Abstract: Soft interlayer in rock and soil mass is a widespread geological structure.In underground mining engineering，the existence of soft interlayer will bring disadvantages to the design and stability analysis of pillar.In view of the ultimate bearing capacity of soft interlayer，based on Prandtl's plastic extrusion failure mode and upper bound limit analysis method，formulae of ultimate pressure of soft interlayer are deduced under plane strain condition and space axisymmetric condition，where different shearing stress conditions of various locations on the surface of soft layer are also taken into account.Analysis of calculated data shows that，the ultimate pressure of soft interlayer has a relationship with width-height ratio of soft interlayer and friction factor on its surface.In general，the ultimate pressure of soft interlayer is enhanced both with the increase of its aspect ratio and friction factor on its surface，and the effectiveness of friction conditions on its surface is even more significant with the increase of aspect ratios.However，as the friction factor increased to a certain degree，its effect on ultimate pressure in various aspect ratio scales will gradually diminish until it vanishes.Besides，comparison with other methods adopted single friction condition on the surface of soft interlayer demonstrates that，the solution in the paper is almost the same with others with the smaller aspect ratio and is higher than others with a larger aspect ratio.Furthermore，as the friction factor and roughness on interlayer surface is extremely large，solutions of other methods including this text are identical approximately.The formula of ultimate bearing capacity could provide a reference to the design and stabilization analysis of a soft interlayer underlying pillars.

Key words: Pillar foundation, Soft interlayer, Plastic extrusion failure, Friction conditions on interlayer surface, Ultimate bearing capacity