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Detection and characterization of the disturbed rock zone in claystone with the complex resistivity method
Journal of Applied Geophysics 57 (2004) 63 – 79 www.elsevier.com/locate/jappgeo
Detection and characterization of the disturbed rock zone in claystone with the complex resistivity method Sabine Kruschwitz*, Ugur Yaramanci1 Technical University Berlin, Department of Applied Geophysics, Ackerstr. 71-76, D-13355, Berlin, Germany Received 1 August 2003; accepted 21 September 2004
Abstract Underground excavations provoke in their vicinity a region, where the rock is disturbed, i.e., loosened due to micro as well as macro fractures. Shape, dimension and properties of this so-called dexcavation damaged zone (EDZ)T are of increasing importance for the planning and construction of geotechnical barriers in underground repositories for toxic and problematic wastes. One way of assessing the EDZ around a drift is the investigation of the geoelectrical complex resistivity. In addition to resistivity, the phase measurements yield information about the pore spaces and not only about the water content. Measurements of resistivity and phase are conducted in a newly built tunnel in the opalinus claystone of the Mont Terri underground rock laboratory. Special ring profiles, as well as a geometry-adapted inversion scheme, are used. Significant local and temporal changes are found in the resistivity as well in the phase, which are directly associated with the disturbed rock zone. Laboratory measurements allow the correlation and quantification of resistivity and water saturation of the rock. A geotechnical modelling and an analysis of the stress pattern around the drift show that the geoelectrical properties agree remarkable with the mechanical state of the rock. D 2004 Elsevier B.V. All rights reserved. Keywords: Disturbed rock zone; Claystone; Complex resistivity method; Spectral induced polarisation; Geotechnics
1. Introduction A good knowledge of the geologic regime, mobilisation, transport and adsorption processes of * Corresponding author. Tel.: +49 30 810 44 259; fax: +49 30 314 72 597. E-mail addresses: [email protected] (S. Kruschwitz)8 [email protected] (U. Yaramanci). 1 Tel.: +49 30 314 72 599. 0926-9851/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2004.09.003
water and solutes as well as contaminants is needed for a successful assessment of the long-term safety of underground repositories. However, no established investigation program for the assessment of the excavation damaged rock zones (EDZ) around underground galleries exists so far. The geometry and characteristics of the EDZ are controlled by the way of backbreaking, the in-situ stress pattern and the particular strength properties of the rock. Different geophysical methods are tested and adapted to find
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out the best assessment strategy for each kind of (host) rock. Seismics, microacoustics and the complex resistivity method turned out to be the most promising methods in claystone. Microacoustic measurements revealed increasing acoustic emission from damaged rock areas (Spies and Eisenbla¨tter, 2001). Emsley et al. (1997) and Alheid et al. (1999) demonstrated that seismic measurements likewise yield reliable results, e.g., when seismic interval velocities are recorded along radial boreholes around a drift. But the drilling of the boreholes itself can damage the rock structure. Recently, Yaramanci and Kiewer (2000) successfully performed electrical measurements along a drift showing the feasibility of EDZ characterization (compare also Kiewer, 2000 and Kruschwitz, 2002). However, all abovementioned methods are rather new. They must be advanced and dovetailed to increase their validity. This study aims at the detection of anomalous rock zones near the tunnel cavity with the complex resistivity method and to assess the suitability of the method. Within a backfill experiment (engineered barrier [EB] project) in Mont Terri, rock failure zones close to the drift are investigated with two circular geoelectrical profiles in the drift crosssection and one horizontal profile along the wall. The complex rock resistivity is recorded in the frequency range of 1 Hz to 12 kHz. This method is known as spectral induced polarisation (SIP). In addition to the dprevalentT quantity, the resistivity of the medium, also the phase shift between the applied voltage and the resulting current are recorded. This phase shift cannot be easily correlated with any petrophysical parameter so far. In general, it has been observed that rocks with fine particles like clay having narrow pore spaces and high internal surfaces reveal higher phase shifts inasmuch as the electrolytic current flow is partly restricted. Hence, large phase shifts are often loosely interpreted as a measure for the dnarrownessT of the pores in a rock. But even so, the phase is deemed to be a promising quantity for the interpretation of loosened and dried rock zones. It is assumed that, in the damaged rock, the geometry of the pore space changes significantly; most likely, larger pores emerge and more narrow pores diminish. The polarisation effect of the medium is reduced in the fractured zones, and therefore, the observed phase shift should decrease. The successful and reliable application of the complex resistivity method for the detection, speci-
fication and monitoring of the EDZ was performed and evaluated. Three extensive field surveys yield the data basis. However, only the results of the first two campaigns will be presented here. The third campaign only repeated and validated the processes and trends found in the last measurements. Besides, resistivity there is a focus on phase measurements, as they were not regarded previously. Moreover, a geotechnical analysis of the modified stress pattern (due to the drift excavation) is calculated as support for the interpretation of the geoelectrical inversion results. In addition, in the laboratory, complex resistivity measurements were conducted on core material of the investigated drift section. These measurements completed the geoelectrical specification of the opalinus clay material.
2. Location and geological setup The investigation drift is located in northern Switzerland in the canton Jura (Fig. 1). It is situated a few kilometres north of St. Ursanne and approximately 100 km SW of Basel. The folded Jura, in which the investigated site is located, adjoins to the Tabular Jura in the NW and to the alpine foreland basin (Molassebecken) in the SE. When the motorway Transjurane A-16 was expanded, this was an opportunity to establish an underground rock laboratory at the same time (Fig. 2b). During the years 2000 and 2001, the DItunnel (diffusion experiment) and the EB-niche (engineered barrier) were excavated, in which the recent investigations took place. The overburden above the rock laboratory is about 300-m thick. The rock laboratory is situated in the opalinus clay, which is part of the folded Jura of Switzerland, embedded between marls and limestones. The opalinus clay is a marine sediment which developed 180 Ma (Jura, Dogger). Besides the 40–80% clay minerals, the rock contains also sand, silt, feldspar and organic carbon. Its water content varies between 4% and 12%. At this particular location, the rock has an overall thickness of about 140 m from which three slightly different facies can be distinguished: a shaly facies in the lower half of the sequence, a 15 m thick sandy–limy facies in the middle and a sandy facies interbedded with the shaly facies in the upper part. At the underground
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Fig. 1. Location of the canton Jura and Mont Terri underground rock laboratory besides the motorway Transjurane A-16.
laboratory, the rock strata has a strike angle of 508 and a dip angle of approximately 30–458SE (Fig. 2a).
3. Measuring technique 3.1. Equipment and installation For the measurements, the SIP-Fuchs device was used. The results of the 1-Hz measurement were inverted and are presented and discussed in the later chapters. The drift has a horseshoe shape with a height of 2.6 m. In the upper half and crown, the diameter measures 3.0 m, and the width along the floor is 2.9 m. In the drift, cross-section two circular profiles dcircular profile 1T and dcircular profile 2T, each consisting of 45 electrodes, were installed. The electrodes of these circular profiles are separated by 88, which corresponds to approx. 21 cm. Additionally, a horizontal profile was mounted along the wall at a height of 1 m from the ground. This profile is 5.5 m long and also uses 45 electrodes in a separation of 12.5 cm (Fig. 3a). 3.2. Geometric factors To calculate the apparent resistivities, geometric factors are needed. For the present geometry of the drift, the suiting geometric factors are not straightforward. However, an approximation to match the true circumstances closely was found appropriate. The difficulty in that particular case is that the measurements were performed underground in a drift, and
therefore, depending on the layout, it had to be considered whether dhalf-space conditionsT (which normally are used for measurements on the surface), dfull-space conditionsT or something in between have to be applied. When the electrode spread is large compared to the excavation (drift or borehole) diameter, full-space conditions become relevant. On the surface of the earth, where the conductive ground adjoins to the nonconductive medium air, the classical dhalf-spaceT condition is given. In this special case, it is something in between of these two extreme situations. For the inversion of the circular as well as the horizontal profile, a 2D inversion routine had been used (compare next chapter). As this algorithm only allows for pseudo sections with slight topographies, the data set was split into subsections, which then had just moderate curvature. However, these small subsections still had to be corrected for the actual nonhalf-space condition, as their length compared to the tunnel diameter could not be neglected. For the calculation of the appropriate geometric, i.e., k-factors of the circular profiles, a finitedifference-model-based scheme was used that was developed by Fan (1998). The k-factors are shown in Fig. 4 (on the left). In fact, the calculated kfactors are for a round drift, but the EB-niche deviates somewhat from this geometry by its horseshoe shape. The distinctive corners of this horseshoe shape act almost as discontinuities for the electrical field. For the calculation of the k-factors (and also for the following inversion), the corners had to be approximated by more or less round shapes, therefore, they were replaced by small pitch circles.
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Fig. 2. (a) Geological profile of the folded and the Tabular Jura of northern Switzerland, (b) map of the Mont Terri underground laboratory. The EB-niche in the lower left corner, where the geoelectrical measurements were conducted, runs 508NE, and the strata have a dip angle of approx. 208–608SE.
