LONG-TERM MONITORING OF ROCK MASS PROPERTIES IN THE UNDERGROUND EXCAVATION

J. Bárta (1), D. Dostál (1), J. Jirků (2), V. Kopecký (3), L. Slavík, (3), J. Vilhelm(2) (1) G IMPULS Praha, Prague, Czech Republic (2) Charles University in Prague, Faculty of Science, Institute of Hydrogeology, Engineering Geology and Applied Geophysics, Prague, Czech Republic (3) Technical University of Liberec, Faculty of Mechatronics, Informatics and Interdisciplinary Studies, Institute of Systems Control and Reliability Management, Liberec, Czech Republic

ABSTRACT:Studying the behaviour of the rock mass is crucial for the development of geophysical monitoring system that allows continuous monitoring of the condition of the geological environment, quantitative and qualitative analysis of the gained knowledge and possibly sending warning signals that the rock environment is getting to situation that exceeds the set safety criteria. Monitoring of the rock mass is for example an essential part of the system of storing hazardous, especially radioactive waste in the geological environment. A team consisting of the above mentioned authors addresses this issue under the project of the Technology Agency of the Czech Republic, see task TA 03020408. The aim of this project is to develop a functional system for long-term monitoring of the rock mass, using non-destructive geophysical methods. The system will use the geoelectric and seismic properties of the rock mass. It will provide long-term monitoring of changes in properties of the rock mass in the immediate vicinity of the underground excavation. Currently, in the Bedřichov gallery, pilot measurements based on the methods of resistivity tomography and microseismics are underway. Measurements using the method of resistivity tomography with spacing of 20 cm between the electrodes (in total 48 electrodes) are performed every six hours on the test site. Every five minutes (or in the another requested interval) a seismic record (one seismic source and three receiving sensors) is acquired. Utmost attention is paid to the behaviour of a significant fracture which is located in the middle of the measuring system. Monitoring apparatus collects data and these data are continuously sent via the Internet to the workplaces of project members. The project Ta03020408 will be finished in December 2016 and the geophysical hardware and software will be ready to commercial utilization.

1. INTRODUCTION Monitoring of the condition of the rock mass and its changes in the vicinity of the underground excavation so far focused on observation of ongoing measurable deformations and showings of occurrence of these deformations. Such deformations occur mainly on geological disturbations. It is possible to directly measure the magnitude of relative shift of the two rock blocks on both sides of the disturbation, or seismoacoustic emission which may occur in the process of formation of brittle disturbance of rocks. For long-term monitoring of the condition of the rock mass in the vicinity of the underground excavation in conditions where we do not expect changes in stress that would give rise to rock disturbances, active tomography by seismic waves can be used. In such case, the transmission properties of the rock mass can be monitored on the long-term basis, specifically through identifying the seismic waves´ variations of velocity in relation to time as well as amplitudes of the seismic waves. Repeated monitoring by the method of electric resistivity tomography can be used as an independent control method, which is able with a high sensitivity to respond to changes in the rock mass caused mainly by changes in saturation with groundwater. Initial thoughts on the draft of the monitoring methodology presented here are mainly based on experiments with monitoring geophysical manifestations of granite rocks in the water conduit gallery leading from the Josefův Důl water reservoir to the water treatment plant in Bedřichov (Liberec Region). This work under the supervision of some members of the research team has been underway since 2003. Monitoring of mechanical and electrical resistivity properties using repeated tomography by the seismic ultrasonic waves and repeated measurements of apparent resistivity indicate the incipient changes in the rock mass, caused by changes in mechanical stress or development of microdisturbance.

