One of the GRC’s main areas of research interest has been the development of an improved understanding of transport processes in soils. Funded in the early days by EPSRC to carry out the underlying research that formed the basis for all subsequent work, the GRC has been heavily involved in addressing the internationally important subject of the “Geological Disposal of High Level Nuclear Waste”. Improved understanding of complex inter-related flow processes were required, focusing on coupled Thermal, Hydraulic and Mechanical (THM) behaviour. Further advances to include chemical/geochemical effects in the coupled formulations were then considered, giving rise to the study of so-called THMC problems. Further developments now being contemplated regarding the inclusion of microbiological influences (THMCB).
To ensure the widespread sharing of the results of this basic scientific work, the GRC has incorporated these advances in a novel geoenvironmental software model (COMPASS). This model provides the ability to analyse coupled heat, moisture (vapour and liquid), air pressure, deformation and stress behaviour. Variations in both spatial (3D) and temporal domains are accommodated. New numerical algorithms have been conceived and novel methods have been developed in order to achieve numerical stability. Further extensions which include both chemical effects and gas flows have recently been incorporated.
COMPASS is based on a theoretical formulation that can be described as a mechanistic approach. The various mechanisms of behaviour are included in an additive manner with inter-related couplings being accommodated. The main features of the behaviour addressed are coupled:
- Heat transfer: conduction, convection and latent heat of vapourisation.
- Liquid transfer: due to pressure and gravitational gradients, and chemical and thermal osmosis.
- Vapour transfer: due to vapour pressure gradients, thermal effects and chemical osmosis.
- Gas (air) transfer: bulk flow of dry air from pressure gradients and dissolved gas.
- Multicomponent transport of chemical species: by advection, dispersion and diffusion in both liquid and gas phases.
- Geochemical reactions: due to heterogeneous and homogenous reactions between/in solid, liquid and gas phases and under both equilibrium and kinetic conditions.
- Mechanical/deformation behaviour: Considering both elastic and elasto-plastic behaviour.
The highly coupled nature of the formulation requires a simultaneous solution of the governing equations. Furthermore, the non-linearity of the problem requires an iterative technique to achieve a converged solution. This solution requires two stages of discretisation:
- Spatial discretisation: finite element method via the Galerkin weighted residual method.
- Temporal discretisation: finite difference method via an implicit mid-interval backward difference algorithm. A sequential (time-splitting) approach is employed to solve the chemical transport and reaction equations.
The computational engine comprises modern, well-documented and well-maintained code in modular format with the following capabilities:
- Extensive library of finite elements, allowing 1D, 2D and 3D geometries to be used.
- Wide range of boundary conditions, including Dirichlet and Neumann conditions and applied load.
- A number of solvers, including direct solvers and indirect solvers and pre-conditioners.
- An advanced High Performance Computation (HPC) version using parallel computing architecture.
A pre- and post processing package (GID) is linked to the code. This allows mesh generation; the definition of initial and boundary conditions; the definition of material parameters; and the processing of the results in a user friendly framework.
Some Typical Examples
Extensive use of COMPASS over a period of years has served to illustrate the robustness of the numerical scheme. The verification programme has consisted of an extensive series of checks against analytical solutions, single element calculations, and numerical solutions from other independent computer codes. The validation program has also consisted of an exhaustive series of checks, this time against well controlled experimental work. The code has been developed and applied to study various aspects relating to the geological disposal of high level radioactive waste, under several international projects and in collaboration with numerous organisations.