Disc Cutters are ubiquitous in hard rock TBM tunnels; however, their performance is anything but.
What can make the difference between frequent downtime to replace the discs and fast excavation with long-lasting cutters is often the small details. In fact, success can be a game of millimetres when it comes to disc cutters. Results that have borne out in the laboratory—a 508mm- (20-inch-) diameter cutter has 60 per cent more wear volume than a 483mm- (19-inch-) diameter cutter, for example—are being backed by more and more studies in the field. A difference like that can result in dramatic increases in cutter life, even in very hard rock. What’s more than that, differences in processing of the tool steel between different manufacturers can further affect cutter life by reducing wear.
At a recent project in Northeastern China, multiple machines were used on one massive, hard rock tunnel project in similar ground conditions. The machines utilised cutters of different sizes and from different manufacturers, affording a unique opportunity to analyse the performance of disc cutters in a more controlled fashion. While differences in TBM operation and maintenance may always be contributing factors that cannot be controlled for, certain generalisations can be made: Larger cutter diameters and more tool steel processing result in better performance, nearly without exception.
Cutters: A Brief History
When disc cutters were first developed by James Robbins at Toronto’s Humber River Sewer Tunnel in 1956, the 292mm- (11.5-inch-) diameter discs were used in relatively soft crystalline limestone. Disc cutter designs have expanded since that first breakthrough, and as recently as the 1980s 356mm- and 394mm- (14- and 15.5-inch-) diameter cutters were the industry standard for hard rock TBMs. The 15.5-inch cutter was seen as an improvement over the 14-inch with a larger bearing set and its corresponding increase in thrust capacity. Shortly thereafter the 15.5-inch cutter was expanded to 432mm (17 inches) by mounting a larger diameter disc on the same bearing set. While the thrust capacity of the cutter hadn’t changed, the premise was that the 17-inch disc, with increased sacrificial material, would increase the mean time between cutter changes. This proved to be the case and the 17-inch cutter became the de facto standard even after the 483mm- (19-inch-) diameter cutter was developed in the late 1980s for the Svartisen Hydroelectric Project in Norway.
The TBMs employed at Svartisen were the first “high performance” TBMs. These were designed with both increased thrust and cutterhead power compared with earlier machines. While the 19-inch cutters featured increased thrust capacity, the material of the disc—the chrome/molybdenum/nickel steel historically used—was not up to the task of boring in hard rock up to 200 MPa UCS. It was not until the introduction of tool steel, and later modified tool steel, that the benefits of increased thrust capacity were able to be fully realised. While the benefits of 19-inch cutters were clear to the contractors actually using them, it would be at least 15 years before they were generally accepted by the industry.
The Benefits of Incremental Increases
In the world of disc cutter design, there are two distinct benefits to employing larger diameter cutters: higher thrust capacity and longer wear life.
Higher thrust capacity enables efficient boring in harder rock formations. To efficiently cut rock, the thrust force applied to an individual cutter must overcome the penetration resistance of the rock and initiate chip formation. Once the critical pressure has been achieved, penetration increases rapidly with a relatively small increase in cutter load. The critical pressure increases with rock strength, and it is primarily for this reason that larger cutters have been developed to bore in harder rock.
Longer wear life is the result of an increased volume of sacrificial material in the larger diameter disc ring. The migration from 19-inch rings to 20-inch rings on the same bearing core is based on the same principle as the improvement made to the 15.5-inch cutter by installing a larger diameter 17-inch disc back in the 1980s.
The cross section of a 17-inch and a 19-inch cutter are effectively identical when considering just the sacrificial portion or “blade” of the disc. The 19-inch disc has just about 12 per cent more wear volume than the 17-inch disc, assuming 30mm of allowable wear. The 20-inch disc, however, has substantially more wear volume (60 per cent) when compared to the 19-inch.
The tip of the 20-inch disc has been extended by 13mm on the radius compared to a 19-inch disc, while the rest of the disc profile remains unchanged.
The same principle of extended tip discs can be applied to the 17-inch cutter and when geology permits, numerous TBM operators have chosen an 18-inch disc or even a 19-inch disc mounted on a 17-inch bearing core in order to take advantage of the added sacrificial material. This can be an effective solution in pressurised face tunnelling to extend the time between cutterhead interventions.
A Case Study in Cutter Design: The Liaoning Now Project
One of the longest tunnels in recent history is Northeastern China’s Liaoning NOW Water transfer project, measuring 120km in length. The government-commissioned tunnel, for irrigation and drinking water, has been divided into nine lots, designated T1 through T9 (for Tunnel No. 1 to 9). Each lot, except for T7, was excavated by TBM. Lot T7 is utilising drill and blast. Lots T1 and T2 utilised new main beam machines from another manufacturer. Contractor China Sinohydro Bureaus 3 & 4, responsible for lots T3 and T4 respectively, elected new Robbins main beam TBMs, 8.53m in diameter. Similarly, T5 contractor Shanxi Hydraulic Engineer Construction Bureau ordered an 8.53m Robbins main beam. Chinese equipment supplier NHI and Robbins supplied main beam machines of the same diameter for T6 and T8, and a rebuilt Robbins machine at 8.03m was provided for lot T9. All eight machines were ordered with Robbins continuous conveyors for muck removal (see Table 1 for summary).
Each of the eight TBMs excavating the Liaoning NOW project bored two consecutive tunnels ranging from 5 to 10km long, totalling about 15km each.
The difficult and long tunnels pass through mainly granite, granite gneiss, and schist geology of varying abrasivity, and this geology was similar for all the drives. Mountainous terrain including valleys and rivers requires versatile ground support. Cover varied widely, from as little as 97m to as high as 590m at T6. Despite their nearly identical designs, some of the TBMs were fitted with 19-inch disc cutters and some with 20-inch cutters at the request of the various contractors.
