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A Discrete Approach to Modelling Hydromechanical Rock Response of FEBEX Tunnel Excavation (Grimsel Underground Research Laboratory, Switzerland)
149 A DISCRETE APPROACH TO MODELLING HYDROMECHANICAL ROCK RESPONSE OF FEBEX TUNNEL EXCAVATION (GRIMSEL UNDERGROUND RESEARCH LABORATORY, SWITZERLAND) Veronique MERRIEN-SOUKATCHOFP, Imad KADIRM, K. SU^, Yves GUGLIELMI S.^ ^) LAboratoire Environnement Geomecanique Ouvrages, Ecole des Mines, Nancy, France 2) ANDRA, Chatenay-Malabry, France ^) Geosciences Azur, Universite de Nice-Sophia Antipolis, France Abstract: LAEGO and ANDRA have participated in the modelling of the task 1A of the "DECOVALEX" III international research project, which was devoted to modelling the hydromechanical effects of FEBEX tunnel excavation at the Grimsel Test Site in Switzerland. After presenting the site, this paper will focus on the 3D discontinuous modelling performed by associating 3DEC and 3FLO codes, which leads to a discontinuous semi-coupled approach. The modelling tools, choice of geomodel, the mechanical boundary and related initial conditions, the hydraulic boundary and related initial conditions, as well as the calibration process of hydraulic properties will all be discussed herein. A purely-hydraulic modelling first yields a rather good estimation of both total flow leading towards the tunnel and the general shape of pressure evolution in a nearby borehole. A hydromechanical computation representing the final stage of excavation reveals a difference of just 5% on the evaluation of total water flow rate and does improve the prediction of water head in the boreholes surrounding the tunnel.
1 INTRODUCTION
2 SITE DESCRIPTION
Within the framework of the international research project entitled DECOVALEX III ("International co-operative project for the DEvelopement of Coupled models and their Validation against Experiments in nuclear waste isolation"), we have participated in the modelling of Task lA, which focuses on modelling the hydromechanical effects of tunnel excavation in crystalline rock. The case examined herein was the FEBEX tunnel, situated at the Grimsel Test Site (GTS), an underground laboratory in Switzerland operated by NAGRA. The purpose of the FEBEX (Full-scale Engineered Barriers Experiment in Crystalline Host Rock) project is to study the various processes occurring in the vicinity of highactivity radioactive waste storage. The specific task undertaken has consisted of predicting changes in water head induced by boring the FEBEX tunnel (pressure variations vs. time) as well as in the total water flow rate into the excavated tunnel over the last 17.40 meters. The data accessed for this purpose were the available geological, hydraulic and mechanical characterizations of GTS, along with the results of hydraulic tests performed on boreholes. In order to accomplish this task, we employed a discontinuous approach. The next section of this paper will describe the test site and subsequent sections will discuss in greater detail the modelling tools, methodology and results.
The GTS is located at an elevation of 1,730 m within the granite rocks of the "Aare Massif in central Switzerland, about 450 m beneath the eastern flank of the Juchlistock Mountain. This experimental site is set up in a tunnel system (see Figure 1) that branches off from a main access tunnel towards the KWO (Kraftwerke Oberhasli AG) underground power station. The GTS tunnel system comprises a laboratory tunnel with a total length of almost 1,000 m plus a central building that houses all the operating facilities (Ortuno, 2000).
Figure I.
Location of the various Grimsel Test Site tunnels, Pardillo et al (1997).
150 2.1 Geographical and geological context The FEBEX drift excavation is located in the northern part of the Grimsel Test Site (GTS) and was completed with a TBM from September 25 to October 30, 1995. Four predrilled pilot boreholes have been positioned in the surrounding area (ROUS 85.001, ROUS 85.002, FEBEX 95.001 and FEBEX 95.002.). The FEBEX tunnel displays a circular section with an average diameter of 2.28 m and a total length of 70.4 m. It was excavated with an ascending slope of 1% to allow for natural drainage. The rock mass is predominantly granite and granodiorite and has been affected by several fracturing phases. Keusen et al. (1989) identified 12 possible discontinuity systems, with the most significant discontinuity families being listed in Figure 2.
