Remote Monitoring of Gauge Cutter Wear in Pressurized Face Tunneling

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Percival Gilbert
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1 Remote Monitoring of Gauge Cutter Wear in Pressurized Face Tunneling Glen Frank, JayDee Contractors, Inc., Seattle, WA Ehsan Alavi Gharahbagh, The Pennsylvania State University, University Park, PA Michael A. DiPonio, JayDee Contractors, Inc., Seattle, WA Michael A. Mooney, Colorado School of Mines, Golden, CO Bryan Walter, Colorado School of Mines, Golden, CO ABSTRACT Cutterhead maintenance is usually required when the gauge cutters (overcutters) wear down to the shield diameter. In soft ground pressurized shielded TBMs, inspection of gauge cutters usually involves complete stoppage of the operation. This is a dangerous, costly, and time consuming process. In this study, a novel approach has been developed to monitor gauge cutter wear by considering the relationship between the overcut length and length of the gauge cutters. By using this testing system, continuous monitoring of gauge cutter length is possible while the excavation is in progress. This monitoring system is being used in an EPB TBM through the excavation of the University Link Light Rail Tunnel in Seattle and the results are presented in this paper. INTRODUCTION Cutterhead inspection and maintenance in Earth Pressure Balance (EPB) TBMs can become dangerous, time consuming and a costly process when the ground is unstable, e.g., under groundwater conditions. In this condition, cutterhead inspection and tool maintenance are performed under pressurized hyperbaric intervention. This involves creating a plug at the face, removing the muck, applying compressed air, and allowing the crew into the pressurized chamber via an air lock. Although, cutterhead inspection and maintenance is important and needs to be conducted in specific intervals through the project, the number of times that humans should be subjected to this environment should be minimized. Cutterhead inspection can be costly because no mining can be performed during inspection. It has become almost a standard procedure that a cutterhead maintenance plan is created and implemented by the contractor. In addition, it is becoming more common for the owners to include a minimum number of cutterhead inspections in the contract documents. For example, the Brightwater West contract mandated 40 inspection stops during 20,000 lineal feet of tunneling. This translated into approximately 120 hours (5 days) of lost production time for inspection purposes. Although, a detailed plan for this project was defined, human entry or replacement of cutters was not mandated. A remote camera (periscope) was used to visually inspect the condition of the cutterhead (Shinouda et al., 2011). Only 11 out of 40 inspections were human entry that one of these 11 entries required hyperbaric intervention. By utilizing this remote camera, the inspection time was shortened to an average 3 hours (2 6 hours time range). To this end, there is considerable benefit from developing cutterhead inspection techniques that can be performed within the normal tunneling operations. In this study, a novel approach is being developed to monitor gauge cutter wear by measuring the overcut. The assumption behind this approach is that the overcut length will decrease as the periphery (gauge) cutter length decreases due to wear. To develop the approach, a new testing device was designed and manufactured which uses concepts similar to the dynamic cone penetration (DCP) test. This device was used during construction of the University Link Light Rail Tunnel in Seattle. By using this DCP like testing device, monitoring of gauge cutter length was possible while the excavation is in progress.

