Problems and research performed

The international FEBEX (Full-scale Engineered Barriers Experiment in Crystalline Host Rock) project, co-financed by ENRESA and the European Commission was in operation from 1994 to 2003. The purpose of the project was the study of the various processes occurring in the near field of a high activity radioactive waste storage site. The experiment was installed at the Grimsel Test Site, an underground laboratory operated by NAGRA, Switzerland, based on the Spanish reference concept of deep geological storage in crystalline host rock, with a multiple Engineered Barrier System (EBS), supported by detailed information on geological and hydrogeological characterization of the Grimsel Test Site, the comprehensive characterization of the bentonite used to fabricate the engineered barrier, and the monitoring performed during the drilling of the FEBEX tunnel, as well as during the test. Figure 1 shows a perspective of the FEBEX drift and associated boreholes.

Figure 1: Layout of the FEBEX in-situ test and associated boreholes.

The modelling work was divided into three parts: Part A – hydro-mechanical modelling of the Grimsel granite host rock; Part B – thermo-hydro-mechanical modelling of the bentonite behaviour; and Part C – thermo-hydro-mechanical modelling of the Grimsel granite rock. A cyclic blind numerical prediction and model calibration with measured data approach was executed through the modelling process for the three parts.

Figure 2: a) Measured and predicted evolutions of water pressure in monitoring borehole SF21 at point 1 (r = 3.03m) and point 3 (r = 13.06m); b) measured and predicted evolutions of total stress components in monitoring borehole SG2 at points 1, 2 and 3 (r = 3.0 m).

Achievements and outstanding issues

The FEBEX test was one of the few large-scale tests available two decades ago to gain an integrated perspective of the behaviour of current concepts for nuclear waste disposal in crystalline rock and provided valuable measured data supports for verifying, validating and studying the capabilities of a number of numerical codes held by the DECOVALEX III project teams to handle coupled problems in geological and porous media. The main scientific finding from the integrated cycles of blind predictions related to:

  • model calibrations and code developments in the subject areas of water inflow into the excavated tunnel with a calibrated hydrogeological model,
  • particular idealization of the rock mass (equivalent porous media, discrete fractures),
  • development and dissipation of excess pore water pressures in the vicinity of the advancing tunnel (at the time of the FEBEX tunnel excavation),
  • predicting the behaviour of the buffer under the combined heating and wetting actions,
  • identifications of necessary physical processes controlling the bentonite behaviour,
  • influence of host rock on hydration of the bentonite buffer, and
  • causes of rock stresses in the vicinity of the FEBEX tunnel by heating.

The entire project was a learning process experienced by the involved participants. A true blind prediction was difficult to make, even for a highly controlled and documented case record such as the FEBEX in-situ test, but could be improved to be more accurate and reliable with incremental comparisons with the real field measurements made available to modellers. Identification of the relevant couplings was a key issue in the modelling for Task 1, such as full HM coupling, 3D initial stress field, and the excavation process for Part A; phase change of water and vapour flow, saturation dependence of water permeability and thermal conductivity and suction-induced deformation for Part B; and thermal dilation of water and skeleton during the HM coupling process for Part C. A number of outstanding issues were identified, such as the long-term behaviour of the system, applicability of modelling results and knowledge at the full scale for a real repository with much more complexities in terms of repository design and the processes involved, and reliable characterization of the mechanical response of the buffer and the blocky nature of the buffer blocks used in the FEBEX test.


Besides the reports published, two research papers by the participating research team members of Task 1 were produced and published in a special issue of the Journal of Rock Mechanics and Mining Sciences, Volume 42, Number 5-6, published on 10 August 2005, as follows.

  • E.E. Alonso, J. Alcoverro, F. Coste, L. Malinsky, V. Merrien-Soukatchoff, I. Kadiri, T. Nowak, H. Shao, T.S. Nguyen, A.P.S. Selvadurai, G. Armand, S.R. Sobolik, M. Itamura, C.M. Stone, S.W. Webb, A. Rejeb, M. Tijani, Z. Maouche, A. Kobayashi, H. Kurikami, A. Ito, Y. Sujita, M. CHijimatsu, L. Bergesson, J. Hernelind, J. Rutqvist, C.-F. Tsang and P. Jussila, The FEBEX benchmark test: case definition and comparison of modelling approaches. Int. J. Rock Mech. Min. Sci., 2005, 42 (5-6): 611-638.
  • T.S. Nguyen, A.P.S. Selvadurai, G. Armand, Modelling the FEBEX THM experiment using a state surface approach, Int. J. Rock Mech. Min. Sci., 2005, 42 (5-6): 639-651.