A second modelling had been done to find out whether, in case of the horizontal profile, half-space conditions are still effective or not. The k-factors for a horizontal profile in a circular drift with a radius of 1.5 m are shown in Fig. 4 (on the right). There is almost no change in k-factors for drift conditions before reaching layouts of up to 18 m (which corresponds to an electrode spacing of a=6 m). Thereafter, the k-
factors for drift conditions significantly differ from those for half-space conditions. Only at large spacings of up to 50 m the drift conditions converge against fullspace conditions. As the maximum electrode spread in the horizontal profile reaches 5.5 m, half-space conditions could be used safely for all measurements, and the well-known geometric k-factors for half-space conditions are applied.
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Fig. 3. (a) 3D-view of the installation of the geoelectrical profiles in the EB-niche. The two circular profiles are separated by 1 m. The horizontal profile is fixed on the NE tunnel wall at a height of 1.20 m from the ground. (b) 2D-view of the assigned locations of the measurements (dots) on a circular profile. The pitch circles mark the 16 single sections that were inverted. For the two sections shown in bold lines, the data fit and inversion are shown in Fig. 4.
3.3. Inversion Scheme Electrode profiles should not have a too strong topography if it is supposed to be inverted reasonably with the widely available commercial software program Res2Dinv from Campus Geophysical Instruments, 1995 as used in this study. Furthermore, a certain number of depth levels must be contained in the data because, otherwise, the information content is
too small. These are conflicting requirements because, with increasing number of depth levels, the topography of the pitch circle becomes stronger and vice versa. Calculations with synthetic data have shown that a pitch circle of about a third part of a circular section is most appropriate. This pitch circle contains 19 electrodes for the first depth level (corresponding to 16 measurement points) and six depth levels. The entire data set was therefore analysed in 16 individual
Fig. 4. Circular profile k-factors (left) and horizontal profile k-factors (right) calculated after Fan (1998). The so-called dWenner angleT means the angle enclosed by the imaginative prolongations of the two potential electrodes M and N to the midpoint of the tunnel cross section. In the right plot, the dashed lines mark the full space and the half space, the straight line the dtunnelT conditions.
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Fig. 5. Data and inversion for two sections contributing to the inversion result of circular profile 1 in July (a and b) show the data of a section from the SW wall, (c and d) the data from a section covering parts of the NE wall and bottom. The measured apparent resistivities in panels (a) and (c) are very well reproduced by the inverted models for both sections. The measured phases in panels (b) and (d) were likewise inverted in models with appropriate fit.
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sections (each having 19 electrodes for the first depth levels). The sections overlap, as it is shown in Fig. 3b. To go from the individual sections to an entire data set again, the inversion results are averaged in the overlapping areas. As an example for two different sections contributing to the inversion, result of circular profile 1 data and inversion are shown in Fig. 5. This way, the measured apparent resistivities and phases can be compared to the ones that are produced by the model. In Fig. 5a and b, a section from the upper right tunnel wall (compare Fig. 3b) is shown as a typical example for sections with smooth resistivity and phase contrasts. The measured values are well reproduced by the inverted model. Typical RMS errors for such sections are between 5% and 8% for the resistivity and 0.048 and 0.078 for the phases. Fig. 5c and d shows the inversion of a section covering a part from the left wall and the tunnel floor. This is one of the sections with the highest resistivity and phase contrasts. Still, both the apparent resistivities and phases can be fitted rather good with the inverted models. The RMS errors of those problematic sections do not exceed 17% for the resistivities and 0.158 for the phases.
4. Measurements and inversion results 4.1. Circular profiles The inversion results for resistivities and phases are shown abreast for two measurement campaigns for each circular profile (Figs. 6 and 7). To enhance the details, the resistivity color scale is limited to the range of 0–64 Vm. Higher resistivities up to 300 Vm occur at the floor of the drift which is paved over. Undisturbed opalinus clay, which reveals resistivities between 8 and 16 Vm and phases around 0.48, was only observed in the drift walls. In the ceiling, the resistivities increase up to values between 16 and 60 Vm (Figs. 6a and 7a). In the same region, rather low phases between 0.38 and 0.28 were recorded (Figs. 6b and 7b). This anomalous rock zone is identified as the excavation damaged zone (EDZ), where the rock fabric is overcome and the formation dehydrated. The dmodifiedT or ddisturbedT rock zone was about 1 m thick in July 1 month after the
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excavation of the drift. It obviously enlarged and incorporated even near surface areas on the upper part of the walls (Figs. 6c, d and 7c, d) in September, when it is at least 1-m thick on top of the ceiling. The floor of the drift covered with concrete of 70 cm thickness shows up with high resistivities of 100–300 Vm and with very low, almost zero, phases as expected. The resistivity and phase variations between July and September are shown in the Figs. 6e, f and 7e, f. The change in the resistivities is given as normalised resistivity difference ½ðresistivity in SeptemberÞ ðresistivity in JulyÞ =ðresistivity in JulyÞ: Normalised differences smaller than 0 show decreased and normalised differences greater than 0 show increased resistivities, respectively. The changes in phases are shown in simple differences. Negative phase differences indicate that larger absolute phases are observed in September, positive phase differences mark lower absolute phases in September. Significant changes of the resistivity are observed mainly on the floor (corners), along the walls and in the surface near parts of the ceiling. This observation holds for both profiles; however, it is more pronounced in profile 2 (Fig. 7). The resistivity increased most prominently, namely up to 100%, in the floor and in the ceiling. At the sides, the upswing is smaller with about 50% in profile 1. In profile 2, the resistivities in the drift walls decreased even by 50%. The different temporal behaviour of the two circular profiles could be attributed to the local fracture network, slightly different rock properties and, therefore, their different drying behaviour. The phase variances between July and September are much smaller than the discussed resistivity changes. Generally, it is expected that increasing resistivities coincide with decreasing phases and vice versa. Thus, in the phase differences (Figs. 6f and 7f), positive values are expected where a resistivity increase had been observed (Figs. 6e and 7e). The observed phase changes are rather small. Only slight positive changes occur around 0.28–0.48 step out in the lower right corner on the floor of the drift as well as in the ceiling. Nearly all the rest of the drift appears with almost zero phase changes (medium grey hues).
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Fig. 6. Circular profile 1, inversion results of resistivity and phase. (a) Resistivity, July; (b) phase, July; (c) resistivity, September; (d) phase, September; (e) resistivity change between July and September; and (f) phase change between July and September.
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Fig. 7. Circular profile 2, inversion results of resistivity and phase. (a) Resistivity, July; (b) phase, July; (c) resistivity, September; (d) phase, September; (e) resistivity change between July and September; and (f) phase change between July and September.
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To gain a better resolution of the very small phase changes, the color scale has been limited to F0.258. Two different cases of complex resistivity change can be noticed. Mainly in the walls, the resistivity increase is accompanied by a simultaneous phase increase (of absolute values). In contrast, in the ceiling, the observed resistivity increase comes along with a phase decrease. To understand these changes and to attribute corresponding pore space variations, two different causing stress mechanisms have to be considered. In the case of dilatation, the grains (solid matrix) are pulled apart, and the pore water can move or mostly even be drained off. The rock dehydrates, air enters into the formation, and water evaporates. Hence, the resistivity increases due to the decreased total water content. The rock loosening and ongoing evaporation dehydrates the originally fully saturated rock. In pores where all the volume water is drained off, even the surface water is affected by the evaporation process (Fig. 8b). Inasmuch as the polarisation ability of a medium is correlated with its amount of surface water, the phases decrease. In the case of compression, the pore space and the volume of water content decrease, whereas the attached surface water remains in the first state about the same (Fig. 8c). The resistivity of the claystone increases due to the less total water content. By the compression, the ratio of surface to volume water increases, and as a consequence, the measured phases increase. Moreover, the pore spaces become even smaller than they are in the normal state. Hence, the movement of the ions and, therefore, the current flow is partly prohibited, and the medium’s polarisation
ability increases. Actually, this is the underlying process of pore space variation observed in the tunnel walls at depth of N0.3 m. 4.2. Horizontal profile The inversion result of the horizontal profile is shown in Fig. 9. Undisturbed clay is found at depths greater than 20 cm. Resistivities higher than 16 Vm are primarily observed on top of the profile. These high resistivities signify the dehydration zone in the uppermost 10–15 cm (Fig. 9a). Decreased absolute phases would have been expected in this area too, but only at some distinct points phases are around 0.28 instead of the 0.48 of unaffected clay (Fig. 9b). In September, the dehydrated rock zone is more continuous and reaches partly a depth of 20 cm (Fig. 9c). Now, likewise, the phase measurements show the dehydration by the relatively continuous band of low phases between 0.258 to 0.18 dominating the uppermost part of the section (Fig. 9d). The changes of the resistivities and the phases are shown on the bottom of Fig. 9. The left section (Fig. 9e) shows the resistivity change from July to September. Most striking are positive changes on top of the profile, which again assign the increased resistivities and the advanced drying in September. At greater depth, the resistivities decreased slightly and show negative normalised resistivity differences around 0 and 0.25. The phase change between the two measurements is shown abreast (Fig. 9f). The resistivity increase on top comes along with small phase reductions around 0.28. Moreover, the somewhat lower resistivities in September are correlated
Fig. 8. Modification of the pore space by different stresses. (a) Normal state of the undamaged clay. (b) Dilatation yields a reduction of the volume water (by air entrance and evaporation of water) and an increase of the resistivity but a decrease of the phase values. (c) Compression yields a reduction of the volume water and therefore an increase of the resistivity and an increase of the phase values.
