![]()
SUMMARY: Strength indices for a wide variety of rocks in dry as well as in water-saturated condition have been measured by the point load test. The general trend is that rocks suffer from a strength reduction when saturated with water. However, a few very fine grained rocks show a significant increase in strength. The results are discussed. A grouping of the rocks based on petrography and water induced changes of strength is presented.
RESUMÉ: Dans cet article plusieurs mesures de tension par pression pointue ont ete conduites sur une variete de roches dans des conditions sèches ainsi que saturee d'eau. La tendence generale indique que la resistance dec rout avec la saturation d'eau, avec l'exception de quelque roche de granulometric tres fine qui montre une augmentation du resistance. On discute les resultats et on montre que les roches peuvent etre divisees en sous-groupes selon leur petrographie ainsi que les variation du resistance provoquee par la saturation d'eau.
ZUSAMMENFASSUNG: Festigkeitsindexen fuer ein breites Auswahl von Felsen in trocken als auch in wassergesattigten Zustand sind mit dem Punktlastapparat gemesst. Die Felsen wollen in Gewöhnlichkeit wenn mit Wasser gesattigt einen Festigkeitsreduktion aufzeigen. Einige wenige, sehr feinkörnige Felsen zelgen doch elne signifikante Vergrösserung der Festigkeit. Die Resultaten sind diskutiert. Eine Gruppierung der Felsen die auf Petrographie und wasserindusierte Anderungen in Felsfestigkeiten basiert ist, ist presentiert.
INTRODUCTION In an earlier paper on the influence water may have on some rock properties, strength reduction as a function of water content was demonstrated for six different rocks, figure 1, BROCH [1974]. For these rocks the reduction in point load strength, from oven dry to fully saturated specimens, varies between 20 and 45% with 34% as an average value. Strength reductions were also observed for rocks tested in uniaxial and triaxial compression. In fact, the reductions in these tests were generally greater than in the point load test. A study of the influence of water saturation on the internal friction, showed that for isotropic, magmatic rocks no change in internal friction was observed. The drop in strength for these rocks must therefore be due to other effects as for instance the reduction in surface energy when the rock Specimens are covered with water. For anisotropic, mica-containing gneisses there was a significant difference in the coefficient of friction from dry to water saturated condition for higher confining pressures. Thus there was no doubt that water reduced the internal friction of such rocks. With a mica content of approximately 15% and with micas being minerals with extreme physical and mechanical properties, it was concluded that the presence of this mineral might explain the great reduction in strength as well as in internal friction for the gneisses. In the present paper the results from a large number of point load strength tests on a wide variety of rocks in dry and water saturated conditions will be reported. This will be followed by a discussion of the petrographical factors that may have an influence on the change of strength from dry to water saturated rocks.
THE TESTING AND THE RESULTS The strength test used was the so-called point load strength test which is described in detail by BROCH & FRANKLIN [1972]. The testing was carried out in accordance with suggestions from the COMMISSION ON STANDARDIZATION OF LABORATORY AND FIELD TESTS OF THE I.S.R.M. [1972] as diametral tests on cores. The core diameter used was 32 mm as this is more convenient for harder rocks. The cores were drilled from blocks brought in to the laboratory. Coring was done both parallel and normal to the bedding or foliation of the rocks. All rocks were Norwegian and of Precambrian or Paleozoic age. Porosities of such rocks are normally in the range of 0,25 - 1,25%, seldomly greater than 1,5%. Dry specimens were obtained by storing the cores in an electrical oven with a temperature of 105°C for two days. Wet specimens, or more pricesely fully water saturated specimens, were obtained by first evacuating the air from dry cores and then carefully adding water to the cores while they were still under low pressure. After that the pressure was raised to normal and the cores kept submerged for 15 to 24 hours. (More detailed test description in BROCH [1974 and 1977]). The results from the testing of 27 different rocks are plotted in figure 2, dots indicating results from tests performed on cores drilled parallel and crosses from cores drilled normal to the bedding or foliation. The inclined lines give the varying relation of point load strengths for saturated rocks in percent of strengths for dry rocks. As the figure shows rocks will in general suffer a strength reduction when saturated with water. The point load strength may for some rocks even be reduced to approximately half of their "dry strength".
Site News