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Similarities in the hydromechanical response of Callovo-Oxfordian clay and Boom Clay during gallery excavation
Physics and Chemistry of the Earth 33 (2008) S343–S349
Contents lists available at ScienceDirect
Physics and Chemistry of the Earth journal homepage: www.elsevier.com/locate/pce
Similarities in the hydromechanical response of Callovo-Oxfordian clay and Boom Clay during gallery excavation Y. Wileveau a, F. Bernier b,* a b
ANDRA – French Radioactive Waste Management Agency, France ESV EURIDICE GIE – European Underground Research Infrastructure for Disposal of Nuclear Waste in Clay Environment, Belgium
a r t i c l e
i n f o
Article history: Available online 14 October 2008 Keywords: EDZ (excavation damaged zone) In situ experiments Fractures Clays Hydromechanical coupling
a b s t r a c t Several Underground Research Laboratories (URLs) in Europe have been dedicated to scientific programs to characterize the confining properties in clay layer geological formations in the frame of geological disposal of high level waste and spent fuel. The URLs are an integral part of national programmes as they provide the scientific knowledge necessary to be gathered in a safety and feasibility case (SFC). An important item for the long-term safety of underground disposal is the assessment of the extent of the damaged zone induced by both the excavation process and the thermal impact of the repository. The paper compares the hydromechanical response and the induced fracture patterns observed during recent excavation works performed at the Bure underground research laboratory in the indurate Callovo-Oxfordian clay and at the HADES underground research laboratory in the plastic Boom Clay. A comparison of the thermo-hydro-mechanical responses is given as well. Despite the important difference in their characteristics the observed thermo-hydro-mechanical response is quite similar for both types of claystone. Ó 2008 Published by Elsevier Ltd.
1. Introduction In order to demonstrate the feasibility of a radioactive waste repository in geological formation, implementing and regulatory organisations have built Underground Research Laboratories since the 1980s. In France, the French National radioactive waste management (Andra) started in 2000 to build an URL in Bure (boundary between the Meuse and Haute-Marne Departments) located nearly 300 km East of Paris (Fig. 1). The target horizon for the laboratory is a 130 m thick layer of argillaceous rocks that lies between about 420 and 550 m below the surface at the URL site. From a lithological view point, the depositional period straddles the Callovian and Oxfordian subdivisions of the middle to upper Jurassic. Argillaceous rocks contain a mixture of clay minerals and clay-sized fractions of other compositions. The clays, which constitute 40–45% on average of the Callovo-Oxfordian argillaceous rocks, offer groundwater isolation and radionuclides retention. Silica and carbonaterich sedimentary components strengthen the rock to contribute to stability of the underground construction (Andra, 2005). The Callovo-Oxfordian clays (COX) are overlaid and underlaid by poorly
* Corresponding author. E-mail addresses: [email protected] (Y. Wileveau), [email protected] (F. Bernier). 1474-7065/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.pce.2008.10.033
permeable carbonate formations. The first URL facilities were finished end of 2005 leaving room to the experimental phase. In Belgium, a Tertiary clay formation, the Boom Clay, present under the Mol-Dessel nuclear site between 190 m and 290 m, was selected as a potential host formation for the disposal of HLW (high level and long lived radioactive waste). The first construction phase of the underground research facility HADES (high-activity disposal experimental site), at a depth of about 223 m, started in 1980. Since then, HADES has been expanded several times. Fig. 2 shows the construction history and Table 1 gives basic information about each phase. The total length of HADES is about 200 m with an average internal diameter of about 4 m. HADES is currently managed by the Economic Interest Grouping EURIDICE (European Underground Research Infrastructure for Disposal of Nuclear Waste in Clay Environment), a joint venture between SCKCEN (Belgian Nuclear Research Centre) and NIRAS (the National Agency for Radioactive Waste and Enriched Fissile Materials). In both URLs the main objective of the research, is to characterize the confining properties of the clay through in situ hydrogeological tests, chemical measurements and diffusion experiments. The URLs aims also to demonstrate that the construction and operation of a geological disposal will not introduce unacceptable preferential pathways for radionuclides migration (Delay et al., 2007). The thermo-hydro-mechanical behaviour of claystones and the EDZ (excavation damaged zone) characterization are two of the
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Y. Wileveau, F. Bernier / Physics and Chemistry of the Earth 33 (2008) S343–S349
Fig. 1. General view of the underground research laboratory at the Meuse/Haute-Marne URL.
Fig. 2. Construction history of the HADES URF (Underground Research Facility).
Table 1 Comparative table: Boom Clay and COX characteristics. Rock parameter
Ind.
COX claystone
Boom Clay
Young’s modulus (MPa) Poisson’s ratio Friction anglea (°) Cohesiona (MPa) Dilation anglea (°) Saturated permeability (m s1) Porosity (%)
E0
4000 0.3 25 7 0° 1 1013 15
300 0.125 18 0.3 0–10 2–4 1012 39
a b
m0 /0 c0
w0 b Kw n
Average out over a range of mean effective stress 2.5–4 MPa. For the numerical analysis w0 = 0 has been adopted.
key issues being investigated in underground experiments. Indeed excavation of underground drifts generally causes damage to the rock in the vicinity of the openings. The level of damage depends, among other factors, on rock properties, stress field, geometry of the openings, excavation method and time. Due to the stress redistribution during the excavation and subsequent rock convergence, an EDZ fracture network consisting of unloading joints and shear
fractures could appear in the vicinity of the openings. Within the EDZ the mechanical and hydraulic rock properties are changed. In the Bure URL, two major experiments were carried out in order to understand the rock response: the REP experiment – a vertical mine by test during the shaft sinking by drill and blast method (Armand et al., 2006) and the SUG-SMR1.1 experiment (Wileveau et al., 2006) – a horizontal mine by test during excavation by classical pneumatic hammer method at the main level 490 m depth. Moreover, the TER experiment consisting to heat the clay rock has been started early in 2006. The process is observed on a long period of time in order that a significant change can take place and allow to observe and to investigate thermal and thermo-hydro-mechanical responses of the Callovo-Oxfordian argillites. In the HADES URL, major progresses were made during the construction of the connecting gallery excavated in 2002. Indeed, the EC CLIPEX instrumentation programme (Clay Instrumentation Programme for the Extension of an Underground Research Laboratory) provided a unique opportunity to monitor the hydromechanical response of Boom Clay during the excavation. Several small scale heater experiments (CACTUS, BACCHUS, CERBERUS and ATLAS – Bernier and Neerdael, 1996) were realised as well to study the thermo-hydro-mechanical behaviour of Boom Clay.
