ASSESSMENT DURING GROUTING

Assessment is essential in order to minimise problems and avoid wastage of money. It is needed

• (a) while the grouting is going on, and
• (b) afterwards.

Dealing with with making assessment while the grouting is in progress, this needs to be carried out on every grout application and to be done by someone familiar with grouting. Experience pays off.

This is because grouting is not a process where a recipe or prescribed procedure can be followed. Every hole has its own peculiar variations on the general theme, and in the case of large foundations, grouting characteristics can vary appreciably over the extent of the site. Successful grouting requires experience and initiative.

The basic concept is, of course, to inject maximum quantities of cement. The grout should be as thick as possible commensurate with reasonable penetration. Furthermore, the time when the peak rate of cement injection takes place should be as early as possible in the application, so as to get maximum mobility before the grout starts to stiffen.

If a plot is made of rate of GROUT take (not just the cement take) against time from start of the application, the ideal shape of the resulting plot should be as shown here

During the first quarter hour or so, the effect of slowly increasing the pressure can give a corresponding increase in grout take.

Once the working pressure has been reached, the rate of take should have reached its peak, and the remainder of the application should show a lesser rate of take until finally refusal is reached.

This ideal state of things does not often happen, though.

Putting some figures on the time scale, the ideal graph is then as shown in the plot above.

Most grout applications last between 1 and 3 hours if carried out by the techniques given in these programmes. Unless there is an obvious reason, such as leaks or connections or rock movements causing a protracted application, finality ought to be reached more or less within about 3 hours. Perhaps this finality might not be refusal: it might be a deliberate "pulling off" because a high take has been reached or for some other reason.

The profiles shown above consider the rate of GROUT take. If looked at a little differently, to show the rate of CEMENT take instead, the smooth curve has jumps in it, as shown here.

Each time the mix is thickened, even if the rate of grout take should happen to remain in a smooth curve, the amount of cement in it automatically changes and so gives a jump in the curve.

In practice, a smooth curve is modified to a series of jerks if readings of grout take are made periodically, such as by taking dipstick readings in an agitator. Continuous readings taken by a flow meter connected to a computer or other recording device can, however, give a smoother curve.

SOME VARIATIONS TO BASIC PROFILE

Returning to plotting of grout take, rather than of cement take, if there is negligible reduction in the take after several hours of grouting, the plot will resemble this.

It can be seen that the hole is continuing to accept grout very freely. In most cases of this, but not all, the hole will accept a thicker grout almost as freely. On the principle that the thickest grout practicable should always be used, it is advisable to thicken rather than to continue with a grout that is too thin.

The situation shown in this figure is undesirable. It indicates a runaway situation where the rate of take will continue to increase if the application is continued. Usually, the cause of this is that rock has moved or leaks are getting worse. Whatever the cause, the application should be given suitable treatment to arrest the situation.

This can take the form of restricting the amount of grout permitted to go down the hole so as to achieve the situation shown here.

The effect of such restriction is to reduce any jacking effect on rock in the process of moving.

A more drastic version of this treatment is to apply the grout intermittently. This takes advantage of the thixotropic stiffening of thick grouts during the periods of 15 minutes or so when the grout is turned off. This stiffening tends to restrict the radius of pressure penetration and hence reduces the jacking effect.