Procedures for calculating rock mass deformability moduli from the results of in situ testings are encumbered with crudity that should be taken into account in all steps of the planning and performance of testings. A critical review of in situ tests most often used is made with emphasis laid on problems concerning an interpretation of results of measurement. Experiences with twenty-eight PL, seventeen ELFJ and more than one hundred dilatometer tests are presented.


Les precedes de calcul du module de deformebilite de la masse rocheuse, bases sur les resultats d'essais in situ, sont encombres de nombreuses imprecisions dont il faut tenir compte dans tous les stades de planification et d'execution d'essais. L'auteur presente une evaluation critique des essais in situ les plus couranunent utilises, en soulignant le problème d'interpretation des resultats de mesure. Les experiences obtenues avec 28 PL, 17 ELFJ, et plus de 100 essais au dilatomètre sont presentees.


Berechnungsmethoden fuer die Ermittlung des Verformungsmoduls des Felsen aus in situ Versuchen sind mit vielen Ungenauigkeiten behaftet, die schon in der Plannungs und Durchfuehrungsphase der Versuche beachtet werden muessen. Bei der kritischen Beurteilung den meisten durchgefuhrten in situ Versuchen treten Probleme hinsichtlich der Interpretation der Ergebnisse auf. Die Erfahrungen aus 28 Lastplattenversuche, 17 Druckkissentest und mehr als 100 Dilatationsverschen werden prasentiert.


The defining rock mass deformability is a complicated process due to the known problems that encumber all kinds of testing in rock mechanics. A scale effect is one of the main problems in rock mechanics and, up to this day, an unsolved one. Aiming to increase the rock volume involved in a test, a great number of in situ tests has been developed. The biggest volume of a loaded zone can be achieved by large pressure tunnel tests and radial jacking tests. However, the use of these tests is limited for two reasons:

  • such tests are very expensive; and

  • a rock mass is in the state of stress similar to that in dilatometer tests, which puts limits to the use of the results of measurement.

The pressure tunnel test and radial jacking test simulate conditions in hydraulic pressure tunnel well. Therefore, for defining deformation properties of the rock mass, only two large in SItu tests are used today. These are a Plate Load Test (PLT) and a Large flat Jack Test (LFJT). As regards small in situ tests for defining rock mass deformability, dilatometer and borehole jack tests should be mentioned. Since with this testings the rock volume is slightly larger than that of a laboratory sample, they cannot serve for determining the rock mass deformability.


All in situ tests are encumbered with considerable problems with the interpretation of results of measurement. Attention is drawn to this problem by authors of some procedures themselves and by numerous users as well. 2.1 Dilatometer Test Deformability determined by dilatometer test should be considered as an index indicator of mechanical properties; therefore, in order to avoid confusion, a modulus of this test is called a dilatometer modulus (Rocha 1974, Charrua Graca 1979). The ratio of dilatometer modulus on one side, and modulus of PL and LFJ test on the other side is similar to that of the strength index of point load strength test and uniaxial compressive strength determined in a laboratory on an intact rock. As the result of the point load strength, test is not strength itself but a strength index, thus the result of the dilatometer test is not a deformability modulus but only an index property called dilatometer modulus. In both cases, i.e. in point load strength test and dilatometer test, the testing is worth doing only if a great number of tests are performed. It should be pointed out that individual results can give a completely wrong information. Valid conclusions about mechanical properties of the rock mass can be made only by statistical analysis of results of a large number of tests. The dilatometer modulus and deformarbility modulus of the rock mass can be correlated when in addition to dilatometer tests PL and LFJ tests were carried out in the rock mass zone that is considered homogenous according to the deformability parameter (GMU Geotechnical Mapping Units).

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