In some mining operations it becomes desirable to have blast holes for explosives extending upwardly from a working chamber. Such upwardly extending blast holes may, for example, be useful in forming raises, blasting to adjacent raises, or for block caving. Suitable blast holes are prepared by drilling upwardly with conventional rock drills to leave a cylindrical blast hole, defined by cylindrical walls of the rock structure that was drilled, within which explosives are placed for subsequent detonation. A number of loading techniques for upwardly extending holes have been developed but none are completely satisfactory for very large scale operations because of costs of the explosives or the effort required for loading. This becomes particularly true as the vertical length of the blast hole is extended.
Prior techniques for loading upwardly extending holes have, for example, included the placement of plugs at intervals in the hole and the pumping of explosive slurries into the hole segments. Such an arrangement can require a plurality of explosive detonators arrayed in the several segments to assure that all portions of the explosive detonate. This may be required because of difficulty in propagating an explosion across the plugs between adjacent segments. The plurality of lead wires coming down the hole creates a particular problem. Dynamite sticks and other prepackaged explosives can be packed into upwardly extending holes but the labor and explosive costs can be high.
A particularly inexpensive explosive for large scale mining operations comprises a mixture of ammonium nitrate and fuel oil commonly known as ANFO. Typically this mixture comprises ammonium nitrate prills mixed with about 5 to 6% of viscous fuel oil that coats the prill surfaces and to some extent works into the prills. Aluminum powder may be included in the mixture for enhanced density and higher energy. Sometimes the ANFO is slightly moistened to enhance sensitivity. Commercial variations of ANFO are available where "dry" reducing materials are mixed with ammonium nitrate. These can also be adapted for use in practice of the invention described herein. Such explosive material is convenient to handle since it is pourable or can be pumped with compressed air. It is extensively used for downhole loading or for lateral holes where it may be blown in with compressed air.
Loading of ANFO into an upwardly extending blast hole has previously been accomplished by blowing a suspension of ANFO in air up through a hose inserted in a blast hole at low velocity, sufficient to convey it through the hose. Some of the ANFO particles would stick at the closed end of the hole and eventually the blast hole would be filled. So far as is known no one has succeeded by such a technique in loading an upwardly extending hole having a length of more than about 30 feet.
Two types of compressed air loading systems have been used in the past. One of these uses a pressurized vessel or "prill pot" wherein the ANFO is partially fluidized and blown through a hose into the blast hole. The other system uses a flow of air through a venturi aspirator to suck up the ANFO from an open vessel and blow it into the hole. Both of these systems are primarily used for loading laterally extending blast holes. Downwardly extending holes are usually loaded by pouring, although compressed air loading may also be used. The compressed air loading systems can load short upwardly extending blast holes, but are not satisfactory for very long holes extending upwardly from the rock face in which they are drilled.
Recent developments in underground mining and/or retorting of oil shale have established the desirability of loading explosives into upwardly extending blast holes extending 70 feet or more above their bottom ends at the face of the rock structure into which the blast hole was drilled. It is therefore desirable to provide a technique for loading ANFO into such long upwardly extending holes in an economical and reliable manner. Such a loading technique should be consistent with the equipment, personnel, and facilities customarily present in mining operations.