The results of laboratory tests, and the consequent values of the mechanical parameters, could be appreciably influenced by the characteristics of the testing device and by the adopted experimental procedure. Some of these, effects, such as those related to the stiffness of the loading frame, are discussed here on the basis of a series of triaxial tests performed on samples of a soft volcanic rock (tuff) from central Italy. In particular, the influence is discussed of the "local" or "global" technique adopted for strain measurement on the overall shape of the stress-strain curve, and on the values of the elastic parameters determined before reaching the "peak" load level and in the "after peak" condition.
La methode experimentale et les caracteristiques de l'appareil peuvent avoir une influence remarquable sur les resultats de l'essais en laboratoire et donc sur les valeurs des parametres mecaniques qui sont derivees, Cette etude presente les resultats d'essais de compression triaxial obtenus sur une roche volcanique ("tufo di Riano", Italie), en utilisant soi un systeme de mesures traditionnelles (mesures globales) soi un systeme avec une instrumentation placee à l'interieur de l'appareil (measures locales). Les valeurs des parametres mecaniques, obtenues des differents essais, ont ete comparees, En plus, des commentaires sont aussi presentes sur l'influence de la rigidite de l'appareil.
Das Pruefverfahren und die Eigenschaften der Pruefeinrichtungen (z.B. die Steifigkeit der Pruefmaschine und die Eigenschaften des Maßverfahrens) können einen nicht vernachlaassigbaren Einfluß auf die Testergebnisse haben. In dieser Arbeit werden die Testergebnisse von triaxialen Versuchen vorgestellt, welche einerseitss mit einem herköommlichen Messverfahren (global Messung) und andererseits mit einem Verfahren, bei dem die Messung innerhalb der Meßzelle durchgefuehrt wird (lokal Messung), gewonnen worden. Abschließend werden die Werte der gewonnenen mechanischen Parameter aus beiden Versuchsserien miteinander verglichen.
The design of major civil engineering structures resting on soft rock formations requires an accurate estimation of the settlements of their foundations in order to avoid negative effects on the overall behaviour of their structural members. This, in turn, requires a proper evaluation of the deformability of the rock mass, as well as of the parameters governing its load bearing capacity (JARDINE et al., 1986; STERPI et al., 1995; CIVIDINI et al., 1992; CIVIDINI & GIODA, 1992).
In rock engineering practice, this evaluation is customarily based on the results of the necessary in situ investigation and of proper laboratory, tests, such as one dimensional, unconfined compression tests and standard triaxial tests (e.g. ADACHI & TAKASE, 1981; KING,'I983; HAIMSON, 1978; JOHNSTON, 1992). Confining our attention to the laboratory investigation, it is known that the experimental procedure and the characteristics of the testing' device could influence the experimental results and, consequently, the values of the derived mechanical parameters (see e.g. YUMLU & OZBAY, 1995; TATSUOKA et al., 1986; 1994; CIVIDINI, 1997).
This work intends to provide an insight into this problem; with particular reference to the behaviour of a soft volcanic rock (tuff) from central Italy, known as "tufo di Riano".
