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Influence of Geological Conditions on Measured TBM Vibration Frequency Michael Mooney, Bryan Walter, John Steele, Daniel Cano Colorado School of Mines ABSTRACT This paper examines TBM vibration as a source of information about geological conditions. An EPB TBM was outfitted with accelerometers to monitor vibration during excavation of the University Link light rail tunnel project (U230) in Seattle Washington. Impact-response testing of the TBM indicated that significant signal over a wide range of frequencies transfers from the cutterhead where vibration due to ground interactions emanate, to the bulkhead where sensors can be installed. Analysis of the vibration data collected during excavation indicates that both amplitude and frequency content appear to be influenced by TBM operating parameters and by geological conditions. INTRODUCTION The vibration characteristics of a system when subjected to external stimulus have long been used as a way to monitor the system itself. For example, the field of vibration based condition or health monitoring uses the measured vibration response and its changes to identify wear and damage of rotating shafts, wind turbines, hydroelectric turbines, bridges and buildings. The vibration characteristics of earth construction equipment have been used in the same manner and also to interrogate the condition of the ground with which the equipment is interacting. Examples include intelligent vibratory soil compactors and smart drilling (Mooney & Rinehart 2009, Richard et al. 2002). In the former case, the elastic stiffness and compacted state of the soil is estimated based on measured drum vibration. In the latter case, the rock hardness and strength is estimated based on measured drill bit vibration. The goal of the study described in this paper is to develop a similar approach where TBM vibration can be used to assess geological conditions. The underlying premise is that the measurable vibration characteristics of equipment interacting with the ground will be influenced by the ground properties under certain operating conditions. The ground conditions, therefore, can be estimated by back-analysis that uses either statistical or physical models of the ground/machine interaction. For example, a vibratory drum operating on soil can be physically modeled with lumped masses, springs and dashpots (see figure 1). The model predicts the contact force vs. deflection response that is a function of the roller parameters, operating frequency and amplitude, as well as the assumed ground stiffness and damping. Through a process of matching experimentally measured vibration response to modeled response, the ground stiffness and damping are estimated. This estimation of soil stiffness and compacted state is provided continuously and in real time, and is now routinely performed in earthwork construction practice. If a physical model is difficult to develop, statistical approaches can also be used. For example, in smart drilling, empirical relationships between measured vibration amplitudes and frequencies with rock types and stress conditions have been developed through statistical analysis. The rock type can then be estimated based on these empirical models.

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Figure 1. Dynamic model of vibratory drum roller interacting with the soil. With this previous work in mind, there is considerable potential benefit to using TBM vibration as a continuous means of characterizing both the ground conditions and the TBM condition. One significant limitation to TBM tunneling, particularly in pressurized face conditions, is the inability to log or otherwise document the geology through which the TBM is excavating. There is currently no way to catalog the as-built geological conditions that could be very beneficial for lifecycle engineering of the tunnel. Further, the lack of documentation of excavated geology makes it impossible to assess the accuracy of the geotechnical baseline report. This has significant implications on differing site conditions, contractor claims, resolution of disputes, etc. Finally, real time characterization of geological conditions could serve to optimize TBM performance and avoid damage, e.g., through identification of boulder impacts, changing ground conditions that require different ground conditioning, face support, etc. This paper examines the potential for TBM vibration based assessment of geology and ground conditions in general. An experimental program was conducted wherein a 6.44 m diameter Hitachi Zosen earth pressure balance (EPB) TBM was outfitted with accelerometers. Vibration data was collected during TBM excavation of the University Link Light Rail Tunnel project (U230) in Seattle Washington. In addition, the vibratory response of the TBM was explored prior to tunneling through impact – response testing to explore the transfer of vibration from the cutterhead through to the bulkhead. PROJECT BACKGROUND Vibration was monitored during EPB TBM operations on the University Link Light Rail Tunnel project (U230) in Seattle, Washington. Monitoring was carried out during southbound tunneling over approximately 1 km from the Capitol Hill Station to Pine Street Stub Tunnel (right to left on Figure 2). The complex soft ground geology (see Figure 1) is divided into fluvial deposits, glacial deposits, lacustrine and glaciolacustrine deposits, all of which have been glacially over‐ridden and are therefore very overconsolidated (Irish 2009). The overburden varied from a minimum of 4.2 m under Interstate 5 (Station 1046) to over 40 m (Station 1060). The entire tunnel alignment has a steep, curved downhill grade (4.8%).

