FEBEX tests

Figure I. Layout of FEBEX test and associated boreholes (Pardillo et al, 1997).

The shear zones of high transmissivity constrain regional groundwater flow and therefore, they constitute boundaries of the FEBEX test area. 

A maximum number of eleven modelling teams have participated in the different benchmark activities related to the FEBEX test. Their names, codes and symbols used in the presentation of results are given in Table 3. 

Figure 15. Results of inflow measurements in part of FEBEX test area arranged in rows (sections) and their relationships to geological structures (Guimerd et al, 1998).

Figure 22. Auxiliary instrumentation boreholes around FEBEX test section...

Alonso, E.E, and J. Alcoverro, FEBEX benchmark test case definition and comparison of different modelling approaches, this volume, 2004,... 

THE FEBEX BENCHMARK TEST. CASE DEFINITION AND COMPARISON OF DIFFERENT MODELLING APPROACHES... 

The Febex in situ test is currently in operation at the Grimsel Test Site, located in the granitic rocks of the Aare Massif in central Switzerland. 

In order to perform the FEBEX in-situ test, it was decided to excavate a new drift. Prior to it, two pilot boreholes (FEBEX 95.001 and FEBEX 95.002) were drilled in the area. Afterwards, the FEBEX drift was excavated between these pilot boreholes. It was parallel to FEBEX 95.002. Figure I shows a... 

Figure S. General scheme of the FEBEX in situ test...

Based on the available geological, hydraulic and mechanical characterizations of the Site as well as on results of hydraulic tests performed on boreholes, a hydro-mechanical model for the zone around the FEBEX tunnel was to be prepared. Using this model, changes in water pressure induced by the boring of the FEBEX tunnel in the near vicinity, as well as the total water flow rate to the excavated tunnel was required. 

Figure 13. Plan view of the test zone of the FEBEX drift and the borehole FEBEX 95.002, showing the intervals P3 and P4. GTS coordinates (in m) are used (north is parallel to the y-axis).

Abstract The Canadian Nuclear Safety Commission (CNSC) used the finite element code FRACON to perform blind predictions of the FEBEX heater experiment. The FRACON code numerically solves the extended equations of Biot s poro-elasticity. The rock was assumed to be linearly elastic, however, the poro-elastic coefficients of variably saturated bentonite were expressed as functions of net stress and void ratio using the state surface equation obtained from suction-controlled oedometer tests. In this paper, we will summarize our approach and predictive results for the Thermo-Hydro-Mechanical response of the bentonite. It is shown that the model correctly predicts drying of the bentonite near the heaters and re-saturation near the rock interface. The evolution of temperature and the heater thermal output were reasonably well predicted by the model. The trends in the total stresses developed in the bentonite were also correctly predicted, however the absolute values were underestimated probably due to the neglect of pore pressure build-up in the rock mass. 

The FEBEX T-H-M experiment is a valuable and important project which should lead to an improvement in the understanding of the behaviour of the bentonite barrier around heat-emitting Nuclear Fuel Waste(NFW) containers. Such large field experiments should always be undertaken with the simultaneous development of constitutive and computational models to interpret the experiments. The FEBEX bentonite possesses strong nonlinear behaviour in the unsaturated state. In order to simulate that behaviour, we have adopted a nonlinear poro-elastic approach. In this approach, the coefficients of the poroelastic equations are assumed to be functions of suction and the void ratio. These functions are derived from the state-surface equation which has been experimentally obtained from suction-controlled oedometric tests performed by the Spanish research organizations UPC and CIEMAT. 

UPC, 2000, Task definition DECOVALEX III, Task 1 Modelling of FEBEX in-situ test. Part B Thermo-hydro-mechanical analysis of the bentonite behaviour. Polytechnical University of Catalonia, Barcelona, Spain. 

Abstract Coupled THM simulation of the FEBEX, which is the full-scale in-situ Engineered Barrier System Experiment performed in Grimsel Test Site in Switzerland, is one Task in the international cooperation project DECOVALEX III. In the Task, the simulation of the thermal, hydraulic and mechanical behaviour in the buffer during heating phase is required, e.g. the evolutions and the distributions of stress, relative humidity and temperature at the specified points in bentonite buffer material. 

The FEBEX is the full-scale in-situ Engineered Barrier System (EBS) Experiment performed in Grimsel Test Site (GTS) in Switzerland (enresa (2000)). The simulation of the coupled thermal, hydraulic and mechanical (THM) behaviour of FEBEX is the task of the DECOVALEX (DEvelopment of COupled models and their VALidation against Experiments) III. 

First, we identified the input parameter for THAMES on properties of FEBEX bentonite, because the fundamental properties of FEBEX bentonite had been obtained by various laboratory tests to identify the input data for the numerical code CODE BRIGHT (enresa (1998)). After calibrations of the all required parameters for THAMES, such as thermal vapour flow diffusivity and intrinsic permeability, the coupled THM simulations were carried out. 

Figure 5 shows the schematic view of the FEBEX. FEBEX has two heaters. Vertical sections, as D, El, E2 and G, in the test tunnel are instrumented sections that are selected for comparison between the prediction and the monitored data for part B. Sections of El and E2 are for relative humidity simulation. Sections of D and G are for temperature simulation. Section of E2 is for total pressure simulation. 

Pardillo J. and Campos R. (1996). FEBEX -Grimsel Test Site (Switzerland) Considerations respect to the fracture distribution. CIEMAT, 70-IMA-L-2-05, Mar. 1996. 

UPC (Technical University of Catalonia), 1999. DECOVALEX HI, Task I Modeling of FEBEX in-situ test General Specifications. Barcelona, Spain. 

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