The use of the flat jack test to identify the residual strength of rock masses is discussed. Test interpretation procedures developed by means of numerical models of the slot are presented. The same procedures are applicable to the determination of original stresses as an alternative to the "cancellation pressure" method recommended by IRSM Suggested Standards. The use of the flat jack for deformability measurements is also analysed and the limitations of the conventional interpretation formula are outlined.


On discute l'emploi des essais au verin plat pour preciser le resistance residuelle des amas rocheux. On presente quelques procedures d'interpretation de ces essais, developpees au moyen de modèles numeriques de la coupe. Les même procedures peuvent être appliquees pour la determination des contraintes naturelles comme alternative à la methode par pression de compensation recommandee par l'ISRM Suggested Standards. On analyse aussi l'emploi des essais au verin plat pour les mesures de deformabilite, et on souligne les limitations de la formule d'interpretation conventionelle.


Es wird die Benutzung von Druckkissen zur Schatzung der Restfestigkeit von Felsen behandelt, und Verfahren zur Deutung der Pruefergebnisse mit Hilfe rechnerischer Modelle des Schlitzes werden dargestellt. Dieselben Verfahren sind auch zur Bestimmung des urspruengllichen Spannungszustands anwendbar, als Alternative zum "cancellation pressure" Verfahren, das vom IRSM Suggested Standards empfohlen wurde. Es wird auch die Benutzung von Druckkissen fuer Verformungsmessungen erörtert, und die Begrenzungen der konventionellen Deutungsformulen werden unterstrichen.


The conventional aim of the flat jack test regards rock deformability and original stress determination. Recently its use in connection to residual strength determination have been proposed (Borsetto, 1980). At the moment the technique appears to be accepted among the set of tests and procedures adopted by ENEL(Italian National Electricity Board) for the geomechanical characterization of rock masses (Dolcetta, 1982). The positive results obtained by designers of the Edolo underground power station, who first adopted the methodology on the field (Forzano et al., 1980), have motivated the Direction of Construction (DCO) and the Direction of Research (DSR) of ENEL to promote a research project on the subject. The activity, carried on at ISMES, is currently intended to improve the theoretical aspects of the interpretation and the experimental methodologies. While it is clear that these two aspects should never be thought undipendent, this paper will mainly be confined on the first as, at the moment of writing, some experimental details are under evolution. Strength characteristics of rock masses are very important for the design of underground structures as they heavily condition upon the safety against collapse, the deformation development and the interaction with stabilization structures. The more promising approach to their determination is the direct observation of the behaviour of ancillary openings or experimental tunnels. Usually displacements are monitored and back analysis procedures are used in order to identify the strength parameters. The principal shortcoming of this approach, alone, is that one has to rely upon a preventive determination of many parameters and he still must handle more than one free parameter, consequently a broad variability field for the parameters to be identified is found. The identification of some strength parameters from the stress measurements is intended to reduce this uncertainty. During the setting up of a reliable interpretation model of the flat jack test for the afore-mentioned purpose, some attentions have been paied to its conventional uses. Some limitations of the common interpretation methods, as those of IRSM Suggested Standards, have been found.


In the following some geomechanical aspects of the context where the test is performed and some definitions on what one would measure are discussed. This provided the guideline for the development of a model of the test and, at the same time, it made clear the actual simplifications one had to accept in order to resort to a conceptually and mathematically treatable matter.

2.1 Undisturbed state

The stress state is denoted by σ0, a six component tensor, which varies from point to point into the rock mass subjected to equilibrium requirements and to boundary conditions. It is assumed to be a continuous function of the position, provided that a sufficiently large observation scale is considered. It is also assumed that this scale is appropriate for describing the behaviour of the rock mass trough the subsequent reference states.

2.2 Opening excavation

A tunnel is usually driven up to the location where the test has to be executed. It is assumed that the excavation of the experimental section is performed under controlled conditions in order to avoid unnecessary disturbance of the rock. The excavation changes the strain-stress state of the rock mass: this new state will be denoted by σ1 (P), ε1(P) assuming that reference is made when gross rheological phenomena, are exhausted - or at least are so slow to be negligible while the test is executed.

This content is only available via PDF.
You can access this article if you purchase or spend a download.