Working at France’s experimental underground laboratory, near the northeastern village of Bure, has presented opportunities to meet several challenges over a number of years. One such challenge recently led Fugro to design an innovative tool to monitor the evolution of radial stress applied around a borehole casing as a result of the convergence of the surrounding rock.
French national radioactive waste management agency, Andra, is studying the feasibility of disposing high-level radioactive waste (HLW) in the deep geological repositories of the underground laboratory. Built 500 metres below the ground, the host formation consists of a claystone (Callovo-Oxfordian argillaceous rock).
The HLW cells consist of horizontal microtunnels, at least 80 metres long with a diameter of almost 0.7 metre. They are lined with a non-alloy steel casing to permit retrieval of the waste packages if necessary (French law requires that storage is reversible for at least 100 years).
In situ experiments enabled analysis of the mechanisms involved in the casing/rock interface evolution and their influence on the mechanical behaviour of the casing. To monitor the evolution of the radial stress applied around a borehole casing due to the convergence of the surrounding rock, Fugro designed an innovative tool. Three tactile pressure sensors were wrapped around the tubing at three different depths in a 10 metre-long borehole.
This innovative equipment based upon the tactile pressure sensor has led to highly accurate monitoring of the convergence of a cased borehole and the induced mechanical load. Successful results led to the installation of sensors in a second borehole.