The dependence of the character of propagation of seismic waves on rock mass stress or its fracturing has been known for many years. Similarly, described in the literature is the dependence of electrical resistivity on the conductivity of the rock. Joint interpretation of both methods in order to predict long-term behaviour of the rock mass, however, has not been used yet. The application of resistivity measurements has brought a new finding that monitoring of resistivity conditions can be reliably performed even in conditions of studying the rock mass, i.e. in conditions with extremely high resistivities (up to tens of thousands ohmm). Under these circumstances, the only fundamental factor of changes in conductivity is water content and mineralization of water in the fractured system. Newly approached is also the realization of seismic tomography, where the original methodology of excitation and registration of seismic waves is used. In addition, the result of years of research is, especially in the aforementioned tunnel in Bedřichov, that even under standard conditions there take place minor geomechanical and hydrogeological changes that are traceable in the measured data. These manifestations, important for understanding the behaviour of the rock mass, show thus properties of time variations that need to be captured by continuous monitoring. This requirement was met by constructing the original geophysical system enabling data storage and transmission via the Internet for further research or early warning of the relevant bodies (construction management, security service, etc.). Realization of research plans proceeds successfully thanks to a grant TA 03020408, provided by the Technology Agency of the Czech Republic. The task was launched in 2013 and will be completed in 2016.

2. DESCRIPTION OF THE GEOPHYSICAL SYSTEM SGI 1 Deliverable of research in the Bedřichov gallery in the Jizera Mountains is particularly geophysical system SGI 1. The system allows continuous measurement of resistivities, involving up to 48 electrodes that are placed in a profile line, in which continuous monitoring of the rock environment is carried out. In the place of interest there are also placed seismic sensors (piezosensors) and a source of seismic excitation (again, working on the principle of piezoeffect), which allows to monitor the characteristics of the propagation of seismic waves. Monitoring system SGI 1 operates with three seismic sensors. Potential capability of the system SGI 1 is a possibility to monitor also the effects of induced polarization (IP) in the time domain. Another potential capability of the system is monitoring of not only actively excited seismic waves, but also passive monitoring of seismicity at high frequencies when the active signal is not excited. These capabilities of the system and possibilities of their use in assessing geotechnical conditions of the rock mass will be studied in the context of other research projects and will subsequently be put into practice. The overall block diagram is shown in Fig. 1.

Figure 1: Configuration of the measuring apparatus

The entire control electronics is housed in Schneider Thalassa PLM switchboard with a transparent door. The brain of the whole system is an industrial PC Advantech ARK 1503, controlled by an Intel Atom processor. In addition, the following components are placed in the switchboard:

• power supply • pulser-receiver Olympus 5077PR

• protection • electronics of seismic apparatus

• distribution channels for wiring

ARESII is measuring apparatus for impedance tomography and induced polarization. Firmware ARES II was supplemented with utility that provides continuous sending of data via RS232 bus into a parent PC. This step allows permanent storage of data in an external PC and then sending data to a remote ftp server. Fig. 2 shows a general view of the concept of the measuring system in the Bedřichov tunnel.

Figure 2: The concept of collecting seismic and geophysical data

Fig. 3 shows an example of the geophysical monitoring with SGI 1 in the field.

Figure 3: Example of placing the geophysical monitoring system SGI 1 in the field.

General view of the workplace in the Bedřichov tunnel

3. DESCRIPTION OF THE TEST SITE IN THE TUNNEL The tunnel in Bedřichov connects Josefov water reservoir and water treatment plant which is situated in the centre of the municipality (see the map in Fig. 4). Bedřichov is situated near Liberec, approx. 10 km to the east. In terms of geology, the site of interest is built by granite, locally called Liberec granite. Liberec granite is a sought-after building material that achieves quality of decorative stone. At present, this granite is extracted at the quarries of Hraničná and Ruprechtice. These quarries, located at a distance of only about 10 km from the research tunnel, are logically very good comparative bases for measurement directly in the tunnel. Water from Josefov reservoir is channeled to the treatment plant via a steel pipe, which occupies part of space of the excavated tunnel (see Fig. 5). The rest of the space can therefore be used for various research works. Currently, the tunnel is subjected to hydrological, thermometric and geomechanical measurements. Since 2003, there have been carried out various geophysical measurements, which focus mainly on issues of resistivity tomography and seismics. Continuous attention paid to this site has resulted in the fact that the measurement has the character of monitoring, which allows to observe the behaviour of the rock mass over time. Experience with the conventional measurements in intervals of several months has led the research to the conclusion that it is necessary to perform measurements in an automated manner, in regular, and preferably short intervals. There was also set a requirement for the data to be systematically sent via the Internet to research bodies (G IMPULS, Faculty of Science of Charles University, Technical University in Liberec), to allow continuous monitoring of the data or timely interpretation of changes in the data. The first