2 UNIVERSITY LINK TUNNELING PROJECT (U230 CONTRACT) The Sound Transit University Link project is a 3.15 mile light rail extension that will run in twin bored tunnels from downtown Seattle north to the University of Washington, with stations at Capitol Hill and the University of Washington campus near Husky Stadium. The project is divided into several parts. The U230 portion deals with the installation of the twin bored tunnels from the Capitol Hill Station to the Pine Street stub tunnel in downtown Seattle. The joint venture of JayDee Contractors, Inc., Frank Coluccio Construction Company, and Michels Corporation was awarded the U230 Project. They are responsible for the construction of twin bored (northbound and southbound) tunnels each with an approximate length of 3880 feet from Capitol Hill Station (CHS) to Pine Street Stub Tunnel (PSST). The tunnels are approximately 21 feet in diameter and are to be finished with pre cast concrete segmental lining, having an outside diameter of 20 feet 7 inches and an inside diameter of 18 feet 10 inches. GEOTECHNICAL PROPERTIES The Seattle area has a complicated geologic history. Six major glacial events have happened during last 2 million years with intervening non glacial erosional and depositional periods. Various interglacial and glacial events have resulted in a very complex soil structure. Each of these events partially eroded and deposited overlapping gravels, sands, silts, clays and tills. The geologic description of this project can be divided into fluvial deposits, glacial deposits, lacustrine and glaciolacustrine deposits, all of which have been glacially over ridden and are therefore highly over consolidated (0.7 < OCR < 4.3 and average of 2.5) Based on the exploration analysis for this project, three glacial and three non glacial cycles are encountered throughout the tunnel alignment based on the observations of sediments, texture, sedimentary structures, etc. These deposits consist of clay, silt, sand, gravel and boulders in various combinations. Soils encountered in the exploration borings were grouped into Soil Groups (SGs) according to soil index properties, particle size, Atterberg Limits, strength and deformation properties. The Soil Groups are identified by color. The Soil Groups defined for this project are Blue SG that represents over consolidated fine grained, plastic soils (USCS soil types: CH, CL, ML, MH, CL ML, OL), Turquoise SG that represents over consolidated fine grained, non plastic soils (USCS soil types: ML, SM, SP, SP SM), Yellow SG that represents over consolidated fine to coarse sand, with varying amounts of gravel, silt, and clay (USCS soil types: SM, SP SM, SP), and Red SG that represents over consolidated course sand and gravel, with varying amounts of fine to course sand, silt, and clay (USCS soil types: GP, GM, and GW). Table 1 summarizes the number of boulders that are expected to be encountered during the excavation of Northbound and southbound tunnels with their representative size. Table 1. Number of boulders for each tunnel based on GBR Boulder Size (ft) 1 to 2 2 to 3 3 to 4 4 to 5 5 to 6 >6 Northbound Tunnel Number of Boulders Southbound Tunnel 57 9 Table 2 shows the information related to the abrasiveness of each soil group. As it can be seen, the ground is not very abrasive.

3 Table 2. XRD analysis and SAT Values for different soil groups Soil Group Blue Turquoise Yellow Quartz (%) Feldspar (%) Hornblende (%) Clay minerals (%) Soil Abrasion Testing (SAT) 2+/ 2 6+/ 1 18+/ 6 DESCRIPTION OF THE DEVELOPED TESTING DEVICE Dynamic Cone Penetration Test (DCP) The DCP test involves the penetration of a small steel cone into the ground using an impact force. The DCP test is performed by dropping a hammer mass from a fixed fall height and measuring the penetration depth per hammer blow. Figure 1 shows the DCP device. Fig.1. Dynamic Cone Penetration Testing Device This test has been widely used for very near surface site investigation (not more than 4 feet deep) in support of analysis and design of geotechnical and foundation engineering, construction and structural control, and pavement design. By using this test the strength, stiffness and density of a wide variety of soils have been evaluated. This test can provide continuous and quick relationship between soil strength and depth.

4 Modification of DCPT and Development of New Testing Device In order to continuously monitor the length of the overcut without interrupting the mining and ring erection process, it was decided to use one of the six available additive injection pipes in the front shield of the TBM. Additive Injection Port utilized for performing the test Fig.2. General arrangement of the TBM in addition to location of the additive injection pipes An initial analysis was performed in order to investigate the possibility of using a standard DCP testing device to measure the length of the overcut by considering the fact that the strength of the overcut material (broken soil) is much less than the strength of the intact ground. In this way, the interface between the overcut soil and intact ground can be recognized and therefore the length of the overcut zone (length of the gauge cutters) can be measured (Figure 3).