Y. Wileveau, F. Bernier / Physics and Chemistry of the Earth 33 (2008) S343–S349
The paper provides a comparative discussion of the coupled hydromechanical processes observed in the Meuse/Haute-Marne and HADES URLs. 2. Characteristics of COX and Boom Clay At Bure, the claystone COX studied is a sedimentary rock called argillite. Main experimental tests have been conducted on samples cored to give the main properties of the rock. At a 490-m depth the argillite shows a rather homogeneous mineralogical composition of quartz (18%), calcite (25%) and clay minerals (55%) together with subordinate feldspars, pyrite and iron oxides (2%). The clay minerals composition is around 65% I/S (illite–smectite interstratified minerals), 30% illite and 5% kaolinite and chlorite. At the microscopic level, quartz and large calcite grains are scattered in a fine matrix of clay minerals and calcite which acts to cement the larger grains. Clay minerals are grouped in clusters of some microns large that can coat very well the grain form. Finally, only a few microcracks were observed in the unstressed samples. Due to a very small average pore diameter (0.02 lm) this material has a low permeability (5 1020–5 1021 m2) even with an average value of porosity between 11% and 14%. Natural water content of samples is estimated to range between 3% and 7%. According to the values of porosity and natural water content, the in situ samples are in nearly saturated condition. Pore-pressure at the level of the main galleries (490 m depth) is measured around 4.5 MPa. The mechanical behaviour is closely coupled with the pore pressure and the degree of saturation. In situ stress in the argillite layer is: rz = q g Z; rh rv, rH/rh close to 1.3, with magnitude varying with depth and with the rheological characteristics of the respective layers (Andra, 2005). The horizontal major stress is oriented NE155°. Geomechanical and hydraulic properties are resumed in Table 1. At Mol-Dessel nuclear site, the Boom Clay is present 190–290 m below ground level. The Boom Clay layer is almost horizontal (it dips 1–2% towards the NE) and water bearing sand layers are situated above and below it. The total vertical stress and pore water
pressure at the level of HADES are respectively some 4.5 and 2.2 MPa. The Boom Clay is characterised by a rather constant chemical and mineralogical composition. The over-consolidation ratio (OCR) is about 2.4 and the UCS (unconfined compressive strength) is some 2 MPa. The hydraulic and geo-mechanical characteristics of the undisturbed Boom Clay are summarized in Table 1. 3. Observations performed in COX In order to observe the hydromechanical behaviour of the argillites the REP zone and the SUG zone have been instrumented to monitor pore pressure evolution around openings. In both cases, the effect of digging has been clearly identified. In REP experiment, one of the goals was to monitor the hydromechanical behaviour of the argillites around the REP zone (460 m to 476 m). A matrix of 20 pore pressure chambers has been installed around the shaft at different location and depth. When the shaft advancing has restarted after initial characterization, a strong hydromechanical coupling was observed during the shaft sinking at the depth of the chambers. Few months later, a second hydromechanical (HM) effect was observed when the northern and southern galleries have been dug and connected. Fig. 3 shows the HM coupling observed in borehole REP2104 located at 13 m from the shaft wall and about 25 m from the galleries excavated at the main level (490 m depth). The reaction to the excavation on both periods indicates a decrease of 1–2 bar due to the first episode (shaft) and 4–5 bar during the second perturbation. In SUG experiment, only one borehole has been dedicated to the HM observation (SUG1102). Five piezometer chambers are installed far away in the rock mass ahead the front face of a gallery excavated in the direction of the minor horizontal stress. The most remote piezometer (PRE1) is located 20 m away to the front face (Fig. 4a). The closest (PRE5) is approximately at 10 m. A smooth over pressure (a few bar) has been monitored at the resumption of the excavation. The sensors are emplaced at 2.2 m from the wall side of the gallery. Fig. 4a depicts the accelerated drop of the pore
Shaft depth (m)
-450 -460
Borehole RE2104 at 13 m from the shaft wall
Chamber depth -470 -480 -490 -500 -510 50 45
19/03/05
17/06/05
15/09/05
14/12/05
14/03/06
15/09/05
14/12/05
14/03/06
Pulse test
Pore pressure (bar)
40 35 30 25 20 15 10 5 0 19/03/05
17/06/05
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Fig. 3. Hydromechanical coupling observed during the REP experiment in borehole REP2104.
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Y. Wileveau, F. Bernier / Physics and Chemistry of the Earth 33 (2008) S343–S349 60
Front Progress
Sensor installed 2.2 m from the wall of drift
25
40
20
(H) Pore Pressure SUG1102_PRE05 (H) Pore Pressure SUG1102_PRE01
30
15 10
Open fractures
20
5
10
Pore pressure (bar)
Pore pressure (bars)
50
0
0 -20
-16
-12
-8 -4 0 4 Distance from the front (m)
8
12
-5
16
0
3
6
9
12
15
18
21
24
27
30
Distance from the excavated front (m)
a) Meuse/Haute-Marne URL
b) HADES URL
Fig. 4. Hydromechanical responses in Callovo-Oxfordian from SUG experiment in borehole SUG1102 (left) – typical pore water pressure evolution ahead of the excavation front in Boom Clay – example from the construction of the connecting gallery (right).
Diameter reduction of the Test Drift ( Dext=4.7m ) 180 160
conv. vert. terrain, sH conv. horiz. terrain, sH
convergence (mm)
140
conv. vert. terrain, sh
120
conv. horiz. terrain, sh
100 80 60 40
Diameter reduction Δ D/D0 [%]
1.8 1.6 1.4 1.2 29
1
43
0.8
52
0.6
71
0.4 0.2
20 temps (j)
0 0
100
200
300
400
500
600
700
0
0.1
1
10
100
Time since 1/1/1987 [year]
a) Meuse/Haute-Marne URL
b) HADES URL
Fig. 5. Mean convergences of galleries observed at the main level of Bure site over a period of 650 days (left) – diameter reduction at four sections located in the central part of the Test-Drift (right).
