SELF-HEALING OF FRACTURES WITHIN THE EDZ AT THE MT. TERRI ROCK LABORATORY : RESULTS AFTER ONE YEAR OF EXPERIMENTAL WORK

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1 SELF-HEALING OF FRACTURES WITHIN THE EDZ AT THE MT. TERRI ROCK LABORATORY : RESULTS AFTER ONE YEAR OF EXPERIMENTAL WORK Peter M. Meier (1), Thomas Trick (), Peter Blümling (3) and Geert Vockaert () (1) ANDRA - Parc de la Croix Blanche 1/7 rue Jean Monnet F 998 Châtenay-Malabry Cedex () SOLEXPERTS Ltd. Schulstrasse 5 CH 803 Schwerzenbach (3) NAGRA Hardstrasse 73 CH 530 Wettingen () SCK.CEN - Boeretang 0 B 00 Mol Abstract After closure of a repository, groundwater of a bounding aquifer might infiltrate along the excavated damaged zone (EDZ) around shafts and galleries, if these zones would be connected. ANDRA, NAGRA and SCK.CEN performed an in-situ experiment at the Mont Terri rock laboratory in order to evaluate if the interaction of clay with infiltrated water can lead to a self-healing of the fractures in the EDZ by processes such as swelling and creep. In a first step the geometry and the air permeability of the fracture network within the EDZ around a 3. m gallery was characterised by pneumatic testing of short radial boreholes drilled into the EDZ. Two experimental sites were selected: a non-saturated single fracture with an extend of about one square meter lying parallel to the wall of the gallery at a depth of about 70 cm, and a non-saturated fracture network within the first 80 cm of the EDZ. The experimental sites were saturated with two types of water in order to check whether the different water chemistry would influence the self-healing capacity significantly. The existence of self-healing processes was clearly detected at both sites : The transmissivity of the single fracture decreased from about E-8 to about 1E-9 m /s within 1 year after artificial saturation, and the transmissivity of the fracture network decreased from about 5E-7 to 1E-8 m /s within 110 days. 1. Introduction Argillaceous formations are widely considered to be suitable as a geological barrier for radioactive waste disposal, and several countries are currently investigating the suitability of various clay formations for this purpose. In Switzerland, a mesozoic shale formation, the Opalinus Clay is being considered as a potential repository host rock. Undeformed Opalinus Clay can be characterised as a stiff, over-consolidated clay with a hydraulic conductivity less than 1E-1 m/s, a Young s modulus varying between '000 MPa (great range due to bedding anisotropy) and a cohesion of greater than 3 MPa. Its clay mineralogy contains mainly sheet silicates (illite, illite-smectite mixed layers, chlorites, kaolinites), framework silicates (albites, K-feldspar), carbonates (calcite, dolomite, ankerite and siderite) and quartz (MAZUREK, 1999). During the construction of a repository in the Opalinus Clay, the host rock properties around the underground openings are expected to be considerably altered. Depending on the mechanical rock properties, initial stress field and applied excavation techniques, plastic deformations around the gallery may occur (plastic in the sense that the Mohr- Coulomb rock law criteria is exceeded and non-elastic deformations occur). Such an altered zone is called an excavation damaged zone (EDZ). Due to the stress re-distribution during the excavation and the proceeding rock convergence, a fracture network of unloading joints and unloading faults is formed in this EDZ. The hydraulic conductivity of these fracture network might be orders of magnitudes higher compared to that in the undeformed Opalinus Clay. Thus, radionuclide transport from the repository to the biosphere might well be likely in the EDZ along repository galleries and shafts. It is of vital interest for the safety analysis and the performance assessment for a potential repository to know whether the interaction of clay with infiltrated water from overlying aquifers can lead to a self-healing of the fractures in the EDZ by processes such as swelling and creep. 1

