Abstract: Based on the SMP failure criterion of clay materials affected by the principal stress, and  considering stress conditions on goaf and roadway, the formula of the coal pillar width under two conditions is obtained. In the meantime, the plastic-zone width of the coal pillar in plane strain is inferred by using the limit equilibrium method. Compared with results obtained by Mohr-Coulomb failure criterion by changing the coal internal friction angle and cohesion, it shows that the traditional approach failed to make fully consideration of ultimate strength of the coal, and coal pillar size was designed conservatively. The calculation on coal pillar width based on SMP failure criterion is not only effective, but it will take an important significance in improving the production efficiency of the coal mine. 

Key words: Coal pillar, SMP failure criterion, Plane strain, Elastic-plastic width