When faced with bad ground or water inflows, mine operators can turn to a wide range of injectable consolidation products. E&MJ asked some of the leading suppliers for details of their products and services.
By Simon Walker, European Editor
While the concept of using injectable grouts to seal water-bearing fissures and rebuild strength in weak rock is nothing new, today’s mine operator has a much wider range of products from which to select an appropriate fix. Traditional cement-based systems have been augmented by resins, and the level of control over setting times for both types of product is now much more precise as formulations have become increasingly sophisticated.
This type of product is suitable for handing two main problem areas: weak rock and water ingress. Taking that a stage further, coal and other soft-rock mining often has a different set of ground control challenges from those found in hard-rock mines, with strata delamination more of a problem that having, say, to drive a heading through a weathered, brecciated fault zone in an otherwise competent rock mass.
The use of ground consolidation products can also vary in terms of the timing of their application. Weak rock conditions near surface can require predevelopment consolidation to enable a shaft or decline to be sunk. Conversely, it may be the case that regular heading or roadway development suddenly intersects water-bearing rocks that require injection before work can continue. Clearly, different technical approaches will be needed in these two scenarios, and a product that may be more suitable for one in terms of application and cost may not be appropriate for the other. A cheaper, slow-setting grout that can bind weak rock together adequately will almost certainly be incapable of dealing with a major water inflow, for which a product that is both water-resistant and fast-acting is essential.
NORMET FOCUSES ON INJECTION RESINS
Lawrence Halls, director and global manager for ground engineering at Normet, provided E&MJ with some background on recent developments in the technology. He explained that the use of silicate-modified high-strength polyurethane injection resins for ground support has increased, as these systems are effective in decreasing convergence in high-stress zones, for setting rock bolts or where rapid consolidation of loose rock is needed. Benefits of these types of resin-based materials include their low viscosity, excellent adhesion even under wet conditions, high strength, fire resistance and non-foaming in contact with water. They are essentially a “high-tech glue” for use in mining and tunneling, he said.
Newtonian-based two-component resins such as Normet’s TamPur 116 are typically injected into fractured or jointed rock through mechanical or inflatable packers, while injection into loose unconsolidated rock is normally done through driven perforated lances, perforated casing or self-drilling anchors. Once injected, the initial reaction (time to gel) is typically 30 seconds, although this will vary depending on the resin and rock formation temperature (the heat-sink environment), something that should always be taken into consideration when planning the works, Halls pointed out.
The time to gel is important in limiting any loss of material outside the injection zone, after which the resin hardens to provide rapid structural integrity to the fractured or loose rock.
Halls stated that specialist resins such as TamPur 116 have proven to be extremely effective, and have become increasingly accepted by the mining and tunneling industry as part of the ground consolidation and support process. It is a rapid, cost-effective system providing high-strength ground support where conventional systems may or cannot be used, he said.
AVANTI: A LEADING US SUPPLIER
Avanti International claims to be the only full-service provider and most experienced supplier of high-quality chemical and cement grouts in the U.S. Since 1978, its product line has grown from acrylamide gel and urethane foam to a comprehensive line of acrylamide, acrylic, acrylate, hydrophilic, hydrophobic, foam, gel and cementitious grouts.
In a white paper written for the geotechnical community, the director of the company’s geotechnical division, Britt Babcock, explained some of the differences between the available systems, in terms of applicability, performance and comparative cost. A key point noted early in the paper is that “a grout’s ability to penetrate a rock fissure largely depends on the particulate size.”
For example, Portland cement grout generally has particulate sizes averaging 15 microns (µm), microfine cements range from 6 to 10 µm, while ultrafine cements can have average particulate sizes of 3–5 µm. “The primary types of chemical grouts (silicates, acrylics and polyurethanes) are each unique in composition,” Babcock continued, pointing out that, because their particulates are so small, sodium silicates can penetrate well into rock, very similar to true solution grouts that have no suspended solids.