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Figure 2. Estimated geological conditions along U230 alignment (based on geotechnical data report by Irish 2009)

The 6.44 m diameter Hitachi EPB TBM was instrumented with four triaxial accelerometers mounted on the bulkhead of the TBM near the main bearings (Figure 3a). Reliable instrumentation and high bandwidth data acquisition are not currently feasible at the ideal location of the cutterhead. The bulkhead was selected with the assumption that vibration occurring at the cutterhead would travel through to the bulkhead. The orientation of the triaxial axes and their relationship to the TBM are shown in Figure 3b. The accelerometers have a bandwidth of 0-600 Hz and were sampled at 2 kHz. TBM operating parameter (OP) data was accessed from the Hitachi PLC every 10 seconds. The OPs used in the study include cutterhead torque (T), axial thrust (F), cutterhead rotational speed (N), average face pressure (σ), and advance rate (AR). Further details about the instrumentation and data acquisition system deployed can be found in Walter (2013).

Figure 3. Schematic of accelerometer locations: (a) side view of the TBM, highlighting the

bulkhead of the machine where triaxial accelerometers 1-4 were mounted; (b) accelerometer coordinate system used in the study.

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TBM IMPACT-RESPONSE BEHAVIOR Impact-response testing was performed on the TBM prior to excavation to characterize vibration signatures and their transmission from the cutterhead to the bulkhead inside the TBM. Testing was performed by striking the cutterhead at locations 0-7 shown in Figure 4 with a hammer. A roving accelerometer (denoted a1) was magnetically mounted next to the impact location (shown in Figure 4) to capture the input signal to the cutterhead. Vibration was recorded at fixed cutterhead locations a2 and a3 as well as fixed bulkhead locations a1-a4 (Figure 3). The aim behind this impact-response testing was to quasi simulate boulder interactions at the cutterhead (cutting tools impacting boulders during advance and rotation), and then assess how vibration carries through the main bearing to the bulkhead where sensors can practically be mounted.

Figure 4. Schematic of Hitachi 6.44 m diameter cutterhead and locations of impacts (0-7) as well as locations of accelerometers (a2-a4)

One advantage of hammer impact testing is that the stiff hammer contacting a steel cutterhead introduces broadband vibration frequency content. The analysis of response observed provides an indication of which frequencies pass through and which are mechanically filtered due to the makeup of the TBM frame. An example set of bulkhead response time histories from five sequential hammer impacts on the cutterhead (same position) is shown in Figure 5. The signals are clear, repeatable and convey vibration response that extends for 200 ms. All time domain signals were analyzed via discrete Fourier transform to explore the frequency content of the cutterhead vibration and bulkhead vibration. As one example, Figure 6a shows the amplitude portion of the frequency response spectrum of longitudinal cutterhead vibration due to an impact at position 4 while Figure 6b shows the amplitude spectrum of longitudinal vibration at the bulkhead. The noise floor amplitude of the accelerometers was found to be 0.1 mg across the frequency spectrum. Therefore, the majority of the signals shown in Figure 6 are well above the noise floor. Figure 6a illustrates some observed resonant modes of the cutterhead. The largest magnitudes occur at 260 and 380 Hz, while smaller magnitudes occur at 180, 340, 400, 460 Hz and 580 Hz. These are considered natural or resonant modes of the cutterhead wherein any excitation signal amplitude (from the impact) would be amplified. Figure 6b illustrates a number of frequencies where significant vibration amplitude was measured at

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the bulkhead, e.g., at 260, 340, 380, 400, 460, 530 and 580 Hz. Some of these bulkhead frequencies are similar to the cutterhead peak frequencies and some are not. A comparison of cutterhead and bulkhead vibration magnitudes at similar frequencies reveals the amount of signal attenuation or amplification.