2 For this purpose ANDRA, NAGRA and SCK.CEN performed a self-healing experiment in the Mont Terri rock laboratory (THURY & BOSSART, 1999), which is situated in the Mont Terri motorway tunnel. The Mont Terri rock laboratory lies in the 10 m thick formation of the Opalinus Clay. In the area where the laboratory is located, the overburden varies between 30 and 3 m and the rock strata dip with an angle of approximately 5 o to the South East. A site map of the northern part of the Mont Terri rock laboratory is shown in Figure 1. It consists of the security tunnel (excavated in 1989), eight side niches along the security tunnel (excavated in 199) and the new gallery (excavated in 1997 and 1998). Three types of excavation techniques were applied during the construction of the rock laboratory: (1) by conventional blasting with electric and electronic detonators, () with a road header and (3) by pneumatic hammering. To avoid swelling and to obtain reliable boundary conditions for future experiments, no water was used during excavation. These excavations lead to the unique opportunity to develop and test structural and hydraulic methods for characterising the excavation disturbed zone in clay formations. Furthermore, structural and hydraulic parameter estimates of the excavation disturbed zone in the Opalinus Clay of the Mont Terri rock laboratory were gained (BOSSART et al., 00). The following chapters present the experimental concept and the results after one year of in-situ experimental work. It must be stressed that the objectives of this first phase was to check whether self-healing processes take place and whether the resulting permeability changes can be monitored in-situ. Further phases should focus on the identification of the processes and techniques to enhance the self-healing capacity. B1 B15 Figure 1 : Map of the Mont Terri Rock Laboratory. Experimental concept The experiment was performed within a section of the new gallery that was excavated with a road header in order to minimise the extend of the EDZ (Figure 1). The experimental procedure consists of the following steps : (1) a search for suitable experimental sites at which the non-saturated part of the EDZ contains high permeable fractures with a lateral extend of about 1 m, () a pneumatic characterisation to determine the initial (air) permeability and the geometry of the fracture system, (3) an artificial saturation of the EDZ (since natural saturation is expected to be slow and would require to control the tunnel climate which is difficult for a small scale experiment), () an instrumentation of some boreholes with extensometers to monitor possible displacements due to swelling or creep perpendicular to the fracture planes, and

3 (5) a monitoring of permeability changes during a long time period (= one year). These steps are described below in more detail. In a first step a total of 80 small diameter (0 mm) radial boreholes with a length between 1.0 and.0 m were drilled through the concrete liner of the gallery to characterise the unsaturated and strongly permeable part of the EDZ by pneumatic testing. Note, that all reported depths along these boreholes include the thickness of the concrete liner which varies between 10 and cm. The strategy was the following : a set of three boreholes with a distance of about cm was drilled together and subsequently tested. More boreholes were added if a high permeable zone connecting the three boreholes was detected by cross-hole pneumatic testing. If the area was not sufficiently permeable for the self-healing experiment a new set of three boreholes was drilled and tested at another site. The pneumatic testing procedure consisted of : (1) An installation of a mechanical double packer with a 10 cm interval into the tested borehole. The surrounding boreholes were instrumented with mechanical single packers such that the contact between the concrete wall and the natural rock was spanned. () Air was then extracted out of the test interval using a vacuum pump. The air flow rates and the pressures in the tested and the observation intervals were recorded with a data acquisition system. A test was stopped when steady state conditions were attained. (3) The double packer system was shifted in increments of 10 cm to obtain a permeability profile for the borehole and a chart indicating the connectivity between the boreholes. A total of six sites were tested (TRICK, 00a). The pneumatic tests revealed a highly heterogeneous EDZ system. The depth of the high permeable non- or partially saturated part of the EDZ varies between and 90 cm. The cross-hole reactions showed abrupt changes of the spatial continuity of high permeable zones. Note, that this does not prove that the high permeable zones are isolated. A regular testing grid would be necessary to investigate the connectivity of these fractures at a larger scale. However, our approach was optimised to find quickly suitable sites for the self-healing experiment. Two suitable sites were found : - Site 0 contains a high transmissive single fracture at a depth of about 70 cm. The orientation of this fracture is parallel to the wall of the gallery and perpendicular to the bedding plane of the Opalinus clay. This orientation indicates that the fracture has been induced as a consequence of the excavation process. The borehole configuration is shown in Figure. Boreholes BEH-1 to BEH-1 with a length of 1.5 m were drilled to perform pneumatic and hydraulic tests. Boreholes BEH-17 to BEH- with a length of about 0-70 cm were drilled for a well distributed artificial saturation of the area. However, it turned out that boreholes BEH-1 to BEH- did not intersect the fracture plane. It is not sure whether this is due to the small borehole length of BEH-17 to BEH- or whether the fracture ends above BEH-1. However, the cross-hole pneumatic tests showed clearly that borehole BEH-1 is not connected to boreholes BEH-1 to BEH Site 1 contains a connected fracture network within the first 80 cm (TRICK, 00b). The predominant fracture orientations seem to be parallel to the wall of the gallery. The air permeability is highly elevated within the first 0 cm and moderate between 0 and 80 cm. The borehole configuration is shown in Figure 3. Two boreholes (BEH-5 and 55) were equipped with high precision extensometers to measure displacements perpendicular to the fracture planes (TRICK, 00c). Such displacements were expected after artificial saturation due to swelling or creep. The conceptual models for the two sites and the experimental layout are shown in Figure. The single fracture was saturated with distilled water and the fracture network with a synthetic formation water in order to check whether a different water chemistry could have a significant influence on the self-healing process. The following saturation procedure was applied at both sites: (1) A plastic filter with an outer diameter of 38 mm was installed in a central borehole prior to saturation to prevent borehole collapse. () The borehole was filled with water and a single packer system was installed. (3) Water was injected through the single packer system using the system shown in Figure. The injection pressure was kept always below 1 bar to avoid an opening of the fractures. Furthermore, the flow rates were kept below 50 ml/min to avoid erosion of the clay. () The breakthrough of water was visually observed in the surrounding boreholes. (5) Plastic filter tubes and single packer systems were installed and the boreholes were fully saturated using a hand pump. () Water was injected every week to ensure saturated conditions during the experimental period of one year. The first hydraulic tests were started immediately after the initial saturation of the sites. Thereafter, the tests were repeated with a decreasing frequency during the initially assumed experimental duration of one year. Water was injected with a constant pressure or with a constant flow rate (Figure ). The injection pressure was always kept below 1 bar to avoid the opening of the fractures. The cross-hole pressure reactions were monitored at all boreholes. 3

