Shani Wallis, TunnelTalk
- Tungsten carbide bit cutters are a standard in the mining and exploration drilling industries, and drill bits for drilling jumbos would be of little use without their bit inserts. However, the crossover of effective bit inserts onto disc cutters for TBM operations has been elusive. From early trials by Robbins and Wirth, the application of bit cutters on the vertical cutterheads of hard rock TBMs failed to impress and the technology was more or less shelved.
Bit cutters on duty in Kuala Lumpur
- The technique, however, is finding new potential in the soft ground TBM industry. In recent months, three EPB projects in different parts of the world have demonstrated experience of fitting bit cutters on the mixed face cutterheads of EPBMs.
- In Kuala Lumpur, the Chinese EPBMs supplied by CRTE to the Klang Valley MRT project are fitted with bit cutters for excavation of the soft clay and sedimentary deposits of the Kenny Hill Formation. On a site visit to the project earlier this year, the two machines had recently broken through into the Sentral Station cut-and-cover box at the end of their first 1.4km drives. A closer inspection of the machines revealed that both had a full set of bit cutters on their 6.67m diameter EPBM cutterheads.
- The TBMs are being operated in Kuala Lumpur on a subcontract with the tunnelling division of CREC, the China Railway Engineering Corporation. It was Chinese managers on the contract who brought experience of the bit cutters from previous projects in China and applied the technology in Kuala Lumpur. The button bit cutters were applied to overcome one of the most common issues for disc cutters on soft ground TBMs – flat spots.
Cutterhead of the 6.67m CRTE EPBM in Kuala Lumpur
- The Kenny Hill Formation of very stiff sandy clay was described as ideal for EPB application with disc cutters fitted to the TBMs for when the material became too hard for efficient excavation by the ripper tools alone; or for coping with possible incidences of mixed ground conditions with either boulders or outcrops of bedrock in the face. Ground water pressures on the alignment remain at 2 bar or below, and EPB control is needed to allow for unpredictability in ground conditions and protect against ground loss and surface settlement as the tunnels progress at about 20-25m beneath the streets of the city.
- Application of the disc cutters when they are needed is a great advantage, but in the softer clay deposits, when their particular services are not required, the experience is that the soft clay causes them to stop turning on their own bearings – causing wear of flat spots on the cutter rings. In such conditions the button bits provide the extra traction needed in the soft materials to ensure that the discs continue to turn, thus extending their service life and avoiding the need to replace them due to wear of flat spots.
This week in San Francisco (June 2014), the two Robbins EPBMs operated by the Barnard/Impregilo/SA Healy JV completed their 2.7km of 6.27m o.d. twin tube running tunnels for the Central Subway project. On breakthrough, it is noticed that these machines are fitted also with tungsten carbide button bit cutters.
- The geology on the alignment comprises mostly of poorly consolidated silty and sandy deposits of the Colma Formation with a section of competent to highly fractured Franciscan Bedrock and a small section of soft clay, with the tunnel horizon for the most part below the water table and with the EPBMs operating at up to 4 bar pressure.
Robbins breakthrough for bit cutters in San Francisco
- In a video report by TunnelTalk, it is reported that crews had to change cutters only once on the drives, with the change carried out at about 1,800m into the 2,700m long drives and with only about half the cutters on the cutterheads requiring replacement.
- Brian Shalk of the Barnard/Impregilo/Healy JV confirmed to TunnelTalk that the cutters, scrapers and bucket lips on the cutterheads of both TBMs are supplied to the project by Herrenknecht. The carbide button tipped disc cutters are reported to have performed well on the project and contributed to an average progress rate of 50-80ft (15-25m)/day, and a best rate of 115ft (35m)/day, in ground that was described as “better than anticipated”.
For its Lot 3 contract for the Metro Los Teques project in Caracas, Venezuela, Brazilian tunnelling contractor Odebrecht used Herrenknecht EPBMs to excavate through highly fractured and weathered rock beneath a high overburden, a high water table, and under operating pressures of up to 4.7 bar. During his presentation at the World Tunnel Congress in Brazil in May, Project Manager Danilo Abdanur of Odebrecht explained that the machines were fitted with button bit cutters in an effort to extend the cutter life of the discs and thereby increase the intervals needed between interventions to undertake necessary cutter changes.
- From the published paper, Abdanur reports that shield friction forces were limited by a sufficient overcut by the cutterhead. To maintain the overcut, frequent inspection and replacement of the gauge cutters was necessary. The paper describes how Herrenknecht engineers proposed using the more wear resistant tungsten carbide bit insert (TCBI) cutters which could work in soft rock conditions of up to 40-50 MPa to a limit of a 70 MPa UCS. An extended cutter lifetime of approximately 250% was the predicted advantage of the TCBI cutters over their normal disc alternatives in the prevailing geological conditions.
Bit cutters applied on the Herrenknecht EPBM in Caracas
- Speaking with Abdanur after the presentation he said that the experience in this particular geology showed mixed results. The button bit discs proved efficient in areas of more homogeneous or softer rock deposits, while in reaches of highly fractured, blocky and harder rock, the conditions had the effect of knocking bits off the cutter ring as the cutters under the required machine thrust, and the rotating cutterhead, connected with the edges of hard blocky rock. “Once a disc was missing a bit,” said Abdanur, “we found that further bit inserts on the same disc would soon also be knocked out and the disc would need replacing.” While adding advantages in some conditions, this kind of damage to discs actually added to the need for interventions.
- As the track record and experience continues to grow, the use of bit cutters may find a sustained application in the civil tunnelling industry on soft ground TBM applications.
- Have you had experience of using button bit insert discs on a TBM project.
Join the discussion via the TunnelTalk Feedback facility.
Breakthroughs line up for Klang Valley MRT - TunnelTalk, Dec 2013
CRTE of China takes over Wirth TBM intellectual property - TunnelTalk, Nov 2013
Video: Central Subway EPBM breakthroughs in San Francisco - TunnelTalk, Jun 2014
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