This article was originally published by the U.S. Department of Transportation.
Cap sensitive explosives, such as dynamite, are initiated by detonating cord. Non-cap- sensitive explosives such as ammonium nitrate. emulsions. and water gels can be affected in many ways by detonating cord passing through the explosive column. If the detonating cord has sufficient energy explosives may detonate or burn. A burning reaction, rather than a detonation, releases only a fraction of the explosive’s available energy. The blast is underloaded because of a low energy release. Ground vibration levels increase while blast holes may vent and produce flyrock.
To prevent the main explosive charge from burning or deflagrating, one must be sure that the detonating cord is not too large for the borehole diameter. Cord grain loads that should not cause deflagration are in Table 1.
Table 1: Maximum cord load
| Borehole diameter (in) | Maximum cord load (grain/ft) |
| 2 – 5 | 10 |
| 5 – 8 | 25 |
| 8 – 15 | 50 |
If the detonating cord is not of sufficient size to cause a reaction in the explosive, it can cause the explosive to be damaged. The location of the cord can be in the center, or side of the hole and its location will control the severity of affects. The damage that results is called dead pressing or pre-compression. Dead pressing increases the explosive density and it will not detonate. This occurs when the detonating cord is of sufficient energy to crush out the air spaces within the explosive or to break the air-filled microspheres placed in some products. Air pockets provide locations to form hot spots for detonation. The adiabatic compression of air is necessary for detonation to proceed throughout the explosive.
When the explosive is partially compressed or damaged by precompression, it may detonate or burn releasing only a fraction of the available energy. This effect can be confusing since the explosive may be totally consumed yet little rock breakage results. Commonly, the blaster who suffers this type of problem believes that the problem is because of hard, tough rock. To obtain a better understanding of this problem, look at the energy loss that results from passing a detonating cord though an explosive column.
