Ins and outs of grouting

“There is no longer a place in our profession for black magicians — although artistry is still to be encouraged — who deliberately obfuscate issues in order to maintain some form of self-serving mystique.”

— Dr. Donald Bruce, Technical Director, Nicholson Construction, Bridgeville, Pa.

At least one major mine in Canada, Rio Agom’s potash mine in Saskatoon, Sask., was forced to close in January, 1987, because of uncontrollable flooding. An “aperture-limited” leak turned into an “aquifer-limited” leak and unsaturated brine inflows were simply too great for the mine pumps to keep pace. The mine closure, which lasted two years, may not have been necessary, however. Had up-to-date grouting technology been used during the early stages of flooding, say grouting experts, the mine may still be operating today as a conventional underground operation. Instead, the company was forced to make an expensive change in its mining method, opting for a solution method at a total cost of C$23 million.

Grouting involves impregnating a porous material with a fluid that becomes hard with time. Conventional cementatious grouts are the ideal materials if properly mixed and installed. However, they pose a number of problems, says Peter White, of Multiurethanes, a leading supplier of grouting materials to the mining industry:

* Water-to-cement ratios are frequently high and, as a result, grouting progress is often slow and time-consuming;

* Very high injection pressures frequently are used, which create potential safety hazards and which may cause hydrofracturing;

* Most mines wait 24 hours for cement grouts to cure before blasting in a grouted area;

* Equipment used for cement grouting is bulky and heavy. Paddle mixers and single-action piston pumps are frequently still used in mining. (Collodial mixers, on the other hand, produce higher mechanical characteristics, enhanced penetration characteristics and less free water on top of the mixture.)

Another way to improve cementatious grouts, says Multiurethanes’ White, is to lower the water-to-ement ratio, add a superplasticizer or deflocculator to reduce initial grout viscosity during grouting, add bentonite clay to maintain the cement particles in suspension and limit bleeding and shrinkage and to use multiple injection techniques. Three or four packers at a time are injected to maintain high grouting rates.

To bridge the gap between these so-called classic grouting techniques and applications using modern materials and methods, Trow Consulting Engineers of Brampton, Ont., recently invited experienced consultants from Europe, the U.S. and Canada to a 2-day seminar in Toronto, Ont. Attendance, at about 170, was poor and Trow lost about $30,000 in the process, but the event attracted 26 exhibitors promoting the latest mixers, pumps, materials and expertise. Also, about 22 technical papers, rich in civil and mining engineering case histories, were presented. (A complete set of the papers is available from Trow, 1595 Clark Blvd., Brampton, Ont. L6T 4V1.)

A classic way to obtain a faster set time in cementatious grouts is to add sodium silicate, which promotes a rapid or flash set, depending on the composition and relative amounts of each component. This 2-component application is, however, very prone to failure. A rapidly expanding list of new, one-and 2-component materials that offer geotechnical engineers a wider range of properties and reaction times is now available. These materials have changed the grouting scene significantly in recent years. Attracting the most attention has been a mixture of two chemcials little-known to mine engineers: polyol and an isocyanate. The resulting material is called polyurethane. “These materials have revolutionized grouting,” says Alex Naudts, manager of grouting and special techniques for Trow. He managed two grouting companies in Belgium for several years before coming to Canada in 1984 and has been involved in thousands of grouting projects.

The sodium silicate/cement grouting establishment in the civil engineering field has been changed significantly by polyurethanes. One reason for the change is the fact that, although they are about five times more costly, polyurethanes are not a granular material and therefore obviate the problem of trying to force the material through hairline cracks in rocks. “Using some cement grouts to fill cracks finer than 160 microns is like trying to force a ping-pong ball through the eye of a needle,” Naudts says. These chemical grouts require smaller, easier-to-handle equipment as well, allowing for fast mobilization and setup. Material is provided in pails and requires a compressed-air-operated pump or a manual hand pump.

“In a typical development heading, about one to two hours are required to drill and grout four to six grout holes per round,” White says. Herein lies the main cost advantage to miners. Although polyurethane materials are more expensive than cement grouting materials, total labor costs per round are much lower because of the faster installation procedures. “In order to maintain maximum heading advance, we recommend grout holes be drilled to a depth of twice the length of the round to be blasted,” White says.

“Mine contractors are interested in grouting the old way,” White says. “And we get involved when the old methods fail. So the initiative to use chemical grouts has to come from the guys who are paying the bills if they’re interested in getting the job done in the most economic manner.”

