“The design concept for shotcrete as a form of ground support has changed completely,” says Dr Peter Kaiser, professor, Chair for Rock Engineering and Ground Control, Geomechanics Research Centre, Laurentian University. “The logical sequence for creating a good skin is to shotcrete first, add a steel mesh, then shotcrete again. But this may not be economic in all cases,” Kaiser says.
Proper shotcreting maintains the interlocking network of blocks of rock intersected by a mine opening. “We should try to create a reinforced unit of rock that is more stable than a number of smaller blocks of material,” he explains. These blocks will consist of bolts, screen and shotcrete.
It is important to shotcrete all the way around an opening, Kaiser says, in order to mobilize the compressive strength of the ring of concrete. In openings with a flat bottom profile, a number of short, low-capacity rock bolts should be installed on the fringes of the concrete to prevent buckling of the shotcrete skin. For a circular opening, a support pressure of 2.5 mpa is the maximum that can be expected from the shotcrete alone. In most mining cases, though, the backs of most openings are flat. So about 0.5 mpa of support pressure is more likely. Therefore, the shotcrete should function another way. “How do we create a shotcrete that will bend and not crack?” Kaiser asks. The objective is to eliminate the incompatibility of the skin cracking in the corners of a square opening by allowing the skin to deform.
One way is to create longitudinal slots (of no shotcrete) in the shotcrete skin so that the skin yields under load, allowing the steel mesh to buckle out. This allows widening of the bolt spacing. Mix Design
Shotcrete is simply concrete pumped through a hose and “shot” at high velocity on to a surface. High- pressure air is added at the nozzle and the material is jetted out. Portland cement is commonly used (although high-heat and sulphate-resistant Portland cements can also be used) in proportions of 18% to 20% by dry weight. Various aggregate gradations (typically American Concrete Institute (ACI) gradations No. 1, 2 or 3) are used, depending on the thickness of the shotcrete layer. (For applying fewer than 5 cm of shotcrete, gradation No. 1 is used; for 5 to 10 cm of shotcrete, No. 2 is used and for more than 10 cm, gradation No. 3 is used). A high- quality material is required to reduce clogging of equipment.
Since 1980, a number of advances in shotcrete material design have strengthened shotcrete and reduced rebound. Some notable research has been done at Sherbrooke University in Quebec.
Discontinuous discrete steel fibres mixed with shotcrete enhances engineering properties of shotcrete. Dosages range from 45 to 75 kg of fibre per cu m of shotcrete, depending on the toughness required. Fracture toughness, flexural and tensile strength, impact resistance, cracking resistance, spalling and abrasion resistance and reduced permeability and porosity are some of the properties enhanced by steel fibres, according to Al Sari, Ontario regional manager of Domecrete Canada, a manufacturer of steel fibres. Unreinforced shotcrete is very brittle, says David Wood, a research engineer at the University of Toronto. “Reinforcing improves shotcrete properties in the post-peak range. After cracks develop, the fibres begin to take up the load because the tensile strength of the fibre comes into play.” Drawn wire is most expensive, followed by chopped wire and melt extract. Cylindrical fibres are more effective than flat ones.
Another increasingly popular additive is a non-toxic substance known as amorphous silica — a material as fine as cigarette smoke, derived from gases produced in ferro-alloy production. When added in amounts of about 10% by weight of the cement fraction, this material makes a thick, cohesive shotcrete which reduces both rebound (by readily sticking to even wet surfaces) and equipment wear. The silica content of the fume is about 86% to 96% silicon dioxide.
On a railway project in British Columbia, shotcrete was applied on rock slopes even during driving rain conditions, says Bruce Semkowski, sales manager for King Packaged Materials of Burlington, Ont. As a result, extremely wet underground conditions shouldn’t pose problems. Various accelerators are also available (in both powder and liquid form). These chemicals are added in the compressed air at the nozzle in proportions of about 2% to 5% by weight of cement added. They allow operators to apply thick layers in one pass and give the strength needed to take a load very quickly. But they do this at the expense of long-term strength and durability. For example, one accelerator (called Scamper 16) gives an initial set in only 40 seconds, Semkowski says. Other accelerators include sodium silicate and various aluminates. Accelerators are often not necessary when silica fume is used.
