Soft interlayer is a widely existing geological body in rock and soil mass. Due to its weak mechanical properties

it usually brings potential safety hazard to engineering construction. Based on the plastic limit equilibrium method

an ultimate-load formulation of soft interlayer is derived by using the modified Prandtl squeezing theory and assuming that the upper and lower layers close to the soft interlayer to be quasi-rigid. In the analysis

we suggests that under the limit state the soft interlayer between quasi-rigid layers displays plastically squeezing failure behaviors. Due to the inaccuracy of Prandtl’s shear stress solution in the x direction

new solutions of stress components and ultimate load of soft interlayer are determined with considering various boundary stress conditions. A simple formulation of the ultimate load on the soft interlayer is derived

which could reflect the comprehensive effects of dead weight

boundary stress conditions

as well as friction condition of boundaries on the ultimate load of soft interlayer. Meanwhile

by comparing results from proposed method to other methods

it is shown that

if the dead weight of interlayer is ignored

the variation of ultimate load with thickness of soft interlayer in this text generally agree with what the Prandtl method predicts; when the dead weight is taken into account

the solutions of the proposed method yields results similar to the method in the construction code. For thick interlayer

however

the proposed method can more properly reflect the influence of width on the ultimate load. In addition

the effect of the boundary friction on the ultimate load varies with width-thickness ratio of soft interlayer

and a critical width is found to exist for the interlayer

which is hardly affected by the friction condition for the interlayer with a certain thickness.