2.2 Hydrogeological data The GTS has been excavated within a fractured rock mass of low permeability. Observation in the laboratory tunnel has revealed that water flow is concentrated almost exclusively in discontinuities (Keusen et al, 1989). Figure 2 demonstrates the relative importance of water-bearing discontinuities. The S families are schistosity regrouped into shear zones of considerable thickness (5 to 20 m) displaying major outflows at the intersection with tunnels, hence indicating their relevance as preferential flow paths. L is a regional structure of large-sized lamprophyre dikes (with thickness reaching several meters). Although their Discontinuity system
Azimuth/dip
relevance as preferential flow paths is not as pronounced as the shear zones, their contact with the host rock yields planes of major transmissivity. A regional numerical model produced by Vobomy et al. (1991) has provided an estimate of the main flows within a vicinity of several kilometres around the GTS, while a second model has allowed deriving more details of the site area. These models have been used as references to infer boundary conditions for the more heavily-localized model performed. Two shear zones (K and S) located within the vicinity of the FEBEX tunnel constrain groundwater flow due to their high transmissivity and therefore constitute FEBEX environment boundaries; in subsequent model, they have been considered as imposed head limits (see Figure 3). Numerous hydraulic tests were cartied out, during the various project phases, either in an isolated borehole or between boreholes. Hydraulic experiments consisted of pulse tests, constant head injection and/or extraction and constant rate injection/extraction using borehole intervals delimited in FBX 95001, FBX 95002, BOUS 85.001 and BOUS 85.002 (Figure 1). Both water flow and piezometric level measurements for borehole intervals are available. Continuous pressure monitoring has allowed detecting crosshole responses during pulse testing; these have then been used to calibrate the hydraulic conductivities of fractures.
Number of water-bearing discontinuities Number of open discontinuities Number of discontinuities observed Number of discontinuities
50 100 10 SI 140/80 S2 155/75 S3 5/155 Kl 55/90 K2 25/80 K3 80/90 K4 120/80 L (Lamprophyre) 40/80 Figure 2. Discontinuity systems in the laboratory tunnel and water flow for each set (according to Keusen etal,1989).
151 2.3 Geomechanical data The principal mechanical properties of granite rock matrix were provided for the purposes of this task; no mechanical characterization of discontinuities was available. The stress measurements presented by Pahl et al. (1989) show that the stress field is triaxial with horizontal stresses 4 to 5 times higher than the lithostatic pressure and a difference of greater than 10 MPa between the minimum and maximum horizontal stresses (see Figure 3).
3 DISCONTINUOUS MODELLING TOOLS EMPLOYED As seen in the previous section, water flow is driven by discontinuity patterns. We have elected to use discontinuous modelling tools to run the simulation of this case and more precisely to combine a 3-code application, known as HM3D. HM3D allows modelling the hydromechanical behaviour of a rock mass by means of successively employing the three following codes: RESOBLOK, a code developed by LAEGO to enable constructing the site's geometrical representation (the "geomodel"); mechanical calculations
performed with the 3DEC distinct element code (ITASCA), flow version 1.34; and the 3FL0 flowmodelling code (ITASCA), version 0.90. Special procedures allow for transferring information from one code to another. The hydraulic computation is influenced by mechanical results via the variation in joint apertures due to the mechanical displacements. Due to the deterministic selection of discontinuities in the FEBEX case, only 3DEC and 3FLO were successively employed. Application of RESOBLOK would have been of interest in the case of statistical knowledge of the fractures. The geometrical representation step, which leads to setting the joints in place, is essential for the mechanical and hydraulic steps and exerts however a sizable impact on subsequent results (Kadiri, 2002;Kadiri^ra/.. 2002). The mechanical phase of the computation has made use of the 3DEC distinct element code, which relies upon an explicit solution of motion equations based on small time steps; the ensuing computation cycles lead to achieving equilibrium. The blocks separated by discontinuities were considered as elastic in the FEBEX case. Joints respect the Coulomb slip criterion characterised by normal and shear stiffnesses, as well as friction, cohesion and tensile strength properties assigned to each joint.
CTK
8,04 MPa < a^ : 10,72 MPa < 13,4 MPa
: 23,5 MPa
^ OH: 34,5 MPa
N
IiTipo^ed head varying Ironi 1930 niU) 1730(1/ Constant head 1730 m
Febex Tunnel lm|X)scd head varying troni 1950 ni to 1730 in Fictive joints limiting the Febex tunnel
Figure 3.
Horizontal cross-section of the geomodel - Initial and boundary hydraulic, mechanical conditions.
