Sarma method: Difference between revisions

Revision as of 13:06, 28 February 2013

Revision as of 03:30, 1 March 2013

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[[Sarada K. Sarma|Sarma]] worked in the area of seismic analysis of earth dams under [[Nicolas Ambraseys|Ambraseys]] at [[Imperial College]] for his doctoral studies in the mid 1960s.<ref>Sarma S. K. (1968) ''Response characteristics and stability of earth dams during strong earthquakes''. PhD Thesis, Imperial College of Science & Technology, University of London</ref> The methods for seismic analysis of dams available at that time were based on the [[Slope stability analysis#Limit equilibrium analysis|Limit Equilibrium]] approach and were restricted to planar or circular failures surfaces adopting several assumptions regarding force and moment equilibrium (usually satisfying one of the two) and about the magnitude of the forces (such as interslice forces being equal to zero).

Sarma looked into the various available methods of analysis and developed a new method for analysis in seismic conditions and calculating the permanent displacements due to strong shaking. His method was published in the 1970s (the very first publication was in 1973<ref>{{cite doi|10.1680/geot.1973.23.3.423}}</ref> and later improvements came in 1975<ref>{{cite doi|10.1680/geot.1975.25.4.743}}</ref> and 1979 <ref>Sarma S. K. (1979), ''Stability analysis of embankments and slopes''. Journal of Geotechnical Engineering, ASCE, 1979, 105, 1511 - 15241511–1524, ISSN: 0093-6405</ref>).

==Method==

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The method is used mainly for two purposes, to analyse earth slopes and earth dams. When used to analyse seismic slope stability it can provide the factor of safety against failure for a given earthquake load, i.e. horizontal seismic force ir acceleration (critical acceleration). Besides, it can provide the required earthquake load (force or acceleration) for which a given slope will fail, i.e. the factor of safety will be equal to 1.

When the method is used in the analysis of earth dams (i.e. the slopes of the dam faces), the results of the analysis, i.e. the critical acceleration is used in the [[Newmark's sliding block]] analysis <ref>Newmark, N. M. (1965) Effects of earthquakes on dams and embankments. Geotechnique, 15 (2) 139-160139–160.</ref> in order to calculate the induced permanent displacements. This follows the assumption that displacements will result if the earthquake induced accelerations exceed the value of the critical acceleration for stability.

==Accuracy==

===General acceptance===

The Sarma method has been extensively used in seismic analysis software<ref>[http://www.finesoftware.eu/geotechnical-software/help/slope-stability/sarma/ GEO 5 Geotechnical Software]</ref><ref>[http://www.slope-analysis.com/html/galena_faq.html slope stability software -– Galena software ]</ref> for many years and has been the standard practice until recently for seismic slope stability for many years (similar to the [[Mononobe-OkabeMononobe–Okabe method]] <ref>Okabe, S. (1926) General theory of earth pressures. Journal of the Japanese Society of Civil Engineers, 12 (1)</ref><ref>Mononobe, N & Matsuo, H. (1929) On the determination of earth pressures during earthquakes. Proceedings of the World Engineering Congress, 9.</ref> for retaining walls). Its accuracy has been verified by various researchers nad it has been proved to yield results quite similar to the modern ''safe'' [[Lower Bound]] numerical stability [[Plasticity (physics)|Limit Analysis]] methods (e.g. the 51st [[Rankine Lecture]] <ref>[http://bga.city.ac.uk/cms/html/51stRankineLecture.pdf 51st Rankine Lecture -– Geotechnical Stability Analysis]</ref>).

===Modern alternatives===

However, nowadays modern [[numerical analysis]] software employing usually the [[Finite element method|finite element]], [[Finite difference method|finite difference]] and [[Boundary element method|boundary element]] methods are more widely used for special case studies.<ref>Zienkiewicz O C, Chan A H C, Pastor M, Schrefler B A, Shiomi T (1999) Computational Geomechanics with Special Reference to Earthquake Engineering. John Wiley & Sons, London.</ref><ref>Zienkiewicz, O. C. & Taylor, R. L. (1989) The Finite Element Method. McGraw-HillMcGraw–Hill, London.</ref> Particular attention has been recently given to the finite element method <ref>Griffiths, D. V. & Lane, P. V. (1999) Slope stability analysis by finite elements. Geotechnique, 49 (3) 387 - 403387–403</ref> which can provide very accurate results through the release of several assumptions usually adopted by the conventional methods of analysis. Special boundary conditions and constitutive laws can model the case in a more realistic fashion.

==See also==