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Fig. 9. Horizontal Profile and inversion results of resistivity and phase. (a) Resistivity, July; (b) phase July; (c) resistivity, September; (d) phase, September; (e) resistivity change between July and September; and (f) phase change between July and September.
with slightly increased phases (+0.18) at greater depths. 4.3. Comparison of the circular profiles To investigate the differences between the two circular profiles, the difference between profile 2 and profile 1 has been normalised in profile 1 (Fig. 10). As expected, due to their spatial proximity, the profiles are very similar for both the resistivities and the phases. This indicates a high homogeneity of the investigated formation and justifies 2D inversions of the individual profiles. Two main areas of higher resistivities in profile 2 attracted attention in July (Fig. 10a). The resistivities at profile 1 are three times larger than those of profile 1 on
the right corner of the floor as well as on the right side of the ceiling. On the left wall of the drift near to the horizontal profile, likewise, a region of higher resistivities (normalised differences up to 0.8) is observed at a depth of about half a meter. Lower resistivities in profile 2 than in profile 1, i.e., negative normalised differences, are observed scarcely anywhere. The differences between the two circular profiles seem to diminish from July to September although the zones of higher resistivities in profile 2 do not disappear completely in September (Fig. 10c). But the anomalies on the lower right corner as well as on the left drift wall become less prominent. Large parts of the two profiles only differ less than F40%. The differences in the phases are shown on the right side in Fig. 10. Most ostentatious are two large
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Fig. 10. Normalised resistivity differences and phase differences between the two circular profiles. (a) Normalised resistivity difference in July; (b) phase difference in July; (c) normalised resistivity difference in September; and (d) phase difference in September.
negative anomalies in the lower left corner of the drift and in the upper part of the right wall in July. Only very few spots of positive phase differences right on the drift surface are found. However, the location of differences, which are presented in the resistivities (compare Fig. 10a), are not identical with those in the phases. As for the resistivities, the differences in the phases are smaller in September. Only in the right corner that slight positive phase differences are observed, but almost the entire profile reveals differences around 08.
5. Laboratory measurements Complex resistivity measurements on core material from the EB-niche of Mont Terri should specify the dependence of saturation and resistivity or phase on saturation, respectively. The laboratory specimens of appropriate size were drilled from existing cores of boreholes. One borehole (wall) runs at a height of 1 m almost horizontally into the left wall, whereas the other one (ceiling) runs vertically up into the roof. The specimens out of the ceiling region were drilled
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almost parallel to the bedding planes, whereas the specimens from the wall are drilled almost perpendicular to the bedding planes. Therefore, differences concerning the porosity or the conductivity could have been expected inasmuch as a clear difference of the resistivity between the walls of the drift and its ceiling emerged from the first outcomes of the geoelectrical survey in July (Figs. 6 and 7). Whereas in the drift wall almost only undisturbed claystone was detected, in the ceiling the excavation damaged zone had built up. The complex resistivity measurements were performed with the Solartron equipment from Solartron Instruments, 1996. Two specimens from two different boreholes were investigated respectively. The specimens had a mean diameter of 15.7 (F0.2) mm at a mean length of 54.5 (F0.5) mm. A four-point layout has been applied using two front current electrodes and two potential electrodes along the specimen. The rock resistivity was determined at different saturations with three different salinities of pore water: distilled, with conductivity of 1 and 5 S/m. The resistivity (and its reciprocal value conductivity) are shown in Fig. 11a and the phase in Fig. 11b. In laboratory electrical work, often, the conductivity is used instead of the resistivity, but to compare the results with the field measurements, each of the plots holds two axes. There is no significant difference in the results for the specimen from the dwallT and dceilingT. Although for higher saturations, the specimens of the ceiling have slightly higher conductivities as those from the wall, but these differences are still in the range of the measuring error. Therefore, a welldefined grouping of the results seems here not reliably significant enough. The phase does not correlate with the saturation degree of the specimen in any way. A basically Archie-type equation (Archie, 1942; Schopper, 1982; Mavko et al., 1988; Danowski and Yaramanci, 1999) was used to fit the measured data (Fig. 11a). The conductivity of the rock is: r ¼ rw S n Um þ rq0 S v
ð1Þ
with U is the porosity, S the saturation degree, r w the conductivity of the pore fluid and r q0 the surface conductivity. The parameters m, n and m are empirically determined quantities basically related to the geometry of the pores and their connectivity. The
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Fig. 11. Rock conductivities for different saturations (pore fluid conductivity 5 S/m) as determined in laboratory measurements. (a) Conductivity measurements of the four specimens and basically Archie-type Eq. (2) fit of the conductivity measurements (continuous curve) together with the two comprising parts volume (dashed line) and surface conductivity (dashed dotted line). (b) Phase measurements of the four specimens.
conductivity of porous rocks varies with their water content, ion concentration, mobility and, last but not least, the volume and arrangement of the pores. The conductivity of rocks increases with higher conductivity of their pore fluids, as well as higher saturation degree and fractional pore space. The surface conductivity becomes important for materials with very large inner rock surfaces, as for example clays.
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Table 1 Results of the laboratory measurements for the parameters of the conductivity Eq. (1) at 1 Hz Specimen
r q0 [S/m] m [] m [] n [] U [%]
Wall, 30 cm (depth) Wall, 90 cm (depth) Ceiling, 30 cm (depth) Ceiling, 90 cm (depth)
0.0663 0.0507 0.1218 0.2004
0.75 0.23 0.41 0.21
1.46 1.46 1.29 1.33
1.93 1.53 1.49 1.95
16.15 11.59 15.02 16.26
The porosities are measured in a usual manner with a pycnometer. The surface conductivity r q0, saturation exponents m and n and the cementation exponent m, which are typically needed for the description after Eq. (2), have been determined in the different saturation work steps. After the specimens were dried and desalinated, they were fully saturated (S=1) with distilled water (r wc0 S/m). Then the measured conductivity is equal to the surface conductivity r q0. By drying the specimen steadily and measuring the conductivity at different saturation degrees, the saturation exponent m was determined. This way, the parameters for the surface conductivity part of the total conductivity were established. In the next step, the two parameters m and n affecting the volume conductivity had to be quantified. Thus, the specimens were fully and partially saturated with two different saline pore water. Measurements with full saturations yield the cementation exponent m and with the partial saturations the second saturation exponent n. The different results for the above-described measurements were averaged for all specimens (Table 1) to obtain a mean equation for the investigated material as (Fig. 11a): r ¼ rw S 1:72 0:151:38 þ 0:11 S 0:4 :
ð2Þ
The pore water conductivity of the saturated claystone in the Mont Terri tunnel has been determined in situ as approximately 2–4 S/m. On the basis of Eq. (2), the different resistivities of the undisturbed rock and the excavation damaged zone can be assigned to different water contents. Such an estimation of the water content obviously is based upon the simplifying assumption of constant porosity and fluid conductivity, which holds quite well here. The undamaged clay according to the in-situ measurements has in the EB-niche a resistivity of 8 Vm, which is only reached in full-saturated rock. In
the ceiling of the drift, where resistivities between 30 and 60 Vm were observed, the rock has a very low water content of 10% down to 2.5%.
6. Geotechnical modelling A modelling of the stress situation around the EBniche was accomplished to study the zones of decreased rock strength or even rock failure due to the excavation of the drift. The phase2 program from Rockscience Incorporation, which has been used for the modelling, is a finite-element-based software. The finite element mesh created around the EB-niche consists of triangles with three inner computation sampling points (knots). In the vicinity of the drift, the triangles are smaller and more densely spaced than at greater distance. As boundary condition, it was required that the strains at the border of the model must be identical to zero. The principal stresses act on the EB-niche very favourably for modelling. That is, r 1 acts vertically with 6–7 MPa and r 3 perpendicular to the drift axis with 2–3 MPa. A dstress blockT that indicated the relative magnitude and direction of the field stresses is charted in the upper right corners of the modelling results. The rock and joint properties needed are listed in Table 2. The final rock behaviour is a composition of the behaviour of the undamaged rock and the included joints. In the EB-niche, the bedding planes, as well as tectonically activated joints, have a strike angle of 1408 (SE) with a dipping of 308–508. The failure of the fabric and the joints has been modelled individually. Fig. 12 shows the stress calculation of the joints. When the interfering stress overcomes the joint
Table 2 Rock and joint properties entered in the phase 2 model Parameter
Rock
Joints
Cohesion [MPa] Friction [8] Poisson number E [MPa] parallel bedding E [MPa] normal bedding Joint Aperture Normal Stiffness [MPa/m] Shear Stiffness [Mpa/m]
2.2 25 0.3 2500 4500 – – –
1 23 0.3 – – 0 3 3
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Fig. 12. Modelled joint failure. Upper right corner: stress block with r 1 vertically and r 3 horizontally. Lower right corner: legend of the strength–stress ratio. The joint failure happens after the rock failure.
strength (strength–stress ratio smaller than 1), it comes to failure. Most notably, the joint strength is exceeded on the right-hand side of the ceiling, as well as on the left-hand side of the floor. This asymmetric shape is ascribed to the dipping of the bedding planes. The rock failure and the total failure (rock including joints) are almost identical, and only the total failure will be presented here (Fig. 13). The transgression of the fabric strength appears as a butterfly shape around the niche. The asymmetric failure mechanism, which emerged out of the joint collapse in Fig. 12, disappeared completely, implying that their failure has nearly no bearing on the total rock failure. Failure of the joints happens not until the rock collapsed itself. The strongest (rock) failure develops in the drift walls and on the sides of the ceiling. Imaginable are three different kinds of failure modes: extension fracturing, bedding plane slip and failure due to swelling and softening. Stress-induced breakouts are mostly observed in boreholes, not in the vicinity of galleries and drifts (Martin and Lanyon, 2004).
