Architecture and management of a geological repository - Andra

The rate of operation of these sensors isvery high even after several decades. Ofthe one thousand sensors that wereembedded in the Daniel Johnson dam inCanada, 90% are still in service after40 years of operation. The same rate of90% has been achieved for the630 sensors embedded in the linings andsegments in the Channel tunnel for the lasttwenty years.In rock or backfilling material,deformation is spread over greaterdistances. It is therefore measured by longbase extensometers, fitted with severalsensors and located in bore-holes or inbackfill material.Figure 10.3.1 Illustration of a vibrating wireextensometer, attached to a reinforcement andintegrated in the concrete structureThese extensometers, whose measurement base can extend to about one hundred metres, provide, atseveral points throughout the bore-hole (in a given direction), the relative displacement of these pointsin relation to each other and in relation to the bottom of the bore-hole. A large number of tunnels anddams have been fitted with bore-hole or back-fill extensometers for over thirty years. Also, vibratingwire extensometers and long base extensometers of different types have been used in a future EPRfeasibility demonstrator at the scale of 1/3 where they withstood pressure cycles of 0.4 Mpa andthermal cycles of from 5°C to approximately 100°C. Despite these difficult conditions, the rate offailure of the sensors remained limited and the measurements obtained in this way can be exploitedwithout difficulty.The overall displacement of a structure along three axes canbe monitored by integrating vertical pendulums into thestructure. Measurement is carried out on a measurementtable located at one end of the pendulum (cf.Figure 10.3.2).The prestressed concrete confinement enclosures in nuclearpower plants are thus monitored using vertical pendulums,capable of providing overall lateral and vertical deformationmeasurements, integrated along the length of the pendulumor in sections between two measurement tables. Thisprinciple is also used to monitor displacement in concretedams (movements downstream, to the banks or swelling)and to monitor displacement in shafts (deformation incertain more deformable layers for example).The displacement measurements only provide relativedisplacements in relation to a point that is rarely fixed(bottom of the extensometer bore-hole, or pendulumattachment point). This disadvantage can in certain cases berectified by including a topographic survey as a complementto the system of sensor or pendulum monitoring. However,topographic measurements are often too imprecisecompared with the displacement variations measured bysensors.Figure 10.3.2 Measuring table for avertical pendulumDOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM395/495