In fractured crystalline rock masses, connected fractures dominate groundwater flow contaminat transport. Therefore, investigating fractured network flow and transport under different stress conditions is of high significance to perfromance assessment in such geologic media. To address these issues, Task C of the Decovalex-2011 project was established and it involved two numerical modelling problems:
a Test Case: a basically hydrogeological problem at the site of the Bedrichov Tunnel in the Czech Republic (Fig. 5), defined as a three-dimensional Test Case involving characterisation and parameterisation of the hydrogeological features of the tunnel site, and modelling the fluid flow and tracer transport processes, with the challenging tasks of data collection, model set-up, parameterisation, and uncertainty evaluation;
a generic Bench Mark Test (BMT) problem of coupled HMC processes in a discrete fracture network model defined in two dimensions, considering the stress effect on fluid flow and particle transport with different retardation mechanisms (advection and matrix diffusion), with challenges relating to the modelling of the fundamental behaviour of interactions between stress and transport processes, and representation and characterisation of a system of 7797 fractures of different lengths and hydraulic apertures (Fig. 6).
Two research teams studied the Bedrichov Tunnel Test Case: Technical University of Liberec (TUL), sponsored by SURAO, Czech Republic; and Imperial College and SERCO Ltd., UK, sponsored by RWM, UK. The 2D BMT problem was studied by five research teams: Institute of Rock and Soil Mechanics, CAS, China; Technical University of Liberec (TUL), sponsored by SURAO, Czech Republic; Lawrence Berkeley National Laboratory (LBNL), USA, sponsored by RWM, UK; Imperial College of London (IC) and Serco Ltd., UK, sponsored by RWM, UK, and the Royal Institute of Technology (KTH), sponsored by SKB, Sweden.
Due to the difficulties in data collecting and the need for on-going site investigation for data identification, the development and modelling of the 3D TC at the Bedrichov Tunnel site could not be completed, despite the progressive achievements that were made. Even so, the research performed made its continuation to the follow up in the next phase of Decovalex –the 2015 project.
Main achievements and outstanding issues
The model development and initial modelling results of the 3D Bedrichov Tunnel Test Case illustrated the importance of understanding, handling and predicting the impact of uncertainties for the site characterisation and reliability of predictive modelling even for apparently ‘simple’ hydrogeological cases, which are in reality the corner stones of understanding and modelling the groundwater flow and solute transport processes for water resources and performance/safety assessments of repositories. The results from the initial modelling of the Bedrichov Tunnel Test case demonstrate that the subject of identification and management of uncertainties for site characterisation has not been understood in proper depth and investigated with adequate achievements and confidence. The research on the 2D BMT successfully investigated the interactions of stress, fluid flow and solute/particle transport in a complex fracture network, considering advective and matrix diffusion mechanisms, and served as a good demonstration of the importance of the stress effect on the flow and transport in fracture systems.
The major outstanding issues related to research for stress, flow and transport of fracture systems were identified through Task C, as the identification and management of the uncertainties in the input data concerning: fracture system geometry, fracture properties: hydraulic and mechanical as required, reliability of the in-situ measured data, development of a systematic approach for identification, quantification and monitoring of the sources and evolution of the uncertainties with the progress of modelling steps, and how to use the current numerical tools for investigating uncertainty issues.
Besides the final report of Task C, two extra research papers were published in the Journal of Rock Mechanics and Geotechnical Engineering, Elsevier, as listed below.
J. Rutqvist, C. Leung, A. Hoch, Y. Wang, Z. Wang, Linked multicontinuum and crack tensor approach for modeling of coupled geomechanics, fluid flow and transport in fractured rock, Journal of Rock Mechanics and Geotechnical Engineering, Volume 5, Issue 1, February 2013, Pages 18-31
Z. Zhao, J. Rutqvist, C. Leung, M. Hokr, Q. Liu, I. Neretnieks, A. Hoch, J. Havlíček, Y. Wang, Z. Wang, Y. Wu, R. Zimmerman, Impact of stress on solute transport in a fracture network: A comparison study, Journal of Rock Mechanics and Geotechnical Engineering, Volume 5, Issue 2, April 2013, Pages 110-123