Low-viscosity true solution grouts (acrylics) include both acrylamide and acrylate compounds. Each requires a base resin to be mixed with a catalyst in order to create a gel matrix within the rock. The gel time is controllable, and can be from 3 seconds to 10 hours for acrylamides, and from a minute to an hour for acrylate gels.
In terms of costs per mixed gallon, Babcock noted that microfine cement grouts cost about twice as much as those using Portland cement, whereas acrylic grouts are three to five times more expensive. Before expansion, polyurethane resins are significantly more costly, although the actual cost of polyurethane foams reduces markedly if the degree of expansion is taken into account.
NEW RESIN FROM STRATA PRODUCTS
Last August, Strata Products Worldwide introduced a new rock consolidation injection product for the U.S. mining market. Designed specifically to meet the demanding conditions in both underground and surface mining applications, Organosol 550 DT is a two-component, non-expanding injection resin for consolidating and stabilizing fractured coal, rock and gravels. Insensitive to water, it can also be used to stop or redirect water inflows.
According to Mike Fabio, engineering manager at the company, two recent applications illustrate how the system can be used. In the first, a longwall panel at a large western U.S. coal mine was out of action with the shields tipping down because of the loss of a hard rock band in the floor. Strata was brought in to stabilize the situation, which involved drilling holes into the weak floor.
Some 175 lb (80 kg) were injected into each hole, for a total of 2,200 lb (1,000 kg) of Organosol. This amount allowed the material to migrate from hole to hole, ensuring total consolidation of the rock layer, Fabio told E&MJ.
In the second application, Strata used Organosol for water displacement. The company was contracted to stop a water flow from around a cased borehole at an underground coal mine in the midwestern U.S. Some 3,750 lb (1.7 mt) of Organosol was pumped into the void, filling any rock fractures and completely sealing the water flow into the mine.
Organosol 550 DT is available in 5-gallon (19-liter) containers or in barrels for large applications, and is injected using a portable, air-powered pump.
CHEMGROUT PUMP HANDLES TAHMOOR CHALLENGE
For more than 50 years, ChemGrout has manufactured what it claims to be the world’s largest selection of grouting equipment. The company told E&MJ about the use of one of its CG-500 diesel-hydraulic mixer pumps on a ground consolidation job at Glencore’s Tahmoor underground coal mine in New South Wales, Australia.
A future mining area lies directly below the main railway line between Sydney and Melbourne. A culvert beneath the rail embankment allows water to flow from one side to the other. Structural reinforcement, needed to prevent further subsidence ahead of the resource below being developed, involved a process of meshing and drilling several hundred rock anchors that were then grouted into position.
Neat cement grout was mixed in the CG-500 unit, with each hole consuming 55 liters of grout pumped into the strata through tubes. According to ChemGrout, a major benefit of the progressive cavity pump used is its variable speed, which allowed the operator to gradually increase grout delivery, preventing line blockages and material wastage. The reversible mix paddles helped produce a superior grout mix quickly with minimal bleed, so ensuring the strength characteristics required for bonding, the company added.
The CG-500 has a twin mix-tank design that permits continuous pumping. This non-stop pumping process maximizes the batching of materials such as neat cement, sand-cement and most commercial re-bagged grouts. Two high-capacity 70-gallon (265-liter) mixing tanks, pump, and a 15-gallon (57-liter) holding hopper are all mounted on a single skid for easy setup and immediate operation.
The open-throat grout pump features a non-pulsating positive displacement rotor-stator that provides a constant discharge of materials. Power options include air, hydraulic, electric-hydraulic, gas-hydraulic and diesel-hydraulic.
RESIN FOR WATER CONTROL
Headquartered in Ontario, Canada, Multiurethanes offers several injection products for ground control and stabilization in underground mines. The company’s Universal Resin is a low-viscosity polyurethane resin designed for single-component injection. It penetrates fine cracks in rock and concrete, and reacts with water to form a semi-rigid foam barrier. Once cured, the hydrophobic seal repels water, will not shrink or expand, and serves as a long-term solution to water infiltration, Multiurethanes stated.
