A three-dimensional numerical investigation of the thermo-hydro-mechanical behaviour of a large-scale prototype repository

This thesis describes the modelling of the thermo-hydro-mechanical behaviour of a large-scale experiment, carried out at SKB's underground research laboratory in Aspo, Sweden. The experiment, known as the Prototype Repository Project, was constructed in highly fractured granite rock and is scheduled to last 20 years. Results from the experiment are collected systematically by SKB from the initial rock characterisation to the highly instrumented installed material. The model applied is the thermo-hydro-mechanical model previously developed at the Geoenvironmental Research Centre (GRC). The GRC's current model was extended to successfully accommodate three-dimensional THM behaviour, including the development of a high-performance computing algorithm using both multi-threaded and message-passing programming paradigms to enable simulations to be completed in significantly reduced time. Model simulations have been conducted of both the pre-placement stage of the experiment and the post-placement operational phase. The results of the pre-placement phase have been used to aid the calibration of the simulation and provide confidence in the development of the operational phase simulation. In the pre-placement phase simulation, a pragmatic approach using a combination of an effective continuum model and a number of key discontinuities was employed. A domain of 100 x 100 x 160m was used, discretised into over 550,000 finite-elements. The simulations were able to reproduce three-dimensional highly anisotropic flow conditions shown in the experimental results. The post-placement operational stage was then simulated in three-dimensions using the same rock domain as for the pre-placement analyses, including the buffer material, and discretised into over 920,000 elements. A number of key features, including the anisotropic hydraulic behaviour, were captured. It was concluded that the geological conditions, backfill re-saturation and buffer re-saturation, including the micro-structural effects of the bentonite, are all important to the simulation of a high-level waste repository. Long term simulation results were also presented. A number of aspects were explored using two-dimensional analyses, including the macro/micro- structural interactions of the bentonite buffer. A time-dependant form of the hydraulic conductivity relationship was developed and yielded significantly improved results in long-term analyses. The behaviour of a fracture intersecting a deposition-hole was also investigated highlighting the importance of discrete fractures on hydration behaviour.