Characteristics of the surface deformation of specimens with a hole during the progressive failure

The leak around the gas drilling borehole is one of the major reasons causing the drainage failure,and three stages (crack initiation stage,stable growth stage and unstable cracking stage) of the crack propagation around the gas drilling borehole are performed under the failure of coal. In order to find out the characteristics of crack propagation at different stages,a study on the crack propagation around the gas drilling borehole during the progressive failure was carried out. Plaster and water are blended in a mass ratio of 7 ∶ 3 to make soft coal like blocks,and uniaxial compres-sion tests were performed with digital speckle correlation method ( DSCM) getting the surface deformation of speci-mens. Based on this,a method determining the stress threshold by DSCM was proposed,and the deformation around the hole in the specimens’ progressive failure was analyzed though this method. The sample failure process was divided in-to five stages by the stress threshold determined by this method,then the full-field deformation data were calculated to obtain the surface relative displacement,displacement around the hole and crack mouth opening displacement. The re-sults show that the development of the surface relative displacement can be divided into the gradually decrease stage,the aggravated decrease stage and the rapidly decrease stage,among that the aggravated decrease stage and the rapidly decrease stage are corresponded with the stable growth stage and unstable cracking stage which are found in stress-stain curve. As the symmetrical movement around the hole,tensile and compression state are formed which finally re-sults in the tensile crack and the normal shear crack formed on the surface.