The use of explosives for rock excavation has been dated way back in time. Nowadays, rock blasting uses many different varieties of explosives with different compositions and performance properties. Higher velocity explosives are used for relatively hard rock in order to shatter and break the rock, while low velocity explosives are used in soft rocks to generate more gas pressure and a greater heaving effect.
Here are other descriptions of various rock excavation methods.
Cushion blasting is done after production blasts. Larger drill holes are used with small diameter, lightly loaded distributed loads and the space around the explosive is filled with crushed rock to cushion the explosive force. Doing so, reduces the amount of radial fracturing around the borehole and also reduces borehole traces. The large diameter holes allow blasting depths up to 30 m (100ft). The procedure produces a ragged final slope face. One great thing about it is that it performs well in all rock types.
But, radial fractures are more abundant than presplit and smooth blasting. Slope face is more prone to raveling and a catchment area is recommended at slope base. This is also more demanding on the driller and borehole traces still apparent in hard, competent rock.
Step drilling involves larger diameter drill holes, drilled vertically and used as production blasting (although spaced closer and loaded lighter to minimize radial fractures) and a slope face is formed along base of blast holes. If properly designed the final slope face shows minimal signs of blasting. It can be used when sloped controlled blasting cannot and is best used in moderately to highly fractured rock.
The cons of such would include producing extensive damage to slope or inadequate base fracturing if not designed properly. Thus, step drilling should only be used with experienced driller and blasting engineer. Also, it is not advisable to be used in hard competent rock.
July 10th, 2017