Tunnelling and Underground Space Technology 38 (2013) 279–286

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Tunnelling and Underground Space Technology

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Technical Note

Periodic inspection of gauge cutter wear on EPB TBMs using conepenetration testing

0886-7798/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.tust.2013.07.013

⇑ Corresponding author. Tel.: +1 214 909 8828; fax: +1 303 273 3413.E-mail address: eia5045@psu.edu (E. Alavi Gharahbagh).

Ehsan Alavi Gharahbagh a,⇑, Michael A. Mooney a, Glen Frank b, Bryan Walter a, Michael A. DiPonio b

a Colorado School of Mines, Golden, CO, United Statesb JayDee Contractors, Inc., Seattle, WA, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 August 2012Received in revised form 30 June 2013Accepted 3 July 2013

Keywords:Periodic inspectionGauge cutterEPB TBMOvercut lengthOvercut CPT testing deviceHyperbaric interventionGauge cutter wear

Cutterhead maintenance is usually required when the gauge cutters (over-cutters) wear down to theshield diameter. In soft ground pressurized shielded TBMs, inspection of gauge cutters usually involvescomplete stoppage of the operation. This can be a dangerous, costly, and time consuming process. In thisstudy, a novel approach has been developed to monitor gauge cutter wear by considering the relationshipbetween the overcut length and the length of the gauge cutters. By using the proposed testing system,frequent monitoring of gauge cutter length is possible at appropriate intervals during ring builds or main-tenance without the need to enter the harsh environment of the excavation chamber. This monitoringsystem was used within an EPB TBM during the excavation of the University Link Light Rail Tunnel(U230) in Seattle, WA.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Cutterhead maintenance is most often required when the gaugecutters are worn down close to the shield diameter. Gauge cuttersmust travel further than face cutters. This tends to increase the rateof wear. Further, the speed that the cutters travel as the cutterheadrotates is highest at the perimeter. This increases the impact load-ing on the gauge cutters when boulders are encountered. Unde-tected wear of the gauge cutters on the Los Angeles ECIS projectresulted in failure and loss of originally fitted chromium carbidewear plate on the cutterhead rim. In this case the cutterhead radiusshowed a loss of 2 cm of carbide plate in addition to 2 cm of struc-tural wear. Extensive underground repair works were required,delaying the project for several months (Nilsen et al., 2007). Thisexample and many others show the importance of inspecting thelength of gauge cutters as the tunneling progress.

Further, cutterhead inspection and maintenance in Earth Pres-sure Balance (EPB) TBMs can become dangerous, time consumingand costly when the ground is unstable, e.g., under groundwaterconditions. In this situation, cutterhead inspection and tool main-tenance are performed under pressurized conditions or so called‘‘hyperbaric intervention’’. This involves supporting the face,removing the muck, applying compressed air, and allowing the

crew into the pressurized chamber via an air lock. Although, cut-terhead inspection and maintenance is important and needs tobe conducted in specific intervals through the project, the numberof times that personnel are subjected to this environment shouldbe minimized.

Cutterhead inspection can also be costly because no mining canbe performed during inspection. It has become almost a standardprocedure that a cutterhead maintenance plan is created andimplemented by the contractor. In addition, it is becoming morecommon for the owners to include a minimum number of cutter-head inspections in the contract documents. For example, theBrightwater West contract in Seattle, WA mandated 40 inspectionstops during 6400 linear meters of tunneling. This translated intoapproximately 120 h (5 days) of lost production time for inspectionpurposes (Shinouda et al., 2011).

The methods for monitoring cutter wear during soft groundmechanized tunneling can be summarized into two groups; instru-mented cutting tools and visual inspection methods. In soft andmixed ground, the only examples of instrumented cutting toolsare the hydraulically pressurized ‘scrapers’ that offer limited resolu-tion and minimal feedback. The pressure in these scrapers is moni-tored and, if a breach occurs, the pressure drops and the operatoris notified. A schematic of a pressurized ripper can be seen in Fig. 2.

In the case of visual inspection, the only method reported in theliterature is the system used in the Brightwater west contract. Inthis implementation, a remote camera (periscope) was used to

Table 1Number of boulders for each tunnel based on GBR.

Boulder size (m) 0.3–0.6 0.6–0.9 0.9–1.2 1.2–1.5 1.5–1.8 >1.8

Number of boulders Northbound tunnel 57 9 6 1 1 0Southbound tunnel 57 9

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visually inspect the condition of the cutterhead (Shinouda et al.,2011). By using this camera, the general condition of the cutter-head was visually inspected without the need for human-entry,therefore shortening the inspection time by an average of 3 h.

