Hydraulic-mechanical (HM) modelling of the saturation of a full-sized concrete and bentonite seal

Document available in extended abstract form only. A full-sized composite shaft seal was designed and installed at the intersection of the 5-m-diameter access shaft with a major inclined fracture zone (FZ) as part of the decommissioning of the Atomic Energy of Canada Limited's (AECL) Underground Research Laboratory (URL). Construction of the shaft seal was funded by Canada's Nuclear Legacy Liabilities Program. Monitoring of the thermal, hydraulic and mechanical (THM) responses for this shaft seal was jointly funded by NWMO (Canada), SKB (Sweden), Posiva Oy (Finland) and ANDRA (France) under the Enhanced Sealing Project (ESP). The performance and behaviour of the shaft seal being monitored is of relevance to repository closure planning and in providing confidence in the functionality of this type of composite shaft seal in a variety of host rock types. The composite shaft seal consists of 6-m-thick clay component that is sandwiched between 3-m-thick upper and lower concrete components. The in situ compacted clay component is a mixture of 40% bentonite and 60% quartz sand. The mid-height of the clay component is located at ∼275 m depth below the surface. The clay component is designed to extend beyond the maximum identified vertical extent of the inclined FZ by at least 1 m. The primary function of the composite seal is to limit the mixing of the deeper, saline groundwater from the nearer-surface, fresh groundwater. The clay component mainly provides the hydraulic sealing component. The concrete components provide mechanical support and confinement to the swelling clay component and also assist in limiting mass transport through the seal. Since the installation of the shaft seal in mid-2009, the thermal, hydraulic and mechanical (THM) evolution of the seal has been constantly monitored. One hundred sensors are installed to measure temperature, total pressure, hydraulic pressure, saturation, strain and displacement at various locations within the shaft seal and nearby host rock. This paper presents the results from hydraulic-mechanical (HM) analyses used to simulate the saturation of the clay component. Of particular interest are the observed differences in the hydraulic pressures above and below the clay component and the development of the swelling pressure throughout the clay component, which provide information on the functionality of the shaft seal. Temperature change was limited to the first 3-month period when curing of the concrete occurred. After this period the temperature in the shaft seal remained relatively constant at 11 to 12 C. Since most of the saturation process occurred under an isothermal condition, the HM analyses alone are suitable for simulating the saturation process. A finite-element software code (COMSOL Multiphysics) is used to analyze 2D-axisymmetric and 3D geometries of the shaft seal. The components in the HM analyses include the clay, concrete, intact rock and the FZ. Their HM parameters are defined from laboratory tests and in situ measurements at the URL. The initial pore water pressure and in situ stress state of the rock are also based on their measured evolution at the URL. The results from the preliminary 2D-axisymmetric model differ from the in situ measurements, since the FZ in the model is assumed to be perfectly horizontal with a uniform thickness of 4 m. The monitoring of the ESP shows that the data vary in the axisymmetrical direction depending on the locations of the sensors with respect to the FZ. This highlights the importance of taking into account the inclination and thickness variation of the FZ in numerical modelling, which is done by analyses using a 3D geometry (Figure 2b). The results of the numerical analyses using both 2D-axisymmetric and 3D geometries are compared to the 3-year ESP monitoring data and future evolutions of the HM responses are predicted from the modelling results. The HM results from the numerical analyses are also used to advance the understanding of the mechanisms during the saturation process of the shaft seal and to examine the functionality of the composite shaft seal. (authors)