The in-situ response of an 8 m3 block of jointed biotitic gneiss to uniaxial and biaxial boundary loading was monitored using two types of borehole gauges; the USBM Borehole Deformation Gauge and the LUT Triaxial Strain Cell. From the displacements and strains recorded by the gauges as loads were applied, point wise stresses were calculated assuming both isotropic and anisotropic rock behaviour. Independent of instrument type, the results obtained are characterized by large variations between the stress magnitudes measured in different parts of the block. The measured stress directions are however in close agreement with the directions of the applied boundary loads.
Le comportement d''un bloc de gneiss à biotite soumis, in-situ, à une charge limite uniaxial et biaxial a été observé en utilisant deux types de jauges pour trous de mines: la jauge USBM de déformation de trous de mines et la cellule LUT de mesure de contrainte triaxiale. A partir des déplacements et déformations enregistrés par les jauges au cours de l''application des charges, on a calculé ponctuellement les contraintes internes subies, en se basant sur la double hypothèse d''un comportement isotropique et anisotropique de la roche. Indépendamment du type d''instrument utilisé, les résultats obtenus se caractérisent par des variations considérables entre les intensités de contrainte mesurées en différents points du bloc. Les directions des Contraintes mesurées sont en corrélation étroite avec l''importance des charges limite appliquées.
Das Verhalten eines 8 m3 zerklüfteten Biotitgneis-Blockes vor Ort bei uniaxialer und biaxialer Grenzzonenbelastung wurde mit zwei Arten von Bohrlock- Messgeräten - dem USBM Bohrloch-Deformationsmesser und der LUT-Messzelle für triaxiale Dehnung - gemessen. Aus den Verschiebungen und Dehnungen, die bei Angringung von Lasten von den Messgeräter ermittelt wurden, berechnete man punktweise Spannungen unter der Annahme von sowohl isotropischem als.auch anisotropischem Gesteinsverhalten. Unabhängig von dem jeweiligen Messgerättyp zeichnen sich die erhaltenen Ergebnisse durch grosse Unterschiede zwischen den in verschiedenen Teilen des Blockes festgestellten Spannungsgrössen aus. Die gemessenen Stressrichtungen stimmen gut mit den Richtungen der angebrachten Grenzzonenbelastungen überein.
Block tests have recently become an important method for determining the mechanical characteristics of rock masses for modeling of planned nuclear waste storage repositories. An important advantage that block tests have over laboratory and borehole tests is that a relatively large, in-situ volume of rock is placed under known boundary conditions. This allows comparisons to be made between a known perturbance at the rock mass scale with a measured disturbance at a localized scale. This testing strategy well applies to study the complex problem of stress distributions in discontinuous rock masses. A series of tests of this category have recently been completed in a load controlled two meter cube of Precambrian gneiss. The primary objective of the tests was to study the distribution of stress in the jointed rock mass constituting the block, when subjecting it to known boundary loadings. Two types of instrument, both based on point deformation measurements in boreholes, were used to monitor the response of the block to loading. One was the USBM Borehole Deformation Gauge (BDG), and the other was the triaxial strain cell used by the Lulea University of Technology (LUT-Gauge). An additional test objective was to compare the results produced by these two instruments. 2 TEST BLOCK DESCRIPTION The testing took place at the Colorado School of Mines'' Experimental Mine in Idaho Springs, Colorado, USA. The block itself is located in the floor of the mine in an underground research laboratory developed for the Office of Crystalline Repository Development (OCRD). The surface of the block is at floor grade while the bottom is continuous with the surrounding rock mass. To minimize the effect of these conditions it was decided that all measurements be made at the block midplane. 2.5 meter deep vertical slots on the four sides define the block perimeter. In each vertical Slot, grouted flat jacks existed providing a means of applying normal stresses of magnitudes up to about 5 MPa to the block boundaries. The rock comprising the block contains fractures on several scales. Three major fractures were evident on the block''s surface as shown in Figure 1. A number of vertical boreholes were drilled in the block as part of an earlier study (Hardin et al., 1983). Both EX (38 mm) and NX (76 mm) boreholes were drilled that provide access to the block interior. Their locations and designations are shown in Figure 1. Additional fracture data was available from TV-logs of these boreholes (Sour, 1985) indicating that the major fractures dip vertically and are continuous throughout the test block. Also evident from these logs were randomly dispersed smaller fractures. 3 DETERMINATION OF MECHANICAL PROPERTIES For each gauge location, detailed definition of the rock deformability was necessary. Unfortunately the cores from the block drill holes were unavailable for those tests.