The discs originated from either the US, Europe, or China, and were manufactured using different processing methods.
Three TBMs on the project utilised 19-inch cutters, while five TBMs utilised 20-inch cutters. The large number of machines on a single project presented a unique opportunity to compare not only 19-inch and 20-inch cutter diameters but also discs manufactured by different suppliers. Data on disc cutter consumption was not made available for the entirety of each tunnel but sufficient data were obtained that revealed some interesting trends. Table 2 illustrates the distances over which disc cutter consumption data were obtained. Most data came from the first drives of each contract.
Cutter diameter analysis
One would expect average cubic meterage rates per disc cutter consumed to be roughly proportional to the additional volume of sacrificial material in a 20-inch cutter when compared to a 19-inch cutter. In addition one would expect that monthly advance rates might be somewhat higher as a result of less frequent cutter changes. These data lend support to both premises when comparing average cubic meterage rates or when comparing the monthly advance rates from all eight tunnels.
Chart 1 summarises the advance rates of all eight TBMs. The first two columns show the monthly advance averages for the portions of each contract where disc cutter data were made available. The 19-inch cutters averaged 446 m/month compared to 554m/month for the 20-inch cutters. In this project, the machines with 20-inch discs had on average a 24 per cent better monthly advance rate than those with 19-inch cutters.
Chart 2 is a summary of cubic metres per cutter disc consumed for all eight TBMs. Cubic metres per cutter disc, the best measure of cutter life and also a good indicator of total cutter cost, are also telling. The averages for all eight machines are shown in the first two columns of the charts. The average for 19-inch cutters is 346 cubic metres bored per cutter and for 20-inch cutters, the number is more than twice as high at 896 cubic metres bored per cutter.
Discs matter
During the execution of this project there were two separate opportunities to compare the relative merits of different manufacturer’s discs against each other: those manufactured in the US, those manufactured in Europe; and in Asia.
More specifically, this comparison could be made on the T1 and T6 contracts.
The most striking difference compares the performance of the T1 TBM on its first drive using 19-inch cutters procured by the contractor from a European supplier with 19-inch cutters made in the US.
At T1, the contractor bored the first 3,614m of the tunnel with cutters manufactured in the EU. For the final 3,710m of the first drive, the contractor switched to a mixture of US-sourced cutters and EU cutters. The US cutters were used in the transition area of the cutter profile. This is the area where the face transitions from being flat to turning out to cut the gauge. It is well understood that the cutters in this area are the most highly loaded of all the cutters on the cutterhead and therefore, as a rule, experience the highest wear.
Despite that, the performance was markedly better during the second half of the first drive, with 543 discs consumed versus 1,479 discs consumed in the first half. Of the 543 discs, 225 were US discs and 318 were EU discs. This equates to a performance increase from 140m3/disc to 390m3/disc by deploying US discs in the transition area. Refer back to the chart where the two bar graphs for T1 are enclosed with a black border and labelled T1. The EU discs are coloured green and the mixed EU and US discs are shown in blue and green. In addition to an improved cubic meterage/disc rate, the monthly advance rate improved significantly on T1 after the changeover. This is clearly illustrated where the two bar graphs for T1 are enclosed with a black border and labelled T1 showing 322m/month for the first 3,614m and 613m/month for the final 3,710m of T1’s first drive.
At T6, the contractor bored the first 3,722m of the tunnel with cutters manufactured in the US. Then the contractor switched completely from the US cutters to Asian- manufactured cutters. Data were only available for 870m after the changeover to Asian discs, but it was long enough to observe two effects.
First, referring back to Chart 1, there is no significant difference in monthly advance rates seen in the two bar graphs enclosed in black and labelled T6.
With U.S. cutters, the average was 555m/month, and this was slightly higher at 563m/month with the Asian discs installed.
The important difference between the US disc cutters and the Asian discs is instead readily apparent in Chart 2, where the cubic metres per cutter disc consumed are compared (refer to the two bar graphs enclosed with a black border and labelled T6). During the first 3,722m of T6, US disc cutters were used and the average rate of rock excavated per disc consumed was 1,131 m3/disc. Compare that to the performance of the Asian disc cutters at 199m3/disc.
Excavation performance of the discs was reduced by a factor of 5.7. This represents a significant cost increase both in discs consumed (even when the discs are substantially cheaper) and in down time for cutter changes.
In Conclusion
Contractors should carefully consider whether the use of larger discs will provide an economic benefit. The choice of 20-inch cutters over 19-inch cutters (coupled with an aggressive cutterhead management program) can provide longer time between cutter changes and longer overall life between rebuilds.
Contractors can also benefit from carefully considering the technology applied by each manufacturer to their disc cutters. Any competent manufacturer can make a disc cutter, but the proof of the quality of the disc will not be apparent until the steel meets the rock. Nearly all disc cutter manufacturers now offer a tool steel disc ring and most have similar composition.
It is, however, less the composition of the disc ring than it is the subsequent processing that makes the difference in performance.
Less expensive disc cutters will not be economical in hard rock when considering the total cost over the duration of a project, and this becomes more and more significant as the rock becomes ever more challenging.
Extra, steps in processing can mean the difference between a swift project and a slow-going one: indeed, this happened at T6 when the performance of the disc was reduced by a factor of 5.7.
More studies such as this one should be conducted in the field to further investigate the relationships between processing, cutter diameter, and cutter life and performance.
However, the overall trends in the data are clear: Extra millimetres can make the difference, and extra care taken in processing add up to fewer discs consumed, reduced costs overall, and faster TBM tunnelling