Mechanism of Rock Breakage under Pressure of Gas Explosion

Abstract

It is believed that rock breakage by blasting is normally completed both by the dynamic breakage under intense stress waves, and by the quasi-static breakage under the pressure of gas explosion. First in this study the stress condition caused by the time dependent pressure in an elastic medium was computed from time to time as it worked on the spherical cavity, and thereby were inferred what were the stresses in the medium that were affected by the variation of the rise time of the pressure in the cavity. It is concluded that the values of the dynamic stresses in the medium caused by the stress wave decrease with increase of the rise time of the pressure, and that the stress condition in the medium is controlled by the quasi-static stress caused by the pressure in the cavity. It is considered therefore that the breakage of rocks caused by the explosion of the explosives having low velocity of defragration is completed chiefly by the quasi-static pressure of the gas explosion.Next the stress condition caused by the quasi-static pressure was analyzed by using the finite element method as it worked on the circular boreholes, assuming the cases in which the boreholes ranged linearly with constant spacing of 10a, where a was the radius of the hole. At first pressure was applied to every other borehole, and the stress condition around the hole where no pressure was applied was carefully observed, and then the pressure was applied to every hole, and probable change in the stress distribution was studied upon the assumed growth of four radial cracks that would be originated from the inner surfaces of the holes, as two cracks grew along the line which connected the centre of each hole, and two other cracks grew along the line perpendicular to the above. These cracks are supposed to have been produced from the inner surface of each hole when the value of the pressure exceeds the tensile strength of the material. It is clarified as the result that the cracks along the line which connects the centre of each hole will grow further while those which grow perpendicular to the above are repressed. By comparing the results of these stress analyses with the strain measured in the material and the pattern of the crack produced actually by the pressure of the gas explosion, it is concluded finally that the quasi-static gas pressure contributes very much to the completion of smooth wall in the smooth blasting operation.