Many sites obviously need grouting so clearly that there is no question, so the subject here is more about marginal cases, where there may be some doubt, and also about standards of grouting. The application is mainly to seepage control.
Can Grouting Be Safely Omitted? Yes, sometimes. But not often at dams if seepage has to be controlled. Hence some guidance is given about when grouting can be and when it can't be omitted.
The main issues are shown below. Don't forget that they are only a guide. Showing them is meant to direct the thinking; they need intelligent adaption to each actual application.
The guidance is on the question "when is grouting warranted?" and is given in terms of lugeons; these represent the standard of grouting to be considered for each of the situations.
Any part of the foundation that has a natural permeability poorer than the standard may need to be grouted; often all of it has to be.
Starting with the issue of economics, how valuable is water lost by seepage? In some desert conditions or in other situations where water is scarce and therefore precious, almost no seepage at all can be tolerated. A tight curtain grouted to a standard of 1 lugeon will probably he necessary. This might also be the case where hazardous fluids or sanitary landfill have to be retained with virtually nil loss by seepage.
Where water will be pumped into the storage at considerable expense, such as at off-stream and pumped storages, water lost by seepage represents money lost. This cost, over the working life of the project, should be reckoned against the cost of grouting.
If thorough grouting is opted for, it will probably need to be to a standard of about 2 or 3 lugeons to keep seepage down to trivial amounts (sometimes even tighter grouting may be needed to achieve this).
However, water lost by seepage usually has negligible value, in which case the chart leads on to the next question.
This refers to possible piping of soft and weak material from seams. If this is likely to happen because of the pressure in seepage, precautions usually are necessary to minimise it. A standard of grouting of 3 lugeons is suggested. Multiple rows in the curtain may also be advisable
If none of the previous issues applies consideration turns to the type of dam. Embankment dams are shown on the left and concrete dams on the right. The embankment dams include (for these purposes) zoned earth/rockfill, homogeneous, fills, and membrane-faced rockfill.
The heading for wide core zoned fills includes these last two types of dams and suggests that if the curtain has a single row (the commonest arrangement ), the standard could range between 5 and 10 lugeons. The actual figure selected within this range depends on local factors. The range may be relaxed to 7 to 15 lugeons if the curtain has three or more rows if these rows are carefully arranged so that penetrations overlap and form a thick region of grouting
Under relatively narrow cores in zoned embankments, hydraulic gradients in seepage can be steep. Tighter standards of grouting are therefore called for, such as 3 to 7 lugeons for a single-row curtain and 5 to 10 for a good multiple-row curtain as described earlier. Gradients can be even steeper in the case of membrane-faced dams, but these do not present the threat from loss of core material into foundation cracks, and hence standards can be relaxed for them a little, as indicated earlier.
Concrete dams of all types are grouped together here; this is a simplification that should be borne in mind. Adequate foundation drainage is necessary, otherwise tighter standards are advisable. Those shown for drained conditions are 3 to 5 lugeons for a single-row curtain and 5 to 7 for good multiple-row curtains.
These considerations primarily apply to surface regions; at lower depths the grouting does not need to be as tight usually. It can be advisable, in important cases, to have widely spaced probe grout holes to check foundations even if the chart indicates no need for grouting.
Having set a standard for the grouting, decisions about when to cease closure can be based on it.
Note particularly that the figures suggested relate to curtain grouting done thoroughly, with extra grouting at deficiencies and weaknesses.
When using these suggestions, other factors must be kept in mind such as particular features and peculiarities of the site in relation, for instance, to cracking frequencies and sizes. A thorough geological and permeability investigation helps greatly when reviewing these things.
On some sites the investigations find that permeabilities are highly variable; possibly the geology may also be. Some generalisations are then necessary and decisions about the grouting must reflect this.
The pragmatic approach to all this emphasises that finesse is futile; common sense is paramount when deciding what to do.
THAT COMPLETES THIS PROGRAMME
SOME INFO....
This is an abbreviated extract from Version 8.0 of the series of ROCKGROUT grouting programmes. The version is on DVD but is not yet publicly available
The DVD contains the book Construction and Design of Cement Grouting together with the contents of this Web site and some additional matter.
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