Blasting techniques have been developed to control overbreak at excavation limits. The operator must decide the ultimate purpose of the control technique, before selection of the technique can be made. Some techniques are used to produce a cosmetically appealing wall with little or no concern for stability within the rock mass.
Other techniques are used to provide stability by forming a fracture plane before any production blasting is conducted. This second technique may or may not be as cosmetically appealing, but from a stability standpoint, performs its function. Overbreak control methods can be broken down into three types: presplitting, trim (cushion) blasting and line drilling.
Presplitting utilizes lightly loaded. closely spaced drill holes. fired before the production blast. The purpose of presplitting is to form a fracture plane across which the radial cracks from the production blast cannot travel, the fracture plane formed may be cosmetically appealing and allow the use of steeper slopes with less maintenance. Presplitting should be thought of as a protective measure to keep the final wall from being damaged by the production blasting.
Trim blasting is a control technique which is used to clean up a final wall after production blasting has taken place. The production blasting may have taken place many years earlier or could have taken place on an earlier delay within the same blast. Since the trim row of holes along 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 the production blast.
Line drilling is an expensive technique. that under the proper geologic conditions. can be used to produce a cosmetically appealing? final wall. It may, under proper circumstances, help 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 presplitting 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.
Reference
U.S. Department of Transportation, Federal Highway Administration, « Rock Blasting and Overbreak Control»