stage of the work performed, until 2012, indicates that the changes that take place in the gallery, at least in the case of resistivity measurements, have the character of daily, weekly, monthly (and yearly?) variations. We note that at the beginning and end of the water conduit there are situated two hydroelectric power plants, which are the source of interfering stray currents. All geophysical measurements had to cope with these issues, on the other hand, stray currents are another interesting subject of research that expands knowledge about the possible effects of stray currents on the construction of underground structures.

Figure 4: Location of the exploratory gallery in the municipality of Bedřichov. Tunnel in the area of the

water reservoir leads partly below it and ends with a vertical shaft with an elevator.

Figure 5: Looking into the tunnel. To the left, water conduit pipe. A view

from the dam to the water treatment plant.

In the gallery there are more test sites which are used by the geophysical group for their research. Of the utmost importance now is the research around tunnel metre 792 from the entrance. Here, according to the project TA 03020408, a prototype of the monitoring system SGI 1 was located and here it is field tested. Attention is paid to the place with a distinctive fracture, see Fig. 6.

Figure 6: A view of the monitored fracture.

The picture is from the period of the first tests. It shows electrodes at a spacing of 10 cm for resistivity measurements. Placed below is a precision analogy meter of movement, belonging to the Institute for Rock Structure and Mechanics of Academy of Sciences of the Czech Republic. Plastic pipes house a wiring required for research operations in the tunnel.

4. CURRENT KNOWLEDGE FROM MONITORING MEASUREMENTS Measurements performed under the task TA 03020408 build on earlier research carried out in the Bedřichov gallery. Issues associated with earlier research are described for example in lit.: Bárta, Jirků (2011, 2013), Bárta et al. (2010), Jirků et al. (2014), Vilhelm et al. (2008, 2010, 2012, 2013, 2015); and also on posters for conferences organized by the European Association of Geoscientists and Engineers (Madrid 2015, Amsterdam 2014). For issues related to microseismic measurements in solid rocks, information was drawn mainly from the articles, see references Petružálek et al. (2013), Vilhelm et al. (2008, 2010, 2012, 2013), Živor et al. (2011). As regards basic literature that relates to the issues of physical properties of solid rocks and also connects these issues with the practical technical field, i.e. tunnelling, reference is made to lit. Barton (2006), or also textbook Butler (2005). Because the goal to be achieved by this work also includes specific technical deliverables (certified methodology, utility model), it may be noted that in the final phase there is the preparation of a European standard Draft prEN 16907-2 ISCS 13.080.99;93.020, English Version Earthworks – Part 2: Classification of materials, in which official classification describing the relationship between the strength (fracturing, weathering) and the velocity of propagation of longitudinal seismic wave is first used. Research team dealing with the task TA 03020408 believes that its research leads to a new way of assessing the behaviour of the rock mass, on the other hand, that the whole issue is currently relevant, and is not detached from what is happening in other geophysical (geotechnical) workplaces. From the data obtained so far by monitoring (also taking into account earlier research) the following conclusions can be arrived to: Rock (granite) mass is characterized by propagation of longitudinal seismic wave at a velocity of around 5,000 to 5,500 m/s. The velocity of the shear wave is around 3,000 m/s. From earlier measurements (larger amount of data) we calculated the average Poisson´s ratio of 0.25. The level of the velocity of the longitudinal wave corresponds to the international experience for measurements in high quality granite in the unloaded condition (measurement on the ground surface). Somewhat surprising is the value of Poisson´s ratio, which is slightly lower than could be expected for virtually intact rock environment. Figures 7a, b show examples of seismic records from the monitoring measurements in Bedřichov. An important observation is that the magnitudes of the seismic velocities in the monitoring station in the long term remain virtually unchanged. It can therefore be concluded that their reduction can be expected only after the recovery or the emergence of a new fracture system.

