DROP RAISING IN MINING (VCR) METHOD
The method of drop raising is an improvement over the method of raising through longholing, described earlier. In India at the Copper Mines of Khetri Copper Complex. This is a method of making a raise connection between two adjacent levels, nearly 60 m apart, by drilling large dia. holes(150 – 165 mm) through, from upper level to lower level and balsting them in stages. The raise is usually vertical but many also be steeply inclined. The charging and blasting of the holes is based on the recently devised method of Vertical Crater Retreat.
The term cratering in blasting used in Craer Method are normally spherical or a geometric equivalent, as research in the application of this breakage mechanism to rock indicates that spherical charges or their equivalent produces optimum results. In blasting practice, , spherical charges have been defined as having a length : diameter (L : D) ratio of 4 : 1 or less, and upto but not exceeding a L : D equal to 6 : 1 or less, and upto but not exceeding a L : D equal to 6 : 1. Thus for holes 165 mm dia. an explosive charge of 165 mm dia. and 990 mm in length would constitute a pherical charge.
At the Kolihan Copper Mines in K C C the raise is 3 m x 3m. The blast holes are 165mm dia, 5 in number (1 in centre, 4 at corners) drilled from the top level to the bottom level, nearly 57 m below. The charging of explosives into blast holes is done from the upper level. A Wooden plug in two pices tied to a nylon rope is lowered into the blast hole to plug the bottom mouth of the blast hole. Aggregate, nearly 12 mm in size, is stemmed for a length of 1 m followed by charging of alurry explosives ( Packaged cartrides) for a length of 1.2. A detonating fuse is tied to the slurry explosives cartideges below they are lowered into the hole. The explosives cartridges is nearly 34 kg. Above it a length of 1.8 m is packed with sand and the rest of the hole is left unstemmed. All the five blast holes are charged and they are blasted in a sequence with the help of delay detonators at a time from the surface when no worker is allowed to remain underground.
The roof of the lower level provides a free face for the explosive charged and the result of blasting is that a cavity of nearly 1.8 m height is formed. The fragmentation is sufficiently good for loading by LHD units. Dressing of the conical cavity is not ester. As the mucking is in progress the upper portions of the blast holes are charged with explosives in the manner as already described and the operation of loading, blasting and mucking follow smilar pattern,. With each blasting a vertical height of 1.8 m is found in the shape of a raise. The top 404.5 m of the blast holes, called plug, is however blasted in one step. The raise so formed has its sides sufficiently strong and stable showing no adverse effect by the heavy blasting.
Normally it is possible to have two blasts in a period of three shifts with reasonably good supervision, and the raise is completed within a period of about 20 days, with due allowance for minor delays.
In this method of blasting the central hole out of the total five blast holes should be of large dia., 150 mm or 165 mm but the corner holes may be of smaller dia, nearly 100 mm. Small holes however result in more deviation. Moreover it is more convement to drill all the holes of the same size once the drilling machine is set up at the site. Hense all the holes are of 165 mm dia.
The main advantage of this method of drop raising is that no workers are employed in the lower level except the operator of the LHD, unit. No sie dressing or roof dressing of the raise is required. The entire work is safe and fast so that a raise nearly 57 m long is completed within a period of about 20 days unless there are undue delays for some reasons.