The hydrojacking and hydrofracturing behaviour of discontinuities in rock is analysed by evaluating water pressure (Lugeon) tests carried out for investigation or grouting at 23 dam sites in Spain. Comparing the results with those obtained in a further 30 dam projects realised in a number of countries allows the assessment of that deformation which occurs in most rock types: Pressures < 15 bar already initiate hydrofracturing in about 80 % of all rock types. The critical pressures provoking hydrofracturing are scattering within an individual scope but do not in-crease with depth. To grout such rock types successfully and economically, grouting pressures should not increase with depth; staggered groutholes are not appropriate; narrow joints are not groutable if the grouting pressures required exceed those cracking latent discontinuities.

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