Figure 7a: Determination of the velocity of P-waves on the seismogram with the marking of traces

according to their distance. Straight line is used to show a time-distance curve of P-wave

Further, Fig. 7b shows the determination of shear wave velocity.

Figure 7b: Determination of the velocity of S-waves on the seismogram with the marking of

traces according to their distance. Straight line is used to show a time-distance curve of S-wave.

Resistivity values in granite range widely from hundreds to tens of thousands ohmm. The lowest resistivity values are typical of heavily crushed and water-bearing rocks in the tunnel invert. Tens of thousands ohmm are typical of entirely solid parts of granite, where there is only minimal fracturing or the rock is entirely dry. Essential finding, however, is the fact that the condition of the rock mass changes, though slightly, in relation to time. As an example we present in Fig. 8 seven resistivity cross-sections, which were selected from the monitored data around metre 792 of the tunnel. Cross-sections with approximately monthly intervals were selected. The measured data have been obtained using the system SGI 1 and interpretation of the data is performed using tomographic programme Res2DInv (Locke). We note that during testing of the geoelectric part of the system, enormously low value of mean error of repeated measurements, namely 1% or less was found. Of course, measurements can also be affected by an unexpected event, such as input of parasitic current field. In this case, however, the geoelectric part of the data is protected by entering in the database of the average value of the five measured data from each observation. These five data may not have the mean error of repeated measurement higher than 5%. The overview of resistivity cross-sections shows well minor, but distinctive changes in resistivities over time, which in our opinion relate to changes in moisture content in fractures and possibly even changes in mineralization of water. Of major importance can be considered the fact that the cracks and fractures on the tunnel wall macroscopically seem to be simply just as one „slot“. In fact, to the fracture there is apparently bound the entire system of microcracks, which in the vicinity of the basic macroscopically visible fracture are wetted (lower resistivity values), while in the centre of the whole system there is a kind of drainage, through which the water can be vented into the tunnel environment (see high resistivity values characterized by blue colour indicating a dry environment). Fig. 8 can be considered illustrative, but it has to be noted that this is an interpretation, i.e. models which are affected by the input conditions of calculations. Currently, also statistical processing of the measured primary

data is underway. For example, primarily measured resistivity values from individual measured positions are compared in time intervals, etc. These issues are, among others, the subject of a pending doctoral thesis. The high variance of values of the measured primary data is shown in Fig. 9 with examples of measured resistivity values over time (selected data are taken from Fig. 8). Individual curves are to the right marked with a position, to which profile metre they relate. Parameters a, n determine the position, to which the measurement is relative to the depth (in more detail, see programme Locke). The depths range between 0.2 and 1 m. Fig. 10 shows the statistical processing of the whole data set used for the overview of resistivity cross-sections. Fig. 10 shows the maximum and minimum values of the individual measurements (cross-sections), medians and lower and upper quartiles. From Fig. 10 it is clearly seen that the measured data are essentially consistent, repeatable over time. However, there are minor shifts in median, which are logically associated with local changes that may be significant in some points.

5. CONCLUSIONS AND DELIVERABLES Research conducted under the task TA 03020408 enabled to better understand the behaviour of the granite rock mass from the perspective of geophysical methods. Some findings, such as the relatively complex hydrological conditions in the immediate vicinity of the open fracture itself (see Fig. 8) detected by the method of resistivity tomography, are obviously a significant complement to the primary macroscopic idea of mapping geologist. Changes in geoelectric (resistivity) properties of the fracture systems are clearly traceable. Obviously, they are related to the change of water content in the fractures (or also change of mineralization). These changes reflect, in our opinion, the changes in stress of the rock environment. Only changes in stress can change the size of the fractures, their closing and opening.