152 For the hydraulic phase, each discontinuity is considered to be crossed by a set of channels (ID hydraulic element) generated either deterministically or statistically. At the intersection of two joints, a hydraulic conduit, called a tube, is produced. Flow is liniited to fractures and the blocks are impervious. The diffusion equation is resolved throughout the finite differences. Initial conductivities are assigned to both channels and tubes (combined within the denoted pipe). Displacements computed with the 3DEC code influence the conductivity of hydraulic elements by the following relationship:
(1)
where Q is the initial conductivity (before stress variation) of the hydraulic element, C the hydraulic element conductivity once displacements have been taken into account, a© the initial aperture (for a channel) or initial radius (for a tube), a the aperture or radius once displacements have been taken into account, and default exp is 3 for a channel or 4 for a tube. This behaviour gives rise to a semi-coupled approach (i.e. the influence of mechanical computation on hydraulic properties).
4 MODELLING OF THE FEBEX CASE 4.1 The "geomodel" In order to perform hydromechanical computations, the choice of which discontinuities to represent must be a judicious one. We only retained the 3 sets of fractures with the most significant hydraulic impact (S1+S2; K2 and L), as shown in Figure 2. Selection of specific fractures to be introduced into the model consisted of: indexing the fractures identified in boreholes within the vicinity of FEBEX and the FEBEX tunnel itself; selecting open fractures that could play a hydraulic role according to descriptive criteria (we have retained those fractures considered as being open by the geophysical log and indicated by the TLV probe);
keeping discontinuities, whose hydraulic role was demonstrated by hydraulic tests and water flow measurements conducted in the boreholes. This choice was also influenced by the fact that HM3D does not allow introducing a large number of fractures. Fractures in the same area with the same orientation were considered as a single fracture (with dip and azimuth being averaged from all individual fractures taken into account): a joint in the model represents a set of in situ fractures. A horizontal view at the altitude of the FEBEX tunnel (1,730 m) of the geomodel obtained is presented in Figure 3. The horizontal dimensions of the model were dictated by the hydraulic boundaries, as will be explained below. The model is 150 m wide in the north-south direction and 200 m wide east-west. Moreover, it is 200 m high (100 m above and below the tunnel altitude).
4.2 Mechanical boundaries and initial conditions The initial stresses and boundary stress conditions were introduced in accordance with the in situ data described in Section 2.3. Vertical stress varies between 8.04 MPa and 13.4 MPa, depending on the lithostatic pressure (with the overburden varying from 300 to 500 m). The maximum horizontal stress is constant, equal to 34.5 MPa (mean of the measured values) and oriented N 130° (i.e. perpendicular to S2 schistosity), whereas the minimum horizontal stress is 23.5 MPa (mean of the measured values) and oriented N 40° (i.e. parallel to S2 schistosity).
4.3 Hydraulical boundary conditions, initial conditions and property calibration On the whole, flow is moving from within the massif towards the laboratory tunnel (from west to east). Before excavation of the FEBEX tunnel, the equipotentials were roughly parallel to the wall of this main tunnel. A zero pressure has been assumed in the laboratory tunnel. To the north and south, the model is constrained by two major shear zones S1+S2, as specified in Section 2.2, Figure 3 displays the hydraulic boundary conditions adopted, in a plane view. The head and gradient adopted were deduced both from measurements conducted within the boreholes intersecting our
153 model boundary and from the regional model developed by Vobomy et al (1991). In the third dimension, the upper and lower surfaces have been considered as impervious. The conductivity and specific storativity of the fracture were obtained by calibration in a purelyhydraulic computation. In each fracture, two perpendicular sets of channels 2 meters apart were generated. At first, the same initial aperture of 10^ was assigned to all fractures, but since a joint in 3FLO represents several in situ discontinuities (see Section 4.1), the conductivity Cond was calculated as follows:
Cond =
n^^a^
(2)
where a is the aperture, n the number of discontinuities represented by a joint in the model, p the water mass density (1,000 kg/m3), g the gravity constant (9.81 m.s-2) and fi the water dynamic viscosity (^i = 10'^ Pa.s). The calibration process was performed iteratively by simulating the test (in general, an imposed zero pressure) for each borehole interval until simulated flow equals measured flow. The first calibration step consists of considering each interval separately and leads to allotting an initial conductivity to each fracture and each test. The iteration then makes it possible to consider the whole intervals and the whole fractures, even those that are not directly in connection with the borehole interval targeted for calibration. Furthermore, in order to simulate the head variation observed in the four boreholes (ROUS 85.001, BOUS 85.002, FEBEX 95.002 and FEBEX 95.002), the conductivity assigned to the fractures was reduced within the western part of the model. The calibration step leads to assigning hydraulic apertures varying from 10 to 20 nm. Let's note herein that the proposed model is not unique: the calibration of another "geomodel" also compatible with the observations could have led to a different set of parameters in good accordance with local information. Pressure Static Recovery tests were modelled (within a purely-hydraulic computation) in order to calibrate fractures storativity. The difference between computed storativity, which takes only joint aperture into account, and calibrated storativity reveals the major role played by rock
mass on the storativity and/or the importance of weak joint stiffness. Since further computations were merely conducted under steady-state conditions, storativities are not subsequently used.