Figure 5. Measured longitudinal bulkhead vibration (a2L) resulting

from five impacts at position 7 of cutterhead

Figure 6. (a) Amplitude response spectrum of cutterhead position 4 longitudinal vibration (impact at 4); (b) the resulting amplitude response spectra at the

bulkhead in longitudinal direction.

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An informative way to assess what frequencies pass from the bulkhead through the TBM main bearing to the bulkhead is through calculation of a transfer function, the ratio of bulkhead (output) FFT to cutterhead (input) FFT. The transfer function amplitudes for the transverse, vertical and longitudinal bulkhead vibration due to impact at cutterhead position 4 are shown in Figure 7. The amplitude at each frequency in Figure 7 is reported in dB where the reference signal is the cutterhead vibration. Therefore the amplitude of the transfer function quantifies how the input signal has changed from cutterhead to bulkhead. For interpretation, -10 dB and -20 dB imply that the vibration amplitude has decreased by a factor of 3 and 10, respectively from cutterhead to bulkhead. Conversely, +10 dB and +20 dB imply that the vibration amplitude has been amplified by a factor of 3 and 10, respectively, from cutterhead to bulkhead. An amplitude of 0 dB implies that the input and output amplitudes are the same. The noise floor for the transfer function is -110 dB; therefore, all amplitudes measured are well above the noise. Figure 7 shows a variety of responses with vibration reduction and amplification depending on direction (T, V or L) and frequency. Transverse vibrations are either amplified or remain unchanged for many frequency bands up until 500 Hz. A number of frequency bands in the vertical and longitudinal directions also show negligible reduction or significant amplification. While the transfer functions are complex, the main takeaway is that considerable and measurable vibration signal generated by cutterhead interactions with the ground can reach the bulkhead where accelerometers can be easily mounted.

Figure 7. Transfer function amplitudes vs. frequency for determined from cutterhead

impact/vibration at position 4 (input) and bulkhead vibration (output). Plots (a), (b) and (c) show the transfer function amplitudes in the transverse, vertical and longitudinal

directions, respectively. VIBRATION RESPONSE DURING U230 EXCAVATION TBM vibration was continuously recorded at bulkhead sensors a1-a4 throughout the southbound drive. To first paint the big picture, the measured response over the 1 km long drive is shown in Figure 8. Here, the root mean square (rms) amplitude of time domain bulkhead vibration recorded by sensor a1 is reported in the L, T and V directions. To provide some perspective regarding tunneling conditions and performance, the key TBM operating parameters (OP) measured are also shown, namely the advance rate (AR), cutterhead torque (T), cutterhead

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rotation rate (N), average face pressure (σ), and axial/thrust force (F). The geology and overburden along the alignment are also shown (see Figure 2 for a geology legend). Figure 8 illustrates that the measured bulkhead vibration amplitude clearly varies along the alignment as do the measured OPs. In fact, a visual assessment of Figure 8 shows that vibration amplitudes often change when OPs change. The broader question is whether vibration characteristics change with ground conditions.

Figure 8. Collective TBM OP and bulkhead vibration data collected along southbound tunneling: (a) OP data; (b) estimate of geological conditions; (c) a1 rms vibration

amplitude in the T, V and L directions To more closely examine the frequency content of TBM vibration measured, data from individual rings were investigated. Figure 9a illustrates the recorded OP data and Figure 9b the a1 vibration data recorded during excavation of ring 502 (Station 1057+13). According to the geological profile (Figure 2), the TBM was mining through full face low plasticity clay during ring 502. Both the raw vibration records and rms amplitude are shown in Figure 9b. OP parameters indicate excavation with a constant advance rate = 100 mm/min through homogeneous ground