Fig. 3. Subsurface geology of the t

Fig. 1. General view of the cutterhead and the location of six gauge cutters.

Fig. 2. Pressurized cutting tool (Trisi, 2011).

There are several reasons that monitoring systems are not morecommon in this environment. The first is that it is more difficult toinspect the cutterhead because of elevated face pressures, thusmaking prototype systems more challenging. The second is thatexamples of instrumented cutting tools such as scrapers and rip-pers have yet to be developed. Lastly, the area behind the cutter-head serves as a soil mixing chamber, making wirelesscommunication nearly impossible. A more detailed review ofinstrumented cutters is presented by Walter et al. (2012).

To this end, there would be considerable benefit from develop-ing cutterhead inspection techniques that can be performed fre-quently and within the normal tunneling operations, i.e., tominimize the down time due to inspection.

The objective of this study was to develop a method to period-ically monitor the length of gauge cutters without stopping thetunneling operation. A novel approach was developed to measurethe overcut. The assumption is that the overcut will decrease withgauge cutter wear and that the overcut region will be discerniblysofter than the undisturbed region outside the overcut region. Anew testing device was designed and manufactured using conceptsof the hydraulic cone penetration test (CPT). The CPT was imple-mented from within through additive injection ports in the TBMshield. This device was implemented during construction of theUniversity Link Light Rail Tunnel in Seattle, WA. This paper intro-duces the tunnel project, describes the components and methodol-ogy of the overcut CPT, and presents results from multiple testsalong the alignment, including measurements during an interven-tion to validate the CPT-based approach.

2. University Link Tunneling project (U230)

The Sound Transit University Link project is a 5.07 km light railextension that will run in twin bored tunnels from DowntownSeattle to the University of Washington, with stations at CapitolHill and on the University of Washington campus near HuskyStadium. The project is broken up into several parts, where theU230 portion deals with the excavation of Capitol Hill Stationand installation of 1.18 km of twin-bored tunnels from the CapitolHill Station (CHS) to the Pine Street stub tunnel (PSST) in Down-town Seattle. The joint venture of Jay Dee Contractors, Inc., FrankColuccio Construction Company, and Michels Corporation wasawarded the U230 Project. The tunnels are approximately 6.44 min diameter and are to be furnished with concrete segmental

unnel alignment (Irish, 2009).

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tunnel lining having an outside diameter of 6.27 m and an insidediameter of 5.74 m.

The Seattle area has a complicated geologic history. Six majorglacial events have happened during last 2 million years withintervening non-glacial erosional and depositional periods. Thegeologic description of this project can be divided into fluvialdeposits, glacial deposits, lacustrine and glaciolacustrine deposits,all of which have been glacially over-ridden and are thereforehighly over-consolidated (1 < OCR < 4.3 and OCR average of 2.5)Geotechnical Data Report, 2009. Around 70% of the alignment is lo-cated in over-consolidated fine grain soils. Fig. 3 shows the subsur-face geology of the tunnel alignment. The Soil Groups defined forthis project in the Geotechnical Baseline Report (GBR) are not veryabrasive with quartz contents ranging from 13% to 42% and soilabrasion testing (SAT) values ranging from 2 to 18. Table 1 summa-rizes the total number of boulders (based on GBR) that wereexpected to be encountered during the excavation of Northboundand Southbound tunnels with their representative size.

The general view of the TBM cutterhead is presented in Fig. 1.The cutterhead has a face opening of 45%, with 6 spokes. Thereare 6 gauge cutters designed in this TBM with the maximum lengthof 10.2 cm. Longer gauge cutters than normal were installed to ex-tend the time between cutter replacement. There is a perception inthe industry that the length of gauge cutters should be very short(on the order of 10–30 mm for small diameter TBMs) in order tominimize the steering gap and associated risk of settlement (forlarger diameter TBMs, even with small overcuts, overcut volumeand the potential for larger ground settlement is much greater ifthe overcut is not filled). This belief is based on the assumptionthat the overcut gap is a void and that ground loss occurs into

Fig. 4. (a) Integrated overcut CPT testing device consisting a laptop, hydraulic jack with linstalled testing device inside the shield.

the gap, causing settlement. This general concept is true foropen-face TBMs where the disturbed ground is not conditionedor pressurized so that it is removed through the cutterhead ratherthan being squeezed into the overcut gap. Monitoring of groundloss with extensometer and deep settlement points as a shield isadvanced has confirmed this behavior for open face shields (Cord-ing, 2010). In contrast, extended overcut can be properly controlledin EPB tunneling in dense soils such that no soil relaxation, loss ofground or additional settlement occurs (DiPonio et al., 2012).

