Rock Grouting

This book deals with grouting and, in particular, with the installation of grout curtains under dams. Why is it necessary to occupy ourselves with this subject?

If previous research and methods of assessment have not prevented unnecessary injection programmes, they are inadequate. The prospect of an improvement in our engineering-geological methods of rock permeability, as well as in a more economical execution of grouting programmes, should be worth attention. Which methods of preparation will be successful?

Fundamental terms used in this chapter (e.g. water pressure tests, WPT) will first be defined and elucidated in subsequent chapters. They are also to be found in the subject index.

In situ tests for determining rock permeability are a part of the fundamental investigation in dam projects, because seepage in the subsoil

In the literature on hydrogeology there are, surprisingly, no detailed examinations of the flow of water in rock which can be applied to the grouting process. This is presumably related to the objective of hydrogeological research, which attempts primarily to explore which volumes of water in which rock can be utilized under which conditions.

The theme of this book is governed by the conviction that many grouting programmes do not end up being practical and thus give rise to considerable excess costs. Additionally, there may also be programmes which are either totally or for the most part unnecessary. The analysis of several grouting programmes in Chap. 3 bears out this belief. This is understandable, because the critical discussion of the previously used methods of studying and judging permeability in Chap. 4 has shown that it is impossible to make reliable judgements with them.

The water pressure test and several important aspects of test execution and evaluation were treated in Chap. 4. The current method of judging rock permeability has also been discussed. In Chap. 6.3.4, the results of the model tests established that small water takes provide unequivocal evidence of the type and scope of the permeability and the hydraulic routing, but that intermediate and larger takes are ambiguous. Even in view of possible misinterpretations of the test results, it is in the author’s opinion more important to investigate the pressure-dependent permeability behaviour rather than to determine only the WPT rates. This is particularly important, because the grouting behaviour, decisively influencing the economy of a grouting programme, can then be studied. Water pressure tests can also be valuable for this, if they are carefully executed and evaluated; whereby, above all, the course of the test has to be considered, because this illuminates also the grouting behaviour. The execution and evaluation of the water pressure tests were dealt with in Chap. 4. Now, after the execution of simulated water pressure tests, whose results are very helpful because they allow a better understanding of the actual tests, the theme of Chap. 4 can be supplemented. In the following the results of actual tests will be interpreted with the aim of finding a way for a better judgement of the scope of the permeability as well as of the pressure-dependent behaviour during testing and grouting.

As a rule, the decision to grout has until now been made when the absorption rates in the water pressure tests exceed certain amounts. The tolerable rates have been set at different rates by several authors, but guiding the decision with the Lugeon criterion (Q_WPT = 1 l min ^-1 m ^-1 at 10 bar) has generally gained the widest acceptance. In Chap. 4.3 it was explained that the application of the Lugeon criterion is not expedient. Firstly, it delineates neither type and scope of the hydraulic routing nor the groutability of the rock surrounding the borehole. Secondly, the decision of implementing grouting measures should be based on the individual conditions concerning the importance of water losses as well as the susceptibility to erosion. Hence, the tolerable permeability is also different from project to project.

The consequence of the rock-type-specific groutability is that the original permeability can be reduced only to a certain specific degree when normal cement grouting is applied. The remaining permeability can be larger than the “tolerable permeability”, which is the measure of conductivity which is still acceptable regarding water loss and stability to erosion. The degree of tolerable permeability is specific to every rock type and the situation of the individual projects, i.e. it can be different from case to case; it can also differ greatly from the previously mentioned groutable permeability, being higher or lower. The results of this study have concluded that the tolerable permeability should not be set according to the criterion of 1 Lugeon.

The behaviour of the subsoil during impoundment first supplies information on whether the seepage through the underground is small enough, and whether the reduction of the uplift pressure is sufficient. Substantial variations from the predicted behaviour are frequently noted while taking measurements during impoundment. Such variations demonstrate that the intended success of the grouting measures is occasionally not achieved, or that an existing grout curtain is not stressed due to hydrogeological circumstances. The dominating influence of the geological conditions on the reduction of the uplift is recognizable particularly in cases of largely anisotropic rocks.

It is considered inadequate to give a summary of all the preceding chapters. Instead, with regard to the application of grouting at dam sites, it seems to be advisable to outline the most important aspects for investigation programmes related to the permeability of the subsoil as well as for the practical execution of grouting work and subsequent control measures.