A great article written by H. K. Verma CSIR-Central Institute of Mining &Fuel ResearchRegional Centre, CBRI Campus.
Line drilling involves the use of closely spaced, small diameter drill holes along the perimeter of final excavation. Line drilling is really not a blasting technique as these holes are left open and not loaded with explosives but provide a defined line along which the final blast can break. The line drilled holes provides a plane of weakness to which final row of blast holes can break. The stress waves of the blast create a plane of breakage between the holes.
The hole diameter for line drilling is usually 50-70 mm. Holes are spaced two to four times the holes diameter. The maximum practical hole depth for effective line drilling depends upon how accurately the holes can be aligned at depth.
Depth of drill holes is seldom more than 10m. As additional preventive measures, the last row of production holes adjacent to line drilling are drilled closely and charged lightly using air decking and detonating cord down the line.
Line drilling is limited to areas where even a light load of explosives in the perimeter holes would cause unacceptable damages. Typically, line drilling is used in very soft material. In hard rock, the hole spacing required is so close that pre-splitting becomes more cost efficient.
Line drilling can be used in conjunction with modified production or trim blast designs. The line drilled row is normally placed between 50 and 100% of the normal production burden from the trim or production row. Line drilling is not often used in mines because the cost is too high.
For those construction jobs where back break may be very costly, this procedure can be used. It is sometimes used in mines for critical situations such as preparing a wall for a crusher installation, in this case half-depth holes may be drilled between the normal pre-split holes to insure that the wall breaks cleanly at the crest.
