Seismic Analysis Without Seismic Data. The relevance of ground observation for seismic interpretation.

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1 Seismic Analysis Without Seismic Data. The relevance of ground observation for seismic interpretation. Jarufe, Juan Andres Assistant Professor at Universidad de Santiago de Chile and Consultant at SRK Consulting Chile Soto, C. Senior Consultant, SRK Consulting Chile Verdugo, J. Geomechanics Principal, Aes-Gener Chile ABSTRACT: Underground mines and tunnels advance at a great pace. In many cases, the transition from stable to seismically active ground occurs faster than the ability of the organization to justify, design, install and implement a seismic monitoring frame, with seismicity occurring only to the eyes of the workers and not being documented in any way. In this paper, the relevance of ground observations is accounted as fundamental part in the understanding of the mechanism that generate seismicity. Empirical relations obtained from seismic data are applied to ground collected information and meaningful conclusions can be drawn about the geologic environment where tunneling is progressing. Key words: Rockburst; Seismicity; Tunnels; Ground Control; Energy Demand; Geology. 1 INTRODUCTION During the development of mines and tunnels in central Chile, ground experience has shown that at depths of 500 m. below surface, rock noises start to be heard by workers. From this point forward, and as result of the abrupt topography that exist in Chilean mountains and the high horizontal compressive stress regime, the effects from excavation induced seismicity increases rapidly with the tunnel advance, in many occasions too fast for mines that originally did not consider a seismic environment, thus the implementation of a system to manage those seismic effects can take up to several months to be fully applicable, and probably, delaying further excavations until a system to evaluate seismic hazard is implemented.

2 This paper describes a method to process ground instabilities observations and make them available as a number that can be used to achieve a better understanding of the failure process that affect the tunnel and relate seismic related instabilities with geological conditions. 2 ALTO MAIPO AND VA-1 TUNNEL. The Alto Maipo Hydropower Project correspond to the construction of two underground hydroelectric power plants along the Colorado River, southeast of Santiago, in central Chile. Together, the power stations will generate up to 531 MW of electricity, supplying the Central electrical grid with an average production of 2350GWh. VA-1 is the name of the adit tunnel that access to one of the power stations. It is a m tunnel starting with 5 meters of overburden but increasing up to 1050m at the end of it (Fig.1). Tunnel section has 42.6 m 2 into a 6.9x6.9 section advanced through drilling and blasting method (Russo and Saragoni 2016) 3 GEOTECHNICAL SETTINGS AT VA1 TUNNEL This project is located in central Chile mountains, specifically in the Cajon del Maipo glacier valley, marked by stratified volcanic rock part of the Abanico rock formation (Eocene-Oligocene), composed by lava, volcanic brechia and andesitic lapilli. The most relevant structural feature corresponds to the homosynclinal rock strata s dipping at NNW-SSE/15 (Figure 2) Fig. 1. Overburden above tunnel VA-1 and cross section specifications (Internal Report) Fig. 2 Regional geological setting at VA-1 tunnel (Armijo et al. 2010, modified by Rauld 2011). Tunnel is driven perpendicular to the stratification thus, several strata layers are expected to be crossed during tunnel. Geologic theory states that in each strata fold there are oblique and tension joints that define discontinuities that the tunnel will face during it advance.

3 Fig. 3 Joint systems associated with rock beddings (modified from Price 1966) Starting in PK 1+950, a decametric thick layer of andesite has been excavated. Intact rock is characterized by high strength and a very stiff rock (Table 1), with intact rock strength as high as 230 MPa. When this data is compared with published from seismically active mines, it is clear the rock material is prone to a relevant seismic response (Figure 4). Table 1. Intact rock properties for main water tunnel Geotechnical Unit Intact Rock Strength Stress orientation is estimated from borehole damage inspection, HI stress cells and Hydraulic Fracturing stress measurement. Based on this data, a stress model that corresponds well with the existing compressive regime controlled by the subduction of the Nazca plate below the American plate was developed. Sigma one is estimated to be horizontal, with a range as shown in Fig. 5. Due to the mountain topography, Sigma 2 and Sigma 3 have very similar magnitudes thus, the orientation of these stresses can be swapped between them. Considering this, the minimum stress calculations resulted in a horizontal orientation, perpendicular to the orientation of the maximim stress (Fig. 5), generating a high anisotropy on tunnels running south, like VA1- tunnel. σci mi Ei µi Andesite Fig. 5 Stress field orientation in tunnel under investigation. Note S2 and S3 orientations overlapping (Internal Report) Fig. 4 Intact rock classification proposed by Deere and Miller According to the stress model accepted on site, stresses magnitudes are governed by rock column heigh, thus by surface topography. The stress field along the tunnel axis is shown in Fig. 6, with values up to 60 MPa at the end of the tunnel.