R502 R473 R425

R545 R586 R503

R665 R625

R685 R485

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as evidenced by the fairly constant torque, thrust and face pressure. The vibration response echoes this homogeneous behavior and constant operation over the 21 minutes of ring excavation. Figure 9c presents the results of a joint time-frequency of a1V vibration. Here, FFT analysis was performed on 10 second segments and stitched together throughout the 21 minutes of excavation. The resulting spectrogram presents both the frequency content and amplitudes at each frequency as a function of time. Figure 9c reveals distinct frequencies in the bulkhead vibration response, with the largest amplitudes present at 460-470, 390 and 290 Hz. Consistent with the OP data and time history vibration data for ring 502, the frequencies and amplitudes remain constant throughout excavation. These frequencies are in the range of those observed during impact-response testing presented earlier but do not match them exactly. This is understandable given that impact-response testing was performed on the TBM without soil pressing on the face, an empty chamber, and with no pressure on the shield or cutterhead.

Figure 9. (a) OP data, (b) bulkhead vibration time history data, and (c) joint time-

frequency response of a1V during excavation of ring 502 (Station 1057+13).

(c)

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Figure 10 explores the amplitude and frequency content more closely by zooming in on the TBM start-up phase at the beginning of ring 502 excavation. The distinct frequencies of 470, 390 and 300 Hz are achieved after 12 sec when the cutterhead rotation speed has reached its constant 2.2 rpm. Prior to this, it is clear that the dominant frequencies increase with cutterhead rotation speed. A similar behavior is evident during the cutterhead rotation ramp down of ring 507 excavation (Station 1056+88). Though not shown here, frequency content and TBM OP values were similar from ring 502 excavation through ring 507 excavation.

Figure 10. (a) Bulkhead vibration time history and (b) joint time-frequency response at the beginning of ring 502 excavation (Station 1057+13) through the end of ring 507

(Station 1056+88) excavation. Bulkhead vibration frequency domain analysis was carried out for data along the southbound alignment. Ten rings were selected in different geological conditions for analysis (see Figure 8 for locations). Ramp up and ramp down operations were not included so that the analysis could focus on steady state data. The dominant frequencies and their amplitudes of a1V vibration observed during excavation of these ten rings are presented in Figure 11. Vibration frequency is normalized by cutterhead rotation speed to enable direct comparison of observed frequencies. Figure 11 shows at a broad scale that the dominant frequencies are consistent throughout data during each ring excavation. These frequencies are evident in primarily three clusters with central frequencies of approximately 280, 180 and 130 Hz/rpm. Individual dominant frequencies and their amplitudes within these clusters were found to be variable across the rings and appear to be sensitive to soil type. For example, TBM vibration while excavating in CH material (rings 665, 586, 425 partial) did not exhibit dominant frequencies within the three cluster areas while TBM vibration in other soil types exhibited clear dominant frequencies. The nature of these frequencies and their relationship with soil type requires further study and verification. Further, the influence of TBM OP values on these frequencies and their amplitude also require more complete characterization.

Ring 507 (b)

Ring 502

(a)

470 Hz

390 Hz 300 Hz

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Figure 11. FFT amplitude spectra of vertical acceleration (a1V) from ten different rings. Note that the x-axis is normalized by the rms cutterhead rotation (N) of each ring.

