Architecture and management of a geological repository - Andra

There are few techniques available to measure stresses in an underground structure directly andcontinuously. Total pressure cells are sometimes used to measure the pressure of the ground on atunnel lining directly, but they are not suitable for use in a repository situation, as they are not reliableover time and they are not easy to use. Often, the pressure of the ground is determined indirectly, frommeasurements of strain in the ground and possibly in the lining. In the same way, the stresses in thelining are determined from the local strain measured in the lining, assuming the lining concrete has aknown elastic behaviour.• Variation of the EDZThe variation of the EDZ is coupledInductive core Coupling ringwith the rate of deformation and therock saturation value. It ismonitored indirectly, by measuringthe rock deformation near to thewalls of the structure and byFigure 10.3.3 Inductive extensometre for boreholesdetecting any desaturated zone nearto the walls.This technique has the advantage of using reliable and proven monitoring equipment, and in particularmulti-sensor inductive extensometers in bore-holes and on interstitial pressure cells. An example of aninductive core (copper rod) and a sensor (opaque coupling ring) is illustrated in Figure 10.3.3.It is also possible to detect the presence and the variations in the EDZ by putting in place acousticmonitoring from the outer wall or microseismic measurement techniques. Several options areavailable, either listening directly for the sound produced when any fractures are created in thedamaged zone, or transmitting a known wave and correlating its transmission properties with thedamage to the rock on the outer wall. In all cases, listening is carried out using geophones oraccelerometers. The microseismic wave propagation measurement technique has been used in theMeuse/Haute Marne laboratory and has proved effective.• Interstitial pressureThe interstitial water pressure is usually measured infoundations, back-fill and in clay cores in all the largeFrench dams, by vibrating wire interstitial pressurecells (Figure 10.3.4). There is no official dataregarding their durability. The experience feedback interms of reliability and longevity gives an 80%operating rate after several decades of operation, suchas for example in the reservoir dam in Marne since1971 or in the oued Mellegue dam in Tunisia since1954. Moreover, this type of instrument can give aminimum indication of the saturation value ofmaterials.Figure 10.3.4 Vibrating wire interstitialpressure cells• Relative humidityThe relative humidity of the air is measured regularly in a number of industrial installations where thisparameter affects the manufacturing process (for example in the food processing industry) and in allmeteorological stations. This measurement is most often carried out by dew-point hygrometers orcapacitive hygrometers. These apparatuses rely upon the dielectric properties of the sensor andmeasure the variation in the relative permittivity as a function of the quantity of water absorbed. Inview of the need for regular maintenance and calibration, their use is reserved for accessible zones.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM396/495