4.4 Purely-hydraulic computation As part of the "DECOVALEX" project, we were asked to predict the total water flow rate to the excavated tunnel over the last 17.40 meters of FEBEX tunnel section (between 54.00 and 71.40 m). A purely-hydraulic computation, using the previous calibration, yields an estimated steadystate flow of between 6 and 7.6 ml/min. This size scale is to be compared with total water inflow estimated from a semi-quantitative hydrogeological map of the FEBEX tunnel: 7.8 ml/min (with an estimation of about 27% of inflow water coming through the matrix). In comparison with the continuous prediction (Alonso et al, 2001), this estimation is rather good and the discontinuous approach allows localizing water output with greater precision. (This kind of comparison was not included within the "DECOVALEX" task.) For the prediction of water head changes induced by FEBEX tunnel boring, it was impossible to simulate a transient evolution due to changes in geometry with HM3D. We chose to model tunnel boring in 4 excavation phases and then simulate the steady state corresponding to the end of each phase. Figure 4 shows the comparison carried out between the measured and simulated pressures. The general shape of the curves obtained with hydraulic simulation is quite similar to measurement results, except there is a constant difference of about 1 bar between the P4 measurements and the simulation. This point will be discussed in Section 4.5 below. Our simulation approach (a hydraulic steady flow analysis with four steady-state steps) was not able to reproduce pore pressure increase (led to a higher horizontal stress than vertical stress) as excavation neared the monitored borehole intervals and as post-tunnel face dissipation was completed.
4,5 Hydromechanical computation A hydromechanical computation, which takes both the mechanical boundary and initial conditions described in Section 4.2 plus the hydraulic initial conditions, boundary conditions and calibration described in Section 4.3 into account, has been performed. Only the final stage of the excavation was simulated and this led to a
154 steady-state flow estimate of between 5.7 and 7.2 ml/min, i.e. 5% less than the flow predicted by a purely-hydrauiic computation. The difference in head between the hydraulic and hydromechanical computations is more significant (see Table 1): 6 m, or 0.7 bar in pressure. This discrepancy may explain the observed difference between measurements and simulation in the purely-hydraulic computation (see Figure 4).
sensitive with respect to pore pressure. Due to the partial coupling along with the inability of tools to simulate transient hydromechanical processes, the precise variations associated with tunnel progression have not been reproduced. The result however incites use of the discontinuous approach even if there is a need of improving the computer codes because they enables to reproduce accurately the localisation of water outflow witch is not the case for equivalent porous medium codes.
REFERENCES *
P4 P3 (nmleO* P4(iw)del)'
Figure 4.
Comparison of measured and simulated (by hydraulic computation) pressures over intervals P3 and P4 of the FBX 95002 borehole.
Table 1. Comparison of measured and modelled heads over the P3 and P4 intervals in both a purelyhydraulic model (H) and a hydromechanical (HM) computation
FBX 95002 Measured Simulated Simulated Intervals head (m) head (m) head(m) H HM P3
1,787
1,781
1,787
P4
1,769
1,778
1,784
5 CONCLUSION FEDEX excavation modelling using discontinuous codes has proven the ability of this conceptual approach to reproduce both the global flow rate towards the excavated tunnel and the general shape of head variation in the surrounding area. The refinement introduced by a hydromechanical computation, in comparison with a purely-hydraulic computation, only mildly influences (5%) flow estimation, yet remains more
Alonso E.E. & Alcoverro J. 2001. DECOVALEX III, Task 1: Modelling of FEBEX in-situ test, Introduction report and comparison of results by UPC. Internal Report. Kadiri I., Merrien-Soukatchoff V. & Su K. 2002, Discontinuous Hydromechanical Modeling of the impact of the FEBEX tunnel (Grimsel, Switzerland). 5th European Conference on Numerical Methods in Geotechnical Engineering (NUMGE) 2002, 4-6 September 2002, Paris, France, pp. 585-590. Kadiri I. 2(X)2. Modelisation hydromecanique des milieux fractures, Ph.D thesis, LAEGO, Ecole des Mines, INPL, France, 28 October 2002, 143 p. Keusen H.R., Ganguin J., Schuler P. & Buletti M. 1989. Grimsel Test Site. Geology. NAGRA, NTB87-14E,Feb. 1989. Ortuno F. 2000. Discrete water inflow measurements at the FEBEX tunnel DECOVALEX III, Task I: Modelling of FEBEX in-situ test. Internal Report. July 20(X). Pahl A., Heusermann St., Brauer V. & Gloggler W. 1989. Grimsel Test Site. Rock stress investigations. NAGRA, NTB 88-39E. Pardillo J., Campos R. & Gimera J. 1997. Caracterizacion geologica de la zona de ensayo FEBEX (Grimsel - Suiza). CIEMAT, 70-IMAM-2-01,May 1997. Vobomy O., Adank P., Hurlimann W., Vomvoris S. & Mishra S. 1991. Grimsel Test Site. Modelling of groundwater flow in the rock body surrounding the underground laboratory. NAGRA, NTB 91-03.
