FEBEX

In Fig. 4 the evolution of pH and Eh (mV) for a Spanish granitic groundwater in contact with FEBEX bentonite is shown, together with the spatial distribution of the bentonite blocks. 

Fernandez, A. M., Cuevas, J. Rivas, P. 2001. Pore water chemistry of the FEBEX bentonite. Materials Research Society Symposium Proceedings, 663, 573—588. 

FAA FA FBC FC FEBEX FFFF FGD FP FSU FT FTIR FUETAP Flame atomic absorption Fly ash Fluidized bed combustion Filter cake Full-scale engineered barriers experiment (in crystalline host rock) Flow-field flow fractionation Flue gas desulphurization Fission products Former Soviet Union Fourier transforms Fourier transformed infrared spectroscopy Formed under elevated temperature and pressure... 

Sedimentary (B-I.b.), and sedimentary-volcano-sedimentary, bentonite samples (B-II.a.) The specific surface area (90-100 m2/g) is in the same range as usual for the bentonite samples (e.g., S Wy-1, MX-80, FEBEX, Serrata de Nijar). The CEC is in agreement with the montmorillonite content. 

FEBEX (Full-Scale Engineered Barrier Experiment)... 

Task 1 FEBEX experiment conducted by ENRESA in Grimsel Mines in Switzerland... 

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... 

Borehole FEBEX 95.002, with a diameter of 86 mm and a length of 132.36 m, was equipped with packers to isolate sections in which pore water pressures could be monitored. 

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

Shear zones are of considerable thickness at the area (5 to 20 m). At the intersection with tunnels, they display major outflows indicating their relevance as preferential flow paths. Lamprophyre dykes have also considerable dimensions (thickness up to several meters), although their relevance as preferential flow paths is not as important as shear zones. A preferential flow path within these dykes is the contact surface between the lamprophyre and the host rock. Figure 2 provides a geological cross section of the Grimsel area and shows the position of the FEBEX drift, bounded by two main shear zones... 

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

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

The properties of FEBEX bentonite have been extensively investigated over the past 8 yrs. Several recent publications describe the thermo-hydromechanical properties of the compacted material (Villar, 2002 Villar and Lloret, 2001, 2002, 2003 Villar et al, 2002 Huertas et al, 20(X) Romero et al. 

Pintado et al, 2002 Lloret et al, 2003). Extensive information is also available for the bentonite S-2 , a material of essentially the same origin (a quarry in Serrata de NIjar, Almeria, Spain) as the FEBEX one. A summary of relevant properties, taken from some of the mentioned references, is given here. 

Figure 7. Swelling pressure of FEBEX bentonite (square dots). Crosses correspond to the "S-2 bentonite.

Figure 8. Saturated hydraulic conductivity as a function of dry density of FEBEX bentonite...

Figure 9. Water retention characteristics of FEBEX bentonite. Bold symbols correspond to wetting paths...

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. 

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. 

This behavior is related to the interaction between rock deformation and water pressure and provides an interesting record of hydro-mechanical interaction in the saturated granitic rock mass. Figure 13 is a Plan view of the Febex tunnel and the Borehole FEBEX 95.002 where observations were made in the intervals P3 and P4. 

The first technique involved discrete measurements at select points on the FEBEX tunnel by means of absorbing pads. The absorbing pads were weighted before and after their placement in order to determine the volume of leaked water. With this information, it was possible to know the distribution of water input flow on the wall of the FEBEX tunnel. 

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).

Pressure (in MPa) and tunnel drilling advance (in m) are plotted as a function of time. The position of the measuring interval (P4) along borehole FEBEX 95.002 is indicated by a vertical segment, which spans the appropriate tunnel metering. Predictions of modelling teams are indicated with the corresponding acronyms and selected format. 

Figure 14a, b. Water Pressure Evolution (borehole FEBEX 95.002, interval P4). 

The long term reaction of the pressure measured in the borehole section P4 is to show a steady decrease since the excavation of the tunnel implies a neighboring boundary at a given relative humidity (the RH prevailing in the FEBEX tunnel prior to the buffer and heater installation). Most of the models show this trend although the rates of water pressure decay may change. 

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...

Note that the values given are increments over the initial state of stress once the FEBEX tunnel was open. They represent therefore the combined effect of temperature effects (the dominant phenomenon), the generation and dissipation of pore pressures and the swelling pressure effects. At the radial distance of 3 m the increments of mean effective stress dominate. Incremental shear stresses tend to be a fraction of the incremental mean... 

For the purposes of the organization of the exercise into specific tasks the Benchmark was divided into three main parts A Rock behaviour during the excavation of the FEBEX tunnel, B Buffer behaviour and C Rock behaviour during the heating and (partial) hydration of the buffer. This distribution has been maintained in the paper. Specific conclusions for each of the mentioned parts have been given before. Only a few concluding remarks will be added here ... 

The development and dissipation of excess pore water pressures in the vicinity of the advancing tunnel (at the time of the FEBEX tunnel excavation) was a clear example of hydromechanical interaction. It was concluded that the development of pore pressures was controlled by the initial stress field state, by the rate of excavation and by the permeability and drainage properties of the granite. However, the available information on the intensity and direction of principal stresses in the area was found inconsistent with the actual measurements. The problem posed by this discrepancy was essentially unsettled since a precise determination of the initial stress state in the vicinity of the FEBEX tunnel was not available. 

The heating of the rock resulted in a significant increase in rock stresses in the vicinity of the FEBEX tunnel. Water pressures remained however essentially unchanged. The relatively... 

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