Control techniques such as presplitting, trim blasting and line drilling work best in massive rock. In massive rock, one can see the half casts or half of each borehole on the final wall. In massive rock, 100% of the holes produce half cast.
Some operators try to assess the success or failure of presplit or cushion blasts by what is called a half cast factor. Half cast factors are the percentage of the total half casts which are visible after the rock has been excavated.
If only 40% of the drill holes remain visible on the final wall as half casts, then the half cast factor would be 40%. This technique could have some merit when blasting in solid homogeneous massive material.
However, half casts may totally disappear in geologically complicated rock. One cannot assume that the lack of half casts indicates a poor blasting job. In geologically complicated material a simple crack does not form.
There is a broken shatter zone formed along the perimeter, and that zone serves as protection for the final wall from the effects of radial cracks emanating from the production blast. Half cast factors only have validity if the rock type in which the half casts are being counted are considered in the evaluation.
When rock has numerous joints between blastholes, and those joints intersect the face at less than a 15⁰ angle. it will be impossible to form a good smooth face with control blasting techniques. In fact. for the wall to be halfway cosmetically pleasing. the joints must intersect the face at greater than a 30⁰ angle. Anything less will cause fractures to intersect the jointing planes having large pieces of material fall out from the face during the excavation Process.
In a weak material, the skill of the excavator operator is extremely critical. Some machines can exert considerable thrust, whereby they can dig into an unbasted wall severely damaging the final contour. Other geologic factors which effect the outcome of control blasting techniques are soft seams or mud seams. If the bench is intersected by numerous muds seams it is difficult to produce good results.