Figure 8: Overview of resistivity cross-sections obtained by SGI 1 around the tunnel metre 792.

Figure 9: Overview of selected primary data (apparent resistivities) and their changes over time.

Figure 10: Box – Whisker graphs, statistically processed data sets, identical also for Fig. 8.

Changes in seismic velocities in the period without significant intervention in the rock mass are probably not manifested, or are very small. If in monitoring there occur significant changes, it can be automatically assumed that they relate to the excavation of a nearby working, to sudden changes in the zone of reduced velocities around the excavation, etc. Geophysical monitoring system IGS 1 shows during pilot tests significant reliability and will further be adjusted in the course of the work planned by the end of 2016. Planned deliverable of the task TA 03020408 is approval of the Certified Methodology (measurement and interpretation workflow) and Utility Model (technical aspect of the product). The entire system is developed especially to help monitoring in carrying out major mining works, namely during the excavation of a deep repository of radioactive waste.

6. ACKNOWLEDGEMENTS The research team would like to thank the Technology Agency of the Czech Republic for the financial support granted for addressing the aforementioned issues under the project TA 03020408.

REFERENCES Professional publications and papers: [1] BARTON, N. Rock quality, seismic velocity, attenuation and anisotropy. Taylor and Francis Group: London 2006.

[2] BÁRTA, J., Jirků, J., Dostál, D., Kněz, J., Slavík, L., Vilhelm, J.: Monitoring chování puklinových systémů horninového masivu geofyzikálními metodami - Informace o probíhajícím úkolu TA 03020408. Hydrogeologická a Inženýrskogeologická conference: Liberec 2014.

[3] Bárta, J., Jirků, J. : Hlubinné úložiště radioaktivních odpadů – výzva pro české geofyziky: EGRSE 2013, No. 1, ISSN 1803-1447.

[4] BÁRTA, J. in PAČES, T., MIKŠOVÁ, J., editoři, Far Field of the Geological Repository in the Bohemian Massif, Czech Geological Survey and Radioactive Waste Repository Authority 2013

[5] Bárta, J., Jirků, J. Resistivity tomography in studying the granite massif in the Bedřichov gallery (the Jizerské hory Mountains). Findings and conclusions gained so far from the measured data: EGRSE 2011, No. 2, CD-ROM, ISSN 1803-1447.

[6] BÁRTA, J. The comparison of certain classical geotechnical terms for underground structures with terminology for deep radioactive waste repositories. A geophysicist´s view. Mezinárodní konference Podzemní stavby: Praha. 2010

[7] BÁRTA, J., KNĚZ, J., BUDINSKÝ, V., JIRKŮ, J. Seven years of experience in experimental testing of granite rocks in the gallery serving as water conduit from the Josefův Důl hydraulic structure to treatment plant in Bedřichov (Northern Bohemia): EGRSE 2010, No 2, ISSN 1803-1447.

[8] Butler, D. K. Near-Surface Geophysics: Society of Exploration Geophysicists 2005.

[9] COSMA, C. Two decades of evolution of hardrock seismic imaging methods applied to nuclear waste disposal in Finland. EAGE 66th Conference & Exhibition: Paris, France, 7 - 10 June 2004.

[10] GEVREAU, E. Etude de l´evolution des versants naturels en mouvements. Prévoir, alerter, Thése LCPC: Paris 1991

[11] Jirků, J. a kol. Monitoring of joint systems time-lapse behaviour via ert. Výroční konference SEG: Denver 2014

[12] JIRKŮ, J., BÁRTA, J. Monitoring of joint systems time-lapse behavior via ERT. Environmental and Engineering Geophysical Society, SAGEEP 2014, , Session E1-Electrical –RES, IP, Self Potential, accepted for oral presentation and proceedings: Boston 2014

[13] JIRKŮ, J. Geoelectric investigations for the assessment of the geotechnical and hydrogeological conditions of the rock massif in the neighbourhood of excavation. Studentská konference EAGE a Geofyzika Krakov: Krakov 2011.