But polyurethanes have their problems too. Adequate ventilation is required because isocyanate vapors are toxic if they exceed certain concentrations. So care has to be taken when installing the grouts. Some conservative mines are leary of using them. Cominco’s potash mine in Vanscoy, Sask., for example, is in the process of doing some grouting using cementatious grouts to create a “dam” to seal off inflow of water from the overlying Dawson formation. The program, which will last about three years, involves hundreds of holes. “I don’t think we’ll end up using polyurethanes,” says General Manager Don Boyle, “because the cracks are large enough to take the cement grout.” Other sources say the project can be done in fewer than three months using an alternative approach.

Martin Jones, president of Groundation Engineering in Georgetown, Ont. (the contracting company involved in the project), says micro-fine cement grouts may be used. These materials are the subject of research spearheaded by Atomic Energy of Canada Ltd. (AECL) at the underground test facility in Whiteshell, Man. In recent years AECL’s research has lead to some important conclusions. Their work has shown that properly formulated cementatious grouts (with no shrinkage or bleeding) have better penetration characteristics than relatively simple, low-viscosity cement/water grouts. Cracks as narrow as 100 microns can be penetrated with the mixed cementatious grout, compared with the 160-micron cracks penetrated by so-called “colored water” grouts.

The new polyurethanes nonetheless have become so popular that more than 100 different types are manufactured by at least four specialized chemical companies, including Denes NV, Spetec NV, 3M, Rhone Poulenq and Rebock Gmbh. Major international chemical companies such as Bayer, Bergwerksverband, BASF and ICI also market polyurethanes. In grouting, three basic types of polyurethanes are used: water-reactive polyurethanes, polyurethane elastomers and polyurethane foams.

Water-reactive Polyurethanes

So-called water-reactive polyurethane prepolymers react with groundwater, expanding up to five to seven times to create a foam or a gel. There are two types: hydrophobic and hydrophillic. Hydrophobic prepolymers react with in-situ water and expand as hydrophillic prepolymers do but are totally inert, non-toxic and stable after the exothermic reaction. The time it takes for these chemicals to react can be adjusted from 45 seconds to several hours by adding a tertiary, amine-based catalyst. Some of the more recently developed prepolymers, such as MME Universal, react r
egardless of pressure and groundwater temperature.

One situation where polymers can be used to advantage is in underground exploration programs. Often, diamond drill holes put down from the frozen surfaces of lakes are not adequately cemented upon completion. So when mining crews begin driving development drifts and crosscuts underground, the flow of water into the openings can occur when drillholes are intersected. Such was the case at the Moss Lake property, 110 km west of Thunder Bay, Ont. Prepolymers were used to grout the inflow of water into the exploration openings and leaking blastholes in each drift round were grounted using prepolymer or a prepolymer/cementacious mixture. Blasting could then be carried out 30 minutes after grouting, compared with 24 hours if classical cement grouting had been used.

Polyurethane Elastomers

Two-component polyurethane elastomers, which are highly flexible and have good mechanical and chemical resistance, are becoming the material of choice in many underground applications in Europe, says Paul Verstraeten, manager & vice-president of Spetec N.V. of Belgium. “PUR 71 X is now the state-of-the-art resin,” he says. It is used extensively to waterproof highway and railway tunnels such as the Liefkenshoektunnel in Antwerp.

Polyurethane Foam

Polyurethane foam, which stabilizes loose and cracked rock, is used extensively in coal mines that use longwall mining. Some polyurethane foams do, however, release lethal gases if ignited by fire and are therefore banned in underground mines in some juristictions.

Despite the growing popularity of polyurethanes, cement-based grouts still have their place in mining applications. And important research work continues at several universities around the world. The aim is to investigate and quantify the effects of the settling of cement particles, of dispersants, water-to-cement ratios, etc. on the penetrability and caking behavior and resulting properties of cementatious grouts. Northwestern University in Evanston, Ill., is one such centre where research on cement grout is carried out. Raymond Krizek and others at Northwestern have conducted lab tests on a microfine cement grout called MC-500, manufactured in Japan and marketed by Geochemical Corp of Ridgewood, N.J., and on two sands. Although preliminary, their results (presented in several tables) give important information on grouting pressures, grouting permeabilities and unconfined compressive strengths of the various grouts. This is an important addition to the body of information for engineers involved in designing grouting projects to consult prior to grouting.

Shaft Applications

So-called grout curtains (overlapping columns of grout) have been used successfully in relatively soft ground applications. A shaft maintenance project at International Minerals & Chemical Corp. (Canada)’s K2 potash mine in Esterhazy, Sask. is an example. That project, carried out in pervious dolomite, was completed in April, 1987.

