Hydro-mechanical Modelling of a Shaft Seal in Crystalline and Sedimentary Host Rock Media Using COMSOL

Abstract

Shaft seals are components of the engineered barriers system considered for closure of a Deep Geological Repository (DGR). These seals would be installed in strategic locations of the shafts, where significant fracture zones (FZ) are located and would serve to limit upward flow of groundwater from the repository level towards the surface.

This paper presents the results of hydro-mechanical (HM) numerical modelling exercises to evaluate the performance of a shaft seal using a finite element computer code, COMSOL. This study considered a variety of host geological media as part of generic assessments of system evolution in a variety of environments including five hypothetical sedimentary and crystalline host rock conditions. Four simulations of a shaft seal in different sedimentary rocks were completed, including: (1) shale with isotropic permeability; (2) shale with anisotropic permeability; (3) limestone with isotropic permeability; and (4) limestone with anisotropic permeability. The other simulation was a shaft seal in crystalline rock with isotropic permeability. Two different stages were considered in these HM simulations. Stages 1 and 2 simulated the groundwater flow into an open shaft and after installation of shaft sealing components, respectively.

As expected, the models were able to simulate that installation of the shaft seal limits groundwater flow through the shaft. Based on the conditions and assumptions defined for the host media and fracture features examined in this study, the following conclusions can be drawn from the results of the numerical modelling exercises. A shaft that remained open for a longer time was beneficial with respect to delaying of seal saturation because it could reduce the groundwater flow rate around the fracture zone. Delaying saturation time indicates slower movement of the groundwater or other substances that may be transported with the groundwater. The core of the shaft seal (i.e., the bentonite-sand mixture (BSM)) became fully saturated after 90-100 years if the shaft remained open for 100 years before a seal was installed. Only a slight difference between five cases was observed from the results of the numerical modelling exercises for different cases. This may, in part, be the result of limitations in the knowledge regarding the HM characteristics of the geological media evaluated. Defining of more site specific conditions (e.g., depth and geometry of fracture, hydraulic properties of rock and fracture feature, and mechanical characteristics of rock) was recommended in order to more effectively simulate HM behaviour of a shaft seal at the location of a fracture zone.