2.1. Design and fabrication of the overcut CPT testing device

Fig. 4 shows the overcut CPT testing device that was designedand built for monitoring the overcut length. As can be seen fromthis figure, a hydraulic jack with a linear displacement sensorwas used to overcome the pressure applied at the overcut zone(outside the shield). The relationship between pressure andpenetration depth was used to determine the length of the overcutarea. This hypothesizes that the pressure remains constant insidethe overcut zone and as soon as the cone enters the intact soil,the pressure increases significantly. A standard DCP hardened conetip with angle of 60� and tip base diameter of 2 cm was welded tothe rod of the utilized tie rod hydraulic cylinder. The hydraulic cyl-inder has 41 cm of stroke and was double-acting so that it could beretracted at the completion of each penetration test. An inline pres-sure sensor was installed on the pump in order to monitor theforce required to push the testing device through the overcut zone.The linear displacement sensor and the inline pressure sensor werethen connected to a data acquisition system, which allowed forrepeatable and continuous measurement of pressure and penetra-

inear displacement sensor, DAQ device, pressure sensor and hydraulic pump and (b)

Fig. 6. Location of the utilized ports to perform the test with respect to location ofthe gauge cutters.

Fig. 5. General arrangement of the EPB TBM in addition to location of the additive injection port.

1 For interpretation of color in Figs. 7 and 8, the reader is referred to the webversion of this article.

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tion length. The precision of the pressure sensor and linear dis-placement sensor are ±1.7 kPa (±0.24 psi) and ±0.1 mm(±0.004 in.), respectively. In addition, because of the pressurizedenvironment and the possibility of flowing material, a sealing sys-tem was designed and attached to the cylinder. By using this seal,the testing device could be connected to one of the six availableadditive injection ports (Figs. 5 and 6) in the front shield of theTBM to prevent material from flowing into the shield. These portswere selected as the location to set up the testing device in order tofrequently monitor the length of the overcut while the TBM wasstationary, e.g., during ring builds or maintenance. The ports onthis TBM are located approximately 3.6 m behind the cutterhead.If there were convergence of the excavated ground behind the faceas a result of the overcut, then this measurement approach wouldaffect gauge cutter wear estimation. However, in EPB tunneling asstated in the previous section, the overcut is pressurized and there-fore, ground loss due to convergence in the annulus and in theshort period of time between the end of excavation and performingthe test is unlikely to happen.

The test was performed in two locations in the front shield of theTBM, namely port 5 o’clock and port 11 o’clock (Fig. 6). Having mea-surements near the crown and near the invert enabled us to assessthe concentric nature of the annulus. In addition, the alignment ofthe excavated tunnel is challenging as there is a continuous curva-ture in the alignment that can affect the testing results significantlyif test is performed in only one location. The goal of performing thetest in two locations (5 and 11 o’clock) and averaging these measure-

ments was to minimize such potential errors. As can be seen in part(b) of Fig. 4, there was limited space between the end of the hydrau-lic jack and the shield of the screw conveyor. The distance from thebeginning of the additive injection port (location of the cone) to theshield is 15.2 cm and the thickness of the shield was 2.5 cm. In addi-tion, the ball valve that originally was attached to the injection portwas replaced with a Guillotine valve in order to ensure that if thetesting system failed, the valve could be closed immediately withoutthe risk of pressure or ground loss.

Penetration length and pressure are recorded continuously byusing a data acquisition (DAQ) system and a laptop. The pressureis applied such that the probe moves slowly outwards with a pen-etration rate of less than 1 cm/s. In addition, the operator can visu-ally monitor the pressure and penetration length in a softwareapplication which utilizes a graphical user interface (GUI). Thecombination of the GUI and the continuous measurements allowsthe operator to halt the program when the pressure increases sig-nificantly, indicating the cone penetrating the undisturbed soil.