From an 11-metre-radius, donut-shaped tunnel centred around the 2.8- metre-radius shaft at a depth of 830 metres, vertical holes drilled 80 metres deep into two water-bearing formations below were grouted to form a continuous seal around the shaft to protect it from water inflows.

Extensive test work was performed prior to the final grouting program. Lab work was conducted to determine the best type of grout to use under various ground conditions. And once the donut-shaped tunnel was excavated, a 6-hole test grouting program was carried out to determine the best grouting procedure.

Flow rates up to 1,000 litres per minute and formation pressures in the 6-MPa range were encountered. In these cases, acrylamides were used to create a continuous grout curtain in the rock matrix but hydrophobic prepolymers were also used where the acrylamides extruded from cracks. Inflow into the shaft has stabilized at around 75 litres per minute and has not increased in the past three years.

Drilling was conducted by American Mine Services, grouting by Haliburton Services and consulting work by ECO Geochemical Consulting.

Grout Bulkheads

Grouting can also be used to advantage in constructing bulkheads to control flooding in underground mines. A room-and-pillar gypsum mine situated in Karst topography near Shoals, Ind., is a good example. It flooded twice, first in 1960 and again in 1965. Both times the mine remained closed for several months before grouting (involving the injection of about 450 cubic metres of cement grout into the formation in 1960 and then 3,600 cubic metres five years later) and dewatering could re-open the operation. In April, 1976, a large concrete bulkhead was constructed to impound seepage from the affected area of the mine. But seepage into an adjacent area raised concerns about safety in that area. So a new underground sulphate-resistant concrete bulkhead was approved to seal off the second area. A polyurethane elastomer was used to seal the area between the concrete bulkhead and the rock and a 2-component polyurethane elastomer was used to grout 5-ft. holes drilled into the rock around the bulkhead. The project was designed and supervised by Naudts and managed by J.S. Redpath.

“The grouting program was completed in February, 1988, and since then there has been no visible seepage from the area and water pressures now exceed 120 lb. per square inch,” says Edward Des Rochers of U.S. Gypsum, operator of the mine.

There is a good possibility that grouted concrete bulkheads will be used to isolate mining blocks in the potash mine being considered for development in southern Manitoba.

Another application of grouting is the containment of toxic chemicals at landfill sites or tailings areas. In one case in the southern U.S., air was used as a grouting material to contain toxic wastes. Compressed air was pumped into holes around a tailings area to create flocs which then clogged the cracks in the deposit.

Closer to home, Trow’s Thunder Bay branch manager Robert Dodds was involved in the design and supervision of a tailings pond grouting project in 1976 at the now-closed Mattawasca Lake uranium mine near Bancroft, Ont. A mixture of local clay, bentonite and cement was used to create a grout curtain across a deep alluvial valley at the end of the mines tailings deposit. The clay has a strong ionic attraction for U226, therefore eliminating a source of radioactivity from the tailings run-off water.

A less-successful example is the Pine Point tailings grouting project in the North West Territories.

Grouting is not used solely for water flow problems, however. It can also be used for ground control. In fact, with a minimum amount of research, the cost of backfilling can be reduced by 30%, Naudts says.

Grouting can also be used to consolidate poorly cemented mine backfill material. At the Kidd Creek mine in Timmins, Ont., this is done in the upper portion of the orebody so that raise bore holes can be safely reamed through unconsolidated material. The raisebore holes are then used to introduce backfill to the stopes below.

Here, a grouting technique known as the multiple packer sleeved pipe (or MPSP) system was used for the first time in North America, says Dr. D.A. Bruce of Nicholson Construction in Pittsburgh, Pa., the contracting company on the job. In this system, a perforated steel casing is installed in the drill hole, secured and centralized by regularly spaced fabric bags that are inflated with cement grout. Grout is then injected into the surrounding rock through the perforations in the casing from the bottom of the hole to the top using a double packer. A cement-based grout was used in this application because it was readily available and would meet the purposes of the grouting progam.

“Typically grout flow rates were 20 to 30 litres per minute with pressures of about 15 to 20 bar,” Bruce says.

Once the backfill had been grouted, a Robbins 34-R raise bore machine was used to drill a 250-mm-diameter pilot hole which was then reamed to a final diameter of 630 mm. A video camera was then lowered down the hole to record the effectiveness
of the grout program. The fill in about half of the length of the hole wall had a porosity of less than 15%. About 22% of the length of the wall had a porosity greater than 30%. “The grouting had reduced the porosity quite substantially in places, but elsewhere the fill was still relatively open-structured, though stable,” Bruce says.