ABSTRACT

The state of stress in a rock mass is an important piece of information for the investigation of High-Level Waste management disposal in deep geological formations. Andra has conducted a large in-situ stress measurement campaign in several boreholes located around the site chosen for the construction of their Underground research laboratory. Reliable data have been collected and have highlighted the high dependency of the state of stress on the formation rheology. As a result of the campaign, the maximum horizontal stress has been estimated to be close to the lithostatic pressure in the host rock a clay formation - at the main level (-490 m depth). This result has important consequences both for the geotechnical design of the galleries of the site and for the long term safety aspect of the site. The role of formation rheology on the state of stress is confirmed by other results like the high ratio - between 1.5 and 2.0 - of the two horizontal stresses in the limestones that sandwich the host rock.

INTRODUCTION

Andra (Agence nationale pour la gestion des dechets radioactifs) is managing in France a feasibility study for storing radioactive high-level waste in a deep hard clay formation. An underground research laboratory (URL) is currently being constructed at the site of Bure (Meuse/Haute-Marne) in North-Eastern France to investigate the adequacy of a 130 m thick layer of clay, which lies between two carbonate units at a depth of about 420–550 m. Formation in situ stresses are part of the parameters to be determined with precision, not only as boundary conditions for rock engineering, but also for long-term safety analyses.

A significant geomechanical program was launched for determining the in situ stresses in the three layers both at the URL location and in neighbouring sites. In total, two separated main phases have been conducted in five different boreholes. The first campaign was carried out in summer 2000, before the construction of the URL shafts. Measurements were gathered from a preliminary borehole (EST205), drilled in the axis of the planned auxiliary shaft. The results, in Particular the contrasts of minimum stress magnitude between layers, led Andra to start a second in situ stress campaign in several boreholes of various orientations (EST312, EST21 0, EST209, and EST361), to complete the understanding on the state of stress in the carbonates/clay/carbonates sequence.

This paper describes a methodology, which combines hydraulic testing with borehole imaging, which has revealed efficient for evaluating the complete stress field in this stratified formation. The results collected from these in situ stress campaigns are commented, and conclusions on the state of stress in the area is given.

INTERPRETATION METHODOLOGY

The micro-hydraulic fracturing technique has been used for decades to estimate the value of the minimum principal stress. It relies on the fact that a hydraulic fracture will eventually propagate normal to the minimum principal stress, and that the pressure at which the fracture closes, deemed closure stress, is thus an estimate of the minimum principal stress (Jaeger & Cook, 1976).

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