Its advantages include its ability to stop high-pressure inflows, it is resistant to corrosive environments and temperature fluctuations, and is solvent-free and non-toxic. Different doses of accelerator can be used to control the reaction time.
Grouting specialist with the company, Peter White, told E&MJ about a call received recently from one of its underground mining clients for emergency engineering assistance after development crews in a 5×5-m heading encountered a water-bearing fault structure. Mining operations were stopped as they attempted to overcome both water inflows and poor ground conditions. The solution involved the use of Multiurethane’s Universal Resin to consolidate the poor ground conditions and gain control of the water inflows, followed by cement grouting using Celbex thixotropic additive to coagulate and minimize cement washout.
White explained that when mining in water-bearing ground conditions, it is common to use 2% calcium chloride as an accelerator when cement grouting under cold-temperature water inflow conditions. In addition, some of the company’s experienced mining clients use a combination of ordinary cement with Ciment Fondu (calcium aluminate) to achieve rapid-setting cement grout mixtures, thereby cutting the time between grouting and being able to restart development operations.
A FULL RANGE FROM BASF
According to Uwe Wyink, global technical manager for mining, BASF is the only company to offer the full range of existing injection systems for consolidation and void filling. These include classic microfine cements and colloidal silica materials that are mainly used for pre-injection for consolidation and water control in tunneling. Reactive polyurethane and polyurea-silicate resins are used for strata and water control in the coal and hard rock mining industries, while low-viscosity acrylic resins are mainly used for repair and consolidation work. To complete the portfolio, BASF has phenolic foam systems for void filling, he added.
The MP 35X series are classic polyurethane resins with additional features to increase safety in mining. The versions for consolidation are hydrophobic and do not react with water. Meanwhile, BASF’s MP 36X products are polyurea-silicate resins that offer extremely fast reaction times for consolidation, with a low exothermic reaction.
Wyink pointed out that when polyurethane resins are used for injection, temperatures of up to 135°C (275°F) occur when the two components react. Since temperatures can be up to 20% higher if water is present, this could be hazardous in coal mining, so the company does not recommend filling large voids with polyurethane under these conditions. However, these materials can be used effectively in other operations, with BASF citing the example of polyurethane resins being injected successfully to seal concrete backfill-retaining walls in a German salt mine.
By contrast, the reaction temperature of polyurea-silicate resins is less than 100°C, with the added advantages that there is little pressure build-up within the rock caused by expansion, and the reinforced rock can be cut cleanly. According to BASF, the use of its MasterRoc MP 364 Flex at a Murray Energy Corp. coal mine in the U.S. to stabilize the floor where permanent seals were to be placed meant that the seals could be built smaller, achieving 50% cost savings in terms of materials, time and labor.
THIN-SPRAY COMBATS INSTABILITY
A global leader in ground stabilization for more than 40 years, Minova is now an independent subsidiary of Orica. As well as supplying steel products for ground control, such as rock bolts, friction rock anchors, welded mesh, straps and self-drilling anchors, the company’s chemical resins and adhesives include Lokset polyester resin capsules, Loosest pourable resin grouts, Capcem cement grout capsules for rockbolting, Capcem cement, the Capcem injection grout system, thin-spray liners and foamed grouts.
The company provided E&MJ with an example of the use of its Tekflex thin-spray liner (TSL) in sealing the surface of vent raises in a South African diamond mine, to prevent long-term deterioration and collapse through weathering. The bored vent raises vary from 1 m to 4 m in diameter and from 50 m to 150 m in depth, with Minova having to develop a system of protecting them without requiring men to work inside.
The solution achieved was to apply the Tekflex as the support and protection medium for the raise walls by means of a spinning disc, rather than by spraying. The disc is mounted on a three-legged carriage—the Spinnekop (“spider” in Afrikaans)—suspended from a variable-speed hoist. The Tekflex is mixed conventionally at the top station, then pumped down to the spinning disc for application as the spider is hoisted slowly up the raise. The liner thickness is controlled by adjusting the hoisting speed, while the projection speed of the Tekflex is controlled by adjusting the disc rotation speed.