Importance

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Abstract

A pilot hole of a preselected diameter is drilled with a suitable bit in an earth formation to a desired depth. The bit is withdrawn from the hole, and an expandable hole opener or underreamer run in the pilot hole with its cutters fully retracted and closely adjacent to each other. When the depth is reached at which enlargement of the pilot hole is to commence to produce a blast chamber, the cutters are forced laterally outwardly by fluid pressure while the hole opener is rotated, without moving the hole opener axially of the pilot hole, to cut the wall of the pilot hole and increase its diameter to the extent determined by maximum expansion of the cutters, thereby forming the upper end of the enlarged blast chamber. Rotation of the hole opener continues while it is progressively lowered to drill the blast chamber down to the bottom or lower end of the pilot hole, the bottom of the blast chamber produced by the hole opener lying in substantially the same plane as the bottom of the pilot hole to collectively form a single blast chamber bottom.

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First Claim