For level pillar stoping under loose medium

determining the critical thickness of roof is difficult. In order to solve the problem

using elastic beam theory

the level pillar is simplified as a mechanical model of a beam fixed at both ends under uniform load; and is considered to be the main failure stress according to the elastic stress solution of beam. Theoretical analysis shows: in level pillar stoping process

the main influence factors of critical thickness of roof is loose medium load

stoping span and tensile strength of rocks. Through using FLAC3D and combining with orthogonal experiment result of numerical simulation

the relationship between single influence factor and critical thickness of roof is revealed. Based on multiple nonlinear regression analysis method

the mathematical forecasting formula of critical thickness of roof under multifactor influence is established. Finally

The critical thickness of the roof is predicted accurately; and applicability of the mathematical forecasting formula is verified by the engineering practice of level pillar stoping in Fenghuangshan Mine.