4 Fig. 7 Expected severity of the brittle failure events based on the geomechanical data collected (Internal Report) Fig. 6 Stress field magnitude increase along tunnel axis 4 SEISMICALLY RELATED ROCKFALLS As the tunnel advanced deep underground, ground personnel started to hear noises and cracks inside the rock in the hours following blasting. As tunnel excavations progressed, these rock sounds were accompanied by rockfalls at unsupported walls, where gravitational fall of ground was the predominating mechanism nevertheless, rock ejection up to some distance from the face was steadily being introduced as an important instability. This ground experiences were supported by rockburst proneness engineering analysis (Fig.7), where the ratio between the maximum stress around excavation and UCS was related to high energy rockburst events with very severe overbreak (Diederichs, 2005, 2010; Hoek, 2010; modif.) Location and time of rockfall, in absolute terms and relative to the last blast. Dimensioning of the rockfall, size and volume. Distance travelled by the failed rock, from the tunnel wall to the ground where it was observed. Location of the rock failure in the tunnel, indicating crown, walls, roof, etc. Damaged rock support Any additional information, as rock noises reported before the fall of ground or some important lithologic change, etc. This information is continuously being documented each time rockfalls occur at VA-1 tunnel. Since STR increased as the tunnel advanced, the number of reports also did, currently forming a robust database of more than 200 actual rockfalls that happened as the tunnel progressed. As seismically triggered rockfalls (STR) were becoming more common with tunnel advance, a report system describing the rock failure was implemented, where important information about rockfalls was documented as:

5 20% 15% 10% 5% 0% 80% 60% 40% 20% 0% TIme of Day Distribution Ejected Weigth (ton) occurring in between blasts. The ejected distance is for most of the events less than one meter, corresponding to a gravitational fall induced by shaking (Kaiser et al. 1996), nevertheless there are rockfalls reported to have been ejected up to 7 meters from the tunnel wall. Supporting the previous chart, the weight of the detached rock in most of the cases is less than one ton (related to the ejection distance lower than 1 meter), but it can be seen also rockfalls up to 130 tons, when large volumes of rock were ejected from the face (Fig. 9) 80% 60% 40% 20% 0% Ejected Distance (m) % 40% 20% 0% Rockfall Heigth (m) Fig. 8 Description of the some of the parameters that describe seismically triggered rockfalls. As shown in Fig.8, the STR database can give some indications about the behavior of rockfalls. While there is some correlation with blast time, usually performed at 06:00 in the morning and 22:00 at night, there is still a great number of rockfalls Fig. 9 Example of two large rockfalls where more than half of the excavation face was ejected, with more than 100 tons of displaced rock. Tunnel section is 7 X 7 m aprox. Also, more than 80% of STR occur from the roof of the excavation, related to a horizontally compressive stress regime.

6 5 BACK ANALYSIS OF SEISMICALLY TRIGGERED ROCKFALL EVENTS The information about distance travelled and size of the ejected block certainly is useful, but the consolidation of these parameters into one single value is much more useful, since only one parameter describes the rock condition at the ejection time. Also, while weight and ejection distance are good parameters that describe the instability, the use of individual parameters does not give a complete picture of the mechanism involved, since high weight ejections not necessarily are ejected a large distance thus, both analysis will not lead to the same conclusion. A method to include all the previous data into one single parameter is the back analysis of the ejection velocity and the associated energy to remove the block from the stable position and eject it to the ground. This method presented by Tannant et al has been use in many individual analysis of rockburst around the world (Jarufe and Vasquez 2014). This method shown in equation 1 and schematically in Fig. 10, v e = d g 2h cos 2 θ+dsin 2θ (1) Based on the ejection velocity (ve) and the mass of the detached block (m), the kinetic energy (Ec) can be calculated according to equation 2 E c = 1 2 m v e 2 (2) In this work, this calculation was applied to each of the STR reported by the construction team, thus a large database covering 2 years of seismically related instabilities along 2 km of deep stressed tunneling was built. This database provides invaluable information about the real, observed behavior of the tunnel, useful to develop empirical relationships between damage and excavation parameters. All the energy results obtained in this analysis will be normalized by the ejected area to get kj/m 2 as the resulting unit, which has been used extensively in rockburst back analysis and support elements energy absorption capacity calculations, thus, reference values for comparison exist. Time History of Energy Release The spatial evolution of the rockfalls can be seen in Fig. 11, where the normalized energy (Total energy /ejected area) is accumulated through the development of the tunnel. It can be seen a clear change in the trend after the 1900 m chainage, where a substantial increase in the accumulative energy occurs. In Fig.11, the energy of each individual rockfall is plotted as a color circle, where the color represents the distance from the blast when the rockfall happened. Red circles correspond to rockfalls that happened more than 50 meters from the blast, while blue circles corresponds to rockfalls that happened at or near the blasted face. Fig. 10 Description of parameters to estimate energy associated to rock ejection (from Kaiser et al. 1986)

