 Rock Grouting Overview
Selecting Grout and Additives Grouting Equipment Grout Mix Formulations Hole Sequencing Grouting is normally done in a closure sequence that is best illustrated in Figure 8-7 which illustrates the sequence of drilling and grouting a radial grout curtain. For holes longer than 3-4 metres, the hole should be grouted in either a downstage sequence or upstage sequence (Figure 8-8). Drilling of a secondary stage can proceed when grouting of two adjacent primary stages has been completed. Figure 7 Sequence of Grouting Between Holes of a Stage Figure 8-8a Downstage Grouting Sequence Figure 8-8b Upstage Grouting Sequence Grouting Procedure for an Individual Stage Grouting of an individual stage is performed using the following procedure. Step 1: Drill all the stages of the sequence of holes to be grouted. Step 2: Flush each hole with water to remove cuttings and mud. This can be achieved by inserting a pipe or hose into the hole, so that the point of discharge of the wash water is at the far end of the hole. Continue flushing until the return water is clean. Step 3: Insert a mechanical or inflatable packer in the hole and install valve assembly. Step 4: Water test each of the holes on the stage that is being grouted. Refer to the section below on water testing. Records of the water testing should be maintained by the grouting crew. The water pressure testing shall never exceed the maximum grout pressure specified. Step 5: Prior to commencement of grouting of any hole, any obvious surface openings such as cracks and fissures, including those which may become apparent during water pressure testing should be caulked or otherwise sealed. Perform the caulking work when the water test is completed. Step 6: Determine a suitable starting mix depending on the results of the water testing. Until experience is developed at a particular site, a reasonable starting mix for a microfine grout job would be a 2:1 (water:cement by weight) mix as indicated in the following table. Materials should be added to the mixer in the following order: Water Bentonite Microfine Cement Superplasticizer
After adding all components to the mixer, the mixture should be mixed for 1-3 minutes, depending on the type of mixer. The mix may be thickened or thinned as grouting proceeds. Generally, cement grouts will be thickened when there is no reduction in the rate of acceptance and the pressure cannot be built up. Step 7: Close the valve to the grout hole and begin circulating grout in the lines. Check there are no blockages by ensuring the grout is returning to the agitator tank. Zero the totalizer on the electromagnetic flowmeter. Step 8: Open the valve to the hole and slowly pinch the valve on the return line to increase the pressure in the hole. Raise the pressure to about 25% of the refusal pressure and maintain that pressure until the flow drops off significantly. Slowly increase the pressure in increments of about 25% until the refusal pressure is reached. Under no condition should the pressure or rate of pumping be changed suddenly or the maximum grouting pressure exceeded. Step 9: During grouting operations, if grout leaks occur to the tunnel, caulking with wedges, burlap, lead wool, etc, will be performed, where the leak is accessible. Systematic thickening of the grout mix should be used to try to stop grout leakage. In this manner it will be possible to maintain the thickest grout mix in the fractures leading to the leak. If grout leakage is excessive, viscosity modifiers should be added to the grout to reduce grout leakage. Alternatively, the grouting may be suspended until the initial injection of grout is adequately set. Step 10: Continue grouting to refusal. The refusal pressure will depend upon the rock quality, confining stress in the rock and other factors. Care should be taken not to exert too high a pressure so as to cause the tunnel walls to become unstable. Holes should be kept valved off until grouting pressure dissipates. Care should be taken to keep equipment and lines clean during grouting operations. Flush out all equipment and lines after grouting each stage in a hole. At the completion of all grouting in each phase of the work, all the grout holes and test holes should be redrilled and cleaned out as necessary and subsequently filled and sealed with a cement grout having a water:cement of 0.7:1, by weight, or less, in a manner to ensure complete filling of the hole. Evaluation of Site Conditions - Water Pressure Testing (Lugeon Test) Water testing (or Lugeon testing) is carried out to: Indicate the hydraulic conductivity of the stage or hole. This helps in deciding the starting mix and special additives that may be necessary; Indicate the effect of previous grouting activities when the hole is a secondary or tertiary hole; and, Find leaks and connections to other holes.
Water testing is usually carried out using the same equipment as the grouting but using clean water rather than grout. The water test is normally carried out at 15 psi. Three 5-minute tests are carried out at the same pressure. The hydraulic conductivity is measured in terms of Lugeon units (UL). 1 Lugeon Unit = a water take of 1 litre per metre per minute at a pressure of 10 bars. The Lugeon unit is not strictly a measure of hydraulic conductivity but it is a good approximation for grouting purposes. 1 Lugeon is approximately equivalent to 1x10-5 cm/s For tests carried out at other pressures: Lugeon value = water take (litres/m/min) x 10 bars/test pressure (in bars)) The three successive test runs, each of 5 minutes duration enable a rough assessment of the water behaviour. If the water take is approximately equal for all three tests, a representative test is being made in all three runs. If the water take decreases on successive tests, the water is probably filling cracks and the last tests should be taken as representative. If the water take increases with successive tests, it indicates the ground may be opening under the pressure or joint infilling is being washed out. In this case, the test pressure should be reduced. | 