Fig. 6. Herringbone patterns observed at Bure site in the gallery parallel to rH (left) and herringbone pattern observed around the connecting gallery at HADES (right).
pressure (reduction of 20–30 bar) due to the mechanical release of the rock around the gallery when the front face reaches the sensor. Even after the passage of the working face, the pressure is still positive and no significant hydraulic connection between chambers and gallery can be deduced. Geotechnical measurements have been carried out during the drifts excavation. The drifts are excavated in the two perpendicular directions of the in situ horizontal stress (rH and rh). The drifts are lined with metallic ribs and rock bolts. The excavated section (surface about 17 m2) is horseshoe shaped. The digging has been performed with classical pneumatic hammer. Measurement sections have been instrumented very close to the front face using conver-
gence meters and radial extensometers. The mechanical behaviour in these perpendicular sections is very different and strongly linked with the in situ stress anisotropy. As example, Fig. 5a shows the mean convergences (vertical and horizontal) in the two directions. The time dependent deformations are very low (few mm per month after 650 days from excavation). EDZ characterization has been deeply studied. Several measuring methods were used to assess the extension of these damaged zones: structural analysis of the core samples, geological survey on the front face and on wall sides, geophysical measurements, hydraulic tests, and resin fracture impregnation in the EDZ zone.
Y. Wileveau, F. Bernier / Physics and Chemistry of the Earth 33 (2008) S343–S349
One can characterize the EDZ as follows: – The herringbone fractures are initiated ahead the working faces during the excavation. They are generally more pronounced when the drifts are parallel to rH. The chevron is created symmetrically to the horizontal plane crossing the gallery axis. The inclination of this pattern with the horizontal plane is around 45° (Fig. 6a). Extension ahead the front face is close to 1 diameter (about 4 m). On the cross section, extension is different for the two main drifts orientations (Armand et al., 2007). – Vertical and oblique fractures are created after the chevron fractures and come against the first one. They are also oriented ahead the working face with a small inclination with the wall side. The combined geophysical methods which have been tested in drifts are microseismic logs along boreholes and tomography sections. The extent of the EDZ zone can be assessed by measuring the velocity decrease corresponding to the damage of the rock. Some microseismic logging has not only revealed the continuous increase of velocity with respect to the radial distance but also some located strong decrease in velocity which can correspond to a located shearing fracture. In the thermal experiment conducted in the COX clay (called TER), a large number of measurements of temperature, pore pressure and deformations were monitored at different heating periods. Main THM coupling parameter values interpreted with axisymmetrical modelling give a satisfactory comparison between the measurements and the results of modelling. Fig. 8a depicts a typical THM response observed in the TER experiment. In both heating regimes the mean pore pressure gradient is calculated with 3 bar/°C. 4. The observations performed in Boom Clay During the construction of HADES, many geophysical measurements have been performed around excavations. Comparison between in situ measurements and modelling results allowed a continuous improvement of our knowledge on the HM behaviour of Boom Clay. This knowledge led to the use of improved excavation techniques which significantly reduce the excavation damaged zone (EDZ). Major progress was made during the construction of the connecting gallery excavated using a tunnelling machine in 2002. The EC CLIPEX instrumentation programme provided a unique opportunity to monitor the hydromechanical response of Boom Clay during the excavation (Bernier et al., 2002). During the excavation of the connecting gallery a progressive increase of the pore water pressure was observed ahead of the excavation front fol-
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lowed by a sharp drop as the excavation front approached very closely (Fig. 4b). The pressure response and mechanical displacement are strongly coupled. The high decompression of the formation nearby the excavation face generates the pore water suction (negative pore pressure). One important finding was the occurrence of measurable hydraulic effects at a distance about 60 m (12.5 tunnel diameters) ahead of the tunnel excavation. This far field behaviour remains difficult to explain (Bernier et al., 2007). Fractures were systematically induced by the excavation during the construction of the connecting gallery. The observations of the front and the sidewalls allowed the fracture pattern in the surrounding formation to be determined. The orientation of the encountered fractures is consistent along most of the excavation. It consists of two conjugate fracture planes: one in the upper part, dipping towards the excavation direction, the other in the lower part, dipping towards the opposite direction (Fig. 6b). The two planes were curved and intersected at mid height of the gallery. Also, some cored borings were carried out after the tunnel was constructed. They indicated a radial fracture extent of about 1 m. It is interesting to note that the observed herringbone fracture pattern is similar to the fracture pattern observed on a smaller scale along cores as a result of the drilling of these cores (Fig. 7). The suction created at about 3 m ahead of the excavation front was followed by an abrupt recovery up to the atmospheric pressure as the front was coming closer. This sudden re-equilibrium with the atmospheric pressure indicated the opening of fractures in the formation ahead of the excavation front up to a distance of about 2– 3 m along the gallery axis. Viscosity of the skeleton (creep, relaxation, etc.) and pore water pressure dissipation imply long-term effects around underground excavations. The convergence of the Test-Drift has now been measured over a period of 19 years. Fig. 5b shows the diameter reductions of 4 sections from the central part of the Test-Drift. The TestDrift was lined with un-reinforced concrete blocks, 64 segments per ring, separated by compressible linex plates. The linex plates are 8 mm thick wooden plates with an assumed E-modulus of 100 MPa. The decrease of the lining diameter was fastest during the first year after construction. The rate of convergence has progressively slowed since the initial excavation but convergence is still continuing, currently (some 18 years after construction) at a rate of about 0.5 mm/year. Since the measurements started, the diameter was reduced by some 60 mm. Several small scale heater experiments (CACTUS, BACCHUS, CERBERUS and ATLAS) were realised at Mol (Bernier and Neerdael, 1996). All experiments show a similar behaviour. During heating, the dilation of the water induces a fast increase in the pore water pressure (0.3 bar/°C) and the total stress. The pore water dissipates with time, inducing a decrease in pore water pressure towards equilibrium (see Fig. 8b). When heating is stopped, the
Fig. 7. Fractures pattern along a COX (left) and a Boom Clay (right) borehole cores induced by the drilling processes.
