The development of the excavation disturbed and excavation damage zone (EDZ) around deposition tunnels and holes is a target of attention in the development of radioactive waste disposal concepts. Aiming at understanding the formation of an EDZ is not required per se from a stability perspective, but mostly motivated by the fact that the formation of an EDZ will change the hydraulic properties of the rock mass in the vicinity of the excavations. Accordingly, there is a potential for increased fluid pathways parallel to the excavations. These induced pathways may act as transport channels for radionuclides. As the excavations might form a continuous network connected to the biosphere, the EDZ may also form a pathway to the surface.
The formation of the EDZ and the associated change of rock mass permeability has been a continuous topic of research, not exclusive but prominently, in former DECOVALEX tasks (Liu et al. 2011; Wang et al. 2011; Rutqvist et al. 2009a; Rutqvist et al. 2009b; Min et al. 2005; Öhman et al. 2005; and many more). Within these studies many individual aspects have been covered, and also sequences of boundary condition changes have been analysed. However, the impact of the evolution of a repository for spent nuclear fuel on the EDZ development and the related transmissivity change has not been fully analysed. Further, it is unclear how rock mass transmissivity should be measured during the construction and operational phases and how these measurement relate to the conditions after closure of a repository.
The aim of this task is to simulate the evolution of hydraulic transmissivity and conductivity throughout the lifetime of a repository for spent nuclear fuel in (sparsely) fractured competent rock mass. In the course of the simulations not only shall the transmissivity change be studied and the best approaches to simulate the change be identified, but also strategies shall be developed on how to monitor the changes. The simulation history shall include the repository excavation phase, backfilling, thermal phase and the effect of a glacial cycle.
The task will require benchmarking the simulations against measured field data to validate the initial transmissivities and permeabilities, developing an integrated concept to be able to numerically simulate the evolution of the EDZ and related transmissivity changes, and identifying possible measurement concepts that reflect the lessons learned from the simulations.
The simulations shall be validated against field data (field reference case). These are taken from the TAS04 experiment of SKB (Ericsson et al. 2014; Ittner et al. 2014; Ittner et al. 2015). The figure below shows an overview to the data.
SKB has set up the TAS04 experiment aiming at defining and developing standards, strategies and methods needed to design and construction, and gathering sufficient specifications to procure underground construction works of the planned repository for spent nuclear fuel. One part of the project has focused on verifying the extension of the EDZ. This included geometrical, geological, geophysical and hydrogeological investigations and blasting design documentations (Ericcson et al. 2014).
The hydraulic properties in water-saturated conditions under the tunnel floor were evaluated. Based on the conducted investigations it has been possible to carry out hydraulic modelling that takes into account a more or less developed EDZ (Ericcson et al. 2014).
The data will be used in Task G as a basis to describe the discrete fracture network (DFN) and to analyse the initial transmissivity. The reported inflow and transmissivity data will be mirrored by the numerical simulations as a result of the included representation of the DFN. Based on this starting knowledge, the evolution of DFN and related transmissivity could be analysed under different conditions.
Task G will be carried out by four teams performing numerical simulations in three main steps. The first step will consist of code validation; the second step will involve inverse simulations of the TAS04 experiment and; the final step will conduct forward simulations of the transmissivity evolution.
In a supporting side-line, the experimental data will be analysed and reinterpreted so to support that the numerical simulations of the complex TAS04 experiment as accurately and comprehensively as possible.
In a second side-line, transmissivity and permeability of the rock mass will be analysed by means of upscaling approaches and equivalent models where numerical simulations of the DFN and its evolution will be complemented with phenomenological and statistical analyses.
For further information, please contact the task leader, Tobias Backers.