Abstract
It is well known that in undrained stability calculations, total stress and effective stress analyses do not give the same calculated factor of safety when FOS >1. This is due to the fact that shear strength is defined differently in these two approaches: In total stress analyses, the mobilised shear stress is compared to undrained shear strength, i.e. strength at failure. In undrained effective stress analyses, the shear strength is defined as corresponding to the mobilised effective stress state. This causes an overestimation of FOS in undrained φ′-c′ analyses. Modelling of excess pore pressure Δu has traditionally been source of most uncertainty in undrained effective stress analyses. Having the correct shear strength along the slip surface can be considered the most crucial detail in all stability analyses. It can be argued that in the context of Limit Equilibrium analyses where deformations are not considered, priority should be given to calculating the shear strength correctly, instead of attempting to obtain a “correct” mobilised Δu value. This paper gives a general introduction to the new HSU (Hybrid s u ) method. For the purposes of LEM analyses, Δu is calculated so that the resulting Mohr-Coulomb shear strength corresponds to the assumed failure state. This approach solves the inherent overestimation of FOS in undrained φ′-c′ analyses. To predict the effective stress at failure, a constitutive effective stress soil model is employed. Also presented is a concept of deriving undrained shear strength s u in LEM, based on an effective stress soil model. This makes it possible to conduct the LEM stability analysis in terms of total stresses, while deriving soil strength from effective strength parameters. The different approaches of calculating Δu and s u with the HSU method are compared using a theoretical stability calculation example. The relative merits of the different approaches are discussed.
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References
Bishop AW (1955) The use of the slip circle in the stability analysis of slopes. Géotechnique 5(1):7–17
Krahn J (2003) The 2001 R.M. Hardy lecture: the limits of limit equilibrium analyses. Can Geotech J 40(3):643–660
Larsson R, Sällfors G, Bengtsson P-E, Alén C, Bergdahl U, Eriksson L (2007) Skjuvhållfasthet – utvärdering i kohesionsjord. Statens geotekniska institut (SGI), Linköping
Lehtonen V (2015) Modelling undrained shear strength and pore pressure based on an effective stress soil model in Limit Equilibrium Method. Dissertation, TUT Publication 1337, Tampere University of Technology
Lehtonen V, Länsivaara T (2013) Two methods for estimating excess pore pressure in LEM. In: Proceedings of the 18th international conference on soil mechanics and geotechnical engineering, 2–6 September 2013, Paris, p 755–758
Lehtonen V, Länsivaara T (2016) Back-calculation of the Saint-Alban A test embankment with a new modelling approach in LEM. In: Proceedings of the 17th Nordic geotechnical meeting, 25–28 May 2016, Reykjavik, p 691–699
Roscoe K, Burland, J B (1968) On the generalized stress-strain behaviour of wet clay. Engineering plasticity, p 535–609
Wheeler SJ, Näätänen A, Karstunen M, Lojander M (2003) An anisotropic elastoplastic model for soft clays. Can Geotech J 40(2):403–418
Wood DM (1990) Soil behaviour and critical state soil mechanics. Cambridge University, Cambridge
Acknowledgments
The authors would like to gratefully thank the reviewer, Professor Leena Korkiala-Tanttu (Aalto University, Finland), for her essential input and comments. This research has been made possible by funding from Finnish Transport Agency.
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Lehtonen, V., Länsivaara, T. (2017). Advances in Determining Δu and su for Limit Equilibrium Analyses. In: Thakur, V., L'Heureux, JS., Locat, A. (eds) Landslides in Sensitive Clays. Advances in Natural and Technological Hazards Research, vol 46. Springer, Cham. https://doi.org/10.1007/978-3-319-56487-6_21
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DOI: https://doi.org/10.1007/978-3-319-56487-6_21
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