Y. Wileveau, F. Bernier / Physics and Chemistry of the Earth 33 (2008) S343–S349 60
6
(H) Pore Pressure (TER1404)
34
5
(T) Temperature (TER1404)
32
4
30
3
28
2
26
1
24
0
22
Temperature (°C)
Pore pressure (MPa)
36
25/09/2005 23/01/2006 23/05/2006 20/09/2006 18/01/2007 18/05/2007 15/09/2007
Pore pressure, total stress (MPa)
4.0
7
55
3.5
M (total stress) 50
3.0
45 2.5 40
H (pore pressure)
2.0
35
T (temperature)
1.5
30
1.0
Temperature (°C)
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25
0.5
20
0.0
15 1
2
3
4
5
6
7
Time since 01/01/92 (year)
Date
a) Meuse/Haute-Marne URL
b) HADES URL
Fig. 8. Typical THM response during a thermal load – example from the TER experiment (left) and ATLAS experiment (right).
inverse phenomenon is observed. The total stress decreases to values near the initial values. The pore water pressure, which is suddenly no more submitted to stress, decreases to low values. 5. Discussion The hydromechanical and the thermo-hydro-mechanical responses around excavations were characterised for two different types of clay: the indurate Callovo-Oxfordian clay and the Boom Clay. For both clays, the pore pressure evolved regularly during excavation of a gallery with the coming excavation front: a progressive increase followed by a sharp drop as the excavation front approached very closely (Fig. 4). The increase of the pore pressure corresponds to the undrained contractant plastic behaviour of the clay. The drop phenomenon is linked to the decompression of the rock, and also to the fracturing ahead the excavation front. Important findings are the unpredicted hydraulic perturbation at large distance (>30 m) from the excavation inside the formation in both clays. An initial attempt has been made to explain this phenomenon. It may be partly associated with the following two factors: 1. The apparent increase of the excavation radius: the fractured zone observed around the connecting gallery (see hereafter) can be seen as an apparent increase of the excavated radius, thereby extending the excavation disturbed zone. The associated increase of the permeability in the highly disturbed zone around the gallery reinforced this far field phenomenon. 2. The viscosity of the skeleton: preliminary numerical research indicated that the hydraulic perturbation zone is larger when using an elastoplastic–viscoplastic model than that obtained with an elastoplastic model (Barnichon and Volckaert, 2003). The viscous characteristic time for clay is much shorter than its hydraulic characteristic time (Rousset et al., 1993; Djeran et al., 1994). This suggests that the viscous effect may appear very soon after the clay has been subjected to a hydromechanical disturbance, such as excavation, and thus contribute to the far field responses. Very similar fracture patterns around galleries and boreholes were observed. In both clays, herringbone fractures were observed ahead the gallery excavation front and boreholes (Figs. 6 and 7). Around boreholes eye-shaped fractures pattern were observed as well (Blümling et al., 2005). Especially the observation of fractures in plastic clay as Boom Clay is surprising. The slight desaturation and suction created around the excavation made the Boom Clay stiffer and therefore favours the development of fractures. These effects show the difficulty of making a clear distinction between
soft and stiff clays as their behaviour strongly depends on the hydromechanical conditions to which they are submitted. Notice that unloading fractures were only observed in the Mont Terri Rock Laboratory (Switzerland) in Opalinus clay where a lower stress state is assumed. The clear difference of fracture pattern can be correlated with the ratio between in situ stress state and compressive strength of rock mass. The thermo-hydro-mechanical behaviour is also quite similar for both clays. The hydraulic response to heating, i.e. the creation of a pore water peak, occurs in a very similar way in both types of clay and is mainly related to their very low hydraulic permeability. In COX, the water peak (3 bar/°C) is more important than in Boom Clay (0.3 bar/°C) due to its lower hydraulic permeability (see Fig. 8). 6. Conclusions Despite the important difference in the characteristics (especially in terms of water content, uniaxial compression strength and hydraulic conductivity), the observed hydromechanical response and the fracture pattern around excavation are quite similar. The thermo-hydro-mechanical behaviour is similar as well. Although the fundamental processes are to some extend the same, their relative importance and response rates are different. The differences are mainly caused by the high compaction and thus low water content and lower hydraulic conductivity in COX. Taking note that two different research teams, working separately and using different instrumentation devices, are able to point out similar hydromechanical processes, built our confidence in the reliability of our results. Acknowledgements Some results presented in the paper were obtained by the CLIPEX project. This project was co-funded by the European Commission within the fourth framework programme, key action: Nuclear Fission. This support is gratefully acknowledged. References Andra, 2005. Dossier 2005 Argile: evaluation of the feasibility of a geological repository.. Armand, G., Mountaka, S., Su, K., Renaud, V., Wileveau, Y., 2006. Hydromechanical response to a mine by test experiment deep claystone. In: Proceedings of the Sea to Sky Geotechnique 2006, Vancouver, October 2006. Armand, G., Wileveau, Y., Morel, J., Cruchaudet, M., Rebours, H., 2007. Excavated damaged zone (EDZ) in the Meuse Haute-Marne underground research laboratory. In: Proceedings of the 11th Congress of the ISRM, Lisbon, Portugal, 9–13 July, 2007, pp. 33–36. Barnichon, J.-D., Volckaert, G., 2003. Observations and predictions of hydromechanical coupling effects in the Boom Clay, Mol Underground Research Laboratory, Belgium. Hydrogeol. J. 11-1, 193–202.
