What to Consider in Pre-Split Design
- Can the pre-split be drilled in advance of the main production blast?
- A minimum of two rows of holes in front of the pre-split should be left
- It is advised that the production blast should be fired parallel to the pre-split and not at right angles, especially if the pre-split holes are angled, to avoid damage to the final pre-split line
- How many pre-split holes are being fired at the same time, does it exceed the site MIC, will overall blast timings be effected
- Can the whole line of pre-split holes be fired instantaneously, check the site Maximum Instantaneous Charge (MIC) and Air Overpressure (AoP) limits
- Bag off or cover all drilled holes in order to avoid blockages, cones are also a good alternative with the hole number written on them
- If the line cannot be fired instantaneously a minimum of 10 holes per. panel is advised be fired per. delay, firing less number of holes goes against the principle of firing a pre-split instantaneously
- What angle is the pre-split to be drilled at?
- Too shallow an angle i.e. > 20’ will make drilling and charging the hole difficult, especially if the ground is broken or fractured and/or water is present
- Ensure the correct azimuth can be achieved
- What type of explosive to use, i.e. detonator cord or a bespoke pre-split explosive product
- Initiation Sequence and Timings of Pre-split holes, even more critical when fired in conjunction with production holes in front of the Pre-split
- If a panel has been drilled and there are more holes to drill at a later date, leave a couple of holes uncharged to avoid damaging the ground on the start of the next set of holes
- If holes are fired in advance ensure they are surveyed and physically marked for future reference, for example use marker posts
If detonating cord is to be used consider the following
- Charge Weight g/m2
- What type of product is used to take the cord to the bottom of the hole, i.e. cast primer, slurry cartridge and how is this coupled to the cord line?
- Is the coupling strong enough and waterproof
- What about the coupling of the main cord line to the trunk cord line, is it strong enough and waterproof, especially when a gas or stemming bag is pushed in and stemming placed in the hole
- Is the main surface trunk line out of water
Whether using Detonator Cord or a Bespoke Pre-split product
- Consider using a separate continuous 10gm Detonating Cord line besides the main product in order to avoid separation and water ingress on joints and avoid misfires and keep MIC limits favorable.
- Use waterproof tape and cable ties for a more secure connection
- Use suitable and safe tools
- Consult the manufacturer or supplier for best use of the product•Cover the main trunk line, especially with site AoP limits
- With dust (damp) at least 0.5m and not chippings or crusher run type material
Pre-Split Drilled and Blasted in Advance
Pre-Split Blasted with Bulk Blast