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ments, the unaffected rock has only been observed in the drift wall. Significantly higher resistivities between 16 to 60 Vd m are observed in the ceiling of the drift, which has been investigated in this study. The increased resistivities in the ceiling are accompanied by obviously lowered phases. Instead of the common 0.48 of opalinus clay that have been measured in the walls, only 0.25 to 0.158 are found in the ceiling. The high resistivities in combination with low phases identify loosened and dehydrated rock. This EDZ (excavation damaged zone) is about 1-m thick on top of the ceiling. Its thickness decreases to about 0.5 m in the side walls. But, different than the resistivities, the phases indicate a slightly asymmetrical ceiling anomaly. At the right-hand side of the ceiling (SW), the phases are much smaller than on the left-hand side (NE). In the EB-niche, the bedding planes dip with approx. 308. The clay particles are aligned in the same direction as the bedding planes, presumably due to tectonic transposition. Transposition yields a geometrical rearrangement of linear elements in the rock. The alignment of the clay particles causes an anisotropy of the rock strength, whereby extension failure preferentially emerges parallel to the bedding, what is here in 308 direction. That means if a loading affects the
7. Discussion and conclusions The presented measurements show that undamaged opalinus clay in the EB-niche of the Mont Terri underground rock laboratory has a resistivity of around 8 Vd m. With complex resistivity measure-
Fig. 13. Simultaneously modelled rock and joint failure. Upper right corner: stress block with r 1 vertically and r 3 horizontally. Lower right corner: legend of the strength–stress ratio.
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particles in the direction of alignment, the cohesion of the particles is broken down very easily. That is the failure mechanism observed in the upper right part of the ceiling. It is on the right-hand side of the ceiling, vertical to the affecting loadings, that it lacks an abutment; therefore, the opalinus clay cracks here at first. A dmigrationT of the fracture zone to the lefthand side of the ceiling is perceived in the repetitive measurements in September, this is the second most fragile region in the drift. Increased resistivities in the ceiling (and on the floor) were found from July to September. Basically, both circular profiles showed this, however, the second circulars profile somewhat more conspicuously. Increased resistivities are usually an indication of advanced drying and failure mechanisms. Simultaneously to the onward loosening and drying, increasing resistivities must be noticed in the drift walls. Inasmuch as every drift excavation necessarily provokes a stress dislocation, the stress trajectories concentrate at the sides of the drift (Fig. 14). At the sides, the stress trajectories point almost perpendicular to the bedding planes and the particle alignment, respectively. The clay material is compressed, which leads to a local decrease in the resistivity when microcracks are closed up. Further on, the compression may lead to a gradual compression of pore space. Thus, the water content within the pores increases relatively, what corresponds to an increasing saturation. With higher saturation a decreased resistivity is observed. The ongoing drying process of the rock is detected in the measurements of October (not further discussed
here). In the ceiling and on the floor, where strong dehydration was found in September, the process slowed down, but the backward walls of the drift behind 0.5 m run dry now, too. Likely, proceeding compression of the clay material in the walls leads not only to a reduction of the pore space (mobile air is squeezed out) but also to a reduction of the water content. On the long run, even water is displaced and probably led away on the drift walls. Not quite the same drying behaviour is monitored at both circular profiles in the observation period. That surely must be attributed to the local fracture network. The joint spacing measures 0.5 m in the EB-niche, and the circular profiles are separated by 1 m. If, for example, joints attach the first circular profile at the walls and dehydrate the rock, it needs not to be all the same with the second profile. The dhydraulic connectionT plays an important role for the time-dependent behaviour of the resistivities. The differences between the resistivities of the two circular profiles remain almost the same over the entire observation period. The most prominent discrepancies between the circular profiles are recorded on the right-hand side of the floor and ceiling. On the floor and in the ceiling of the second circular profile, much higher resistivities are observed. The strongest differences are found in the comparison of the data sets collected in July. In September, the resistivities of the two circular profiles adjust somewhat. Except for the abovementioned discrepancies, merely differences of F20% are calculated between the two data sets. More obvious is the by-
Fig. 14. Rock fabric anisotropy in the opalinus clay. Extension failure due to main stresses affecting in the direction of the alignment of the clay particles.
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and-by dharmonisation effectT of the circular profiles in the phase measurements. On the left-hand side of the drift floor, a large area of lower phases in the second circular profile was detected, whereas this enormous discrepancy utterly vanishes in September. More than one reason can be found to explain unequal resistivities and phases observed in the two circular profiles. First, profile 2 is located more closely to the drift face, hence, the affecting loads could be at some extent smaller as in profile 1 nearly in the middle of the drift. Clearly, near to the drift face, the rock is more supported by the dintactT rock. Furthermore, the local fracture network plays a significant role in the drying process, in the walls particularly. Maybe even local differences in the rock effect permeability. All in all, on the long run, it must be assumed that the circular profiles more and more adjust in both, resistivity and phase. Most probably, the mentioned effects just delay the assimilation of the resistivities and phases measured at the two profiles, but they do not stop it at all. Besides the geoelectric measurements, also seismic down-hole investigations have been conducted in the EB-niche (Schuster et al., 2001). Inasmuch as the two methods geoelectric and seismic are heading for different material properties, a combined evaluation of the measurements yielded a still more comprehensive understanding of the loosening processes in the vicinity of the drift. With the aid of seismic borehole measurements, the existence and position of a compacted rock region with high velocities in the wall of the drift could be validated. Slightly increased resistivities together with low phases definitely identify the EDZverified with seismics beyond any doubt. Borehole measurements obviously give detailed information over the entire length of the wells, but with the resistivity method, the whole volume can be covered. So, the simultaneous application of both methods provided an increase of the assessment certainty. The risk of possible misinterpretations could be reduced.
Acknowledgements The authors like to thank to the Federal Institute for Geosciences and Natural Resources (BGR) for the funding of this work within the frame of the European Union project dEngineered Barrier Emplacement
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Experiment in Opalinus Clay.T Furthermore, we especially thank Dr. H.-J. Alheid and Dr. K. Schuster (BGR) for the cooperation and advice. P. Bossart and H. Steiger from the Mont Terri Consortium supported us in the field and helped us through their experience underground. We thank Dr. T. Wonik and M. Grinat (Leibniz Institute for Applied Geosciences) for the disposal of the measurement device in July 2001. References Alheid, H.J., Knecht, M., Boisson, J.Y., Homand-Etienne, F., Pepa, S., 1999. Comparison of in-situ hydraulic and seismic measurements in the excavation damaged zone of underground drifts. 9th International Congress on Rock Mechanics, Paris, France, Expanded Abstracts. , pp. 1263 – 1266. Archie, G., 1942. Electrical resistivity as an aid in core analysis interpretation. Transactions of American Institution of Mining and Metallurgical Engineers 146, 54 – 61. Dannowski, G., Yaramanci, U., 1999. Estimation of water content using combined radar and geoelectrical measurements. European Journal of Environmental and Engineering Geophysics 4, 71 – 86. Emsley, S., Olsson, O., Stenberg, L., Alheid, H.J., Falls, S., 1997. A study of damage and disturbance from tunnel excavation by blasting and tunnel boring. Technical report, SKB-Swedish Nuclear Fuel and Waste Management Co., Stockholm. Fan, X., 1998. Modellierung und Inversion von gleichstromgeoelektrischen Bohrlochmessungen mit 2D und 3D Finite Differenzen. PhD Thesis, Technical University Berlin, Germany. Kiewer, M., 2000. Geoelektrische Charakterisierung von Tonformationen. MSc Thesis, Technical University Berlin, Germany. Kruschwitz, S., 2002. Detection and characterization of the disturbed rock zone in claystone with complex valued geoelectrics. MSc Thesis, Technical University Berlin, Germany. Martin, C.C., Lanyon, G.W., 2004. Excavation disturbed zone (edz) in clay shale: Mont Terri. Technical Report 2001-01, Mont Terri Project (Paul Bossart), Switzerland. Mavko, G., Mukerji, T., Devorkin, J., 1988. The Rock Physics Handbook. Cambridge University Press. Schopper, J.R., 1982. Electrical conductivity of rocks containing electroloyts. Landolt-Bfrnstein, Group V, Physical Properties of Rock, Subvolume B, vol. 1, pp. 276 – 291. Schuster, K., Alheid, H.-J., Bfddener, D., Eichhorn, P., Spies, T., Heidrich, D., 2001. ED-C Experiment: seismic investigations of the edz and acoustic emission measurements in the new gallery. Technical Report 2000-05, Mont Terri Project (Paul Bossart), Switzerland. Spies, T., Eisenbl7tter, J., 2001. Acoustic emission investigation of microcrack generation at geological boundaries. Engineering Geology 61, 181 – 188. Yaramanci, U., Kiewer, M., 2000. Geoelectrical characterization of the disturbed rock zone in opalinus clay (Mont Terri, Switzerland). Proceedings of 6th EEGS-ES Meeting, Expanded Abstracts EL 14, Bochum, Germany.