Y. Wileveau, F. Bernier / Physics and Chemistry of the Earth 33 (2008) S343–S349 Bernier, F., Li, X.L., Bastiaens, W., 2007. Twenty-five years geotechnical observation, testing in the Tertiary Boom Clay formation, Thomas Telford. Géotechnique 57 (2), 229–237. Bernier, F., Li, X.L., Verstricht, J., Barnichon, J.D., Labiouse, V., Bastiaens, W., Palut, J.M., Ben Slimane, J.K., Ghoreychi, M., Gaombalet, J., Huertas, F., Galera, J.M., Merrien, K., Elorza, F.J., Davies, C., 2002. CLIPEX: clay instrumentation programme for the extension of an Underground Research Laboratory. EUR 20619, Commission of the European Communities, Luxembourg. Bernier, F., Neerdael, B., 1996. Overview of in situ thermomechanical experiments in clay: concept, results and interpretation. Eng. Geol. (41), 51–64. Blümling, P., Bernier, F., Lebon, P., Martin, D., 2005. The excavation-damaged zone in clay formations – time-dependent behaviour and influence on performance assessment. In: International Meeting ‘‘Using Natural and Engineered ClayBased Barriers for the Containment of Radioactive Waste”, Tours 14–18 mars, 2005, Physics and Chemistry of the Earth, vol. 32(1–7).
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Delay, J., Vinsot, A., Krieguer, J.M., Rebours, H., Armand, G., 2007. Making of the underground scientific experimental programme at the Meuse/Haute Marne URL, North Eastern France. Phys. Chem. Earth 32, 2–18. Djeran, I., Bazargan, B., Giraud, A., Rousset, G., 1994. Etude expérimental du comportement Thermo-Hydro-Mecanique de l’argile de Boom. Rapport final. Rapport No. 94-002, SCKCEN, Mol, Belgium. Rousset, G., Bazargan, B., Ouvry, J.F., Bouilleau, M., 1993. Etude du comportement différé des argiles profondes, Rapport final. EUR 14438 FR, 1993, Commission of the European Communities, Luxembourg. Wileveau, Y., Renaud, V., Kazmierczak, J.-B., Armand, G., 2006. Rheological characterization of a clay formation from drifts excavation: elastic and elastoplastic approach. In: Proceedings of the Sea to Sky Geotechnique 2006, Vancouver, October 2006.
Contents lists available at ScienceDirect
Physics and Chemistry of the Earth journal homepage: www.elsevier.com/locate/pce
Similarities in the hydromechanical response of Callovo-Oxfordian clay and Boom Clay during gallery excavation Y. Wileveau a, F. Bernier b,* a b
ANDRA – French Radioactive Waste Management Agency, France ESV EURIDICE GIE – European Underground Research Infrastructure for Disposal of Nuclear Waste in Clay Environment, Belgium
a r t i c l e
i n f o
Article history: Available online 14 October 2008 Keywords: EDZ (excavation damaged zone) In situ experiments Fractures Clays Hydromechanical coupling
a b s t r a c t Several Underground Research Laboratories (URLs) in Europe have been dedicated to scientific programs to characterize the confining properties in clay layer geological formations in the frame of geological disposal of high level waste and spent fuel. The URLs are an integral part of national programmes as they provide the scientific knowledge necessary to be gathered in a safety and feasibility case (SFC). An important item for the long-term safety of underground disposal is the assessment of the extent of the damaged zone induced by both the excavation process and the thermal impact of the repository. The paper compares the hydromechanical response and the induced fracture patterns observed during recent excavation works performed at the Bure underground research laboratory in the indurate Callovo-Oxfordian clay and at the HADES underground research laboratory in the plastic Boom Clay. A comparison of the thermo-hydro-mechanical responses is given as well. Despite the important difference in their characteristics the observed thermo-hydro-mechanical response is quite similar for both types of claystone. Ó 2008 Published by Elsevier Ltd.
1. Introduction In order to demonstrate the feasibility of a radioactive waste repository in geological formation, implementing and regulatory organisations have built Underground Research Laboratories since the 1980s. In France, the French National radioactive waste management (Andra) started in 2000 to build an URL in Bure (boundary between the Meuse and Haute-Marne Departments) located nearly 300 km East of Paris (Fig. 1). The target horizon for the laboratory is a 130 m thick layer of argillaceous rocks that lies between about 420 and 550 m below the surface at the URL site. From a lithological view point, the depositional period straddles the Callovian and Oxfordian subdivisions of the middle to upper Jurassic. Argillaceous rocks contain a mixture of clay minerals and clay-sized fractions of other compositions. The clays, which constitute 40–45% on average of the Callovo-Oxfordian argillaceous rocks, offer groundwater isolation and radionuclides retention. Silica and carbonaterich sedimentary components strengthen the rock to contribute to stability of the underground construction (Andra, 2005). The Callovo-Oxfordian clays (COX) are overlaid and underlaid by poorly
* Corresponding author. E-mail addresses: [email protected] (Y. Wileveau), [email protected] (F. Bernier). 1474-7065/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.pce.2008.10.033
permeable carbonate formations. The first URL facilities were finished end of 2005 leaving room to the experimental phase. In Belgium, a Tertiary clay formation, the Boom Clay, present under the Mol-Dessel nuclear site between 190 m and 290 m, was selected as a potential host formation for the disposal of HLW (high level and long lived radioactive waste). The first construction phase of the underground research facility HADES (high-activity disposal experimental site), at a depth of about 223 m, started in 1980. Since then, HADES has been expanded several times. Fig. 2 shows the construction history and Table 1 gives basic information about each phase. The total length of HADES is about 200 m with an average internal diameter of about 4 m. HADES is currently managed by the Economic Interest Grouping EURIDICE (European Underground Research Infrastructure for Disposal of Nuclear Waste in Clay Environment), a joint venture between SCKCEN (Belgian Nuclear Research Centre) and NIRAS (the National Agency for Radioactive Waste and Enriched Fissile Materials). In both URLs the main objective of the research, is to characterize the confining properties of the clay through in situ hydrogeological tests, chemical measurements and diffusion experiments. The URLs aims also to demonstrate that the construction and operation of a geological disposal will not introduce unacceptable preferential pathways for radionuclides migration (Delay et al., 2007). The thermo-hydro-mechanical behaviour of claystones and the EDZ (excavation damaged zone) characterization are two of the
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Y. Wileveau, F. Bernier / Physics and Chemistry of the Earth 33 (2008) S343–S349
Fig. 1. General view of the underground research laboratory at the Meuse/Haute-Marne URL.
