The observational method includes the idea that rock engineering design can be modified as construction progresses, and that identification (confirmation or rejection and updating) of e.g. rock classes can be used in order to choose from predefined design classes. To facilitate rock engineering design, we suggest that geological and hydrogeological reference conditions are used as a basis. Fundamental to a reference condition approach is the identification and sampling of reference sites. Attributes, in this case geological and hydrogeological attributes, are selected and used to define the reference conditions. The reference conditions should be defined and described early on in a project, using relevant engineering parameters, and grouped together into rock classes with similar engineering characteristics. The reference conditions are subsequently used to define e.g. grouting design classes, to adapt to project-specific flow requirements. In this paper we suggest and give examples of geological and hydrogeological reference conditions for grouting, and present an example where reference conditions were used to define the grouting design for a site in crystalline rock, located at a depth of 400 m at the Äspö Hard Rock Laboratory, Sweden.


Constructions in rock below the groundwater table will influence both flow and groundwater levels. Sealing these constructions using e.g. grouting will improve working conditions and limit the impact on groundwater levels in the surroundings. Successful sealing will decrease the flow into the construction, resulting in a decrease in the pumping capacity required at the site. If the water quality is unacceptable, smaller volumes of water will need to be treated. The aim of this paper is to present a systematic way of handling grouting, starting from geological and hydrogeological reference conditions for crystalline rock, continuing with examples of these reference conditions for a Swedish field site, and concluding with an example from the same site, where one of the reference conditions was used for the grouting design.

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