The thickness reduction method in forecasting the critical safety roof thickness of gob area
Zhongming He · Meitan xuebao · 2009
According to the idea of slope safety factor calculated by the method of strength reduction,the thickness reduction method was proposed to forecast the safety roof thickness of gob area.In the procedure of thickness reduction method,the roof thickness was reduced gradually until it reached the critical failure state,which marked the minimum safety thickness.If roof reached the critical failure state,displacement of the grid in the roof would mutate,and the calculation would be divergent.So whether or not the static equilibrium equations was of solvability and the calculation was of convergence can be chosen as the standard of roof failure.The proposed method was applied in the forecasting of one roof safety thickness in the gob area.In the calculation,the Mohr-Coulomb criterion was adopted,which can consider the tensile failure as well as shear failure of material.And the stress and deformation of the gob area were analyzed.The results show that the roof safety thickness are 2.1,5.2,9.2,12.3,16.6,22.9 m,responding to the gob area span of 4,7,10,15,20,25 m;area of plastic zones is small and failure mode is clicking for small span of gob area;with the increase of span,the area of plastic zones increases at the same time,while the failure mode transfers to caving;while the failure of roof takes place,the unbalance force of calculation can not convergent with the perpetual increase of displacement,and the maximum displacement zones are in the gob upper areas.