Pre-investigation data in grouting design: recommendations regarding fracture transmissivity distributions

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In grouting design a design methodology can be used where transmissivity and fracture aperture distributions form the basis for selecting grouting parameters. The calculations are based on section transmissivity data calculated from hydraulic tests in boreholes. These are linked to individual, mapped fractures using a statistical distribution function called the Pareto distribution. The design methodology has been used in several research and infrastructure projects in the Nordic countries. However, there are still uncertainties regarding requirements for pre-investigation data and how data should be handled in the calculation process. This article presents a study of how different types of test set-ups and interpretations of mapped geological domains can affect a Pareto-analysed fracture aperture distribution. Sensitivity analyses have been performed by testing various existing datasets from tunnel projects and modifying parameters such as measurement limit, section length, number of fractures and geological domains. The sensitivity analyses form a basis for recommendations regarding the applicability of the calculation method, test set-ups and model assumptions. Fundamental to producing reliable fracture transmissivity distributions is the collection of sufficient and representative amounts of data. Mapping of drill cores and execution of hydraulic tests must be of good quality, and it is recommended that weakness zones, the boundary between shallow rock and host rock and recorded flows near the measuring capacity of the test equipment are the subject of deeper analyses.

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