1 INTRODUCTION
2 SITE DESCRIPTION
Within the framework of the international research project entitled DECOVALEX III ("International co-operative project for the DEvelopement of Coupled models and their Validation against Experiments in nuclear waste isolation"), we have participated in the modelling of Task lA, which focuses on modelling the hydromechanical effects of tunnel excavation in crystalline rock. The case examined herein was the FEBEX tunnel, situated at the Grimsel Test Site (GTS), an underground laboratory in Switzerland operated by NAGRA. The purpose of the FEBEX (Full-scale Engineered Barriers Experiment in Crystalline Host Rock) project is to study the various processes occurring in the vicinity of highactivity radioactive waste storage. The specific task undertaken has consisted of predicting changes in water head induced by boring the FEBEX tunnel (pressure variations vs. time) as well as in the total water flow rate into the excavated tunnel over the last 17.40 meters. The data accessed for this purpose were the available geological, hydraulic and mechanical characterizations of GTS, along with the results of hydraulic tests performed on boreholes. In order to accomplish this task, we employed a discontinuous approach. The next section of this paper will describe the test site and subsequent sections will discuss in greater detail the modelling tools, methodology and results.
The GTS is located at an elevation of 1,730 m within the granite rocks of the "Aare Massif in central Switzerland, about 450 m beneath the eastern flank of the Juchlistock Mountain. This experimental site is set up in a tunnel system (see Figure 1) that branches off from a main access tunnel towards the KWO (Kraftwerke Oberhasli AG) underground power station. The GTS tunnel system comprises a laboratory tunnel with a total length of almost 1,000 m plus a central building that houses all the operating facilities (Ortuno, 2000).
Figure I.
Location of the various Grimsel Test Site tunnels, Pardillo et al (1997).
150 2.1 Geographical and geological context The FEBEX drift excavation is located in the northern part of the Grimsel Test Site (GTS) and was completed with a TBM from September 25 to October 30, 1995. Four predrilled pilot boreholes have been positioned in the surrounding area (ROUS 85.001, ROUS 85.002, FEBEX 95.001 and FEBEX 95.002.). The FEBEX tunnel displays a circular section with an average diameter of 2.28 m and a total length of 70.4 m. It was excavated with an ascending slope of 1% to allow for natural drainage. The rock mass is predominantly granite and granodiorite and has been affected by several fracturing phases. Keusen et al. (1989) identified 12 possible discontinuity systems, with the most significant discontinuity families being listed in Figure 2.
2.2 Hydrogeological data The GTS has been excavated within a fractured rock mass of low permeability. Observation in the laboratory tunnel has revealed that water flow is concentrated almost exclusively in discontinuities (Keusen et al, 1989). Figure 2 demonstrates the relative importance of water-bearing discontinuities. The S families are schistosity regrouped into shear zones of considerable thickness (5 to 20 m) displaying major outflows at the intersection with tunnels, hence indicating their relevance as preferential flow paths. L is a regional structure of large-sized lamprophyre dikes (with thickness reaching several meters). Although their Discontinuity system
Azimuth/dip
relevance as preferential flow paths is not as pronounced as the shear zones, their contact with the host rock yields planes of major transmissivity. A regional numerical model produced by Vobomy et al. (1991) has provided an estimate of the main flows within a vicinity of several kilometres around the GTS, while a second model has allowed deriving more details of the site area. These models have been used as references to infer boundary conditions for the more heavily-localized model performed. Two shear zones (K and S) located within the vicinity of the FEBEX tunnel constrain groundwater flow due to their high transmissivity and therefore constitute FEBEX environment boundaries; in subsequent model, they have been considered as imposed head limits (see Figure 3). Numerous hydraulic tests were cartied out, during the various project phases, either in an isolated borehole or between boreholes. Hydraulic experiments consisted of pulse tests, constant head injection and/or extraction and constant rate injection/extraction using borehole intervals delimited in FBX 95001, FBX 95002, BOUS 85.001 and BOUS 85.002 (Figure 1). Both water flow and piezometric level measurements for borehole intervals are available. Continuous pressure monitoring has allowed detecting crosshole responses during pulse testing; these have then been used to calibrate the hydraulic conductivities of fractures.