Research is also being conducted on super plasticizers, such as polypropylene fibres, to reduce water requirements in the mix. The plasticizer changes the electric forces in the mix, dispersing the fine particles of cement and aggregate. The thin film of water on the grains acts as a lubricant. Plasticizers are essential when using silica fume.
To confirm long-term strength projections, Wood recommends shotcreting a cardboard box interior, curing it and sending it off to a lab (like that of Trow Consulting Engineers in Sudbury) for testing. This should be done before production shooting begins and regularly during production.
As for the water used in shotcrete mix, ambient temperatures are usually fine, Wood says, but in some cases water is heated. The same water temperature in pre-production and actual production must be used. Shotcrete Equipment
Most shotcreting equipment available to Canadian mine operators is manufactured in the U.S. and Europe. Dynatec Mining of Richmond Hill, Ont., markets machines manufactured in the U.S. by Reed Manufacturing of Walnut, Calif.; Ground Control of Sudbury, also markets a unit called the Meyco Piccola; and Normet Canada of Sudbury markets machines built by its parent company in Finland.
Basically two types of machines are available: wet-mix and dry-mix machines. Choosing the appropriate machine will depend on the material- handling capability of the underground mine.
Most dry-mix machines are air- operated; however, electric-powered machines are also available. But these machines require compressed air as well. To produce significantly less dust, operators use a pre-dampening machine, consisting of a feed auger and a mixing auger and three jet nozzles for water sprays. These units increase the moisture content of the mixture to about 5% to 6%. If the mixture is too wet, it will plug the hose (which comes in 50-ft lengths or shorter). Anti-static liners are standard equipment, but a ground wire is recommended (otherwise you will “light up” the nozzleman, says Dennis Wrixon, general manager of Dynatec). Dry shotcrete mix is supplied in large (up to 1,680-kg) sacks.
Dry-mix nozzzles are equipped with water jets that eject the material. No matter how much water is used, dust is always a problem, unless a pre- dampener is used,
in which case dust is reduced significantly.
A wet-mix machine is basically a concrete pump. The typical machine can deliver 45 cu yd of shotcrete per hour. Concrete is loaded into a hopper and is pumped to the end of the line. At the end of the line, air (instead of water) is injected to blow the material out of the nozzle. A stand pipe can be installed in the line to eliminate pulses, but some nozzlemen prefer pulses.
Hoses measuring 2 1/2 inches in diameter can deliver about 10 cu yd of material per hour. The material delivered to a wet-mix machine is already mixed, which poses problems for some mining operations. If the concrete is delivered to the shotcreting machine in a mine car, load-haul- dump machine or underground truck, it will settle out in transit (with cement paste and finer material floating on top of coarser material). So, often, mixers or re-mixers are needed. Most shotcrete research today focuses on testing nozzle designs, says Dr Evert Hoek, professor of Rock Engineering, University of Toronto. Accessory equipment includes voice- activated communications built into the nozzleman’s hard hat, face shield, eye protection, oilers and adequate lighting. Minimizing the Human Factor’
Since it is virtually impossible for a man to hold on to a shotcrete nozzle all day, machines have been developed to take the nozzle away from the man. U.S. manufacturer Reed has developed a hydraulic trailer-mounted spray arm that consists of a telescoping boom and oscillation nozzle controlled by a joy stick. A prototype is being tested at Inco Ltd.’s Frood mine in Sudbury.
Normet Canada, on the other hand, is marketing a shotcreting truck equipped with an automated, hydraulic boom that has been used successfully in Finland for many years. A demonstration unit is expected to arrive in Sudbury late this summer, Sales Manager John Morissette says. It will tour several mines there before going to the Kidd Creek mine in Timmins, Ont.
“Mines have to commit themselves to assembling crews that are adept at shotcreting,” Wrixon says. “The material and equipment have cleaned up the image of shotcreting. Now it is up to the companies to do that little extra to make the operator proud of his job.” An unskilled operator can more than double the rebound rate. An experienced nozzleman, on the other hand, will keep the nozzle about one metre from the face and maintain the nozzle at right angles to the rock surface, thus cutting rebound to about 10%, sometimes 5%.
