Evaluation of Geological Conditions Ahead of Tunnel Face Using Seismic Tomography between Tunnel and Surface

Transcription

1 Evaluation of Geological Conditions Ahead of Tunnel Face Using Seismic Tomography between Tunnel and Surface Y. Yokota a *, T. Ymamoto a and K. Kurihara a a Kajima Technical Research Institute, 19-1, Tobitakyu 2-Chome, Chofu-shi, Tokyo, Japan, * yokotaya@kajima.com Abstract It is difficult to understand the geological condition accurately ahead of the tunnel face during tunnel excavation. Accordingly, for the safe and economical tunnel excavation, particularly in complicated ground conditions, survey techniques should be effectively used because they would enable us to avoid large costs and unexpected collapses. In recent years, a seismic refraction survey and a seismic reflection survey, which are conducted on the ground surface and inside a tunnel, has been customarily carried out as a preliminary survey method. However it does not sufficiently provide proper information of faults and fracture zones. In order to solve this problem, the authors developed the Tunnel to surface seismic tomography, which is the new survey method using seismic tomography technique between the inside of a tunnel and the ground surface above the tunnel. This new method enables us to grasp P-wave velocity and more proper prediction over a wide area ahead of the tunnel face. In order to conduct seismic tomography analysis, arrival time of seismic wave has to be precisely calculated and, for that reason, the wave -generation and reception systems must be synchronized with each other. It is the simplest to use wired channels, but, in reality, it is difficult to synchronize them with wire connection because the receivers on the ground are usually located far away from the source points inside a tunnel. The authors consequently developed a new time synchronization system using GPS satellite time with wireless data transmission solutions. Furthermore, the authors have improved this system so that vibrations from blasting for excavation could be used as a survey source. This new system contributed to extending the survey limit for depth and improving the survey precision. Furthermore, it is remarkable that this system can be performed without interrupting tunnel excavation. Keywords: Tunnel, Seismic tomography, GPS, Blasting excavation 1. Introduction Geological surveys, such as boring surveys and seismic refraction survey, are commonly conducted from surface before excavating a tunnel. However, when there is a large earth covering of the tunnel, it is difficult to conduct multiple boring surveys. Furthermore, in seismic refraction surveys, it is hard to obtain an accurate elastic wave velocity for the area near the tunnel as the exploratory wave could not reach the tunnel excavation depth. Therefore, the appearance of unexpected faults can cause problems such as the collapse of a tunnel face. It may also cause differences between the design support pattern based on preliminary surveys and the execution support pattern, and additional changes may have to be made to the design. In terms of such problems with preliminary surveys, construction methods using the results of surveys of the ahead of a tunnel face during construction have come into widespread use in recent years, to proceed safely and efficiently excavation. Typical methods to survey the ahead of a tunnel face include horizontal boring and seismic reflection survey (Yamamoto, 2010). In seismic reflection surveys, seismic waves are generated near the tunnel face, and the waves reflected from a geologically discontinuous surface are analyzed in order to predict the locations of faults, fracture zones, hard rock masses, etc. With this method, it is possible to gain a general understanding of the locations of fracture zones, but it is difficult to make quantitative evaluations. Therefore, the authors developed new survey technique, the Tunnel to surface seismic tomography (Yokota, 2012). In this survey, seismic wave from the inside of tunnel is measured at the ground surface and their data are subjected to tomography analysis. As a result, it is possible to predict of precise ground conditions in a wider area ahead of the tunnel face. On the other hand, it is necessary to have an accurate understanding of the arrival time of the observed seismic waves for the tomography analysis. This means that times of