25 Penetration/ Blow (blow/min) 20 15 10 5 0 Interface between intact ground and overcut material 0 1 2 3

5 25 Penetration/ Blow (blow/min) Interface between intact ground and overcut material Penetration distance (in) Fig.3. Example of analyzing the data from a DCP test Several problems were encountered which made the use of a standard DCP device impractical. The first issue was that there is no vertical injection pipe available (6 o clock location), therefore the device was required to be at an angle of approximately 57 degrees from the vertical. This is challenging for a device that relies upon a falling mass. The second issue was related to the pressurized environment that this device was subjected to (maximum EPB pressure through the tunnel alignment was ~ 50 psi). In order to overcome the outside pressure and secure the device in its location, 90 kg of clamping would have been required before the test even starts. In addition, a retaining system is required to keep the cone in place and to counteract the pressurized environment (Figure 4). The third issue was related to the limited available space for the device. There is only 28 inches of clearance from the pipe to the screw conveyor shield. The combination of these issues made it unrealistic to use a standard DCP device. Fig. 4.Free body diagram of the DCP device

Design and Fabrication of the New Testing Device Figure 5 shows the new testing device that was designed and built for monitoring the overcut length.

The relationship between pressure and depth is used to correlate the length of overcut area.

6 Design and Fabrication of the New Testing Device Figure 5 shows the new testing device that was designed and built for monitoring the overcut length. As it can be seen from this figure, a hydraulic jack is used to overcome the pressure applied from the overcut zone (outside the shield). The relationship between pressure and depth is used to correlate the length of overcut area. This hypothesizes that the pressure remains constant inside the overcut zone and as soon as the cone enters the intact soil, the pressure increases significantly. As shown in Figure 5, a standard DCP hardened cone tip with angle of 60 degrees and tip base diameter of 0.79 inches (from Kessler soils engineering) was welded to the rod of a tie rod hydraulic cylinder. The hydraulic cylinder has 16 inches of stroke and is double acting so that it can be retracted at the end of each penetration out into the overcut. A tape measurement is attached to the base of the cone in order to monitor the overcut length. In addition, because of the pressurized environment and possibility of flowing material, a sealing system was designed and attached to the cylinder. By using this sealing, the testing device could be connected to the additive injection pipe to prevent material from flowing into the shield. A pressure gauge with a resolution of 20 psi was installed on the pump in order to monitor the force required to push the DCP through the overcut zone. Figure 6 shows the device attached to the pipe in the front shield of the TBM for U230 project. Fig.5.The new testing device for measuring the gauge cutters length

7 Fig.6. The new testing device attached to the pipe in the TBM shield As can be seen in Figure 6, there is only 1.5 inches of space between the end of the hydraulic jack and the shield of the screw conveyor. The distance from the beginning of the additive injection pipe (location of the cone) to the shield is 9 inches and the thickness of the shield is 1 inch. In addition, the ball valve that originally was attached to the injection pipe was replaced with a Guillotine valve in order to make sure that if the testing system failed, the valve could be closed immediately without the risk of pressure loss and the potential for ground loss. RESULTS AND DISCUSSION In order to minimize the number of cutterhead maintenance events, longer gauge cutters should be installed so that the rate of wear that can be tolerated by gauge cutters before cutter replacement or repair is required increases. On the other hand, there is a perception in the industry that the length of gauge cutters should be very short, on the order of 10 to 30 mm for small diameter TBMs, in order to minimize the risk of settlement. For larger diameter TBMs, the length of overcut is more limited and more accurate control of machine functions are required in order to control the volume of ground loss into the annular space that can cause surface settlement. This industrial belief is based on the assumption that the overcut is void and regular ground loss (settlement) occurs into the overcut gap. This general concept is true for open face TBMs where the overcutters cut the ground and the disturbed ground falls to the invert where it can be removed and transferred to the outside of the tunnel. In contrast to open face TBMs and the existed perception, it has been the authors experience in 50,000 linear feet of EPB tunneling in glacial soils that the extended overcut can be properly controlled in EPB tunneling in dense soils such that no soil relaxation, loss of ground or additional settlement occurs. In this project, the contractor has decided to use relatively long gauge cutters (4 inches). In several locations during the excavation of the northbound tunnel, the testing device was used to measure the length of the overcut and finally during one of the mandatory inspection stops the length of the gauge cutter was measured by man entry and compared to the results of testing. In order to