Detection and characterization of the disturbed rock zone in claystone with the complex resistivity method Sabine Kruschwitz*, Ugur Yaramanci1 Technical University Berlin, Department of Applied Geophysics, Ackerstr. 71-76, D-13355, Berlin, Germany Received 1 August 2003; accepted 21 September 2004
Abstract Underground excavations provoke in their vicinity a region, where the rock is disturbed, i.e., loosened due to micro as well as macro fractures. Shape, dimension and properties of this so-called dexcavation damaged zone (EDZ)T are of increasing importance for the planning and construction of geotechnical barriers in underground repositories for toxic and problematic wastes. One way of assessing the EDZ around a drift is the investigation of the geoelectrical complex resistivity. In addition to resistivity, the phase measurements yield information about the pore spaces and not only about the water content. Measurements of resistivity and phase are conducted in a newly built tunnel in the opalinus claystone of the Mont Terri underground rock laboratory. Special ring profiles, as well as a geometry-adapted inversion scheme, are used. Significant local and temporal changes are found in the resistivity as well in the phase, which are directly associated with the disturbed rock zone. Laboratory measurements allow the correlation and quantification of resistivity and water saturation of the rock. A geotechnical modelling and an analysis of the stress pattern around the drift show that the geoelectrical properties agree remarkable with the mechanical state of the rock. D 2004 Elsevier B.V. All rights reserved. Keywords: Disturbed rock zone; Claystone; Complex resistivity method; Spectral induced polarisation; Geotechnics
1. Introduction A good knowledge of the geologic regime, mobilisation, transport and adsorption processes of * Corresponding author. Tel.: +49 30 810 44 259; fax: +49 30 314 72 597. E-mail addresses: [email protected] (S. Kruschwitz)8 [email protected] (U. Yaramanci). 1 Tel.: +49 30 314 72 599. 0926-9851/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2004.09.003
water and solutes as well as contaminants is needed for a successful assessment of the long-term safety of underground repositories. However, no established investigation program for the assessment of the excavation damaged rock zones (EDZ) around underground galleries exists so far. The geometry and characteristics of the EDZ are controlled by the way of backbreaking, the in-situ stress pattern and the particular strength properties of the rock. Different geophysical methods are tested and adapted to find
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out the best assessment strategy for each kind of (host) rock. Seismics, microacoustics and the complex resistivity method turned out to be the most promising methods in claystone. Microacoustic measurements revealed increasing acoustic emission from damaged rock areas (Spies and Eisenbla¨tter, 2001). Emsley et al. (1997) and Alheid et al. (1999) demonstrated that seismic measurements likewise yield reliable results, e.g., when seismic interval velocities are recorded along radial boreholes around a drift. But the drilling of the boreholes itself can damage the rock structure. Recently, Yaramanci and Kiewer (2000) successfully performed electrical measurements along a drift showing the feasibility of EDZ characterization (compare also Kiewer, 2000 and Kruschwitz, 2002). However, all abovementioned methods are rather new. They must be advanced and dovetailed to increase their validity. This study aims at the detection of anomalous rock zones near the tunnel cavity with the complex resistivity method and to assess the suitability of the method. Within a backfill experiment (engineered barrier [EB] project) in Mont Terri, rock failure zones close to the drift are investigated with two circular geoelectrical profiles in the drift crosssection and one horizontal profile along the wall. The complex rock resistivity is recorded in the frequency range of 1 Hz to 12 kHz. This method is known as spectral induced polarisation (SIP). In addition to the dprevalentT quantity, the resistivity of the medium, also the phase shift between the applied voltage and the resulting current are recorded. This phase shift cannot be easily correlated with any petrophysical parameter so far. In general, it has been observed that rocks with fine particles like clay having narrow pore spaces and high internal surfaces reveal higher phase shifts inasmuch as the electrolytic current flow is partly restricted. Hence, large phase shifts are often loosely interpreted as a measure for the dnarrownessT of the pores in a rock. But even so, the phase is deemed to be a promising quantity for the interpretation of loosened and dried rock zones. It is assumed that, in the damaged rock, the geometry of the pore space changes significantly; most likely, larger pores emerge and more narrow pores diminish. The polarisation effect of the medium is reduced in the fractured zones, and therefore, the observed phase shift should decrease. The successful and reliable application of the complex resistivity method for the detection, speci-
fication and monitoring of the EDZ was performed and evaluated. Three extensive field surveys yield the data basis. However, only the results of the first two campaigns will be presented here. The third campaign only repeated and validated the processes and trends found in the last measurements. Besides, resistivity there is a focus on phase measurements, as they were not regarded previously. Moreover, a geotechnical analysis of the modified stress pattern (due to the drift excavation) is calculated as support for the interpretation of the geoelectrical inversion results. In addition, in the laboratory, complex resistivity measurements were conducted on core material of the investigated drift section. These measurements completed the geoelectrical specification of the opalinus clay material.
2. Location and geological setup The investigation drift is located in northern Switzerland in the canton Jura (Fig. 1). It is situated a few kilometres north of St. Ursanne and approximately 100 km SW of Basel. The folded Jura, in which the investigated site is located, adjoins to the Tabular Jura in the NW and to the alpine foreland basin (Molassebecken) in the SE. When the motorway Transjurane A-16 was expanded, this was an opportunity to establish an underground rock laboratory at the same time (Fig. 2b). During the years 2000 and 2001, the DItunnel (diffusion experiment) and the EB-niche (engineered barrier) were excavated, in which the recent investigations took place. The overburden above the rock laboratory is about 300-m thick. The rock laboratory is situated in the opalinus clay, which is part of the folded Jura of Switzerland, embedded between marls and limestones. The opalinus clay is a marine sediment which developed 180 Ma (Jura, Dogger). Besides the 40–80% clay minerals, the rock contains also sand, silt, feldspar and organic carbon. Its water content varies between 4% and 12%. At this particular location, the rock has an overall thickness of about 140 m from which three slightly different facies can be distinguished: a shaly facies in the lower half of the sequence, a 15 m thick sandy–limy facies in the middle and a sandy facies interbedded with the shaly facies in the upper part. At the underground
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Fig. 1. Location of the canton Jura and Mont Terri underground rock laboratory besides the motorway Transjurane A-16.
laboratory, the rock strata has a strike angle of 508 and a dip angle of approximately 30–458SE (Fig. 2a).
3. Measuring technique 3.1. Equipment and installation For the measurements, the SIP-Fuchs device was used. The results of the 1-Hz measurement were inverted and are presented and discussed in the later chapters. The drift has a horseshoe shape with a height of 2.6 m. In the upper half and crown, the diameter measures 3.0 m, and the width along the floor is 2.9 m. In the drift, cross-section two circular profiles dcircular profile 1T and dcircular profile 2T, each consisting of 45 electrodes, were installed. The electrodes of these circular profiles are separated by 88, which corresponds to approx. 21 cm. Additionally, a horizontal profile was mounted along the wall at a height of 1 m from the ground. This profile is 5.5 m long and also uses 45 electrodes in a separation of 12.5 cm (Fig. 3a). 3.2. Geometric factors To calculate the apparent resistivities, geometric factors are needed. For the present geometry of the drift, the suiting geometric factors are not straightforward. However, an approximation to match the true circumstances closely was found appropriate. The difficulty in that particular case is that the measurements were performed underground in a drift, and
therefore, depending on the layout, it had to be considered whether dhalf-space conditionsT (which normally are used for measurements on the surface), dfull-space conditionsT or something in between have to be applied. When the electrode spread is large compared to the excavation (drift or borehole) diameter, full-space conditions become relevant. On the surface of the earth, where the conductive ground adjoins to the nonconductive medium air, the classical dhalf-spaceT condition is given. In this special case, it is something in between of these two extreme situations. For the inversion of the circular as well as the horizontal profile, a 2D inversion routine had been used (compare next chapter). As this algorithm only allows for pseudo sections with slight topographies, the data set was split into subsections, which then had just moderate curvature. However, these small subsections still had to be corrected for the actual nonhalf-space condition, as their length compared to the tunnel diameter could not be neglected. For the calculation of the appropriate geometric, i.e., k-factors of the circular profiles, a finitedifference-model-based scheme was used that was developed by Fan (1998). The k-factors are shown in Fig. 4 (on the left). In fact, the calculated kfactors are for a round drift, but the EB-niche deviates somewhat from this geometry by its horseshoe shape. The distinctive corners of this horseshoe shape act almost as discontinuities for the electrical field. For the calculation of the k-factors (and also for the following inversion), the corners had to be approximated by more or less round shapes, therefore, they were replaced by small pitch circles.
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Fig. 2. (a) Geological profile of the folded and the Tabular Jura of northern Switzerland, (b) map of the Mont Terri underground laboratory. The EB-niche in the lower left corner, where the geoelectrical measurements were conducted, runs 508NE, and the strata have a dip angle of approx. 208–608SE.
A second modelling had been done to find out whether, in case of the horizontal profile, half-space conditions are still effective or not. The k-factors for a horizontal profile in a circular drift with a radius of 1.5 m are shown in Fig. 4 (on the right). There is almost no change in k-factors for drift conditions before reaching layouts of up to 18 m (which corresponds to an electrode spacing of a=6 m). Thereafter, the k-
factors for drift conditions significantly differ from those for half-space conditions. Only at large spacings of up to 50 m the drift conditions converge against fullspace conditions. As the maximum electrode spread in the horizontal profile reaches 5.5 m, half-space conditions could be used safely for all measurements, and the well-known geometric k-factors for half-space conditions are applied.