2 the wave-generation and wave-reception systems have to be synchronized to a high degree of accuracy. Although the wave-generation and reception systems are connected by wire to synchronize their respective times in the cross hole tomography, it is difficult to synchronize them with wire connection at an actual tunnel site, because the wave-reception points on the surface are usually located several kilometers away from the wave-generation points inside a tunnel. This has made it difficult to apply the new survey technique. To solve this problem, the authors developed a highly accurate, wireless time synchronization system using GPS satellite time signals. This system made it possible to carry out the tomography between the inside of tunnel and the surface above. Furthermore, the authors improved this system to use the vibrations from blasting for excavation as a survey source in order to apply Tunnel to surface seismic tomography to the tunnel with a large earth covering. Consequently, this system contributed to extending the survey limit for depth and improving the survey precision without interrupting tunnel excavation. 2. Verification by numerical simulation In order to verify the applicability of tomography analysis to predict geological conditions ahead of a tunnel face, a numerical simulation using a simulated geological profile was performed as shown in Fig.1. The target profile was located in bedrock having a P-wave velocity of 4.0 km/s where four poor geological zones (2.5 to 3.0 km/s) occur. (m) Bed rock Poor rock Surface :s :Sources Tunnel (m) Fig.1. Simulated geological profile As the face advanced, vibrations were generated at 25-meter intervals, and received by geophones installed on the ground surface with 25 meter spacing. Fig.2 shows the analysed P-wave velocity distribution of the ground where the tunnel face advanced at 50 meters, 275 meters, and 500 meters, respectively. As shown in Fig.2(a), when the tunnel face was located at 50 meters, a decrease in the velocity was observed in the poor geological zone near the tunnel face. However, due to the small number of wave rays passing through the zone, a decrease in seismic wave velocity can be observed all over, not only in the poor geological zone. On the other hand, as shown in Fig.2(b) and (c), as the number of passing wave rays increased, the ability to detect both the width of the poor geological zone and the decreased velocity value also increased. In particular, it is clear in Fig.2(b) that the width and the velocity of fault fracture zones distributed ahead of the tunnel face can be estimated with high precision. Tunnel face (a)tunnel face : 50m (b)tunnel face : 275m (c)tunnel face : 500m Fig.2. Results of analyses 3. Development a new survey technique 3.1 Outline of Tunnel to surface seismic tomography In order to conduct tomography analysis in Tunnel to surface seismic tomography, the clocks of the wave-generation and reception systems must be synchronized. Therefore, a GPS receiver and a high-precision GPS satellite time marker were installed in each system, and the wave-generation and reception systems were synchronized within 1.0 msec error. As GPS data cannot be received inside the tunnel, an optical transmission device (consisting of an optical transmitter, a junction cable, and an optical receiver) was used to receive the data. Fig.3 shows a principle of Tunnel to surface seismic tomography and Fig.4 shows an overview of this surveying system. Fig.5 is a photograph of the surveying system.

3 s GPS Satellite Tunnel to Surface Seismic Tomography Ray path Time Tunnel Source Fault Survey area Time GPS GPS High-Precision GPS Time Marker Data Logger Surface Seismic s Vp=4.5 Tunnel Vp=2.0(Low velocity zone) Fig.3. Principle of new survey Optical Transmitter Portal Junction Cable Optical Tunnel High-Precision GPS Time Marker Data Logger Fig.4. Overview of new survey Face Source Seismic Cable GPS Source (Hammer) Seismic s GPS Data Logger High-Precision GPS Time Marker () Optical Junction Cable Optical Transmitter High-Precision GPS Time Marker & Data Logger (Source) Ground surface Tunnel portal Tunnel face Fig.5. High-precision time synchronization device and survey devices 3.2 Test for checking the accuracy of time synchronization system The time error in synthesizing the wave forms at wave-generation and reception systems was kept within 1.0ms because there would be no engineering effects on the analytical results at the error. As the authors compared the time error of wave form data acquired from their newly-developed wireless time synchronization system with wave form data acquired from a conventional wired system, the time error of both systems was verified. Fig.6 shows the test conditions and the arrangement of transmitting and receiving points, and Fig.7 shows typical acquired wave forms. In these results, no major differences between the waveforms from new systems with GPS and from conventional systems could be seen. This finding indicates that the time synchronization and wave pattern synthesis could be done accurately. In the test, the average difference value and its standard deviation were calculated, 0.20msec and 0.18msec, respectively. From these results, it could be confirmed that the wireless time synchronization system can be applied in the actual site.

