The strip coal pillar method is widely used in deep coal seams mining. The reasonable strip coal pillar size is of great significance to improve the coal recovery rate and protection of surface topography. Under the plane strain state

a unified formula is firstly summarized for five strength criteria

such as Wilson criterion

Mohr-Coulomb (M-C) criterion

generalized Matsuoka-Nakai (M-N) criterion

generalized Lade-Duncan (L-D) criterion and circumcircle Drucker-Prager (D-P) criterion. Then

the unified formula of limit strength

yielding width and remained width for strip coal pillar is presented. Finally

the effect of strength criterion is investigated and parametric studies are discussed. The results show that the obtained unified formula covers existing solutions in the literature and its application is very convenient; the effect of strength criterion on the strip coal pillar design is significant; the results of Wilson and M-C criteria are in too conservation

while the use of circumcircle D-P criterion is need to be cautious; moreover

the generalized M-N criterion or generalized L-D criterion is recommended to be adopted; the influence of changes in strength parameters (cohesion and inner friction angle) of coal seam on the strip coal pillar design is remarkable and should be fully considered. The obtained results can provide some theoretical guidance in reasonable design and construction of strip coal pillars.