| In tunneling operations, the perimeter holes are typically drilled with a tighter spacing and the holes loaded more lightly in order to reduce overbreak. The use of perimeter blasting techniques will also reduce the amount of scaling and ground support required to make the tunnel safe after blasting. There are a number of techniques in use at mines and the use of each is usually a function of cost, effectiveness, and worker preference. These techniques include: Loading perimeter holes with a heavy detonating cord having a load in the range of 85 grams per metre. Use of decoupled explosives. Use of a low density explosive such as ANFO mixed with polystyrene. B-Line tracing of ANFO
Heavy Detonating Cord The use of a heavy detonating cord (85 grams per metre) is often effective and is very fast to load the perimeter holes. A length of cord is cut and inserted into the hole. A lighter detonating cord is used to ignite the cord in each of the holes. A clove hitch knot is used to tie the cords together. 
Perimeter blasting using heavy detonating cord loaded into perimeter holes and connected with a smaller diameter detonating cord trunkline and initiated by a No. 17 LP delay Provided there are no vibration restrictions, the holes are detonated at the same time at the end of the blast as indicated in the figure. In order to get a heavier charge at the toe of the hole, the detonating cord can be folded back on itself as shown in the figure. In difficult rock breaking conditions, the detonating cord may be folded back completely to form a double length in a hole. A common problem with the use of detonating cord or small diameter explosives for perimeter blasting is the charges get "sucked out" of the holes when the preceding holes are detonated. This can be avoided by using wood or plastic wedges in the holes. Decoupled Explosives
The use of decoupled explosives involves using an explosive that is smaller in diameter than the perimeter hole or the use of explosive cartridges that are separated by some inert material such as wood dowels. A common product is 25 mm diameter slurry cartridges that come in long continuous lengths or paper wrapped cartridges that can be fitted together at the ends with small plastic connectors. Low Strength ANFO ANFO blended with polystyrene is available for loading perimeter holes. The addition of polystyrene yields a low energy explosive with reduced shock for minimizing overbreak. It can be pneumatically loaded into horizontal holes or poured into down-holes. The poured density is typically about 0.53 g/cm3 while the pneumatically loaded density is about 0.70 g/cm3. The use of low strength ANFO is not recommended in wet conditions since it has no water resistance. | 