DISCUSSION While there is significant measurable TBM vibration during excavation, the nature of the vibration, particularly the frequency content, is quite complicated. The observation of dominant frequencies in the 300-500 Hz range, the range that is consistent with harmonic modes of the cutterhead and bulkhead based on impulse-response testing and modeling efforts (see Walter 2013 for finite element analysis results), suggests that the TBM vibration is influenced by free vibration response during excavation. The strong influence of cutterhead rotation speed on the dominant frequencies, however, suggests that the TBM is exhibiting forced vibration response, where the cutterhead rotation speed serves as the root forcing frequency. That the observed frequencies are five orders of magnitude greater than the 0.02-0.04 Hz cutterhead rotation frequency is related to the multi-degree of freedom complexity of the TBM. A number of other sources of forcing frequency, including electric motors turning the cutterhead and the main thrust bearing, may also influence TBM vibration. A detailed assessment of these sources, however, revealed that these sub-100 Hz frequencies are evident in the TBM bulkhead response but at amplitudes much less than those observed between 300-500 Hz (Walter 2013). The complex characteristics of TBM vibration and the desire to use TBM vibration as a continuous monitoring approach are likened to wind turbine vibration monitoring. Modern wind turbines are driven by motors that turn the blade assembly at sub 1 Hz forcing frequencies. Vibration, however, is often observed in the 100s of Hz, creating the same phenomenon as observed in TBMs.

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CONCLUSIONS An experimental program was conducted to relate measurable TBM bulkhead vibration to geological conditions. A used during the Seattle University Link light rail project was outfitted with triaxial accelerometers. Vibration data was collected from a 6.44 m diameter Hitachi-Zosen EPB TBM during the 1 km long southbound excavation. Impact-response testing revealed that measurable vibration signature up to 500 Hz propagated from the cutterhead where geology-related vibrations would originate to the bulkhead where accelerometers can be easily mounted. For a number of frequency bands, cutterhead vibration amplitudes were either amplified at the bulkhead or attenuated only slightly. This is important because to date accelerometers cannot reliably be placed at the cutterhead. The results show that the bulkhead is a suitable surrogate location for ground-cutterhead vibration measurements. The analysis of bulkhead vibration data collected revealed clearly measurable vibration signals with amplitude and frequency content changing along the alignment. Vibration response was found to be influenced by TBM OPs, namely cutterhead rotation speed, that varied considerably throughout the alignment. When normalized by cutterhead rotation speed, changes in the resulting frequency content can be related to geological conditions. Vibration amplitude was also found to vary with geological conditions. ACKNOWLEDGEMENTS This research was partially funded by an IGERT grant from the National Science Foundation (DGE-0801692). The authors would like to acknowledge Jay Dee Contractors, Inc. for their support in this research. REFERENCES Carden, E.P. (2004) “Vibration based condition monitoring: A review,” Structural Health Monitoring, 3(4), 355-377. Cooper, G. Lesage, M. Sheppard, M. and Wand, P. (1987) “The interpretation of tricone drill bit vibrations for bit wear and rock type,” Proc. RETC, June, 14–18. Farrar, C.R and Worden, K. (2007) “An introduction to structural health monitoring,” Phil. Trans. R. Soc. A, 365(1851), 303-315. Hameed, Z., Hong, Y.S., Cho, Y.M., Ahn, S.H. and Song, C.K. (2009) “Condition monitoring and fault detection of wind turbines and related algorithms: A review,” Renewable and Sustainable Review, 13(1), 1-39. Irish, R.J. (2009) “Pre-Bid Engineering Geologic Evaluation of Subsurface Conditions for the University Link Light Rail Tunnels, Capitol Hill Station to Pine Street Stub Tunnel, and Capitol Hill Station Excavation and Supprt, Contract U230 Seattle, Washington.” Ledgerwood, L.W., Hoffmann, O.J., Jain, J.R., Herbig, C., and Spencer, R.W. (2010) “Downhole Vibration Measurement , Monitoring and Modeling Reveal Stick-Slip as a Primary Cause of PDC Bit Damage in Today’s Applications,” SPE Annual Technical Conference, 10. Mooney, M.A. and Rinehart, R.V. (2009) “In Situ Soil Response to Vibratory Loading and Its Relationship to Roller-Measured Soil Stiffness,” J. Geotechnical & Geoenvironmental Engineering, ASCE, 1022–1031. Walter, B. (2013) Detecting changing geologic conditions with tunnel boring machines by using passive vibration measurements, PhD Dissertation, Colorado School of Mines, 163 pp.