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Fig. 3. (a) 3D-view of the installation of the geoelectrical profiles in the EB-niche. The two circular profiles are separated by 1 m. The horizontal profile is fixed on the NE tunnel wall at a height of 1.20 m from the ground. (b) 2D-view of the assigned locations of the measurements (dots) on a circular profile. The pitch circles mark the 16 single sections that were inverted. For the two sections shown in bold lines, the data fit and inversion are shown in Fig. 4.
3.3. Inversion Scheme Electrode profiles should not have a too strong topography if it is supposed to be inverted reasonably with the widely available commercial software program Res2Dinv from Campus Geophysical Instruments, 1995 as used in this study. Furthermore, a certain number of depth levels must be contained in the data because, otherwise, the information content is
too small. These are conflicting requirements because, with increasing number of depth levels, the topography of the pitch circle becomes stronger and vice versa. Calculations with synthetic data have shown that a pitch circle of about a third part of a circular section is most appropriate. This pitch circle contains 19 electrodes for the first depth level (corresponding to 16 measurement points) and six depth levels. The entire data set was therefore analysed in 16 individual
Fig. 4. Circular profile k-factors (left) and horizontal profile k-factors (right) calculated after Fan (1998). The so-called dWenner angleT means the angle enclosed by the imaginative prolongations of the two potential electrodes M and N to the midpoint of the tunnel cross section. In the right plot, the dashed lines mark the full space and the half space, the straight line the dtunnelT conditions.
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Fig. 5. Data and inversion for two sections contributing to the inversion result of circular profile 1 in July (a and b) show the data of a section from the SW wall, (c and d) the data from a section covering parts of the NE wall and bottom. The measured apparent resistivities in panels (a) and (c) are very well reproduced by the inverted models for both sections. The measured phases in panels (b) and (d) were likewise inverted in models with appropriate fit.
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sections (each having 19 electrodes for the first depth levels). The sections overlap, as it is shown in Fig. 3b. To go from the individual sections to an entire data set again, the inversion results are averaged in the overlapping areas. As an example for two different sections contributing to the inversion, result of circular profile 1 data and inversion are shown in Fig. 5. This way, the measured apparent resistivities and phases can be compared to the ones that are produced by the model. In Fig. 5a and b, a section from the upper right tunnel wall (compare Fig. 3b) is shown as a typical example for sections with smooth resistivity and phase contrasts. The measured values are well reproduced by the inverted model. Typical RMS errors for such sections are between 5% and 8% for the resistivity and 0.048 and 0.078 for the phases. Fig. 5c and d shows the inversion of a section covering a part from the left wall and the tunnel floor. This is one of the sections with the highest resistivity and phase contrasts. Still, both the apparent resistivities and phases can be fitted rather good with the inverted models. The RMS errors of those problematic sections do not exceed 17% for the resistivities and 0.158 for the phases.
4. Measurements and inversion results 4.1. Circular profiles The inversion results for resistivities and phases are shown abreast for two measurement campaigns for each circular profile (Figs. 6 and 7). To enhance the details, the resistivity color scale is limited to the range of 0–64 Vm. Higher resistivities up to 300 Vm occur at the floor of the drift which is paved over. Undisturbed opalinus clay, which reveals resistivities between 8 and 16 Vm and phases around 0.48, was only observed in the drift walls. In the ceiling, the resistivities increase up to values between 16 and 60 Vm (Figs. 6a and 7a). In the same region, rather low phases between 0.38 and 0.28 were recorded (Figs. 6b and 7b). This anomalous rock zone is identified as the excavation damaged zone (EDZ), where the rock fabric is overcome and the formation dehydrated. The dmodifiedT or ddisturbedT rock zone was about 1 m thick in July 1 month after the
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excavation of the drift. It obviously enlarged and incorporated even near surface areas on the upper part of the walls (Figs. 6c, d and 7c, d) in September, when it is at least 1-m thick on top of the ceiling. The floor of the drift covered with concrete of 70 cm thickness shows up with high resistivities of 100–300 Vm and with very low, almost zero, phases as expected. The resistivity and phase variations between July and September are shown in the Figs. 6e, f and 7e, f. The change in the resistivities is given as normalised resistivity difference ½ðresistivity in SeptemberÞ ðresistivity in JulyÞ =ðresistivity in JulyÞ: Normalised differences smaller than 0 show decreased and normalised differences greater than 0 show increased resistivities, respectively. The changes in phases are shown in simple differences. Negative phase differences indicate that larger absolute phases are observed in September, positive phase differences mark lower absolute phases in September. Significant changes of the resistivity are observed mainly on the floor (corners), along the walls and in the surface near parts of the ceiling. This observation holds for both profiles; however, it is more pronounced in profile 2 (Fig. 7). The resistivity increased most prominently, namely up to 100%, in the floor and in the ceiling. At the sides, the upswing is smaller with about 50% in profile 1. In profile 2, the resistivities in the drift walls decreased even by 50%. The different temporal behaviour of the two circular profiles could be attributed to the local fracture network, slightly different rock properties and, therefore, their different drying behaviour. The phase variances between July and September are much smaller than the discussed resistivity changes. Generally, it is expected that increasing resistivities coincide with decreasing phases and vice versa. Thus, in the phase differences (Figs. 6f and 7f), positive values are expected where a resistivity increase had been observed (Figs. 6e and 7e). The observed phase changes are rather small. Only slight positive changes occur around 0.28–0.48 step out in the lower right corner on the floor of the drift as well as in the ceiling. Nearly all the rest of the drift appears with almost zero phase changes (medium grey hues).
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Fig. 6. Circular profile 1, inversion results of resistivity and phase. (a) Resistivity, July; (b) phase, July; (c) resistivity, September; (d) phase, September; (e) resistivity change between July and September; and (f) phase change between July and September.
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Fig. 7. Circular profile 2, inversion results of resistivity and phase. (a) Resistivity, July; (b) phase, July; (c) resistivity, September; (d) phase, September; (e) resistivity change between July and September; and (f) phase change between July and September.
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To gain a better resolution of the very small phase changes, the color scale has been limited to F0.258. Two different cases of complex resistivity change can be noticed. Mainly in the walls, the resistivity increase is accompanied by a simultaneous phase increase (of absolute values). In contrast, in the ceiling, the observed resistivity increase comes along with a phase decrease. To understand these changes and to attribute corresponding pore space variations, two different causing stress mechanisms have to be considered. In the case of dilatation, the grains (solid matrix) are pulled apart, and the pore water can move or mostly even be drained off. The rock dehydrates, air enters into the formation, and water evaporates. Hence, the resistivity increases due to the decreased total water content. The rock loosening and ongoing evaporation dehydrates the originally fully saturated rock. In pores where all the volume water is drained off, even the surface water is affected by the evaporation process (Fig. 8b). Inasmuch as the polarisation ability of a medium is correlated with its amount of surface water, the phases decrease. In the case of compression, the pore space and the volume of water content decrease, whereas the attached surface water remains in the first state about the same (Fig. 8c). The resistivity of the claystone increases due to the less total water content. By the compression, the ratio of surface to volume water increases, and as a consequence, the measured phases increase. Moreover, the pore spaces become even smaller than they are in the normal state. Hence, the movement of the ions and, therefore, the current flow is partly prohibited, and the medium’s polarisation
ability increases. Actually, this is the underlying process of pore space variation observed in the tunnel walls at depth of N0.3 m. 4.2. Horizontal profile The inversion result of the horizontal profile is shown in Fig. 9. Undisturbed clay is found at depths greater than 20 cm. Resistivities higher than 16 Vm are primarily observed on top of the profile. These high resistivities signify the dehydration zone in the uppermost 10–15 cm (Fig. 9a). Decreased absolute phases would have been expected in this area too, but only at some distinct points phases are around 0.28 instead of the 0.48 of unaffected clay (Fig. 9b). In September, the dehydrated rock zone is more continuous and reaches partly a depth of 20 cm (Fig. 9c). Now, likewise, the phase measurements show the dehydration by the relatively continuous band of low phases between 0.258 to 0.18 dominating the uppermost part of the section (Fig. 9d). The changes of the resistivities and the phases are shown on the bottom of Fig. 9. The left section (Fig. 9e) shows the resistivity change from July to September. Most striking are positive changes on top of the profile, which again assign the increased resistivities and the advanced drying in September. At greater depth, the resistivities decreased slightly and show negative normalised resistivity differences around 0 and 0.25. The phase change between the two measurements is shown abreast (Fig. 9f). The resistivity increase on top comes along with small phase reductions around 0.28. Moreover, the somewhat lower resistivities in September are correlated
Fig. 8. Modification of the pore space by different stresses. (a) Normal state of the undamaged clay. (b) Dilatation yields a reduction of the volume water (by air entrance and evaporation of water) and an increase of the resistivity but a decrease of the phase values. (c) Compression yields a reduction of the volume water and therefore an increase of the resistivity and an increase of the phase values.
