MULTISTAGE PORTFIRE

PROBLEM TO BE SOLVED: To produce an allowable ground vibration by interposing an impact pressure releasing means between the inner wall of a plurality of initializing/detonator tubes and the detonator and on the preceding explosion side between the tubes and disposing a gas explosion pressure enclosing means on the following exploding side between the tubes thereby decreasing the maximum amplitude, speed and acceleration. SOLUTION: The multistage portfire 1 is loaded in series with a plurality of initializing/detonator tubes 10 from the bottom of holes 3 made in the ground 2 with a spacer 20 being inserted between respective initializing/detonator tubes 10. The initializing/detonator tube 10 is provided, between the inner wall of the tube and a detonator 14 to be loaded, with a gap as an impact relaxing means. The spacer 20 comprises a bored tubular body 23 provided, on the bottom thereof, with a lower cover 24 and, in the outer circumferential part thereof, with a plurality of side holes 25 and being filled, on the inside thereof, with a closing material 26 for a gas expansion pressure enclosing means, and a can body part 22 having an air gap 22a for the impact relaxing means disposed in the opening of the bored tubular body 23. An allowable ground vibration can be ensured even it an impact pressure or a gas expansion pressure of various magnitude acts in various directions, by decreasing the maximum amplitude, speed and acceleration of ground vibration factors incident to blasting method.