Fig. 2. Construction history of the HADES URF (Underground Research Facility).
Table 1 Comparative table: Boom Clay and COX characteristics. Rock parameter
Ind.
COX claystone
Boom Clay
Young’s modulus (MPa) Poisson’s ratio Friction anglea (°) Cohesiona (MPa) Dilation anglea (°) Saturated permeability (m s1) Porosity (%)
E0
4000 0.3 25 7 0° 1 1013 15
300 0.125 18 0.3 0–10 2–4 1012 39
a b
m0 /0 c0
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key issues being investigated in underground experiments. Indeed excavation of underground drifts generally causes damage to the rock in the vicinity of the openings. The level of damage depends, among other factors, on rock properties, stress field, geometry of the openings, excavation method and time. Due to the stress redistribution during the excavation and subsequent rock convergence, an EDZ fracture network consisting of unloading joints and shear
fractures could appear in the vicinity of the openings. Within the EDZ the mechanical and hydraulic rock properties are changed. In the Bure URL, two major experiments were carried out in order to understand the rock response: the REP experiment – a vertical mine by test during the shaft sinking by drill and blast method (Armand et al., 2006) and the SUG-SMR1.1 experiment (Wileveau et al., 2006) – a horizontal mine by test during excavation by classical pneumatic hammer method at the main level 490 m depth. Moreover, the TER experiment consisting to heat the clay rock has been started early in 2006. The process is observed on a long period of time in order that a significant change can take place and allow to observe and to investigate thermal and thermo-hydro-mechanical responses of the Callovo-Oxfordian argillites. In the HADES URL, major progresses were made during the construction of the connecting gallery excavated in 2002. Indeed, the EC CLIPEX instrumentation programme (Clay Instrumentation Programme for the Extension of an Underground Research Laboratory) provided a unique opportunity to monitor the hydromechanical response of Boom Clay during the excavation. Several small scale heater experiments (CACTUS, BACCHUS, CERBERUS and ATLAS – Bernier and Neerdael, 1996) were realised as well to study the thermo-hydro-mechanical behaviour of Boom Clay.
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The paper provides a comparative discussion of the coupled hydromechanical processes observed in the Meuse/Haute-Marne and HADES URLs. 2. Characteristics of COX and Boom Clay At Bure, the claystone COX studied is a sedimentary rock called argillite. Main experimental tests have been conducted on samples cored to give the main properties of the rock. At a 490-m depth the argillite shows a rather homogeneous mineralogical composition of quartz (18%), calcite (25%) and clay minerals (55%) together with subordinate feldspars, pyrite and iron oxides (2%). The clay minerals composition is around 65% I/S (illite–smectite interstratified minerals), 30% illite and 5% kaolinite and chlorite. At the microscopic level, quartz and large calcite grains are scattered in a fine matrix of clay minerals and calcite which acts to cement the larger grains. Clay minerals are grouped in clusters of some microns large that can coat very well the grain form. Finally, only a few microcracks were observed in the unstressed samples. Due to a very small average pore diameter (0.02 lm) this material has a low permeability (5 1020–5 1021 m2) even with an average value of porosity between 11% and 14%. Natural water content of samples is estimated to range between 3% and 7%. According to the values of porosity and natural water content, the in situ samples are in nearly saturated condition. Pore-pressure at the level of the main galleries (490 m depth) is measured around 4.5 MPa. The mechanical behaviour is closely coupled with the pore pressure and the degree of saturation. In situ stress in the argillite layer is: rz = q g Z; rh rv, rH/rh close to 1.3, with magnitude varying with depth and with the rheological characteristics of the respective layers (Andra, 2005). The horizontal major stress is oriented NE155°. Geomechanical and hydraulic properties are resumed in Table 1. At Mol-Dessel nuclear site, the Boom Clay is present 190–290 m below ground level. The Boom Clay layer is almost horizontal (it dips 1–2% towards the NE) and water bearing sand layers are situated above and below it. The total vertical stress and pore water
pressure at the level of HADES are respectively some 4.5 and 2.2 MPa. The Boom Clay is characterised by a rather constant chemical and mineralogical composition. The over-consolidation ratio (OCR) is about 2.4 and the UCS (unconfined compressive strength) is some 2 MPa. The hydraulic and geo-mechanical characteristics of the undisturbed Boom Clay are summarized in Table 1. 3. Observations performed in COX In order to observe the hydromechanical behaviour of the argillites the REP zone and the SUG zone have been instrumented to monitor pore pressure evolution around openings. In both cases, the effect of digging has been clearly identified. In REP experiment, one of the goals was to monitor the hydromechanical behaviour of the argillites around the REP zone (460 m to 476 m). A matrix of 20 pore pressure chambers has been installed around the shaft at different location and depth. When the shaft advancing has restarted after initial characterization, a strong hydromechanical coupling was observed during the shaft sinking at the depth of the chambers. Few months later, a second hydromechanical (HM) effect was observed when the northern and southern galleries have been dug and connected. Fig. 3 shows the HM coupling observed in borehole REP2104 located at 13 m from the shaft wall and about 25 m from the galleries excavated at the main level (490 m depth). The reaction to the excavation on both periods indicates a decrease of 1–2 bar due to the first episode (shaft) and 4–5 bar during the second perturbation. In SUG experiment, only one borehole has been dedicated to the HM observation (SUG1102). Five piezometer chambers are installed far away in the rock mass ahead the front face of a gallery excavated in the direction of the minor horizontal stress. The most remote piezometer (PRE1) is located 20 m away to the front face (Fig. 4a). The closest (PRE5) is approximately at 10 m. A smooth over pressure (a few bar) has been monitored at the resumption of the excavation. The sensors are emplaced at 2.2 m from the wall side of the gallery. Fig. 4a depicts the accelerated drop of the pore
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Fig. 3. Hydromechanical coupling observed during the REP experiment in borehole REP2104.
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Fig. 5. Mean convergences of galleries observed at the main level of Bure site over a period of 650 days (left) – diameter reduction at four sections located in the central part of the Test-Drift (right).