4 Mont Terri EH Self Healing Figure 1: Schematic Borehole Configuration - Site EH1 BEH- BEH-9 35 BEH-3 BEH-8 BEH-53 1 BEH BEH-55 BEH BEH BEH- 5 1 BEH-3 7 BEH-71 BEH BEH- 35 BEH-5 BEH-70 Tunnel Meter 8.5 (distances in cm) Figures and 3: Configuration of sub-horizontal boreholes that penetrate the single-fracture at a depth of about 70 cm (upper Figure) and a fracture network within the first 80 cm (lower Figure).

5 Figure : Conceptual model of the two experimental sites and hydraulic testing configuration. 5

6 3. Results The drawdown reactions of each test were normalised with the injection flow rates and were plotted for each observation borehole on a semi-log plot. Examples for such plots are shown in Figure 5 for the hydraulic tests within the single fracture and in Figure for the tests in the fracture network. Similar plots were obtained for the other observation boreholes. The curves are different for each consecutive test indicating a changing transmissivity. Dupuit s formula for steady-state and the Cooper-Jacob method for transient conditions were used for the interpretation of the data (see MARSILY (198) for a detailed description of the methods). A rearrangement of these formulas shows that transmissivity is inversely proportional to flow rate normalised drawdown at the end of the tests and to the slope of the straight line segment of the?s/q data curves : 1 s π = T Q r ln o r w Dupuit s formula (1) where T is transmissivity, Q is the pumping rate,?s is the drawdown at an observation well for steady-state conditions, r o is the radial distance between the pumping and the observation well and r w the radius of the pumping well. And 1 sb sa π = T Q t ln b t a Cooper-Jacob formula () where?s b and?s a are the drawdown data for times t b and t a during the straight-line period on a semi-log plot. Figures 5 and show that the normalised total drawdown (?s/q) at the end of the tests, and the slope of the intermediate time data (from about 100 to 1000 seconds) increase for each consecutive test. This indicates a decreasing transmissivity according to equations 1 and. The data were analysed with the Cooper-Jacob method because it yields estimates for the effective transmissivity even in strongly fractured systems (MEIER et al., 1998). Figures 7 and 8 show the decreasing trend of estimated transmissivity. The transmissivity of the single fracture decreased from about E-8 to about 1E-9 m /s within 1 year after artificial saturation, and the transmissivity of the fracture network decreased from about 5E-7 to 1E-8 m /s within 110 days. The flattening of the?s/q data curves at large testing times (see Figures 5 and ) can be attributed to constant head boundary effects. These occur because the saturated part of the experimental site is surrounded by the non-saturated EDZ in which atmospheric pressure prevails since the high permeable fracture networks are strongly connected to the tunnel walls. Note, that the start of the flattening of the curves is slightly retarded for each of the consecutive hydraulic tests due to the decreasing transmissivity. The extensometer measurements in borehole BEH-55 at site 1 indicate an extension (swelling?) of the area by about 0.1 mm during the first two months after artificial saturation. Figure 8 shows that a large part of the permeability reduction took place during the same period. However, no significant (< 0.0 mm) displacements were detected by the extensometer in BEH-5.

