ABSTRACT
Initiation of a single blasthole near a rock surface induces a bulk acceleration of the surface material, which acts like a large piston, initially compressing the air and producing a significant airblast radiation away from the surface. The general solution for radiation from any single piston source can be obtained directly by using a Green’s Function approach (Morse and Ingard, 1968). Freedman (1971, 1997) gave simple, specific solutions for the case of rectangular and circular pistons whose radiation is detected in the far field. However, all such solutions are invariably applied to oscillating pistons and not a piston (rock surface) that only moves in an outward direction. For an oscillating piston, both the velocity and acceleration are necessarily non-zero. However, for a non-oscillating piston appropriate to surface movement it is possible to have constant velocity (zero acceleration). Nevertheless, Freedman’s solutions for far field radiation from a moving piston are still applicable to vertical blastholes in the front row of a free-face shot, which produces movement of the vertical face. These solutions are also applicable to all blastholes in the body of a shot, which produces vertical heave of the horizontal surface. Blair (2004) used the far-field rectangular piston solution to
2 THE AIR PISTON MODEL FOR VERTICAL FACES
Figure 1 shows a schematic of the vertical face divided into N rectangular piston elements, each sequentially delayed in time according to the delay interval between the face blastholes.
