effect and minimize dilution application_number US 69752110 A description fig1 illustrates an underground ore body 10 or vein extending between two horizontal drifts 12 and 14 accessible from the ground surface through a ramp or a vertical opening known as a shaft not shown as known in the art the shaft is equipped with elevators by which the workers machines and material can gain access to the galleries or drifts 12 and 14 the drifts 12 and 14 are tunnels made in the rock with a size and shape to permit mining of the ore body 10 each drift defines a working level running through the ore body the working levels can for instance be vertically spaced by about 18 m 59 feet as shown in fig1 and 2 boreholes 16 only two being shown in fig1 and 2 are drilled at regular intervals along the vein 10 each borehole extending vertically though the vein 10 from the top drift 12 to the bottom drift 14 the diameter of these holes is typically in the range of 6 inches 15 2 cm the diameter of the holes 16 is not critical but is preferably kept as small as possible to minimize drilling costs and time the diameter must however be sufficient to permit hole enlargement by thermal fragmentation each borehole 16 can be drilled haft way through from the top drift 12 and the bottom drift 14 in order to limit hole deviation it is however understood that the holes 16 can be completely drilled from the top drift 12.