Mohr – Coulomb failure criterion continued

Things to remember when using the Mohr – Coulomb failure criterion:

  • The linear failure envelope is just an approximation to simplify calculations
  • The failure envelope is stress dependent and will produce some kind of curvature if shear strength tests are executed in much different confining stresses (fig 1, from Duncan and Write, 2005).


  • According to Lade, 2010 the failure envelope is curved and at low effective stresses which can be found in superficial failures on slopes, the use of linear Mohr – Coulomb may be in the unsafe side. Soils without cementation do not provide any effective cohesion in very low effective stresses (fig 2, from Lade, 2010).


  • When the linear Mohr – Coulomb criterion is used it must be evaluated for the expected stress range in the field.
  • Small cohesion values will not produce significant errors when high effective stresses are anticipated in the calculation.
  • In low effective stress even minimum values of effective cohesion (in cohesionless soils) can produce significant errors in factor of Safety (FS) calculations.


Duncan J. M.,  Wright S. G., (2005). “Soil Strength and Slope Stability”. Wiley, New York.

Lade P. V. (2010). “The mechanics of surficial failure in soil slopes”. Engineering Geology 114, pp 57-64.

About Chrys Steiakakis

Chrys Steiakakis is a practicing geotechnical engineer with more than fifteen years of experience in the field of geotechnical engineering. He earned his bachelor and master in mining engineering from the Technical University of Crete, Greece and a second master’s degree in Civil Engineering from Virginia Polytechnic Institute and State University, USA. He has been the technical director of engineering department of General Consulting ISTRIA for four years and now he is a partner and also provides his own consultancy services via Geosysta ltd. He has been involved in numerous highway, railway and mining projects. Chrys with his long term collaboration with the Technical University of Crete has participated in numerous research projects in the field of geotechnical engineering and rock mechanics and has provided self sustained seminars of geotechnical engineering in related areas for the Industry. His main field of experience covers all aspects of tunnel design, earthworks design and monitoring (slope stability, embankment in difficult ground, reinforced embankments and retaining walls), landslide investigation and mitigation, foundations for bridges and structures, risk assessment in geotechnical projects and value engineering in large projects.