Fig. 6. Herringbone patterns observed at Bure site in the gallery parallel to rH (left) and herringbone pattern observed around the connecting gallery at HADES (right).
pressure (reduction of 20–30 bar) due to the mechanical release of the rock around the gallery when the front face reaches the sensor. Even after the passage of the working face, the pressure is still positive and no significant hydraulic connection between chambers and gallery can be deduced. Geotechnical measurements have been carried out during the drifts excavation. The drifts are excavated in the two perpendicular directions of the in situ horizontal stress (rH and rh). The drifts are lined with metallic ribs and rock bolts. The excavated section (surface about 17 m2) is horseshoe shaped. The digging has been performed with classical pneumatic hammer. Measurement sections have been instrumented very close to the front face using conver-
gence meters and radial extensometers. The mechanical behaviour in these perpendicular sections is very different and strongly linked with the in situ stress anisotropy. As example, Fig. 5a shows the mean convergences (vertical and horizontal) in the two directions. The time dependent deformations are very low (few mm per month after 650 days from excavation). EDZ characterization has been deeply studied. Several measuring methods were used to assess the extension of these damaged zones: structural analysis of the core samples, geological survey on the front face and on wall sides, geophysical measurements, hydraulic tests, and resin fracture impregnation in the EDZ zone.
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One can characterize the EDZ as follows: – The herringbone fractures are initiated ahead the working faces during the excavation. They are generally more pronounced when the drifts are parallel to rH. The chevron is created symmetrically to the horizontal plane crossing the gallery axis. The inclination of this pattern with the horizontal plane is around 45° (Fig. 6a). Extension ahead the front face is close to 1 diameter (about 4 m). On the cross section, extension is different for the two main drifts orientations (Armand et al., 2007). – Vertical and oblique fractures are created after the chevron fractures and come against the first one. They are also oriented ahead the working face with a small inclination with the wall side. The combined geophysical methods which have been tested in drifts are microseismic logs along boreholes and tomography sections. The extent of the EDZ zone can be assessed by measuring the velocity decrease corresponding to the damage of the rock. Some microseismic logging has not only revealed the continuous increase of velocity with respect to the radial distance but also some located strong decrease in velocity which can correspond to a located shearing fracture. In the thermal experiment conducted in the COX clay (called TER), a large number of measurements of temperature, pore pressure and deformations were monitored at different heating periods. Main THM coupling parameter values interpreted with axisymmetrical modelling give a satisfactory comparison between the measurements and the results of modelling. Fig. 8a depicts a typical THM response observed in the TER experiment. In both heating regimes the mean pore pressure gradient is calculated with 3 bar/°C. 4. The observations performed in Boom Clay During the construction of HADES, many geophysical measurements have been performed around excavations. Comparison between in situ measurements and modelling results allowed a continuous improvement of our knowledge on the HM behaviour of Boom Clay. This knowledge led to the use of improved excavation techniques which significantly reduce the excavation damaged zone (EDZ). Major progress was made during the construction of the connecting gallery excavated using a tunnelling machine in 2002. The EC CLIPEX instrumentation programme provided a unique opportunity to monitor the hydromechanical response of Boom Clay during the excavation (Bernier et al., 2002). During the excavation of the connecting gallery a progressive increase of the pore water pressure was observed ahead of the excavation front fol-
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lowed by a sharp drop as the excavation front approached very closely (Fig. 4b). The pressure response and mechanical displacement are strongly coupled. The high decompression of the formation nearby the excavation face generates the pore water suction (negative pore pressure). One important finding was the occurrence of measurable hydraulic effects at a distance about 60 m (12.5 tunnel diameters) ahead of the tunnel excavation. This far field behaviour remains difficult to explain (Bernier et al., 2007). Fractures were systematically induced by the excavation during the construction of the connecting gallery. The observations of the front and the sidewalls allowed the fracture pattern in the surrounding formation to be determined. The orientation of the encountered fractures is consistent along most of the excavation. It consists of two conjugate fracture planes: one in the upper part, dipping towards the excavation direction, the other in the lower part, dipping towards the opposite direction (Fig. 6b). The two planes were curved and intersected at mid height of the gallery. Also, some cored borings were carried out after the tunnel was constructed. They indicated a radial fracture extent of about 1 m. It is interesting to note that the observed herringbone fracture pattern is similar to the fracture pattern observed on a smaller scale along cores as a result of the drilling of these cores (Fig. 7). The suction created at about 3 m ahead of the excavation front was followed by an abrupt recovery up to the atmospheric pressure as the front was coming closer. This sudden re-equilibrium with the atmospheric pressure indicated the opening of fractures in the formation ahead of the excavation front up to a distance of about 2– 3 m along the gallery axis. Viscosity of the skeleton (creep, relaxation, etc.) and pore water pressure dissipation imply long-term effects around underground excavations. The convergence of the Test-Drift has now been measured over a period of 19 years. Fig. 5b shows the diameter reductions of 4 sections from the central part of the Test-Drift. The TestDrift was lined with un-reinforced concrete blocks, 64 segments per ring, separated by compressible linex plates. The linex plates are 8 mm thick wooden plates with an assumed E-modulus of 100 MPa. The decrease of the lining diameter was fastest during the first year after construction. The rate of convergence has progressively slowed since the initial excavation but convergence is still continuing, currently (some 18 years after construction) at a rate of about 0.5 mm/year. Since the measurements started, the diameter was reduced by some 60 mm. Several small scale heater experiments (CACTUS, BACCHUS, CERBERUS and ATLAS) were realised at Mol (Bernier and Neerdael, 1996). All experiments show a similar behaviour. During heating, the dilation of the water induces a fast increase in the pore water pressure (0.3 bar/°C) and the total stress. The pore water dissipates with time, inducing a decrease in pore water pressure towards equilibrium (see Fig. 8b). When heating is stopped, the
Fig. 7. Fractures pattern along a COX (left) and a Boom Clay (right) borehole cores induced by the drilling processes.