4 GPS signal receiver Hammer striking Time GPS satellite Time Source disposition (distance from 1CH of receiver) 0.5m, 5.0m, 10m, 15m Direct waves GPS signal receiver Survey device disposition 0.5m*23CH Amplitude 振幅 Amplitude 振幅 Time(second) 時間 (s) Time(second) 時間 (s) GPS 合成 system waveform Wired 有線式 system waveform GPS合成 system waveform 有線式 Wired system waveform Fig.6. Survey condition Fig.7. Typical waveform 4. Application to an actual site using hammer shot source 4.1 Overview of the survey The authors applied Tunnel to surface seismic tomography using the hammer strike to a site. Around the survey point, there are mudstones that are fractured finely and softened inhomogeneously, indicating that large deformation would occur. Thus, the authors decided to perform the Tunnel to surface seismic tomography in order to verify the applicability of the new method by comparing the results with those of vertical boring survey and actual excavation conducted around the point. Fig.8 is a photograph of a generation of seismic waves inside the tunnel, while Fig.9 is a photograph of the measurement system on the ground surface. Hammer striking Measurement system Fig.8. Generation of the seismic wave Fig.9. Measurement system 4.2 The survey results Fig.10 shows a comparison of the results of tomographic analysis with vertical boring survey and the observed geology. It is clear in the results of the vertical boring around TD 655 meters that fractured mudstones are distributed down to around 15 meters below the ground surface. Taking into account the results of preliminary boring conducted in the vicinity of the survey point, it appears that fractured mudstones occur in a ridge-like shape at this point. The results of the tomographic analysis also show an increase in the seismic wave velocity around this point, which is consistent with the results of the preliminary boring. Furthermore, around TD 680 meters to 700 meters ahead of the tunnel face, the results of the tomographic analysis show that the seismic wave velocity decreases to around 2.0 km/s in the fractured mudstone zone (P-wave velocity = around 2.5 km/s), which indicates that fragmentation led to further softening of the geology. In fact, the observed geology turned worse at this point. By applying the newly-developed the Tunnel to surface seismic tomography to predicting the geological conditions ahead of the tunnel face, it becomes possible to use seismic wave velocity to evaluate the poor geological zone and the hard rock zone distributed ahead of and around the tunnel face with high precision.

5 :Sources :s Vertical Boring(TD650m) :Embankment :Alluvial soil deposit :Fractured mudstone Vp(km/s) Tunnel Face TD: Tunnel Distance (m) Elevation (m) Poor Very Poor Observed Geology Fig.10. Comparison of the results of tomography with the vertical boring and the observed geology 5. Application to an actual site using vibrations from blasting excavation 5.1 Overview of the survey By establishing a wireless system between the the wave-generation and reception systems, tomography survey could be undertaken in tunnels with a high earth covering, which had not been possible before. However, in some cases using conventional sources as hammer striking, the seismic waves cannot be observed at the surface as the vibration energy is too small. Therefore, Tunnel to surface tomography was conducted using vibrations generated when the tunnel was being excavated with a blasting method. Fig.11 is a geological profile of the tunnel where the Tunnel to surface seismic tomography was applied. The length of the tunnel is L=1637.0m, with a maximum earth covering of 170m. The geology consists mainly of granodiorite and sandy gneiss. In the tunnel with a high earth covering, become geological conditions at the tunnel depth could not be predicted by the preliminary seismic refraction survey from the surface. From the above, Tunnel to surface seismic tomography using vibrations from blasting is effective. In addition, plural faults were estimated to exist near the portals of this tunnel. Therefore, in order to excavate the tunnel rationally and safely, it was necessary to conduct a detailed geological survey. This survey used a total of 6 excavation blasts that were carried out during the tunnel excavation. Fig.12 shows the equipment to detect ignition signals for blasting, the blasting apparatus, and the measuring conditions on the ground surface. Ground surface Geological legend Granodiorite Predicted fault lines Predicted fault lines Sandy gneiss Elevation (m) TD; Tunnel Distance (m) Fig.11. Geological profile

