AFTER REFUSAL

Once refusal has been obtained the pressure should be kept on the hole for a further 15 minutes or thereabouts. This holds the grout firmly in place in the cracks to ensure that thixotropic stiffening, aided perhaps by part of the cement setting process (but not all the way to initial set), stiffens the grout sufficiently to resist removal by groundwater or by water from nearby grouting operations.

After this the valve at the top of the standpipe is closed and left closed until wash-out time.

MORE APPLICATIONS OF GROUT

The process of grouting a hole or stage from start to refusal is termed an "application." Quite often one application is enough to produce the required standard of grouting, but at times it is not enough. One example is when rather a lot of water has been injected with the grout; it can be anticipated that some of this water has been trapped in the cracks and produced voids.

The amount of water injected with grout is a function of the w : c ratio (thin grouts have a lot of water in them) and the quantity of grout taken. Another application of grout should be given if the CEMENT take has exceeded :

• 0.25 cu ft of cement/ft [0.25 litres of cement/cm] when the final mix has been 2 : 1 or thinner or
• 0.50 cu ft of cement/ft [0.50 litres of cement/cm] when the final mix has been 1 : 1 or thicker.

The calculation of cement take per unit length of hole should be available from the Report Form at the conclusion of the grouting. Any need for another application can then be decided well before hole wash-out time. In some cases the hole would not need wash-out were it not going to be given another application.

At least 24 hours should elapse between applications. This lets the grout harden and water escape. Note that this is not really long enough for the thinner ( 2 :1 and thinner) grouts to reach hard set but is a practical compromise. There is no need to do a water test before second and subsequent applications.