Number of water-bearing discontinuities Number of open discontinuities Number of discontinuities observed Number of discontinuities
50 100 10 SI 140/80 S2 155/75 S3 5/155 Kl 55/90 K2 25/80 K3 80/90 K4 120/80 L (Lamprophyre) 40/80 Figure 2. Discontinuity systems in the laboratory tunnel and water flow for each set (according to Keusen etal,1989).
151 2.3 Geomechanical data The principal mechanical properties of granite rock matrix were provided for the purposes of this task; no mechanical characterization of discontinuities was available. The stress measurements presented by Pahl et al. (1989) show that the stress field is triaxial with horizontal stresses 4 to 5 times higher than the lithostatic pressure and a difference of greater than 10 MPa between the minimum and maximum horizontal stresses (see Figure 3).
3 DISCONTINUOUS MODELLING TOOLS EMPLOYED As seen in the previous section, water flow is driven by discontinuity patterns. We have elected to use discontinuous modelling tools to run the simulation of this case and more precisely to combine a 3-code application, known as HM3D. HM3D allows modelling the hydromechanical behaviour of a rock mass by means of successively employing the three following codes: RESOBLOK, a code developed by LAEGO to enable constructing the site's geometrical representation (the "geomodel"); mechanical calculations
performed with the 3DEC distinct element code (ITASCA), flow version 1.34; and the 3FL0 flowmodelling code (ITASCA), version 0.90. Special procedures allow for transferring information from one code to another. The hydraulic computation is influenced by mechanical results via the variation in joint apertures due to the mechanical displacements. Due to the deterministic selection of discontinuities in the FEBEX case, only 3DEC and 3FLO were successively employed. Application of RESOBLOK would have been of interest in the case of statistical knowledge of the fractures. The geometrical representation step, which leads to setting the joints in place, is essential for the mechanical and hydraulic steps and exerts however a sizable impact on subsequent results (Kadiri, 2002;Kadiri^ra/.. 2002). The mechanical phase of the computation has made use of the 3DEC distinct element code, which relies upon an explicit solution of motion equations based on small time steps; the ensuing computation cycles lead to achieving equilibrium. The blocks separated by discontinuities were considered as elastic in the FEBEX case. Joints respect the Coulomb slip criterion characterised by normal and shear stiffnesses, as well as friction, cohesion and tensile strength properties assigned to each joint.
CTK
8,04 MPa < a^ : 10,72 MPa < 13,4 MPa
: 23,5 MPa
^ OH: 34,5 MPa
N
IiTipo^ed head varying Ironi 1930 niU) 1730(1/ Constant head 1730 m
Febex Tunnel lm|X)scd head varying troni 1950 ni to 1730 in Fictive joints limiting the Febex tunnel
Figure 3.
Horizontal cross-section of the geomodel - Initial and boundary hydraulic, mechanical conditions.
152 For the hydraulic phase, each discontinuity is considered to be crossed by a set of channels (ID hydraulic element) generated either deterministically or statistically. At the intersection of two joints, a hydraulic conduit, called a tube, is produced. Flow is liniited to fractures and the blocks are impervious. The diffusion equation is resolved throughout the finite differences. Initial conductivities are assigned to both channels and tubes (combined within the denoted pipe). Displacements computed with the 3DEC code influence the conductivity of hydraulic elements by the following relationship:
(1)
where Q is the initial conductivity (before stress variation) of the hydraulic element, C the hydraulic element conductivity once displacements have been taken into account, a© the initial aperture (for a channel) or initial radius (for a tube), a the aperture or radius once displacements have been taken into account, and default exp is 3 for a channel or 4 for a tube. This behaviour gives rise to a semi-coupled approach (i.e. the influence of mechanical computation on hydraulic properties).
