PERIODICAL REVIEWING

Every 15 minutes, or at whatever other regular time intervals may have been chosen, the progress of the grouting should be reviewed for the purpose of discerning whether any changes are necessary. This is done by comparing the quantity of grout taken in the latest interval with takes in earlier intervals, making allowance for any relevant factors such as pressure changes or the outbreak of problems such as leaks, connections, or movements.

The quantity of grout taken in each interval should reach a peak and then decrease, generally as indicated by this graph.

• A. At the start of the injection, fresh grout should be moving fairly easily through the cracking system.
• B. Then, when the easily reached cracks have become full with grout, the grout slows down and becomes sluggish, and the pressure drops off at locations away from the hole. The overall effect of this is to reduce the take gradually.
• C. Eventually, the grout near the fringe becomes stiff and pressure becomes zero there. Thixotropic stiffening is commencing in adequately thick grouts.
• D. After that, the remainder of the application is confined to injecting fresh grout into passages and voids that have formed where bleed water has risen up out of grout injected earlier in the application.
• E. Eventually refusal is reached when all bleed passages accessible from the hole are filled with fresh grout. Good practice is then to keep the hole under pressure for a further 15 minutes to ensure that full thixotropic stiffening has been reached. This avoids erosion of some of this grout by pressured water such as groundwater or water testing, in nearby holes.

Sudden Takeup

The sudden takeup early in the application after the first 15 minute interval shown in this sketch indicates that the mix has been too thick. It has choked off the cracks.

Sudden Increase of Take

The sudden increase in the second interval shown in this sketch probably indicates that trouble has occurred, such as rock has moved or leaks or connections have broken out. Vigilant observation should have noticed the trouble before it became apparent in the quantity measurements.

Protracted Equal Takes

Unless refusal is near, a succession of almost equal takes, as shown here indicates that the mix is probably too thin, because no limit to the penetration is being reached over an hour or so. This means that the grout is excessively mobile due to having too much water. If the mix is not thickened, a patchy job can be expected, and the application will take longer than would otherwise be the case.

In general, the thickest possible grout should be injected into the cracks as quickly as possible so as to reach the limit of penetration while the grout is still able to move freely and transmit pressure effectively. The time available for this ranges from 0.5 hours for small takes to 3 hours for fast, high takes.

The sketches of grout in an agitator, shown in the preceding figures, have been simplified to omit the topping up actually necessary usually.