Architecture and management of a geological repository - Andra

Cell atmosphere monitoring is limited to one temperature measurement near the liner. A vibrating wiresensor is embedded in the filling concrete, the sensitive part protrudes inside the repository chamber.Other characteristic data for this atmosphere, i.e. hygrometry and possible hydrogen concentration, arecollected in the ventilation air leaving the cell. In fact, these parameters are difficult to measure in-situwithout having access to the sensors to carry out maintenance on them (see below).The monitoring of the lining can be carried out by a set of vibrating wire extensometers, a controlcompensating extensometer and two vibrating wire temperature sensors. All these sensors are installedin the lining compression ring, using a reinforcing cage (of approximately one metre in length), beforeconcreting. They are therefore integral with the concrete once it has set.The vibrating wire extensometers measure local deformations in all three directions. Deformation ismeasured in the center of the liner thickness, except for tangential deformation which is usuallymeasured at the intrados and the extrados. In addition, redundant measurements will be madesystematically. The control compensating extensometer, also set in the concrete, is dissociated fromthe strength ring so that it is not subject to stresses due to the pressure of the ground. It is thereforeused for the precise measurement of deformation due to the contraction of the concrete during settingand to the temperature variations or saturation rates in the immediate vicinity of the otherextensometers. It is then possible to calculate the deformation specific to the ground pressure in allthree directions.Each extensometer also provides a temperature measurement so that the acquisition system can correctthe raw measurement of "parasite" deformation from the sensor due to temperature variations. Thesemeasurements are complemented by two temperature probes placed at the intrados and the extrados todetermine the radial thermal gradient, and from there, to calculate the heat flow transferred to the rockfrom the package or to the ventilated air.The monitoring of the near-field rock can be carried out by a set of sensors made up of (i) amultisensor inductive core extensometer fitted in a short bore-hole made perpendicular to the cell'saxis, (ii) a fiber optic allowing distributed measurement of the temperature in the same bore-hole, and(iii) two interstitial pressure sensorsThe multisensor extensometer provides the deferred displacement of the rock in the radial direction.These measurements can be correlated with the deformation in the lining caused by the groundpressure. The length of the bore-hole is limited to a length equivalent to that of the short rock bolts, soas to comply with the long-term safety functions. Consequently, the extensometer anchor point at thebottom of the bore-hole is not fixed, and the displacements measured are relative displacements ofpoints in the rock distant to each other on the order of one metre.Rock temperature monitoring complements the radial distribution measurement of the temperaturefield in the atmosphere and in the lining. Variations in the thermal gradient may make it possible toconfirm the monitoring of the gradual desaturation of the rock in the walls, with which it is correlated.The monitoring of the interstitial pressure allows us to monitor rock drainage in the walls and to detecta change in the desaturated state. The sensors are positioned near to the cell wall and at the end of thebore-hole, so as to detect the moment when the rock starts to desaturate, then the radial progression ofthe desaturation front in the microfissured zone.• Distributed measurement fiber optic systemsFiber optic systems embedded in concrete all along the structure (one at the intrados, one at theextrados and one in the filling concrete) can enable us to monitor the axial dimension of the thermalfield. One of the fibers can be extended to the end of the bore-hole at the bottom of the cell so as tocomplement the thermal field monitoring in the near-field.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM405/495