3. Results and discussion

In several locations during the excavation of the southboundtunnel, the testing device was used to measure the length of theovercut. During the only man-entry inspection stop, the length ofthe gauge cutter was precisely measured and compared to the re-sults obtained during testing. The locations of performed tests insouthbound tunnel were at advances 67, 195, 291, 415, 478, 555,631, and 684 after 102 m, 297 m, 443 m, 632 m, 728 m, 846 m,962 m, and 1042 m of excavation respectively. The final inspectionwas performed at advance 684, which was exactly at the last testlocation. This inspection was performed under free air and insidea controlled density fill (CDF) shaft. This was an open shaft thathad a concrete roof constructed above the tunnel crown and thenwas filled with CDF. During this inspection the overcut lengthwas measured precisely.

The pressure and penetration rate vs. penetration length resultsfor advance 67 and advance 291 are reported in Figs. 7 and 8. Thevertical red1 lines in Figs. 7 and 8 illustrate the shield thickness andan estimate of where the overcut – undisturbed soil interface liesbased on the data. In the analysis, the threshold between overcut-undisturbed soil is selected whenever the recorded pressure in-creases to a level higher than the recorded pressure through the test.

Fig. 7. The results of testing for advance 67, after 102 m of tunneling.

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As you can see from Fig. 7, there is a slight difference between therecorded length of overcut in port 5 o’clock (8.1 cm) and 11 o’clock(8.6 cm). The average of these two readings, 8.4 cm, is indicated asthe length of overcut at advance 67.

Table 2 summarizes the results of testing during the excavationof southbound tunnel. Reported average EPB pressure is represent-ing the average recorded pressure by the two installed pressuresensors in the TBM plenum at spring-line of the tunnel (Fig. 9).As you can see from Table 2, the results of testing from advance67 to advance 631 are quite consistent. The two readings in ports5 and 11 o’clock show slightly different readings; however, thereis not consistent pattern, e.g., the invert overcut is not consistentlyless than the crown overcut. As the data in Table 2 confirms byreducing the face pressure, the applied pressure to the testing de-vice was reduced as well.

The result of testing for advance 684 is presented in Fig. 10. Thereason for the large measured overcut in port 11 o’clock of advance

684 and low measured overcut in port 5 o’clock is because the copycutter in the left side of the TBM is used in this location in order toguide the TBM through the curved alignment. A free-air man-entrycutterhead inspection was performed at advance 684. The inspec-tor confirmed the 13–14 cm of measured overcut at the left side ofthe shield (port 11 o’clock) because of using copy cutters and 5–6 cm of overcut in the right side of the shield (port 5 o’clock). Inaddition the inspector reported around 7.5 cm as the real remainedlength of the gauge cutter which confirms the previous testing re-sult at advance 631. In addition, at the end of the tunneling and re-moval of the cutterhead, a detailed measurement was conductedon the gauge cutters remained length. The measurement showed7.4 cm as the maximum length of the gauge cutters.

Please note that the first test is being performed after 102 m oftunneling and after passing through the most abrasive groundthrough the alignment. This portion of the alignment mostly con-sists of an abrasive ground with high quartz content (�42%), and

Fig. 8. The results of testing for advance 291, after 443 m of tunneling.

Table 2The results of testing by using the optimized device in southbound tunnel of the U230project.

Advanceno.

Mininglength

Measuredovercut (cm)

Average(cm)

Ave EPBpressure

Ave testpressure

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Soil Abrasion Testing (SAT) value of 18 ± 6. So, based on the test re-sults and available data in the GBR, it is concluded that the most ofthe wear that happened to the gauge cutters (1.8 cm of wear) wasduring the excavation of this section of the tunnel and after thatwear rate was very low.

(m) (bar) (bar)At 11o’clock

At 5o’clock

67 102 8.6 8.1 8.4 1.2 2.1195 297 8.1 7.6 7.9 1.7 2.2291 443 7.9 8.1 8 2.0 3.4415 632 7.9 8.1 8 2.2 3.4478 728 7.4 7.4 7.4 1.3 2.3555 846 7.4 7.1 7.3 1.6 2631 962 7.4 7.4 7.4 1.3 2.0

684a 1042 13.4 5.6 9.6 0.6 1.8

a The copy cutter was used in this advance in order to guide the system in thecurved alignment of the tunnel.