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Fig. 9. Horizontal Profile and inversion results of resistivity and phase. (a) Resistivity, July; (b) phase July; (c) resistivity, September; (d) phase, September; (e) resistivity change between July and September; and (f) phase change between July and September.
with slightly increased phases (+0.18) at greater depths. 4.3. Comparison of the circular profiles To investigate the differences between the two circular profiles, the difference between profile 2 and profile 1 has been normalised in profile 1 (Fig. 10). As expected, due to their spatial proximity, the profiles are very similar for both the resistivities and the phases. This indicates a high homogeneity of the investigated formation and justifies 2D inversions of the individual profiles. Two main areas of higher resistivities in profile 2 attracted attention in July (Fig. 10a). The resistivities at profile 1 are three times larger than those of profile 1 on
the right corner of the floor as well as on the right side of the ceiling. On the left wall of the drift near to the horizontal profile, likewise, a region of higher resistivities (normalised differences up to 0.8) is observed at a depth of about half a meter. Lower resistivities in profile 2 than in profile 1, i.e., negative normalised differences, are observed scarcely anywhere. The differences between the two circular profiles seem to diminish from July to September although the zones of higher resistivities in profile 2 do not disappear completely in September (Fig. 10c). But the anomalies on the lower right corner as well as on the left drift wall become less prominent. Large parts of the two profiles only differ less than F40%. The differences in the phases are shown on the right side in Fig. 10. Most ostentatious are two large
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Fig. 10. Normalised resistivity differences and phase differences between the two circular profiles. (a) Normalised resistivity difference in July; (b) phase difference in July; (c) normalised resistivity difference in September; and (d) phase difference in September.
negative anomalies in the lower left corner of the drift and in the upper part of the right wall in July. Only very few spots of positive phase differences right on the drift surface are found. However, the location of differences, which are presented in the resistivities (compare Fig. 10a), are not identical with those in the phases. As for the resistivities, the differences in the phases are smaller in September. Only in the right corner that slight positive phase differences are observed, but almost the entire profile reveals differences around 08.
5. Laboratory measurements Complex resistivity measurements on core material from the EB-niche of Mont Terri should specify the dependence of saturation and resistivity or phase on saturation, respectively. The laboratory specimens of appropriate size were drilled from existing cores of boreholes. One borehole (wall) runs at a height of 1 m almost horizontally into the left wall, whereas the other one (ceiling) runs vertically up into the roof. The specimens out of the ceiling region were drilled
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almost parallel to the bedding planes, whereas the specimens from the wall are drilled almost perpendicular to the bedding planes. Therefore, differences concerning the porosity or the conductivity could have been expected inasmuch as a clear difference of the resistivity between the walls of the drift and its ceiling emerged from the first outcomes of the geoelectrical survey in July (Figs. 6 and 7). Whereas in the drift wall almost only undisturbed claystone was detected, in the ceiling the excavation damaged zone had built up. The complex resistivity measurements were performed with the Solartron equipment from Solartron Instruments, 1996. Two specimens from two different boreholes were investigated respectively. The specimens had a mean diameter of 15.7 (F0.2) mm at a mean length of 54.5 (F0.5) mm. A four-point layout has been applied using two front current electrodes and two potential electrodes along the specimen. The rock resistivity was determined at different saturations with three different salinities of pore water: distilled, with conductivity of 1 and 5 S/m. The resistivity (and its reciprocal value conductivity) are shown in Fig. 11a and the phase in Fig. 11b. In laboratory electrical work, often, the conductivity is used instead of the resistivity, but to compare the results with the field measurements, each of the plots holds two axes. There is no significant difference in the results for the specimen from the dwallT and dceilingT. Although for higher saturations, the specimens of the ceiling have slightly higher conductivities as those from the wall, but these differences are still in the range of the measuring error. Therefore, a welldefined grouping of the results seems here not reliably significant enough. The phase does not correlate with the saturation degree of the specimen in any way. A basically Archie-type equation (Archie, 1942; Schopper, 1982; Mavko et al., 1988; Danowski and Yaramanci, 1999) was used to fit the measured data (Fig. 11a). The conductivity of the rock is: r ¼ rw S n Um þ rq0 S v
ð1Þ
with U is the porosity, S the saturation degree, r w the conductivity of the pore fluid and r q0 the surface conductivity. The parameters m, n and m are empirically determined quantities basically related to the geometry of the pores and their connectivity. The
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Fig. 11. Rock conductivities for different saturations (pore fluid conductivity 5 S/m) as determined in laboratory measurements. (a) Conductivity measurements of the four specimens and basically Archie-type Eq. (2) fit of the conductivity measurements (continuous curve) together with the two comprising parts volume (dashed line) and surface conductivity (dashed dotted line). (b) Phase measurements of the four specimens.
conductivity of porous rocks varies with their water content, ion concentration, mobility and, last but not least, the volume and arrangement of the pores. The conductivity of rocks increases with higher conductivity of their pore fluids, as well as higher saturation degree and fractional pore space. The surface conductivity becomes important for materials with very large inner rock surfaces, as for example clays.
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Table 1 Results of the laboratory measurements for the parameters of the conductivity Eq. (1) at 1 Hz Specimen
r q0 [S/m] m [] m [] n [] U [%]
Wall, 30 cm (depth) Wall, 90 cm (depth) Ceiling, 30 cm (depth) Ceiling, 90 cm (depth)
0.0663 0.0507 0.1218 0.2004
0.75 0.23 0.41 0.21
1.46 1.46 1.29 1.33
1.93 1.53 1.49 1.95
16.15 11.59 15.02 16.26
The porosities are measured in a usual manner with a pycnometer. The surface conductivity r q0, saturation exponents m and n and the cementation exponent m, which are typically needed for the description after Eq. (2), have been determined in the different saturation work steps. After the specimens were dried and desalinated, they were fully saturated (S=1) with distilled water (r wc0 S/m). Then the measured conductivity is equal to the surface conductivity r q0. By drying the specimen steadily and measuring the conductivity at different saturation degrees, the saturation exponent m was determined. This way, the parameters for the surface conductivity part of the total conductivity were established. In the next step, the two parameters m and n affecting the volume conductivity had to be quantified. Thus, the specimens were fully and partially saturated with two different saline pore water. Measurements with full saturations yield the cementation exponent m and with the partial saturations the second saturation exponent n. The different results for the above-described measurements were averaged for all specimens (Table 1) to obtain a mean equation for the investigated material as (Fig. 11a): r ¼ rw S 1:72 0:151:38 þ 0:11 S 0:4 :
ð2Þ
The pore water conductivity of the saturated claystone in the Mont Terri tunnel has been determined in situ as approximately 2–4 S/m. On the basis of Eq. (2), the different resistivities of the undisturbed rock and the excavation damaged zone can be assigned to different water contents. Such an estimation of the water content obviously is based upon the simplifying assumption of constant porosity and fluid conductivity, which holds quite well here. The undamaged clay according to the in-situ measurements has in the EB-niche a resistivity of 8 Vm, which is only reached in full-saturated rock. In
the ceiling of the drift, where resistivities between 30 and 60 Vm were observed, the rock has a very low water content of 10% down to 2.5%.
6. Geotechnical modelling A modelling of the stress situation around the EBniche was accomplished to study the zones of decreased rock strength or even rock failure due to the excavation of the drift. The phase2 program from Rockscience Incorporation, which has been used for the modelling, is a finite-element-based software. The finite element mesh created around the EB-niche consists of triangles with three inner computation sampling points (knots). In the vicinity of the drift, the triangles are smaller and more densely spaced than at greater distance. As boundary condition, it was required that the strains at the border of the model must be identical to zero. The principal stresses act on the EB-niche very favourably for modelling. That is, r 1 acts vertically with 6–7 MPa and r 3 perpendicular to the drift axis with 2–3 MPa. A dstress blockT that indicated the relative magnitude and direction of the field stresses is charted in the upper right corners of the modelling results. The rock and joint properties needed are listed in Table 2. The final rock behaviour is a composition of the behaviour of the undamaged rock and the included joints. In the EB-niche, the bedding planes, as well as tectonically activated joints, have a strike angle of 1408 (SE) with a dipping of 308–508. The failure of the fabric and the joints has been modelled individually. Fig. 12 shows the stress calculation of the joints. When the interfering stress overcomes the joint
Table 2 Rock and joint properties entered in the phase 2 model Parameter
Rock
Joints
Cohesion [MPa] Friction [8] Poisson number E [MPa] parallel bedding E [MPa] normal bedding Joint Aperture Normal Stiffness [MPa/m] Shear Stiffness [Mpa/m]
2.2 25 0.3 2500 4500 – – –
1 23 0.3 – – 0 3 3
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Fig. 12. Modelled joint failure. Upper right corner: stress block with r 1 vertically and r 3 horizontally. Lower right corner: legend of the strength–stress ratio. The joint failure happens after the rock failure.
strength (strength–stress ratio smaller than 1), it comes to failure. Most notably, the joint strength is exceeded on the right-hand side of the ceiling, as well as on the left-hand side of the floor. This asymmetric shape is ascribed to the dipping of the bedding planes. The rock failure and the total failure (rock including joints) are almost identical, and only the total failure will be presented here (Fig. 13). The transgression of the fabric strength appears as a butterfly shape around the niche. The asymmetric failure mechanism, which emerged out of the joint collapse in Fig. 12, disappeared completely, implying that their failure has nearly no bearing on the total rock failure. Failure of the joints happens not until the rock collapsed itself. The strongest (rock) failure develops in the drift walls and on the sides of the ceiling. Imaginable are three different kinds of failure modes: extension fracturing, bedding plane slip and failure due to swelling and softening. Stress-induced breakouts are mostly observed in boreholes, not in the vicinity of galleries and drifts (Martin and Lanyon, 2004).
