ABSTRACT

Stiffness controlled test procedures performed with servo-controlled shear systems are the most versatile for simulating the wide range of boundary conditions expected in nature. Examples of a function controlled procedure in addition to dilatation controlled procedures are discussed for different characteristic behaviours.

ZUSAMMENFASSUNG

Steifigkeitskontrollierte Scherversuche mit servohydraulischer Steuerung eignen sich gut unterschiedliche Bettungen, wie auch in der Natur vorkommen, zu simulieren. Beispiele fuer spezielle Steuerungsfunktionen und dilatationsgesteuerte Prozeduren mit verschiedenen charakteristischen Eigenschaften werden diskutiert.

RESUMÉ

La rigidite a commande des methodes d'essai executees avec les systèmes asservis de cisaillement sont la plus souple pour simuler l'eventail d'etats de frontière prevus en nature. Des exemples d'un procede commande par fonction en plus des procedures commandees par dilatation sont discutes pour differents comportements caracteristiques.

Introduction

Characterizing the shear strength and behaviour of both intact rock and joints is an important aspect of rock engineering. One of the most important aspects of a proper characterization is that the boundary conditions most representative of the in situ conditions are applied to the sample.

In order to simulate the range of boundary conditions possible in nature, stiffness controlled test procedures performed with a servo controlled shear system are the most versatile. If the boundary conditions are changed the same rock mass may exhibit different modes of failure, and thus deformability. The same is true for laboratory conditions. If the control mode is changed from a load controlled procedure to a stiffness controlled procedure a different strength and deformation envelope would result. The mobilized strength of both intact rock and joints (thus the rock mass) is not an independent parameter. The stress-strain behaviour results from a complex interaction between the breakdown of the cohesion (or tensile strength), dilation, the rate at which friction is mobilized, and the stiffness of the material surrounding the region of interest. The influence of two of the above mentioned factors can be investigated using stiffness controlled shear tests, the stiffness of the surround rock mass and the dilatational characteristics of a given sample.

To demonstrate these characteristics the results of several stiffness controlled tests will be discussed.

Shear System Description

The shear system used to perform the discussed tests is a digitally controlled servo-hydraulic test system with the capability to use programmable control processes, as well as acquire measurements of the displacements and forces with high accuracy. The normal load capacity is 500 kN while the maximum shear force is 250 kN. The maximum shear displacement is +/−50 mm. The shear box is 20 cm by 20 cm. This system provides considerable flexibility for evaluating the shear behaviour of intact and jointed rock specimens (Bluemel and Bezat 1998). Figure 1 shows the direct shear assemble which is positioned inside a load frame.

(Figure in full paper)

Function Controlled Shear Test Procedure

In order to investigate the influence of the joint angle and roughness on removable blocks for tunnelling a function controlled test procedure was developed (Pötsch 2002).

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