4. Alternative testing system

A simple, less expensive testing system was used during theexcavation of the Northbound tunnel of U230 project in order toconfirm the hypothesis that the relationship between pressureand penetration depth can be used to measure the length of over-cut and therefore length of gauge cutters. A simple tie rod hydrau-lic jack was used to overcome the pressure. A standard DCPhardened cone tip with angle of 60� and tip base diameter of

Fig. 10. The results of testing for adv

Fig. 9. Location of two pressure sensors at the spring-line of the TBM plenum(yellow circles), Red circles display the location of four pressure sensors on top andbottom of the plenum. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

E. Alavi Gharahbagh et al. / Tunnelling and Underground Space Technology 38 (2013) 279–286 285

2 cm was welded to the rod of a tie rod hydraulic cylinder. A tapemeasurement was attached to the base of the cone in order tomonitor the overcut length. A pressure gauge was installed onthe pump in order to monitor the force required to push the testingdevice through the overcut zone and finally a sealing system wasdesigned and attached to the cylinder (Fig. 11).

In several locations during the excavation of the Northbound tun-nel, the testing device was used to measure the length of the overcutand during the only man-entry inspection stop the length of thegauge cutter was measured and compared to the results of testing.

In order to perform the test, two operators are required. One ofthe operators reads the penetration distance from the tape mea-surement as the cone penetrates through the pipe, overcut and in-tact ground continuously and the other operator records thepressure corresponding to the penetration distance. The pressureis applied such that the probe moves slowly outwards with thepenetration rate of 2.5 cm/5 s. When the pressure increases signif-icantly the test is stopped. The locations of performed tests are atadvance 256 (after 390 m of excavation), advance 395 (after

ance 684, 1042 m of tunneling.

Fig. 11. The new testing device for measuring the gauge cutters length.

Fig. 12. 7–7.5 cm (2.75–2.95 in.) of overcut was recorded in the inspection stop.

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602 m of excavation), advance 470 (after 716 m of excavation),advance 581 (after 885 m of excavation) and advance 690 (after1052 m of excavation) of the Northbound tunnel. The final inspec-tion was performed at advance 697, which was 11 m after the lasttest location. Two to three tests were performed at each location.All tests are performed in the port located at 5 o’clock.

An inspection was performed during advance 697, after 1062 mof excavation of the Northbound tunnel. Around 7–7.5 cm of over-cut was recorded through the inspection that completely con-firmed the recorded results of the approach that was used duringthe study (Fig. 12).

In this test, the uncertainty in pressure measurement is 1.4 bars(20 psi) and the uncertainty in penetration distance is between 1.3

and 2.6 cm (0.5–1 in.). Comparison of the testing results with theactual length of the gauge cutters measured directly during theinspection stop confirmed the hypothesis that the length of gaugecutters can be measured by using the proposed testing device butthe uncertainty is fairly high. Furthermore, the test was operatorsensitive since one of the operators records the length and theother operator records the pressure and the time-lapse betweentwo readings can influence the final result. Despite the fact thatthe original length of the gauge cutters was 10.2 cm (4 in.) and1.3–2.6 cm (0.5–1 in.) of resolution was acceptable for demonstrat-ing the feasibility of this study, however in many TBMs the lengthof gauge cutters are very short in order of 10–30 mm which re-quires higher resolution and overcut CPT testing device will pro-vide better results in compare to the alternative testing device.

5. Conclusion

Cutterhead maintenance is usually required when the gaugecutters wear down to the shield diameter. In soft ground pressur-ized shielded TBMs, inspection of gauge cutters usually involvescomplete stoppage of the operation by performing man-entryinspection in free air or in hyperbaric intervention. This can be adangerous, costly, and time consuming process. In this study, a no-vel approach was developed to monitor gauge cutter wear by con-sidering the relationship between the overcut length and length ofthe gauge cutters. The results of the proposed approach for indicat-ing the length of the gauge cutters used during the excavation ofthe northbound and southbound tunnels of University Link LightRail (contract U230) confirmed the hypothesis that the length ofgauge cutters can be measured by measuring the length of theovercut. The approach developed allows rapid and frequent inspec-tion, i.e., possible during any TBM stoppage such as ring builds. Inaddition, it allows remote monitoring of gauge cutters length, i.e.,minimizing the number of times that human-entry inspection (infree-air or hyperbaric intervention) to the unpleasant environmentof the cutterhead chamber is required.

One possible limitation to this approach stems from the loca-tion of the measurements approximately 3.6 m behind the cutter-head. If there were convergence of the excavated ground behindthe face as a result of the overcut, then this measurement approachwould affect gauge cutter wear estimation.

References

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Geotechnical Baseline Report, 2009. University Link Light Rail TBM Tunnels (CHS toPSST) Link Contract U230. IFB No. RTA/LR 60-09. Vol. 6 of 7.

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