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ments, the unaffected rock has only been observed in the drift wall. Significantly higher resistivities between 16 to 60 Vd m are observed in the ceiling of the drift, which has been investigated in this study. The increased resistivities in the ceiling are accompanied by obviously lowered phases. Instead of the common 0.48 of opalinus clay that have been measured in the walls, only 0.25 to 0.158 are found in the ceiling. The high resistivities in combination with low phases identify loosened and dehydrated rock. This EDZ (excavation damaged zone) is about 1-m thick on top of the ceiling. Its thickness decreases to about 0.5 m in the side walls. But, different than the resistivities, the phases indicate a slightly asymmetrical ceiling anomaly. At the right-hand side of the ceiling (SW), the phases are much smaller than on the left-hand side (NE). In the EB-niche, the bedding planes dip with approx. 308. The clay particles are aligned in the same direction as the bedding planes, presumably due to tectonic transposition. Transposition yields a geometrical rearrangement of linear elements in the rock. The alignment of the clay particles causes an anisotropy of the rock strength, whereby extension failure preferentially emerges parallel to the bedding, what is here in 308 direction. That means if a loading affects the
7. Discussion and conclusions The presented measurements show that undamaged opalinus clay in the EB-niche of the Mont Terri underground rock laboratory has a resistivity of around 8 Vd m. With complex resistivity measure-
Fig. 13. Simultaneously modelled rock and joint failure. Upper right corner: stress block with r 1 vertically and r 3 horizontally. Lower right corner: legend of the strength–stress ratio.
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particles in the direction of alignment, the cohesion of the particles is broken down very easily. That is the failure mechanism observed in the upper right part of the ceiling. It is on the right-hand side of the ceiling, vertical to the affecting loadings, that it lacks an abutment; therefore, the opalinus clay cracks here at first. A dmigrationT of the fracture zone to the lefthand side of the ceiling is perceived in the repetitive measurements in September, this is the second most fragile region in the drift. Increased resistivities in the ceiling (and on the floor) were found from July to September. Basically, both circular profiles showed this, however, the second circulars profile somewhat more conspicuously. Increased resistivities are usually an indication of advanced drying and failure mechanisms. Simultaneously to the onward loosening and drying, increasing resistivities must be noticed in the drift walls. Inasmuch as every drift excavation necessarily provokes a stress dislocation, the stress trajectories concentrate at the sides of the drift (Fig. 14). At the sides, the stress trajectories point almost perpendicular to the bedding planes and the particle alignment, respectively. The clay material is compressed, which leads to a local decrease in the resistivity when microcracks are closed up. Further on, the compression may lead to a gradual compression of pore space. Thus, the water content within the pores increases relatively, what corresponds to an increasing saturation. With higher saturation a decreased resistivity is observed. The ongoing drying process of the rock is detected in the measurements of October (not further discussed
here). In the ceiling and on the floor, where strong dehydration was found in September, the process slowed down, but the backward walls of the drift behind 0.5 m run dry now, too. Likely, proceeding compression of the clay material in the walls leads not only to a reduction of the pore space (mobile air is squeezed out) but also to a reduction of the water content. On the long run, even water is displaced and probably led away on the drift walls. Not quite the same drying behaviour is monitored at both circular profiles in the observation period. That surely must be attributed to the local fracture network. The joint spacing measures 0.5 m in the EB-niche, and the circular profiles are separated by 1 m. If, for example, joints attach the first circular profile at the walls and dehydrate the rock, it needs not to be all the same with the second profile. The dhydraulic connectionT plays an important role for the time-dependent behaviour of the resistivities. The differences between the resistivities of the two circular profiles remain almost the same over the entire observation period. The most prominent discrepancies between the circular profiles are recorded on the right-hand side of the floor and ceiling. On the floor and in the ceiling of the second circular profile, much higher resistivities are observed. The strongest differences are found in the comparison of the data sets collected in July. In September, the resistivities of the two circular profiles adjust somewhat. Except for the abovementioned discrepancies, merely differences of F20% are calculated between the two data sets. More obvious is the by-
Fig. 14. Rock fabric anisotropy in the opalinus clay. Extension failure due to main stresses affecting in the direction of the alignment of the clay particles.
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and-by dharmonisation effectT of the circular profiles in the phase measurements. On the left-hand side of the drift floor, a large area of lower phases in the second circular profile was detected, whereas this enormous discrepancy utterly vanishes in September. More than one reason can be found to explain unequal resistivities and phases observed in the two circular profiles. First, profile 2 is located more closely to the drift face, hence, the affecting loads could be at some extent smaller as in profile 1 nearly in the middle of the drift. Clearly, near to the drift face, the rock is more supported by the dintactT rock. Furthermore, the local fracture network plays a significant role in the drying process, in the walls particularly. Maybe even local differences in the rock effect permeability. All in all, on the long run, it must be assumed that the circular profiles more and more adjust in both, resistivity and phase. Most probably, the mentioned effects just delay the assimilation of the resistivities and phases measured at the two profiles, but they do not stop it at all. Besides the geoelectric measurements, also seismic down-hole investigations have been conducted in the EB-niche (Schuster et al., 2001). Inasmuch as the two methods geoelectric and seismic are heading for different material properties, a combined evaluation of the measurements yielded a still more comprehensive understanding of the loosening processes in the vicinity of the drift. With the aid of seismic borehole measurements, the existence and position of a compacted rock region with high velocities in the wall of the drift could be validated. Slightly increased resistivities together with low phases definitely identify the EDZverified with seismics beyond any doubt. Borehole measurements obviously give detailed information over the entire length of the wells, but with the resistivity method, the whole volume can be covered. So, the simultaneous application of both methods provided an increase of the assessment certainty. The risk of possible misinterpretations could be reduced.
Acknowledgements The authors like to thank to the Federal Institute for Geosciences and Natural Resources (BGR) for the funding of this work within the frame of the European Union project dEngineered Barrier Emplacement
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Experiment in Opalinus Clay.T Furthermore, we especially thank Dr. H.-J. Alheid and Dr. K. Schuster (BGR) for the cooperation and advice. P. Bossart and H. Steiger from the Mont Terri Consortium supported us in the field and helped us through their experience underground. We thank Dr. T. Wonik and M. Grinat (Leibniz Institute for Applied Geosciences) for the disposal of the measurement device in July 2001. References Alheid, H.J., Knecht, M., Boisson, J.Y., Homand-Etienne, F., Pepa, S., 1999. Comparison of in-situ hydraulic and seismic measurements in the excavation damaged zone of underground drifts. 9th International Congress on Rock Mechanics, Paris, France, Expanded Abstracts. , pp. 1263 – 1266. Archie, G., 1942. Electrical resistivity as an aid in core analysis interpretation. Transactions of American Institution of Mining and Metallurgical Engineers 146, 54 – 61. Dannowski, G., Yaramanci, U., 1999. Estimation of water content using combined radar and geoelectrical measurements. European Journal of Environmental and Engineering Geophysics 4, 71 – 86. Emsley, S., Olsson, O., Stenberg, L., Alheid, H.J., Falls, S., 1997. A study of damage and disturbance from tunnel excavation by blasting and tunnel boring. Technical report, SKB-Swedish Nuclear Fuel and Waste Management Co., Stockholm. Fan, X., 1998. Modellierung und Inversion von gleichstromgeoelektrischen Bohrlochmessungen mit 2D und 3D Finite Differenzen. PhD Thesis, Technical University Berlin, Germany. Kiewer, M., 2000. Geoelektrische Charakterisierung von Tonformationen. MSc Thesis, Technical University Berlin, Germany. Kruschwitz, S., 2002. Detection and characterization of the disturbed rock zone in claystone with complex valued geoelectrics. MSc Thesis, Technical University Berlin, Germany. Martin, C.C., Lanyon, G.W., 2004. Excavation disturbed zone (edz) in clay shale: Mont Terri. Technical Report 2001-01, Mont Terri Project (Paul Bossart), Switzerland. Mavko, G., Mukerji, T., Devorkin, J., 1988. The Rock Physics Handbook. Cambridge University Press. Schopper, J.R., 1982. Electrical conductivity of rocks containing electroloyts. Landolt-Bfrnstein, Group V, Physical Properties of Rock, Subvolume B, vol. 1, pp. 276 – 291. Schuster, K., Alheid, H.-J., Bfddener, D., Eichhorn, P., Spies, T., Heidrich, D., 2001. ED-C Experiment: seismic investigations of the edz and acoustic emission measurements in the new gallery. Technical Report 2000-05, Mont Terri Project (Paul Bossart), Switzerland. Spies, T., Eisenbl7tter, J., 2001. Acoustic emission investigation of microcrack generation at geological boundaries. Engineering Geology 61, 181 – 188. Yaramanci, U., Kiewer, M., 2000. Geoelectrical characterization of the disturbed rock zone in opalinus clay (Mont Terri, Switzerland). Proceedings of 6th EEGS-ES Meeting, Expanded Abstracts EL 14, Bochum, Germany.