7 10x s/q [sec/m] h min 5 h 9 h 0 min 1 h 7 h min 8 days 15 days days 3 days days 3 days 105 days 13 days 39 days 39 days 33 days t (Seconds) 1x10 1 P/ Q (sec/m ) 8 crosshole response BEH-3: Test 1 (HI; 1h after saturation) Test (HI; 1d after 1. saturation) Test 3 (HI; 5d after 1. saturation) Test (RI; 8d after 1. saturation) Test 5 (RI; d after 1. saturation) Test (RI; 110 d after 1. saturation) t (Seconds) Figures 5 and : Semi-log plots of drawdown reactions of several tests after the first saturation at the same observation borehole. The flow rate normalised drawdown as well as the slopes of the curves between 100 and 1000 seconds indicate a decreasing transmissivity with time for a single fracture (upper Figure) and for a fracture network (lower Figure). 7

8 Transmissivity (m /s) BEH-13 (HI) BEH-13 (RI) Crosshole response (injection borehole BEH-15) Calculated transmissivity vs. elapsed time (after first saturation): log-log plot t (d): time since 1. saturation Transmissivity (m /s) Test 1 Test Calculated transmissivities vs. elapsed time (after first saturation): log-log plot Test 3 BEH-30 BEH-53 BEH-3 BEH-8 BEH-71 Test Test 5 BEH-3 BEH- BEH- BEH-9 Test t (d): time since 1. saturation Figures 7 and 8: Decreasing transmissivity estimates for a single fracture (upper Figure) and for the fracture net work (lower Figure).. Discussion and conclusions The existence of a self-healing process has been demonstrated at two sites within the new gallery of the Mt. Terri Rock Laboratory. However, the process is slow and the high initial transmissivity of the fractures within the EDZ decreased 8

9 less than two orders of magnitude within one year. The continuation of the decreasing trend is difficult to predict because the governing processes could not be identified with the current experimental approach which focused primarily on the in-situ detection of the self-healing phenomenon. Nevertheless, the experimental results and the developed technology constitutes an important basis for further research in the area of safety assessment and the design of engineered barrier systems. Future work should first focus on the identification of the processes by a post mortem analysis after the stabilisation of permeability, and then on techniques to enhance the self-healing capacity. References BOSSART, P., MEIER P.M., MÖRI R., TRICK, Th., and MAYOR, J.-C. (00), Geological and Hydraulic Characterisation of the Excavation Disturbed Zone in the Opalinus Clay of the Mont Terri Rock Laboratory, manuscript in preparation. MARSILY, G. de (198), Quantitative Hydrogeology, Groundwater Hydrology forengineers, 0 pp., Academic Press, San Diego. MAZUREK, M. (1999), Mineralogy of the Opalinus Clay. In Thury and Bossart (eds)., Results of the Hydrogeological, Geochemical and Geotechical Experiments, perfomred in 199 and Geological report No. 3, Swiss National Geological and Hydrogeological Survey. MEIER, P.M., CARRERA J. and SANCHEZ-VILA X. (1998), An Evaluation of Jacob s method for the Interpretation of pumping tests in heterogeneous formations: Water Resources Research, Vol. 3, No. 5, Pp THURY, M. and BOSSART, P. (1999), The Mont Terri rock laboratory, a new international research project in a Mesozoic shale formation, in Switzerland. Engineering Geology 5 (1999), TRICK, T. (00a), Tecnical Note 00-: Mt Terri EH-Selfhealing Experiment: Pneumatic extraction tests for characterisation of two additional sites for the EH-Selfhealing Experiment. TRICK, T. (00b), MT. TERRI: EH EXPERIMENT: Pneumatic Extraction Tests and periodic hydraulic testing in site EH-1 to evaluate the self-healing capacity of fractures in the EDZ Solexperts report 19-5 TRICK, T. (00c), Tecnical Note 00-7: Mt Terri EH-Selfhealing Experiment: Extensometer Installation in two boreholes at the EH1 site as part of the EH (Selfhealing) Experiment. 9