[14] PETRUŽÁLEK, M., VILHELM, J., RUDAJEV, V., LOKAJÍČEK, T., SVITEK, T., Determination of the anisotropy of elastic waves monitored by a sparse sensor network, International Journal of Rock Mechanics and Mining Sciences, 60, 208-216. 2013.

[15] SPILLMANN, T., BLÜMING, P., MANUKYAN, E., MARELLI, S., MAURER, H. R., GREEN, A. G. (2010): Geophysics applied to nuclear waste disposal investigations in Switzerland. First Break, Vol. 28, Issue 8, August 2010, 39-50, ISSN 0263-5046.

[16] VILHELM, J., SLAVÍK, L. High frequency measurement of P- and S-wave velocities on crystalline rock massif surface - methodology of measurement. European Geosciences Union, General Assembly 2014: Vienna 2015.

[17] VILHELM, J., RUDAJEV, V., LOKAJÍČEK, T., ŽIVOR, R. Velocity dispersion in fractured rocks in a wide frequency range, Journal of Applied Geophysics, 90, 138-146. 2013.

[18] VILHELM, J., RUDAJEV, V., ŽIVOR, R., LOKAJÍČEK, T. Seismic Measurement on a rock massif surface at short distances, Acta Geodynamica et Geomaterialia, 9 (2), 143-153. (2012).

[19] VILHELM, J., RUDAJEV, V., ŽIVOR, R. Assessment of fracture properties from P-wave velocity distribution, In: Idziak, A. F., Dubiel, R.: Geophysics in Mining and Environmental Protection, 109-116: Springer-Verlag Berlin Heidelberg 2011.

[20] VILHELM, J., RUDAJEV, V., ŽIVOR, R., LOKAJÍČEK, T., PROS, Z., Influence of crack distribution of rocks on P-wave velocity anisotropy - a laboratory and field scale study: Geophysical Prospecting 2010, 58 (6), 1099-1110

[21] VILHELM, J., RUDAJEV, V., ŽIVOR, R., LOKAJÍČEK, T., PROS, Z. Comparison of field and laboratory seismic velocity anisotropy measurement (scaling factor): Acta Geodynamica et Geomaterialia 2008, V. 5, No. 2(150), 161-169.

[22] ŽIVOR, R., VILHELM, J., RUDAJEV, V., LOKAJÍČEK, T. Measurement of P- and S-wave velocities in a rock massif and its use in estimating elastic moduli: Acta Geodynamica et Geomaterialia 2011, 8 (2), 157-167.

RNDr. Jaroslav Bárta, CSc. G IMPULS Praha, Prague, Czech Republic E-mail address: barta@gimpuls.cz RNDr. Dušan Dostál Charles University in Prague, Faculty of Science, Institute of Hydrogeology, Engineering Geology and Applied Geophysics, Prague, Czech Republic E-mail address: dostal@gimpuls.cz Mgr. Jaroslav Jirků Charles University in Prague, Faculty of Science, Institute of Hydrogeology, Engineering Geology and Applied Geophysics, Prague, Czech Republic E-mail address: jirku@natura.cuni.cz prof. Ing. Václav Kopecký, CSc. Technical University of Liberec, Faculty of Mechatronics, Informatics and Interdisciplinary Studies, Institute of Systems Control and Reliability Management, Liberec, Czech Republic E-mail address: vaclav.kopecky@tul.cz Ing. Lubomír Slavík, Ph.D. Technical University of Liberec, Faculty of Mechatronics, Informatics and Interdisciplinary Studies, Institute of Systems Control and Reliability Management, Liberec, Czech Republic E-mail address: lubomir.slavik@tul.cz doc. RNDr. Jan Vilhelm, CSc. Charles University in Prague, Faculty of Science, Institute of Hydrogeology, Engineering Geology and Applied Geophysics, Prague, Czech Republic E-mail address: vilhelm@natur.cuni.cz