Ground support operations are critical in most underground mines, not only for the longer-term durability of tunnels, but also for the immediate safety of people working in new drives who may be conducting various operations at a heading or barricade.
Shotcrete has been used for decades either solely, or in combination with other stability support methods, such as mesh and rock bolting. Shotcreting must be applied to specification for safety and commercial reasons, and making sure there is a real-time, safe and ‘hands-off’ way to ensure thicknesses are correct has many advantages.
Traditional shotcrete thickness measuring techniques have been fraught with issues. Various methods such as:
- Fixed volume used
- Covering the mesh or measuring rods
- ‘Eye-balling it’
These methods suffer from some or all the many problems associated with reduced safety, material wastage and the length of time taken to measure the applied thickness.
Using stamps may bring inconsistent results if their placement is incorrect, biased or simply too few. They do not provide a digital, traceable, record of the location or thickness of measurements collected. Probes have similar issues and in addition may not actually be used until some hours after the shotcreting rig has moved on, increasing the cost to return if too little has been applied, (or worse a safety issue if it does not return at all), or material wastage if too much has been sprayed.
In some mines a fixed volume of shotcrete is provided using a theoretical calculation of the requirement. This is loaded to the sprayer and once it is used then no further application is made. This however has possible disadvantages with uniformity and excess rebound. If the objective is purely to cover the mesh, rockbolts, or any placed measurement rods to determine the thickness of shotcrete applied, this works until the item is covered at which point the amount by which it is covered becomes indeterminate by default, leading to possible excess material.
Many mechanised shotcrete rigs are operated by very experienced personnel who are sometimes left to spray exact thicknesses by eye. However, since under-spraying can be a safety or regulatory issue and over-spraying a commercial loss, almost all operators need additional guidance to differentiate between applying set thicknesses of only a few millimetres.
Laser scanner based shotcrete thickness measuring systems fortunately provide:
- a safe,
- simple to use,
- digital technique to measure sprayed concrete in combination with mechanised sprayer units.
The thickness measuring process with the laser scanner is a ‘non-contact system’ whereby the laser scanner taking measurements is located away from the area to be stabilised. This removes the need for manual measurements to be taken in un-stabilised areas or on newly sprayed areas that may suffer from fall outs causing injuries. The system is operated in real-time by the sprayer ‘nozzleman’ negating the need for additional personnel or technical staff, and the easy to learn process and simple operation is done with a single button push.
Laser scanning technology has been used for mapping and monitoring since the 1990’s and has come to be appreciated for the high volume of accurately measured data created. The applied shotcrete thickness, as mentioned, is critical for safety and cost reasons and even a few millimetres less or more, can have serious consequences, so accurate measurements acquired on-site are important to the process. After the spraying is complete, as the data is digital, various reports and information may be produced immediately. Not only the placement and thickness is recorded but also the volume of material is calculated, a useful quality control check. Both client and contractor can see and agree on contractual specifications being met and a digital archive created for quality assurance purposes.
Modern efficient shotcrete thickness measuring systems therefore have numerous benefits;
- reducing job time by improving efficiency of the process
- lowering direct costs by reducing material wastage, and personnel and machinery time
- generates digital data which can be used to show compliance with specifications
- increases safety by operating the system remotely from the shotcrete application area
- is very simple and easy to learn and use.
For mines using even moderate quantities of shotcrete, the savings of applying the correct thickness can be enormous, leading to a short return on investment. That coupled with enhanced safety makes the choice of a real-time laser based measuring solution highly attractive.