An automatic spraying boom eliminates “the human factor” in excessive rebound, according to Normet. Mechanized shotcreting machines also have a higher capacIty (about 20 cu m per hour) than manual machines. The Mining Research Directorate, in conjunction with Cambrian College in Sudbury, is preparing a demonstration video tape on the various shotcreteing machines available. Operators can arrange to recieve copies of that video by contacting mrd’s David Wood at (705) 673-6512. Shotcreting at Kidd Creek
Shotcreting on a full-time basis was introduced at the Kidd Creek mine in Timmins, Ont., in the fall of 1988. This mine had a problem transporting the concrete mix underground in trucks and scoops, so a mobile re- mixer (a 5-cu-yd mixer on a Getman carrier) was purchased in 1988. Shotcrete is pumped by two Reed pumps and blown on to surfaces to a thickness of 1 1/2 inches. As already mentioned, an automated shotcreting boom (manufactured by Normet of Finland) is expected to arrive in late 1989.
The mine uses about 840 lb of 3/8-inch coarse aggregate per cubic yard of concrete, a superplasticizer and a sodium silicate-type accelerator. Thicknesses of two to three inches can be achieved in a single pass. Rebound has been reduced to between 5% and 10% from 40% to 50%, says Senior Engineer Robert Currie.
“I concur with the recommendation that the nozzle be kept at about one metre from the back and at right angles to it,” Currie says. “But I’d add that a circular motion of the nozzle be used for the best results. Shotcreting has gained acceptance at Kidd Creek and we are now turning our attention to applications in drifts and drawpoints.”
The mine has one truck mixer now and another is to be delivered soon. Concrete is discharged from the truck on to a small, mobile conveyor and into the hopper of a wet-mix machine. The Geco Experience
Both dry-mix and wet-mix shotcreting have been tried at the Geco mine in Manitouwadge, Ont. Between Dec 15, 1988 and April 15, 1989, about 277 cu yd of shotcrete were placed at a cost of $307.22 per cu yd, says Research and Contracts Technologist Dennis Blier. Rates averaged about 3 cu yd per shift using a 3-man crew.
The main application in this mine is not ground control but muck control. Box holes, through which 50,000 to 1,000,000 tonnes of ore are pulled, are in constant need of being rebuilt. The main cause is blasting down hangups in the box holes.
“For an inexperienced crew, it was a challenge to keep the material flowing (in the shotcreting machine),” Blier says. He recommends operators use an air-strainer to take impurities and moisture out of the compressed air supply. An air-powered chainblock was used to unload bags of shotcrete mix from a mine car into the shotcreting hopper. Geco also used shotcrete to build a 6-inch-thick ventilation bulkhead. A rigid back form was constructed, then the shotcrete was blown into place at a rate of about 5 cu yd per hour. This procedure saved considerable time by eliminating the need to build another front form to contain the concrete. Ground Control of Sudbury is marketing a temporary wall system called Gun-Quip. The system consists of 4×7-ft panels of brattice cloth and a 3-dimensional wire mesh that can fit any shape. By applying shotcrete, the panels become a solid, reinforced concrete wall. Last Resort for Hemlo Gold
Operators of the Golden Giant mine in Hemlo, Ont., turned to shotcrete to solve some problems with marginal ground conditions encountered during development work. The problems arose on the 4235 and 4335 levels because of the orientation of joint planes, bedding planes and dykes intersected by the development headings. Resin rock bolts and screen and 12-m cable bolts were used initially, but localized problems persisted.
“Shotcreting through screen is not the way to go,” Mechanical Technician Peter Case says. “The problem of leaving voids was too great, as was the amount of rebound we experienced. So we opted for a more expensive nozzle.”
An area measuring 800 to 1,000 sq ft and about three inches thick was shot. (That’s equivalent to the weight of ore the company hoists in one day.) Inco Looks to Automation
One company that will be looking very seriously at automating shotcreting in the near future is Inco Ltd. The top sills of all of Inco’s vertical retreat mining stopes are being shotcreted so that they do not have to be re-conditioned after each blast, says Mines Research Technologist Fred Beaudry.
The company originally used the wet-mix method but now uses dry- mix. However, dust levels have proved unacceptable, Beaudry says. A pre- dampener has reduced dust to acceptable levels.
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