For a two-laver slope with a known geometry,unit weight and pore water pressure distribution,the position of critical slip surface from a limit equilibrium stability analysis will remain unique for a given setof (λ1=c'1/tanΦ'1,λ2=c'2/tanΦ'2,λ3=c'1/c'2) values regardless of the magnitude of individual strengthparameters (c'1,Φ'1,c'2,andΦ'2) of two soil layers. This theoretical relationship between c' and Φ' and the criticalslip surface is used to develop a back analysis method by which the shear strengths of soils can be estimated froma known position of the failure surface observed in the field two-layer slope. In this method,an objective functionis first defined to describe the difference between the actual failure surface and theoretical critical slip surface,and then a nonlinear programming technique is used to minimize the difference (i.e. the objective function) soas to determine an optimal solution of (λ1,λ2,λ3) that corresponds to the failure surface. When the magnitudeof(λ1,λ2,λ3) are obtained,a unique set of(c'1,Φ'1,c'2,Φ'2) values can easily be computed by considering the factthat the factor of safety is equal to unity. Results back-calculated from a failure surface involved in a two-layerslope are presented to demonstrate the effectiveness of the proposed back analysis method.
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