Evaluation of coupled hydro-mechanical (H-M) behaviour of in situ shaft sealing components for used nuclear fuel

This thesis examines hydraulic-mechanical performance of a full-scale 5-m diameter by 12-m long shaft seal installed at Canada’s Underground Research Laboratory (URL). This installation spans a major water-bearing fracture zone located at a depth of 275 m and is intended to isolate two hydrogeochemcial regimes. Performance of the hydraulic-mechanical behaviour of the shaft seal was examined using a Finite Element Method (FEM) program, CODE_BRIGHT (Vaunat and Olivella 2002). Coupled H-M constitutive models were developed using CODE_BRIGHT (Vaunat and Olivella 2002). An elastoplastic constitutive model was used to describe the behaviour of the unsaturated soil. Hydraulic-mechanical parameters of the shaft seal components were determined using laboratory test data. By matching van Genuchten’s model (van Genuchten 1980) to the experimental laboratory data, hydraulic parameters of the clay component were determined. The relationship between unsaturated hydraulic conductivity and degree of saturation for the clay component was developed and used in this numerical modelling. The Basic Barcelona Model (BBM) proposed by Alsonso et al. (1990), an elastoplastic constitutive model, was used and the BBM’s mechanical parameters of the clay component were determined using laboratory test data. Three numerical models were simulated. The first model assumed fracture zone 2 (FZ2) to be horizontal, the second model assumed FZ2 to be inclined 16 degrees from the horizontal plane and the last model applied seepage of groundwater into the system based on the first model. ii The numerical results were compared with field readings regarding water uptake and development of total pressure in the system. Numerical prediction from the three models showed that the hydration process of the clay seal was dominantly affected by the groundwater flow from FZ2. Overall, numerical prediction showed reasonably good agreement with field readings; however, it underestimated evolution of total stresses. The possible reason could be that the simplified groundwater flow system did not take into account radial groundwater flow toward the perimeter of the clay seal. Prediction models of swelling pressure and swelling pressure estimates of the unsaturated clay seal have been discussed. The first model using crystalline and osmotic swelling mechanisms estimated a swelling pressure of 0.86 MPa, which was close to what was expected (0.8 MPa) in the field. The second model applying confined conditions estimated a swelling pressure of 0.41 MPa. This difference in swelling pressure prediction could be attributed to different microstructures developed under different confining conditions. The model predictions were then verified in terms of the water retention capacity of the unsaturated clay seal. Although the first model showed some deviation, overall predictions using the two models showed reasonably good agreement with the measured data. Sensitivity analyses were conducted to assess effects of variations of H-M parameters on evolution of hydraulic-mechanical behaviour of the sealing components. Several sets of hydraulic and mechanical parameters of the clay seal were determined using available laboratory test data. The numerical results were compared in terms of changes in suction, degree of saturation and hydration time of the sealing components. The numerical prediction indicated that the hydraulic iii behaviour of the clay seal was sensitive to variations of the water retention parameters P0 and . From sensitivity analyses of the BBM mechanical parameters, numerical simulation showed the development of compressive horizontal and vertical stresses. Different sets of the BBM parameters were obtained for the same clay material in this study and they resulted in slightly different predictions of mechanical behaviour of the clay seal. For this reason, sensitivity analysis of the parameters and validation of the predictions were required to improve the possibility to match field and modelled behaviour.