure with a known groundwater source. The intent of this literature review was to bring together concepts from several areas

Access this article:
Canadian Geotechnical Society members can access to this article, along with all other Canadian Geotechnical Conference proceedings, in the Member Area. Conference proceedings are also available in many libraries.

Cite this article:
Charlie Harrison (2014) On the mechanics of seepage induced cohesionless soil slope instability in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper390,author = Charlie Harrison,title = On the mechanics of seepage induced cohesionless soil slope instability,year = 2014}