Rock Grouting Overview

Selecting Grout and Additives

Grout Mix Formulations

Grouting Procedure

The most suitable arrangement of gear for most underground grouting jobs is shown in Figure 1. It provides the greatest chance of avoiding difficulties during grouting. The grouting crew will generally consist of three people; one person to control the grout flow rate and pressure, one to mix the grout, and one to connect and disconnect the grout hose from the various holes. All of the grout crew must be able to communicate with each other quickly so it may be necessary to provide telephones if the grout hole or mixing tank is a long distance or out of visible range from each other.

 

 

Figure 1 Typical Grouting Equipment Arrangement

Drilling Equipment

Equipment for drilling is generally percussion type capable of drilling holes to the length required. Diamond drilling can be used although it is more expensive. Hole diameters can range from and 32 mm to about 55 mm. Holes should be continuously flushed with water during drilling. The drill should be capable of drilling a radial fan of holes 360° around the tunnel perimeter. The drilling equipment must be capable of being rapidly moved from one hole to the next. When drilling a deep grout curtain, a drilling accuracy of better than 1 in 50 should be achieved.

Rod dope, grease or other lubricants should not be used on the drill rods. In addition, drilling water additives of any kind should not be used.

Circulation Lines

Circulation lines should be pipes or hoses at least 25 mm diameter and should not contain any constrictions that could cause blockages. Grout line pipe, hoses and fittings should have pressure ratings at least twice the injection pressure.

Packers and Valve Assembly

Grouting shall be carried out through mechanical grout plugs, properly secured into the rock or concrete of the plug. The length of the rubber seals shall be at least 300 mm, but longer in weaker ground conditions. Packers can be easily constructed at mine sites from readily available materials or purchased from drilling supply companies. A typical packer and valve arrangement is shown in Figure 8-2.

 

 

Figure 8-2 Typical Mechanical Packer Assembly

Pressure Gauges

Grouting pressure should be measured with an air filled or oil filled pressure gauge. To prevent damage to the gauges from the grout, oil filled gauge savers should be used on the line. The use of large expensive gauges is not considered suitable for grouting work. Smaller, throw-away gauges that are checked and replaced often are more appropriate.

Grout Flow Meters

The grout flow rate is best measured using an electromagnetic non-intrusive type flow meter. The flowmeter should have a system accuracy of at least 1% of the full range over the full range of flow rates. These instruments can come complete with a digital readout that measures flow rate and counts the total flow.

Water Flow Meters

Water meters to measure the water addition to the mixer should have their calibration checked before each grouting job. They should be capable of measuring to 1 litre accuracy and be equipped with screens and clean-outs.

Grout Mixers

Thorough mixing and wetting of the cement particles is very important when grouting rock joints. Paddle type mixers are not suitable for this type of work. A "high shear" or "colloidal" mixer is necessary. The most common type of colloidal mixer is the "Colcrete" or "Cemix" variety shown in Figure 3. Colloidal mixing develops a vortex as it mixes which acts as a centrifugal separator. Unmixed cement and grout is thicker than the average grout and is spun to the outside of the vortex. This thicker material moves down the walls of the tank and passes through the mixing rotor where the high shearing action:

• Breaks up lumps into individual particles;

• Removes adsorbent gas layers;

• Thoroughly wets each cement particle; and,

• Produces a grout that resembles a colloidal solution rather than a mechanical suspension.

The treated thick fraction is recirculated back into the vortex and the mix becomes progressively thinner until a uniform thickness is reached. A good mixer can achieve this mixing in as little as 15 seconds. Mixing should be done in batches. Do not attempt to carry out mixing as a continuous process.

Mixing the grout should be performed in the following steps:

1. Fill the mixer with the required amount of water. Use a mechanical flow meter to control the amount of water.

2. Turn on the mixer to full speed before adding any cement.

3. Add the cement in whole bags. Avoid using half bags.

4. Do not leave the mixer running for more than a few minutes between batches as it will add a lot of heat to the mix, reducing the set time.

5. There should always be enough water in the mixer to cover the rotor whenever it is running.

6. Clean the mixer thoroughly after each day's work, or even during the work if long delays occur. The rotor and other internal parts can be cleaned by mixing clean water through the machine.

To ensure that proper mixing is being carried out, regular maintenance of the mixers and efficiency checks should be performed. To carry out an efficiency check, first check that the rpm of the mixing rotor is close to rated rpm. Collect a sample of grout from the mixer in a small plastic or styrofoam cup about 1 inch deep. When the grout is set, break it apart and observe the grain structure on a vertical face. If the mixing is efficient, the vertical face will be uniform in colour and have no banding. If the vertical face has horizontal banding of different colours, it indicates some segregation of different particle sizes and inefficient mixing.

Figure 3 Colloidal Mixing

Grout Agitators

The purpose of agitators is simply to keep the grout stirring before it is injected into the holes. They typically consist of a paddle mixer revolving at approximately 100 rpm (Figure 8-4). The outlet to the tank should have a removable screen that can take out any lumps or other debris from the tank before it enters the pump. The return grout circulation line will feed into the agitator tank.

Figure 4 Typical Grout Agitator

Grout Pumps

For small grouting operations that may consist of one or two days work, a small pneumatic piston pump such as a Speidel or Monarch may be adequate. These pumps are especially suited for low quantity, high pressure applications. The action of a single-acting piston pump is shown in Figure 8-5. On the upstroke, grout is drawn into the lower chamber of the pump and forced out of the pump through the upper chamber. On the downstroke, grout, the lower chamber is sealed by a spherical seating valve and grout is pushed out of the lower chamber into the upper chamber. These pumps can have output pressures of up to 10 times the compressed air pressure operating them (>1000 psi). Advantages and disadvantages of piston pumps are as follows:

Figure 5 Piston Grout Pumps

Progressive Cavity Pumps

Progressive cavity type pumps consist of a single pitch steel spiral with a circular cross section and a degree of eccentricity. The single helix rotor turns eccentrically within a rubber coated double helix steel stator. The action of a progressive cavity type pump is shown in Figure 8-6. As the rotor turns, it rotates concentrically around its own axis and eccentrically as well, creating a complete seal along the length of the stator at all times and cavities which progress in a continuous way along the pump. The continuous nature of the pumping means that the pressure does not pulsate like piston pumps.

Common brand names of progressive cavity pumps are Moyno, and Mono.

Figure 8-6 Progressive Cavity Pumps

Grouting Technical Papers Available Online

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