Y. Wileveau, F. Bernier / Physics and Chemistry of the Earth 33 (2008) S343–S349 60
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Fig. 8. Typical THM response during a thermal load – example from the TER experiment (left) and ATLAS experiment (right).
inverse phenomenon is observed. The total stress decreases to values near the initial values. The pore water pressure, which is suddenly no more submitted to stress, decreases to low values. 5. Discussion The hydromechanical and the thermo-hydro-mechanical responses around excavations were characterised for two different types of clay: the indurate Callovo-Oxfordian clay and the Boom Clay. For both clays, the pore pressure evolved regularly during excavation of a gallery with the coming excavation front: a progressive increase followed by a sharp drop as the excavation front approached very closely (Fig. 4). The increase of the pore pressure corresponds to the undrained contractant plastic behaviour of the clay. The drop phenomenon is linked to the decompression of the rock, and also to the fracturing ahead the excavation front. Important findings are the unpredicted hydraulic perturbation at large distance (>30 m) from the excavation inside the formation in both clays. An initial attempt has been made to explain this phenomenon. It may be partly associated with the following two factors: 1. The apparent increase of the excavation radius: the fractured zone observed around the connecting gallery (see hereafter) can be seen as an apparent increase of the excavated radius, thereby extending the excavation disturbed zone. The associated increase of the permeability in the highly disturbed zone around the gallery reinforced this far field phenomenon. 2. The viscosity of the skeleton: preliminary numerical research indicated that the hydraulic perturbation zone is larger when using an elastoplastic–viscoplastic model than that obtained with an elastoplastic model (Barnichon and Volckaert, 2003). The viscous characteristic time for clay is much shorter than its hydraulic characteristic time (Rousset et al., 1993; Djeran et al., 1994). This suggests that the viscous effect may appear very soon after the clay has been subjected to a hydromechanical disturbance, such as excavation, and thus contribute to the far field responses. Very similar fracture patterns around galleries and boreholes were observed. In both clays, herringbone fractures were observed ahead the gallery excavation front and boreholes (Figs. 6 and 7). Around boreholes eye-shaped fractures pattern were observed as well (Blümling et al., 2005). Especially the observation of fractures in plastic clay as Boom Clay is surprising. The slight desaturation and suction created around the excavation made the Boom Clay stiffer and therefore favours the development of fractures. These effects show the difficulty of making a clear distinction between
soft and stiff clays as their behaviour strongly depends on the hydromechanical conditions to which they are submitted. Notice that unloading fractures were only observed in the Mont Terri Rock Laboratory (Switzerland) in Opalinus clay where a lower stress state is assumed. The clear difference of fracture pattern can be correlated with the ratio between in situ stress state and compressive strength of rock mass. The thermo-hydro-mechanical behaviour is also quite similar for both clays. The hydraulic response to heating, i.e. the creation of a pore water peak, occurs in a very similar way in both types of clay and is mainly related to their very low hydraulic permeability. In COX, the water peak (3 bar/°C) is more important than in Boom Clay (0.3 bar/°C) due to its lower hydraulic permeability (see Fig. 8). 6. Conclusions Despite the important difference in the characteristics (especially in terms of water content, uniaxial compression strength and hydraulic conductivity), the observed hydromechanical response and the fracture pattern around excavation are quite similar. The thermo-hydro-mechanical behaviour is similar as well. Although the fundamental processes are to some extend the same, their relative importance and response rates are different. The differences are mainly caused by the high compaction and thus low water content and lower hydraulic conductivity in COX. Taking note that two different research teams, working separately and using different instrumentation devices, are able to point out similar hydromechanical processes, built our confidence in the reliability of our results. Acknowledgements Some results presented in the paper were obtained by the CLIPEX project. This project was co-funded by the European Commission within the fourth framework programme, key action: Nuclear Fission. This support is gratefully acknowledged. References Andra, 2005. Dossier 2005 Argile: evaluation of the feasibility of a geological repository.
Y. Wileveau, F. Bernier / Physics and Chemistry of the Earth 33 (2008) S343–S349 Bernier, F., Li, X.L., Bastiaens, W., 2007. Twenty-five years geotechnical observation, testing in the Tertiary Boom Clay formation, Thomas Telford. Géotechnique 57 (2), 229–237. Bernier, F., Li, X.L., Verstricht, J., Barnichon, J.D., Labiouse, V., Bastiaens, W., Palut, J.M., Ben Slimane, J.K., Ghoreychi, M., Gaombalet, J., Huertas, F., Galera, J.M., Merrien, K., Elorza, F.J., Davies, C., 2002. CLIPEX: clay instrumentation programme for the extension of an Underground Research Laboratory. EUR 20619, Commission of the European Communities, Luxembourg. Bernier, F., Neerdael, B., 1996. Overview of in situ thermomechanical experiments in clay: concept, results and interpretation. Eng. Geol. (41), 51–64. Blümling, P., Bernier, F., Lebon, P., Martin, D., 2005. The excavation-damaged zone in clay formations – time-dependent behaviour and influence on performance assessment. In: International Meeting ‘‘Using Natural and Engineered ClayBased Barriers for the Containment of Radioactive Waste”, Tours 14–18 mars, 2005, Physics and Chemistry of the Earth, vol. 32(1–7).
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Delay, J., Vinsot, A., Krieguer, J.M., Rebours, H., Armand, G., 2007. Making of the underground scientific experimental programme at the Meuse/Haute Marne URL, North Eastern France. Phys. Chem. Earth 32, 2–18. Djeran, I., Bazargan, B., Giraud, A., Rousset, G., 1994. Etude expérimental du comportement Thermo-Hydro-Mecanique de l’argile de Boom. Rapport final. Rapport No. 94-002, SCKCEN, Mol, Belgium. Rousset, G., Bazargan, B., Ouvry, J.F., Bouilleau, M., 1993. Etude du comportement différé des argiles profondes, Rapport final. EUR 14438 FR, 1993, Commission of the European Communities, Luxembourg. Wileveau, Y., Renaud, V., Kazmierczak, J.-B., Armand, G., 2006. Rheological characterization of a clay formation from drifts excavation: elastic and elastoplastic approach. In: Proceedings of the Sea to Sky Geotechnique 2006, Vancouver, October 2006.