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.

References:

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.