8 perform the test, two operators are required. One of the operators reads the penetration distance from the tape measurement as the cone penetrates through the pipe, overcut and intact zone continuously and the other operator record the pressure corresponds to the penetration distance. The pressure is applied such that the probe moves slowly outwards with the penetration rate of 1 inch/ 5 sec. When the pressure increases significantly the test is stopped. The locations of performed tests are at advance 256 (after 1280 ft of excavation), advance 395 (after 1975 ft of excavation), advance 470 (after 2350 ft of excavation), advance 581 (after 2905 ft of excavation) and advance 690 (after 3450 ft of excavation) of the northbound tunnel. The inspection was performed in advance 697, 35 ft after the last tests location. The pressure vs. penetration results are presented in Figures 7 11 for each location. Two to three tests were performed at each location. All tests are performed in the port located at 5 o clock. The vertical black lines in Figures 7 11 illustrate the shield thickness and our estimate of where the overcut undisturbed soil interface lies. The uncertainty in each pressure measurement is 20 psi and the uncertainty in each penetration distance is between inches. Pressure (psi) Test 1 Test Depth (in) Length of the pipe (Inside the shield) Shield Overcut Thickness Intact Soil Fig.7. The results of testing for advance 256, 1280 ft of tunneling

9 Pressure (psi) Test 1 Test Depth (in) Fig.8. The results of testing for advance 395, 1975 ft of tunneling Pressure (psi) Test 1 Test 2 Length of the pipe (Inside the shield) Depth (in) Shield Overcut Intact Soil Thickness Length of the pipe Shield Overcut (Inside the shield) Thickness Fig.9. The results of testing for advance 470, 2350 ft of tunneling Intact Soil

10 Pressure (psi) test 1 Test 2 test 3 Depth (in) Length of the pipe (Inside the shield) Shield Overcut Thickness Intact Soil Fig.10. The results of testing for advance 581, 2905 ft of tunneling Pressure (psi) test 1 Test 2 Depth (in) Length of the pipe (Inside the shield) Shield Overcut Thickness Intact Soil Fig.11. The results of testing for advance 690, 3450 ft of tunneling An inspection was performed during advance 697, after 3485 ft of excavation of the northbound tunnel. This inspection was performed under free air and inside a CDF shaft. This was an open shaft that had a concrete roof constructed in it above the tunnel crown and then it was filled with CDF. During this inspection the overcut length was measured precisely. Around 2.75 inches of overcut was recorded through the inspection that completely confirmed the recorded results of the approach that was used during the study. Figure 12 shows the overcut length during the inspection. Please notice that the first

test is being performed after 1280 ft of tunneling and after passing through the most abrasive ground through the alignment (Yellow Soil Group).

11 test is being performed after 1280 ft of tunneling and after passing through the most abrasive ground through the alignment (Yellow Soil Group). This portion of the alignment was mostly consists of an abrasive ground with 21 42% quartz, and SAT value of 18±6. So, based on the test results and available data in the GBR, it is concluded that the most of the wear that happened to the gauge cutters was during the excavation of the first 1000 ft of the tunnel and after that wear was very low (Blue Soil Group and Turquoise Soil Group). In addition, the average face pressure at the location of each test and the corresponding pressure that is applied to the penetrometer during the test are reported in table 3. As the data in the table 3 confirms by reducing the face pressure, the applied pressure to the testing device was reduced as well. Table 3. Face pressure and approximate pressure recorded during the test Advance Number Face Pressure (psi) Approximate Pressure during the test (psi) ~ ~ ~ ~ ~60 Fig to 75 mm ( in) of overcut was recorded in the inspection stop

