Taking precautions against flyrock
Flyrock is the undesirable throw of debris from a blast. Because flyrock can cause severe injury and property damage, shotfirers must take precautions to prevent it, especially if blasting occurs near property or people.
Nearly all previous flyrock incidents could have been avoided. Shotfirers and blast designers must consider flyrock prevention from the early stages of blast planning and take all necessary steps to minimise it. You may need to close off roads, walkways, etc., during a blast to remove the risk.
Causes of flyrock
Flyrock is usually caused by the incorrect selection or application of burden, insufficient stemming length or blast holes initiated out of sequence.
Trench blasting and blasting where bench heights are small compared with the burden usually provide the greatest potential for flyrock because the degree of confinement and the specific charge are both high, and the charge is sometimes placed close to the surface.
Precautions to take
The geology of the rock being blasted is also important to consider, including joints, fissures and other planes of weakness. When designing a shot, you must account for the type of rock and local variability in rock properties, including the presence of more highly weathered rock near the surface.
Good blast design is the best way to avoid flyrock. This includes:
- selecting the direction of the blasting face to place people and buildings behind the face
- avoiding shallow blasts less than 1m in depth, and using high blast ratio and small hole diameters where possible
- ensuring burden is one-third to one-half the depth of holes
- using a stemming depth either no less than 25 times the diameter of the hole or no less than the burden, whichever is greater
- ensuring stemming is free from rocks and properly tamped
- using sequential or delay firing so all shots initiate from the face backwards
- avoiding overcharging when conducting secondary blasting.
Previous excavations and the driller of the blast holes can provide important information about the rock's structure.
The area surrounding a blast site must be inspected to determine the distances to residences, roads, public places and dangerous goods storages. Consider the presence of these areas when deciding how much protection you need.
Drilling and loading
While a blast pattern determined by experience may be satisfactory, inaccuracies in drilling, including incorrect angle of blast holes, can cause large deviations from the planned pattern. This can create flyrock, excessive noise and vibration. You should use instruments to determine the drilling pattern. It will be difficult to estimate the burden in hilly terrain.
You should also use the correct charge weight in the blast hole. When using ANFO or other free-running explosive, pour measured quantities into the hole and check using a tape measure or wooden pole to monitor the build-up of the explosive column. This avoids overcharging due to explosives concentrating in fissures or chambers in the rock.
Precautions when covering the blast
If you need to cover blasting that is shallow or near residences, roads, public places, etc., place enough cover on the blast to further minimise the possibility of flyrock.
If you use backfilling to cover loaded holes, ensure the material is sorted and free from stones. Carefully place the cover to avoid damaging connecting wires, shock tube or detonating cord trunklines. Use only the minimum thickness required to eliminate flyrock.
You can use blasting mats as additional cover. If you do this:
- cover the rock in a layer of sandbags at least in the area of the hole collar to protect the mat from damage
- load and fire only the number of holes that blasting mats can adequately cover at one time, and anchor mats where possible
- don't place rock or debris on top of the mats, as they may become missiles
- don't use long-delay or multiple-delay safety fuse firing where you're depending on mats to contain flyrock, as the first firing holes may strip the mats off the remaining unfired holes
- don't use metal mats (e.g. wire rope or steel ring) near overhead power lines
- don't damage connecting wires, shock tube or detonating cord trunklines when laying mats.
- Read Explosives information bulletin 69 - flyrock incidents for more information.
- Refer to Explosives safety alert 61 - flyrock damage outside the blast-exclusion zone.