Since 2000, the French National Radioactive Waste Management Agency (Andra) has been constructing an Underground Research Laboratory (URL) at Bure (east of the Paris Basin) to perform experiments in order to obtain in situ data necessary to demonstrate the feasibility of geological repository in the Callovo-Oxfordian claystone. Different drift construction methods were tested and the continuous measurements of displacements are available, allowing a better understanding of the time-dependent response of the claystone at natural scale. In this context, an experiment, DPC (preliminary déconfinement before tunnel excavation to the final diameter), was conducted at the URL where a drift was excavated in three steps: digging of the pilot drift, wait times and over excavation to the final diameter. Using the differed behaviour of the claystone, the objective is to observe if the final Excavation Damaged Zone (EDZ) extension evolved in comparison with a drift excavated in one step. The first results are presented and discussed.

1 Introduction

Since the year 2000, the French national radioactive waste management agency (Andra) started to build the Meuse/Haute-Marne underground research laboratory (M/H-M URL) at Bure nearly 300 km East of Paris. Technology demonstration, performance and safety assessment are main objectives of the research program, as a crusader for feasibility studies for a deep geological repository. The host formation consists of a claystone (Callovo-Oxfordian argillaceous rock) lying between 420 m and 550 m in depth, which is overlain and underlain by poorly permeable carbonate formations. The main level of the URL at -490 m is the most representative level of the future disposal in terms of mechanical behaviour. An important experimental program is planned to characterize the response of the rock to different drift construction methods at the main level.

It is of first importance to well understand the impact of the excavation and support methods on behaviour of the claystone around the drift and particularly on the induced fracture formation. Numerous experiments and direct measurements have been performed in the laboratory drifts during their construction. Figure 1 presents the extent of the URL in March 2015. Indeed, the construction of about 150 m of new galleries is planned each year: an overview on the essential of the instrumented galleries in the URL can be found in Armand et al. (2013).

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