12 CONCLUSION The results of the proposed novel approach for measuring the length of the gauge cutters that is being used during the excavation of the Northbound tunnel of the University Link (contract U230) are confirmed the hypothesis that the length of gauge cutters can be measured by measuring the length of the overcut. The novel approach can be used for remote monitoring of gauge cutter length while the excavation is in progress. By using this approach, the number of times that human entry into the risky and dangerous environment of the cutterhead chamber (in pressurized or free air condition) for monitoring the gauge cutter length can be minimized. In addition, in order to monitor the length of gauge cutters, no stoppage to the mining operation is required in this approach. FUTURE WORK As it was mentioned in the conclusion section of the paper, comparison of the testing results with the actual length of the gauge cutters measured directly during the inspection stop confirmed the hypothesis that the length of gauge cutters can be measured by using the proposed testing device and by considering the relationship between the pressure and the penetration depth. It should be noted that there are some shortcomings with respect to the testing procedure and the precision of measurements. One of the most important shortcomings of this study was that the tests were performed at only one position through the pipe that was located at 5 o clock position in the shield. It is clear that the lower portion of the shield penetrates inside the ground because of the shield weight. In addition, the alignment of the excavated tunnel was very challenging and there is a sharp curvature involved in the alignment that can affect the testing results significantly. This problem should be addressed in a more detailed analysis. In the meantime, the resolution for measuring the penetration depth and pressure in this analysis were only inch and 20 psi. Despite the fact that the original length of the gauge cutters was 4 inches and inch of precision was quite promising for confirming the applied hypothesis but in general the length of the gauge cutters are much shorter therefore a more precised measurement should be done. In order to address these shortcomings a more detailed analysis is designed for the excavation of the southbound tunnel of the University Link light rail tunnel. In order to have a more precise analysis, a new hydraulic cylinder with a linear displacement transducer is utilized and a pressure sensor is installed into the system as well. In this way, a continuous and more accurate measurement of pressure and penetration depth can be recorded. In addition, another injection additive pipe that is located in 11 o clock position of the front shield is utilized for performing the test. In this way, by performing the test through 5 o clock and 11 o clock positions at the same mining location and averaging the two measurements the possible error that could be caused by performing the test in only one location can be addressed (Figure 13).

13 Fig.13. Utilized ports for the planned analysis during the excavation of southbound tunnel REFERENCES 1 Shinouda, M.M., Gwildis,G.U., Wang, P., Hoddar, W., Cutterhead Maintenance for EPB Tunnel Boring Machines. Proceedings Rapid Excavation and Tunneling Conference, San Francisco, CA 2 Luo, Xiadong; Salgado, Rodrigo; and Altschaeffl, A. G., Cone Penetration Test to Assess the Mechanical Properties of Subgrade Soils..Joint Transportation Research Program. Paper Amini, F., Dynamic Cone Penetrometer in Quality Control of Compaction, State of the Art Report. Geotechnical Engineering for Transportation Projects, proceedings from Geo Trans 2004, Los Angeles, CA 4 Chen, D., Lin, D., Liau, P., Bilyeu, J., A Correlation Between Dynamic Cone Penetrometer Values and Pavement Layer Moduli. Geotechnical Testing Journal, Vol. 28, No. 1, ASTM International, Pennsylvania 5 Siekmeier, J.A., Young, D., Beberg, D., Comparison of the Dynamic Cone Penetrometer with Other Tests During Subgrade and Granular Base Characterization in Minnesota, Nondestructive Testing of Pavements and Backcalculation of Moduli: Third Volume, ASTM STP 1375, S.D. Tayabji and E. O. Lukanen, Eds., American Society for Testing and Materials, West Conshohocken, PA 6 University Link 230 Geotechnical Baseline Report (volume 6) 7 University Link 220 Geotechnical Baseline Report (volume 6) 8 University Link 230 Geotechnical Data Report (volume 7)

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