4 MODELLING OF THE FEBEX CASE 4.1 The "geomodel" In order to perform hydromechanical computations, the choice of which discontinuities to represent must be a judicious one. We only retained the 3 sets of fractures with the most significant hydraulic impact (S1+S2; K2 and L), as shown in Figure 2. Selection of specific fractures to be introduced into the model consisted of: indexing the fractures identified in boreholes within the vicinity of FEBEX and the FEBEX tunnel itself; selecting open fractures that could play a hydraulic role according to descriptive criteria (we have retained those fractures considered as being open by the geophysical log and indicated by the TLV probe);
keeping discontinuities, whose hydraulic role was demonstrated by hydraulic tests and water flow measurements conducted in the boreholes. This choice was also influenced by the fact that HM3D does not allow introducing a large number of fractures. Fractures in the same area with the same orientation were considered as a single fracture (with dip and azimuth being averaged from all individual fractures taken into account): a joint in the model represents a set of in situ fractures. A horizontal view at the altitude of the FEBEX tunnel (1,730 m) of the geomodel obtained is presented in Figure 3. The horizontal dimensions of the model were dictated by the hydraulic boundaries, as will be explained below. The model is 150 m wide in the north-south direction and 200 m wide east-west. Moreover, it is 200 m high (100 m above and below the tunnel altitude).
4.2 Mechanical boundaries and initial conditions The initial stresses and boundary stress conditions were introduced in accordance with the in situ data described in Section 2.3. Vertical stress varies between 8.04 MPa and 13.4 MPa, depending on the lithostatic pressure (with the overburden varying from 300 to 500 m). The maximum horizontal stress is constant, equal to 34.5 MPa (mean of the measured values) and oriented N 130° (i.e. perpendicular to S2 schistosity), whereas the minimum horizontal stress is 23.5 MPa (mean of the measured values) and oriented N 40° (i.e. parallel to S2 schistosity).
4.3 Hydraulical boundary conditions, initial conditions and property calibration On the whole, flow is moving from within the massif towards the laboratory tunnel (from west to east). Before excavation of the FEBEX tunnel, the equipotentials were roughly parallel to the wall of this main tunnel. A zero pressure has been assumed in the laboratory tunnel. To the north and south, the model is constrained by two major shear zones S1+S2, as specified in Section 2.2, Figure 3 displays the hydraulic boundary conditions adopted, in a plane view. The head and gradient adopted were deduced both from measurements conducted within the boreholes intersecting our
153 model boundary and from the regional model developed by Vobomy et al (1991). In the third dimension, the upper and lower surfaces have been considered as impervious. The conductivity and specific storativity of the fracture were obtained by calibration in a purelyhydraulic computation. In each fracture, two perpendicular sets of channels 2 meters apart were generated. At first, the same initial aperture of 10^ was assigned to all fractures, but since a joint in 3FLO represents several in situ discontinuities (see Section 4.1), the conductivity Cond was calculated as follows:
Cond =
n^^a^
(2)
where a is the aperture, n the number of discontinuities represented by a joint in the model, p the water mass density (1,000 kg/m3), g the gravity constant (9.81 m.s-2) and fi the water dynamic viscosity (^i = 10'^ Pa.s). The calibration process was performed iteratively by simulating the test (in general, an imposed zero pressure) for each borehole interval until simulated flow equals measured flow. The first calibration step consists of considering each interval separately and leads to allotting an initial conductivity to each fracture and each test. The iteration then makes it possible to consider the whole intervals and the whole fractures, even those that are not directly in connection with the borehole interval targeted for calibration. Furthermore, in order to simulate the head variation observed in the four boreholes (ROUS 85.001, BOUS 85.002, FEBEX 95.002 and FEBEX 95.002), the conductivity assigned to the fractures was reduced within the western part of the model. The calibration step leads to assigning hydraulic apertures varying from 10 to 20 nm. Let's note herein that the proposed model is not unique: the calibration of another "geomodel" also compatible with the observations could have led to a different set of parameters in good accordance with local information. Pressure Static Recovery tests were modelled (within a purely-hydraulic computation) in order to calibrate fractures storativity. The difference between computed storativity, which takes only joint aperture into account, and calibrated storativity reveals the major role played by rock
mass on the storativity and/or the importance of weak joint stiffness. Since further computations were merely conducted under steady-state conditions, storativities are not subsequently used.
