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    Effect of Water Saturation History on the Strength of Low-Porosity Rocks

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    The purpose of the tests described in this paper was to investigate how the mechanical properties of rock observed in the tests are influenced by the whole saturation history of the specimen prior to testing. Three aspects of the saturation history were studied in this paper: the effect of drying and resaturating the specimen prior to testing, the effect of resaturation method, and the effect of the chemical nature of the resaturating fluid. Three rock types were used in the tests: a gneiss, a cemented sandstone, and a fine grained limestone. All three rocks had apparent porosities below 2 percent.

    Results of triaxial and splitting tests are reported in the paper. One series of specimens was brought from the site in its natural saturated state and tested without drying while the others were either air or oven dried and then resaturated prior to testing. The resaturation was performed either by immersing the specimen in water under vacuum, or by injecting the saturation fluid, under pressure, through a thin channel drilled along the specimen axis. Either distilled or seawater were used as the resaturating fluid.

    The results show that the inclusion of a drying and wetting cycle prior to testing has a clear overconsolidation effect on the rock behavior, that is, it increases its apparent strength. On the other hand, the channel saturation technique gives a better saturation of the specimen and results in a strength decrease. Finally, the results show that the chemical composition of the saturation fluid has also a significant effect on the measured rock strength.

    The practical conclusion to be drawn from this study is that representative rock samples, taken in connection with a given project, should, from the moment of coring until they are tested, be held under environmental conditions that are as close as possible to those which will prevail after the completion of the project. This implies that no drying and wetting cycles should be included if they are not expected to occur in practice. If this condition cannot be met, specimens should be saturated using a natural saturation fluid and using an efficient saturation technique such as the described axial channel saturation method.


    soils, rock mechanics, rock sampling, splitting tests, triaxial tests, saturation methods, pore pressure

    Author Information:

    Ballivy, G
    Geotechnical engineer, Lalonde, Girouard, Letendre and Associates, Montreal, P.Q.

    Ladanyi, B
    Professor and associate professor, Ecolé Polytechnique, Montreal, P.Q.

    Gill, DE
    Professor and associate professor, Ecolé Polytechnique, Montreal, P.Q.

    Committee/Subcommittee: D18.12

    DOI: 10.1520/STP39072S