Broadly controlled blasting is done for controlling overbreak (in opencast and tunnel) and ground vibration. It can be catagorised into four types: pre-splitting, trim blasting, line drilling, and smoothing blasting. Muffle blasting is done to restrict fly-rock.
This method is not new blasting technique. It became a recognized blasting technique for wall control when it was used in the mid-1950s on the Niagara Power Plant. The purpose of pre-splitting is to form a fracture plane across which the radial cracks from the production blasting can't travel. This method may cause a fracture plane which may be cosmetically appealing and allow the use of steeper slope with less maintenance. Pre-splitting uses lightly loaded, closely spaced drill holes, and is fired before the production holes.
2) Trim (or Cushion) Blasting
Trim blasts are designed to produce a final wall similar to a pre-split blast, but they are fired after the production holes. The idea is to eliminate costly small diameter blasthole and work along with the associated hole loading difficulties. The spacing normally larger than in pre-splitting because there is relief towards which the holes can break. Since the trim row of holes along the a perimeter is the last to fire in a production blast, it does nothing to protect the stability of the final wall. Radial fractures from production blasting can go back into the final wall. Mud seams or other discontinuities can channel gasses from the production blast areas into the final wall. The sole purpose of a trim blast is to create a cosmetically appealing, stable perimeter. It offers no protection to the wall from production blast.
3) Line Drilling
This system involves a single row of closely spaced uncharged holes along the neat excavation line. This provides a plane of weakness. It also causes some of the shock waves generated by the blast to be reflected, which reduces shattering and stressing in the finished wall of the host rock. Thus, preserving, to a great extent, the original strength of the host rock is possible. This technique gives maximum protection to the host rock to preserve its original strength.
It may, under proper circumstances, help to protect the final contour from radial fractures by acting as stress concentrators causing the fracture to form between line drill holes during the production blasting cycle. If, on the other hand, the wall contour was extremely important, one could not depend on line drilling to necessarily protect the final wall. Line drilling is more commonly used in conjunction with either pre-splitting or trim blasting rather than being used alone. Although the use of control blasting is more common for surface excavations, it has been successfully used underground, residual stress conditions permitting.
4) Smooth Blasting (or Contour or Perimeter Blasting)
A technique used (rarely in surface and mostly in underground blasting) in which a row or closely spaced drill holes are loaded with decoupled charges (charges with a smaller diameter that drill hole) and fired simultaneously to produce an excavation contour without fracturing or damaging the rock behind or adjacent to blasted face. In this technique, perimeter or contour holes are drilled along specified final excavation limits and are lightly loaded than that of buffer holes and production holes. The spacing is kept closer than buffer holes and production holes. Unlike production drill hole blast where higher charge concentration is required, contour drill holes require low charge concentration and explosives should be lightly distributed all along the length of the bore hole. Sometimes the use of high grammage Detonating Fuse (about 40 gm/m core wt., to 60 gm/m core wt.) for contour blasting can give effective result in tunneling. This results in an air cushion effect, which prevents over-break and reduces in-situ rock damage for preservation of strength of host rock.
4) Muffle Blast
In case of blasting in congested areas, muffling or covering of blast holes properly before blasting, is the common solution to prevent fly-rock from damaging human habitats and structures.
Figure 1: Blast Doors