In the Mont Terri rock laboratory, nearly 20 years of experience with long-term monitoring could be gained. Long-term monitoring under harsh conditions, such as enhanced pressure, temperature and corrosive chemical environment is one of the key issues of the Mont Terri Project research program. Two experiments specifically focusing on monitoring are presented. Numerous long-term pressure time series are available, from overpressure systems, where the pressure transducer is installed at the borehole head. Long-term stability of pressure transducers proved to be very good. The measurement of temperature is regarded as a simple parameter to be measured; however, long-term experience with conventional temperature sensors has revealed their vulnerabilities. Long-term performance of different fiber optical sensors is evaluated in a further borehole. The three-year dataset clearly highlights the importance of independent conventional parameter control and the possibility for later recalibration.
Since 1996, numerous experiments have been conducted in the Mont Terri rock laboratory, located in the northwestern part of Switzerland. Monitoring and especially long-term monitoring of different parameters under harsh conditions in argillaceous rocks with highly saline pore waters, e.g. up to 20,000 mg/l of solutes, Cl-/Br- -ratios similar to recent sea water, anaerobic conditions, negative redox potential, and subjected to increased pressure and temperature conditions is one of the key issues of the research conducted in the Mont Terri rock laboratory. The research on monitoring is closely linked to the Swiss concept for a HLW repository, which foresees a Pilot repository located close to the main repository, where a well-defined part of HLW can be monitored over decades in the early phase of repository evolution. Experience about the behavior and long-term performance of different kinds of sensors could be gained from numerous experiments. Common parameters measured in the 130 experiments at Mont Terri rock laboratory, of which 46 are currently running, are pore water pressure, total pressure, temperature, humidity, deformation, climatic data, geochemical parameters, such as pH, Eh, electric conductivities, corrosion rates, gas concentrations, seismic and geo-electric signals. There are several experiments yielding datasets of up to nearly 20 years duration. From these experiments three examples, where monitoring and data evaluation including interpretation are of great importance are presented and discussed. The results and experience from numerous additional experiments are finally summarized, building the basis for future instrumentations of underground facilities.