International Journal of Rock Mechanics and Mining Sciences
The FEBEX benchmark test: case definition and comparison of modelling approaches
Section snippets
The FEBEX “In situ” test
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 borehole. Fig. 1 shows a perspective of the FEBEX drift and
The benchmark
The benchmark was divided into three parts, described as follows in general terms:
Part A: Hydro-mechanical modelling of the rock.
Based on the available geological, hydraulic and mechanical characterizations of the site as well as on results of hydraulic tests performed in 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
Modelling approaches for Part A
In the zone surrounding the FEBEX tunnel it is possible to identify macroscale features at a scale comparable to the scale of the problem, such as lamprophyre dykes and shear zones, and microscale features at a scale smaller than the scale of the problem, such as secondary fractures and microfractures.
In order to model the macroscale features, two types of approaches have been used:
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equivalent 3D continuum (CNS, DOE, IPS, JNC, SKB, SKI),
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equivalent 2D continuum (ANG, ANN).
All modelling teams
Comparison of field data with model calculations for Part A
Total water inflow in the test zone (which extends along 17.40 m along tunnel axis, from coordinates 54.00–71.40) was measured by two techniques (absorbing pads on selected points of the tunnel wall and a small gauge measuring overall leaked water) at different dates in the period January–May 1996, once the tunnel was fully excavated.
The first technique involved discrete measurements at selected points on the FEBEX tunnel by means of absorbing pads. The absorbing pads were weighted before and
Discussion—Part A
Widely different models for water inflow were used. Some teams (ANN, CNS, and SKB) used uncoupled hydraulic transient models to solve the first part of the exercise, whereas others (ANG, DOE, SKI) used a coupled HM modelling. It does not seem that the mechanical coupling introduces any advantage in this case. In fact, the reason for some of the better predictions (such as SKB calculation) may be associated with previous calibration of the model using other hydraulic data in the same area. Some
Overview of physical processes in the bentonite barrier
Fig. 16 shows a general scheme of the main hydro-thermal processes taking place in the buffer. The figure shows a cross-section of the buffer and the near field. A heat flow is introduced at the canister–bentonite interface and it is conducted away through the buffer and the rock towards the outer boundary. The saturated granite provides the water to progressively saturate the bentonite, initially unsaturated. A net inflow of liquid water is sketched flowing in opposite direction to the heat
Modelling approaches for Part B
Each Modelling Team used a particular model whose parameters were determined on the basis of the information provided with the case definition, references found in the literature and its own modelling experience. Main features of different models are given in Table 9. A few relevant physical phenomena have been isolated to prepare this table. The first column describes the main couplings considered. A fully coupled thermo-hydro-mechanical approach built in the calculation procedure is indicated
Comparison of field data with model calculations for Part B
Fig. 17 shows the evolution of measured input electrical power on heater 1 and the set of predictions. Most of calculations reproduce the progressive decrease of required power but only in one case (SKB) the reverse trend experienced after some time is well reproduced.
The variation of relative humidity with radial distance in Section E1 is shown, in Fig. 18a, for days. Measured data of four radial directions (two vertical, two horizontal) are lumped together in the figure. The measured
Discussion—Part B
The heating power is a global variable of the barrier performance dominated by the heat conductivity of buffer and rock. The initial strong drying of the inner part of the bentonite leads to a reduction of the heat conduction coefficient and, therefore, a reduction in power is required to maintain a constant temperature at the heater–buffer contact. As the buffer becomes progressively saturated in the mid and long term, the average heat conductivity of the bentonite increases and the required
Features of the experimental data
Several instruments were located, at increasing depth, in the 19 auxiliary boreholes, perforated in radial directions from the FEBEX tunnel (Fig. 1). An enlarged view of the borings is given in Fig. 23. The length of these radial boreholes does not exceed 15 m. Typically, readings are available in three or four positions: close to the tunnel wall (say at 1–3 m distance from the origin of the boring, one or two intermediate distances and a distant position (13–14 m deep). The Cartesian co-ordinate
Comparison of field data with model calculations for Part C
The model developed for Part B was used for Part C by most of Modelling Teams. A summary of the modelling approaches is given in Table 10. The inclusion of thermal dilation properties of rock and water is now necessary to tackle Part C. A full THM coupling is also needed to make meaningful predictions.
Measured and computed evolutions of temperature in Boring SF 21, at a radial distance of 1.20 m are shown in Fig. 24. Most predictions are quite accurate. Two of them, however, depart in a
Discussion—Part C
Temperature distributions are well reproduced in general terms. In fact, some of the calculations are very precise, unlike the case of temperature prediction for the buffer. In the case of the (saturated) rock heat flow is essentially controlled by constant parameters: the granite heat conduction and heat capacity coefficients. Convective heat transport is very small because of the negligible flow rate towards the bentonite. In the bentonite buffer, besides the effect of degree of saturation on
Conclusions
The FEBEX test is one of the few large-scale tests available to gain an integrated perspective of the behaviour of current concepts for nuclear waste disposal in crystalline rock. The comprehensive instrumentation installed in the rock and in the compacted bentonite buffer has yielded vast amounts of data over the past 6 years. Part of this data, the data corresponding to the first 3 years of heating, has been used to conduct a Benchmark exercise to evaluate the capabilities of a number of
Acknowledgements
The authors wish to acknowledge and thank the support provided by the Research Project DECOVALEX III during the development of the work partially reported in this paper. Discussions with all the modelling teams participating in the benchmark exercise have been very useful to increase the level of understanding of the observed behaviour of the FEBEX “in situ” test. A particular appreciation is extended to ENRESA, the Spanish National Agency for Nuclear Waste Disposal, owner and manager of the
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