An instrument designed and built at the LNEC for measuring the deformability of rock masses along boreholes with a small diameter is presented. The Authors discuss the advantages and disadvantages of this instrument with respect to the methods in current use for determining deformabilities, present some expressions for interpretating the results of measurements with the instrument and describe calibrations and some results obtained in the field.


On decrit un appareil, conçu et fabrique au LNEC, pour mesurer la deformabilite des massifs roeheux dans des trous de sondage à petit diamètre. On discute les avantages et les inconvenients de son emploi par rapport aux methodes courantes dans la determination de la deformabilite, on presente quelques expressions analytiques pour l'interpretation des resultats des mesures faites avec l'appareil et on decrit les etalonnages effectues et quelques-uns des resultats dejà obtenus sur place.


Die Verfasser beschreiben ein im LNEC entworfen und gebautes Gerat zur Bestimmung der Gebirgsverformbarkeit in Bohrlöchern kleinen Durehmessers. In der vorliegenden Arbeit werden Vor- und Nachteile der Methode gegenueber anderen zur Bestimmung der Verformbarkeit benutzten Metho den diskutiert, einige Formeln zur Auswertung der Messergebnisse erwahnt, sowie die durgefuerhrten Eichungen und einige im Feld erhaltenen Ergebnisse angegeben.

1. Introduction

The first instrument for measuring the deformability by observing deformations in boreholes was, as far we know, developed by MENARD[1] (¹) for soils. Subsequently, JANOD and MERMIN[2, 3] in France (Electricite de France) and KUJUNDŽIĆ and STOJAKOVIĆ[4] in Yugoslavia (JAROSLAV ČERNI INSTITUTE)constructed instruments specially designed for determining the deformability of rock masses. All these instruments apply a uniform pressure along a certain length of the borehole by means of a liquid contained between a metallic cylinder and a very deformable rubber or steel jacket, which is applied against the wall of the borehole. As is known, the deformability characteristics of rock masses are usually determined by means of jack loading tests, in which a surface is loaded with jacks and the resulting displacements are measured. It is thus possible to load large surfaces, i. e. to affect large volumes of rock and therefore, in principle, to obtain representative values of the deformability which take into account the fissuring and jointing usually found in rock masses. An additional advantage of these tests, of great interest in the interpretation of the results, lies in the possibility of a direct visual observation of the loaded area, which later can even be excavated for that purpose. Nevertheless loading tests have two serious disadvantages: the first is that, however careful the excavation of the gallery or trench for the tests, it inevitably disturbs the rock mass; the second is the need to limit the loaded area, and therefore to decrease the representativeness of the results, and the number of tested spots in the rock mass, due to the cost and the time required for each test. Due to these two serious objections, the possibility of developing the instruments called dilatometers for measuring deformability inside boreholes has been considered with great interest in later years. In comparison with the current load tests, measurements inside boreholes have the following advantages: first, provided diamond drilling is used, the rock mass remains almost undisturbed; second, due to the reduced cost and time required, many tests can be carried out. It is thus possible to obtain statistically relevant data on the distribution of deformability in the rock mass, including its anisotropy, so that the results of dilatometer tests can even be considered as a quality index of the rock mass. Additional advantages of dilatometer tests are the possibility of performing deformability tests under water, such as in river beds, and at great depths, which is of special interest in the study of tunnels, and also of using much higher pressures than in the current load tests. A disadvantage to be emphasized is the small volume of rock involved in each test, so that frequently the results obtained may not be representative of the deformability of the rock mass in the tested zone. Nevertheless, given the possibility of making many tests, amounting to an almost continuous determination of deformability along the boreholes, it should be possible, at least in some cases, to subject the results obtained to a statistical processing by means of which the average deformability of the different zones of the rock mass could be determined with the accuracy currently required in engineering, which is not high. As for the state of stress in the tested zone of the rock mass, it should be noted that whereas a zone subjected to a jack loading test is free from stresses, i. e. decompressed in the direction of the applied loads, the volumes involved in the dilatometer tests are in a state of radial compression (zero at the surface of hole) when the rock mass exhibits residual stresses.

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