7 Fig. 11 History of energy associated to rockfalls during tunnel construction. Color indicate the distance to the blasted face. There are some zones where rockfalls persisted as the tunnel advance, i.e., zones with red circles indicating that the rockfall happened away from the blast are clustered around chainage 500, 1000 and with more occurrences in chainage This result is consistent with the rockfall timing after blast (Fig.12), where individual rockfall energies are plotted as colored diamonds where red correspond to rockfalls that happened more than 9 hours after blast. These red labeled rockfalls occur at chainage 500 and 1900, coincident with the zones with persistent rockfall, even when the blast were done more than 50 meters from damage location. After Blast STR attenuation Along with the energy, the time difference between the rockfall occurrence and the last blast was measured. When all this information is merged together, the rate of seismicity after blast and the distance of events to the blasted face can be plotted as in Fig.13. there high normalized energy (kj/m 2 ) rockfalls occur not only during blast, but up to several hours later. The case presented in Fig. 13 show two groups of large events; those that occur at the same time than the blast, induced by the stress change at the excavation face, and those large events that occur several hours after the blast, more related to a fault slip mechanism (Potvin & Hudyma 2012) triggered by the near blast (Woodward 2016). It can also be seen, as the color scale, that several rockfalls occur several meters (more than 50m) from the current excavation face, symptom also related to the existence of faults that accumulate and transfer stresses. Fig. 13 Relative energy accumulation after blast (dashed line) and normalized energy of each rockfall in the hours following each blast. Fig. 12 History of energy associated to rockfalls during tunnel construction. Color indicate the time after last blasted face. Frequency of energy released: When plotting the accumulative frequency/ energy chart (Fig. 14), considering the total energy associated to each STR (not normalized by area), an important part of the data can be adjusted by a linear relationship. If the same linear relationship that exist in earthquake seismology si applied, the maximum energy to be released through rock fracturing is

8 around 1000 KJ, which normalized by half of the tunnel face area (largest rockburst at the face), correspond to 47KJ/m 2, defined as a severe rockburst incident in the Canadian Rockburst Handbook (Kaiser et al. 1996). In the same way, the minimum energy associated with the linear trend correspond to 2KJ, value associated with small rockfalls at the face, that occur in the first few hours after blast, thus, not all observed since the sector is under after blast restriction period i.e. many small rockfalls that occur, are not measured in this time window. This information suggests that there are specific zones in the rock mass where the rock responds differently to the induced stresses and, starting from chainage 1900, a different rock type occurs, marked by more violent rockfall occurrences. The fact that there are large instabilities meters behind the face and that this corresponds with the zones where instability happened long time after the blast is an indication of a fault slip mechanism, recognized by large event occurrence (Gibowicz and Kijko 1990, Ortlepp 1992, Hasegawa 1984), away from blasts (Swanson 1992, Swan and Semanedi 1992, Mckinnon 2006, etc..) and with a considerable time difference in respect to the last blast (Gibowicks and Kijko 1990, Beneteau 2012) Based on this, it is recommended that these structures or lithological changes must be recognized and characterized as this features may be present in the future construction of the tunnel. 7 ACKNOWLEDGMENT Fig. 14 Accumulative frequency of energy occurrences based on the proposed method, identifying a emin and emax value. 6 CONCLUSIONS FROM ESTIMATED ROCKFALL ENERGY. From the previous results, it can be stated that: 1. There are sharp changes in the behavior of the accumulative estimated energy, most noticeable in chainage Only 60% of the total seismic energy after blast occurs in the first 7 following hours 3. Rockfalls that occur far away from the blast face are clustered in specific zones along the tunnel chainage. 4. Rockfalls that occur more than 9 hours after the blast are clustered in the same zone as those in point 3. Authors would like to thanks AES-GENER and SRK Consulting Chile for their permission to publish this work. Also to the Australian centre for Geomechanics and the mxrap international Consortium. 8 REFERENCES 1. Armijo, R. et al., The West Andean Thrust, the San Ramon Fault, and the seismic hazard for Santiago, Chile. Tectonics, 29(2). 2. Diederichs, MS, Carter, T. Martin Practical Rock Spall Prediction in Tunnel. Proceedings of World Tunneling Congress '10 - Vancouver. 3. Gibowicz, S. J., & Kijko, A. (1990). An Introduction to Mining Seismology. Elsevier. 4. Jarufe, J. A. & Vásquez, P., Numerical modelling of rock-burst loading for use in

9 rock support design at Codelco s New Mine Level Project. Mining Technology, 123(3), pp Kaiser, P. K., McCreath, D. R. & Tannant, D. D., Canadian rokburst support handbook. 1 ed. Sudbury: Elsevier. 6. McKinnon, S. D. (2006). Triggering of Seismicity Remote from Active Mining Excavations. Rock Mechanics and Rock Engineering, 39(3), Ortlepp, W. D. (1992). Note on fault-slip motion inferred from a study of microcataclastic particles from an underground shear rupture. Pure and Applied Geophysics. 8. Price, N. J., Fault and Joint Development in Brittle Rock and Semi- Brittle Rock. 1 st ed. Oxford: Pergamon Press Ltd. 9. Swan, G., & Semadeni, T. (1992). Rockbursts in a development drift: field observations. Workshop on Induced Seismicity. 10. Tannant, D.D., McDowell, G.M., Brummer, R.K., Kaiser, P.K. (1993). Ejection Velocities measured during a rockburst simulation experiment. 3 rd Int. Symp. On Rockburst and Seismicity in Mines. A.A. Balkema, Rotterdam,