4.4 Purely-hydraulic computation As part of the "DECOVALEX" project, we were asked to predict the total water flow rate to the excavated tunnel over the last 17.40 meters of FEBEX tunnel section (between 54.00 and 71.40 m). A purely-hydraulic computation, using the previous calibration, yields an estimated steadystate flow of between 6 and 7.6 ml/min. This size scale is to be compared with total water inflow estimated from a semi-quantitative hydrogeological map of the FEBEX tunnel: 7.8 ml/min (with an estimation of about 27% of inflow water coming through the matrix). In comparison with the continuous prediction (Alonso et al, 2001), this estimation is rather good and the discontinuous approach allows localizing water output with greater precision. (This kind of comparison was not included within the "DECOVALEX" task.) For the prediction of water head changes induced by FEBEX tunnel boring, it was impossible to simulate a transient evolution due to changes in geometry with HM3D. We chose to model tunnel boring in 4 excavation phases and then simulate the steady state corresponding to the end of each phase. Figure 4 shows the comparison carried out between the measured and simulated pressures. The general shape of the curves obtained with hydraulic simulation is quite similar to measurement results, except there is a constant difference of about 1 bar between the P4 measurements and the simulation. This point will be discussed in Section 4.5 below. Our simulation approach (a hydraulic steady flow analysis with four steady-state steps) was not able to reproduce pore pressure increase (led to a higher horizontal stress than vertical stress) as excavation neared the monitored borehole intervals and as post-tunnel face dissipation was completed.
4,5 Hydromechanical computation A hydromechanical computation, which takes both the mechanical boundary and initial conditions described in Section 4.2 plus the hydraulic initial conditions, boundary conditions and calibration described in Section 4.3 into account, has been performed. Only the final stage of the excavation was simulated and this led to a
154 steady-state flow estimate of between 5.7 and 7.2 ml/min, i.e. 5% less than the flow predicted by a purely-hydrauiic computation. The difference in head between the hydraulic and hydromechanical computations is more significant (see Table 1): 6 m, or 0.7 bar in pressure. This discrepancy may explain the observed difference between measurements and simulation in the purely-hydraulic computation (see Figure 4).
sensitive with respect to pore pressure. Due to the partial coupling along with the inability of tools to simulate transient hydromechanical processes, the precise variations associated with tunnel progression have not been reproduced. The result however incites use of the discontinuous approach even if there is a need of improving the computer codes because they enables to reproduce accurately the localisation of water outflow witch is not the case for equivalent porous medium codes.
REFERENCES *
P4 P3 (nmleO* P4(iw)del)'
Figure 4.
Comparison of measured and simulated (by hydraulic computation) pressures over intervals P3 and P4 of the FBX 95002 borehole.
Table 1. Comparison of measured and modelled heads over the P3 and P4 intervals in both a purelyhydraulic model (H) and a hydromechanical (HM) computation
FBX 95002 Measured Simulated Simulated Intervals head (m) head (m) head(m) H HM P3
1,787
1,781
1,787
P4
1,769
1,778
1,784
5 CONCLUSION FEDEX excavation modelling using discontinuous codes has proven the ability of this conceptual approach to reproduce both the global flow rate towards the excavated tunnel and the general shape of head variation in the surrounding area. The refinement introduced by a hydromechanical computation, in comparison with a purely-hydraulic computation, only mildly influences (5%) flow estimation, yet remains more
Alonso E.E. & Alcoverro J. 2001. DECOVALEX III, Task 1: Modelling of FEBEX in-situ test, Introduction report and comparison of results by UPC. Internal Report. Kadiri I., Merrien-Soukatchoff V. & Su K. 2002, Discontinuous Hydromechanical Modeling of the impact of the FEBEX tunnel (Grimsel, Switzerland). 5th European Conference on Numerical Methods in Geotechnical Engineering (NUMGE) 2002, 4-6 September 2002, Paris, France, pp. 585-590. Kadiri I. 2(X)2. Modelisation hydromecanique des milieux fractures, Ph.D thesis, LAEGO, Ecole des Mines, INPL, France, 28 October 2002, 143 p. Keusen H.R., Ganguin J., Schuler P. & Buletti M. 1989. Grimsel Test Site. Geology. NAGRA, NTB87-14E,Feb. 1989. Ortuno F. 2000. Discrete water inflow measurements at the FEBEX tunnel DECOVALEX III, Task I: Modelling of FEBEX in-situ test. Internal Report. July 20(X). Pahl A., Heusermann St., Brauer V. & Gloggler W. 1989. Grimsel Test Site. Rock stress investigations. NAGRA, NTB 88-39E. Pardillo J., Campos R. & Gimera J. 1997. Caracterizacion geologica de la zona de ensayo FEBEX (Grimsel - Suiza). CIEMAT, 70-IMAM-2-01,May 1997. Vobomy O., Adank P., Hurlimann W., Vomvoris S. & Mishra S. 1991. Grimsel Test Site. Modelling of groundwater flow in the rock body surrounding the underground laboratory. NAGRA, NTB 91-03.