Architecture and management of a geological repository - Andra

Dams are of particular interest when designing the monitoring of a repository. Their inspection andmonitoring has indeed been mandatory for several decades. The improvement of knowledge ofmonitoring equipment, and the large increase in the number of dams in France between 1930 and1970, led in 1970 and 1983 to a change to the 1927 circular. Regulations thus require continuousmonitoring of the entire structure and its foundation during its filling, regular monitoring (on a weeklybasis) by the operator, an annual inspection and a complete inspection every ten years by thegovernment inspection department [99]. On certain dams, the sensors have thus been supplyingmeasurements for several decades. Statistical analysis tools of measurement have also been developedover the last twenty years or so in order to assist the operator in detecting any slow changes liable toalter the behaviour of the structure [100] (for example swelling of the concrete or damage to the claycore’s seal). The implementation of similar monitoring policies in many countries has made it possibleto reduce the worldwide rupture rate of 2.2 % for dams built before 1950 to 0.5 % for those builtbetween 1951 and 1986 [101].In addition, the problem of ensuring an operational life of at least a hundred years for major structures(tall buildings, bridges or viaducts) has led to the introduction of monitoring of the behaviour of thesestructures and their foundations. The purpose of this monitoring is to measure the gradual strains ofthe structure, detect any delayed settling of the foundations, that may be damaging to the structure, orto assess the impact of work carried out nearby (as, for example, in the case of the monitoring of theMinistry of Finance in Paris during the construction of the new “Metro” line N°14). In this respect, themetal foundation piles of certain of these structures are the subject of special monitoring.As a reminder, the experiments conducted in underground research laboratories provide feedback thatis somewhat different from that provided by civil engineering. These installations, dedicated toscientific research, have the advantage of using monitoring equipment that is specific to the rocksstudied for deep disposal and specific to the particular phenomena associated with disposal. Theknowledge base comes principally from laboratories on clay sites: at Mol in Belgium [102], MontTerri in Switzerland and the Meuse/Haute-Marne in France. However, certain experiments conductedin laboratories on granite sites also provide interesting feedback like, for example, sealing experiments(named TSX) in Canada [67] or back-filling experiments at Äspö in Sweden [103]. As part of theexperiments conducted in these laboratories, research was also carried out on measuring instruments,for example by comparing several redundant technologies or by exploring innovative approaches.10.3.5.2 Lessons learnt from feedback for the design of a monitoring systemThe “standard professional practices” that have developed based on feedback from observation andsurveillance of numerous civil engineering structures make it possible to establish a series of goodpractices.The first rule is redundancy of equipment. This redundancy enables a check to be made of theconsistency of measurements made by different instruments. It also makes it possible to limit the lossof data when a sensor is defective and cannot be replaced. In inaccessible structures, severalmeasurements of the same parameter can be made, either using different technologies or by doublingup the same type of sensor. In accessible structures, it is possible to combine a visual inspection withgeometrical monitoring carried out using topographical equipment, and with measurements obtainedusing sensors buried in the structure. These methods are supplemented as required by taking andanalysing solid or fluid samples. This procedure is used, notably, for dams, rail tunnels and nuclearpower station containment envelopes. Another means of contributing to redundancy consists ofsupplementing a spread of point measurements with an overall measurement on a larger scale (forexample local thermal expansion strain and overall deformation).The choice of monitoring equipment and the quality of their implementation are also very importantelements in developing an appropriate monitoring system, that is reliable over the long-term, andyields useable data [32]. The sensors must be specially selected according to the magnitude of theparameter to be measured and the required accuracy. Moreover, incorrect implementation of a sensormay lead either to its loss or to measurements with high interference levels.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM393/495