6 Lighting signal detector Exploder Survey line Measurement system Survey status Fig.12. Lighting signal detector & Exploder & Survey line & Measurement system & Survey status 5.2 The survey results When conducting the tomography analysis, the data that were obtained in this survey were used in conjunction with a seismic refraction data from the ground surface. Fig.13 shows a diagram of the paths of the seismic waves and Fig.14 shows the distribution of seismic wave velocity from the tomography analysis. Looking at Fig.13, the acquired wave paths traversed the area ahead of the tunnel face and increased vicinity tunnel in each time blasting vibration data were acquired as the tunnel face progresses. This means that more detailed distribution of seismic wave velocity could be estimated at the tunnel excavation depth which could not be obtained in the preliminary survey. The results of the tomography analysis indicated that there were almost no areas with a noticeable decrease in seismic wave velocity except for slightly low seismic wave velocity area of the upper part of the tunnel at around T.D.1480m. From this result, it was understood that the faults that had been of concern in the preliminary study were small in scale, and that they would have no effect on the efficiency of excavation work, so construction could proceed without lowering the support patterns. By utilizing blasting excavation vibrations in Tunnel to surface seismic tomography, it was possible to get a detailed understanding of the geological structure at the tunnel depth, even in the tunnel a high earth covering without interrupting tunnel excavation. In addition, because of vibration-generating energy was much greater than the conventional hammer-striking method, accuracy of survey could be improved from betterment in the S/N ratio. :Sources :s Ground surface Blasting positions Elevation (m) TD; Tunnel Distance (m) Fig.13. Ray tracing of the seismic wave

7 :Sources :s Vp(km/s) Air;Vp=0.35(km/s) Ground surface Predicted fault lines Blasting positions Elevation (m) TD; Tunnel Distance (m) weathered Poor Medium-hard -Predicted geology- Poor weathered Poor Medium-hard -Observed geology- Fig.14. Comparison of the results of tomography with the predicted and observed geology 6. Conclusion This paper has reported about newly developed the Tunnel to surface seismic tomography with a brief description of the exploration and a case study of its application in the field. Tunnel to surface seismic tomography made it possible not only to obtain the locations of faults, fracture zones and hard rock masses, but also to provide quantitative evaluations. Because this method measures at the direct waves that penetrate through fracture zones or hard rock sections from ground surface. In addition, as GPS satellite time were used when observing the seismic waves, could be realized of a wireless system between the wave-generation system in a tunnel and the wave-reception system on the ground surface, which previously had to be connected by wire. As a result, it has become possible to conduct wider-ranging surveys even at tunnel sites where it had been difficult to utilize the conventional system. Furthermore, by utilizing tunnel blasting excavation vibrations as a source of exploration vibrations, it has now also become possible to conduct wide-area exploration in tunnels with high earth-covering without stopping the progress of excavation. However, it will still be necessary to accumulate more data on the utilization of this system at actual sites. Furthermore, the exploration system should be improved so that surveys using tunnel blasting excavation vibrations can be conducted on a continuous basis, and efforts should also be made to further improve the accuracy of analysis and the practicality of the system.

8 References Yamamoto, T., Shirasagi, S., Yokota, Y. and Koizumi, Y., 2010, Imaging geological conditions ahead of a tunnel face using Three-dimensional Seismic Reflector Tracing System, International Journal of the JCRM., Volume 6, Number 1, Yokota, Y., Yamamoto, T., Shirasagi, S. and Koizumi, Y., 2012, Evaluation of Geological Conditions Ahead of Tunnel Face Using Seismic Reflector Tracing and New Seismic Tomography between Tunnel and Surface, Ground Engineering in a Changing world, 2012 ANZ Conference Proceedings., pp