【文章內(nèi)容簡介】 r degree. During the last few years semiautomatic chargers have been taken into use, primarily in underground work. Compressed air chargers for blasting powder in the form of loose material have also e into use on a large scale. As far as slurry blasting is concerned, special pumping methods have been developed through which charging capacity in the case of large diameter drill holes is practically good. A tamping rod must be made of wood or plastic. It must not be too thick in relation to the drill hole diameter since 17 this can crush and damage fuse or electric detonator cables during charging work. If a good degree of packing is to be obtained during charging with a tamping rod then only one cartridge at a time should be charged and tamped. The detonator must be correctly fed into the drill hole during charging work. Compressed air chargers have been in use is Sweden for about 20 years. The first type consisted of aluminum pipes connected together and the cartridges were blown into the hole with an air pressure of 42 pounds per square inch .since that time the charging tube has been replaced by antistatic treated plastic hose of a special design. A charger includes a footoperated valve, reduction vavle with air hose, breech, connecting tube and charging hose. The semiautomatic charger permits the continuous insertion of explosive cartridge at the same rate as they are charged in the hole by the hose .Instead of a valve being used ,the cartridges pass through an air lock between two flaps. The air pressure in the charging hose is retained while cartridges are pressure in the charging hose is retained while cartridges are beins inserted .The semiautomatic charger permits considerably higher charging capacity than the normal type of charger. Explosives in the form of the form of loose material, usually ammonium nitrate explosives(ANFO), require special chargers. Two types can be differentiated: pressrure vessel machines and ejector units. Pressure vessel machines are particularly suitable for crystalline An 18 explosives with good charging capacity. Ejector units are operate by an ejector sucking up explosive from a container through a charging hose. The explosive is then blown through the charging hose into the drill hole .There are, also bined pressure ejector machines. The charging hose used for ANFO charging operations must conduct electricity and have a resistance of at least 1K?/m and ?/M. Nitro Nobel has developed a special pumping procedure which consists of a tanker vehicle which is used to pump explosive directly the drill holes. The charging capacity is very high in the case of large diameter drill holes. CONTROLLED BLASTING TECHNIQUTES Controlled blasting is used to reduce overbreak and minimize fracturing of the rock at the boundary of an excavation. The four basic controlled blasting techniques are: line drilling, presplitting, cushion blasting and smooth blasting. Line drilling, the earliest controlled blasting technique, involves drilling a row of closely spaced holes along the final excavation line, providing a plane of weakness to which to break. Line drill holes, 2or 4 diameters apart and contain no explosive. The blastholes adjacent to the line drillholes normally are loaded lighter and are on closer spacing than the other blastholes. The maximum depth for line drilling is about 30 ft .Line drilling involves no blasting in the final row of holes, and thus minimizes 19 damage to the final wall. Presplitting, sometimes called preshearing ,involves a single row of boreholes ,usually 2 to 4 in .in diameter ,drilled along the final excavation at a spacing of 6 to 12 borehole diameters .Dynamite cartridges 1to in . in size on 1 to 2 ft .centers usually are stringloadde on detonating cord ,although special smalldiameter cartridges with special couplers are available for total column loading .In unconsolidated formations ,closer spacings with lighter powder loads are required .The bottom 2 to 3 ft .of borehole usually is loaded somewhat heavier than the remainder .Stemming between and around the individual charges is optional .The top 2 to 3 ft . of borehole is not loaded ,but is stemmed. The depth that can bu presplit is limited by hole alignment ,with 50 ft .being about maximum .The presplit holes are fired before before the adjacent primary holes to provide a fracture plane to which the primary blast can break .In presplitting it is difficult to determine the results until the adjacent primary blast is shot .For this reason ,presplitting too far in advance is not remended .Presplitting seldom is done underground. Cushion blasting involves drilling a row of 2 – to 6in .diameter boreholes along the final excavation line ,loading with a light welldistributed charge ,pletely stemmed and firing after the main excavation is removed rather than before ,as in presplitting. The burden on the holes is slightly larger than the spacing .Wedges may be used to abut the charges to the excavation 20 side of the borehole and minimize damage to the final wall .Eeplosive loading is similar to that in presplitting .Cushion blasting has been done to depths near 100 ft .in a single lift with the largerdiameter boreholes because alignment is more easily retained .Cushion blasting seldom is done underground. Smooth blasting is the underground counterpart of cushion blasting .At the perimeter of the tunnel or drift ,closely spaced holes with a burdentospacing ratio near :1 are loaded with light welldistributed charges .Smooth blasting differs from cushion blasting in that (1) except at the collar ,the charges are not stemmed and (2) the perimeter holes are fired on the last delay in the same round as the primary blast .Total column loading is most mon ,although spacers may be used .T