This article deals mainly with disc cutters, which have become a key element in the design of TBMs, to achieve greatly improved rates of progress in hard rock and mixed ground with hard inclusions. First developed in 1956 by James Robbins, disc cutters became a key factor in the development of the Robbins range of hard rock TBMs.

The use of a rotating ring of high-quality steel to apply forward thrust to the rock being excavated allows much greater pressure to be applied, rapidly crushing rock in the immediate area and effectively cutting the face into manageable lumps, or chips, according to the spacing of the cutters. This is aided by the greater thrust available from modern TBMs, necessitating robustly designed mountings and bearings for the cutters to handle the forces involved.

This action is in contrast to the use of rollers with tungsten carbide inserts that adopt a more grinding type of action. Roller cutters are more commonly used in small-diameter drives such as microtunnelling or raiseboring where the application of disc cutter principles has been more difficult, mainly because of the higher rating of bearings necessary.

The development of mini-disc cutters however holds the promise of the effective use of the same principles in small-diameter excavations.

Durability and performance

Opinions tend to vary on the relative importance of specific rock properties on cutter choice, but influencing factors include quartz content (generally the most abrasive mineral), hardness (UCS) and jointing.

Alastair Biggart, an international tunnelling consultant with vast experience of TBM methods says, “I personally put a lot of importance onto quartz content. Discontinuities are also extremely important. You can, for instance, have a very hard material that would be difficult to cut, but if there are lots of discontinuities it will break more easily. In fact, a soft rock with no discontinuities may be worse than a hard rock with lots of discontinuities.”

Stein Harvested, the developer of Split-Ring cutters, agrees: “The distance between jointing varies considerably. Widely spaced joints require much more force to break out the material, and this can be very important. It may influence advance considerably more than quartz content.”

Cutters will be of little use unless they can last the expected course, but many factors affect performance and durability. Alastair Biggart lists the main factors as: “abrasivity, the load per cutter, and rock hardness.” However, he also points out that performance can be optimised through:

  • regular maintenance and cutter change
  • use of good quality cutters
  • correct cutter spacing
  • use of a cutterhead with good gathering performance to avoid muck collecting in the invert of the chamber
  • setting the cutter in a saddle with adequate clearance, and with the addition of shrouding at the front. This is to prevent muck blocking the box, which stops the cutter turning
  • good seals on the cutter, especially in closed-face TBMs where the muck pressure can penetrate the bearing
  • monitoring of cutter rotation. “The technology for this is not available at present, so regular inspection for flats must be done,” says Biggart.

Mixed ground, of whatever form, is likely to apply extra duties on the cutterhead, even though the softer ground may be easier to penetrate. “Boulders in clay can be cut by discs,” explains Biggart, “although discs in a clay face increases the torque requirement. Boulders in sand can also be cut by discs, as long as the earth or slurry pressure is sufficient to hold the boulder in the matrix whilst it is cut. I had direct experience of this on the Cairo Wastewater Project.”

Vittorio Guglielmetti, TBM tunnelling specialist with Geodata of Italy, lists steel quality, wear resistance, general cutter design, and the design of the overall cutterhead (in terms of the number of cutters and spacing) as very important factors affecting cutter performance. “Cutter failure due to rock characteristics,” says Guglielmetti, “depends on the properties. In abrasive, quartzose rock, cutter wear is the most important factor, but in very hard rock bearing failure and/or metallurgical failure may be predominant.”

He is more wary of mixed ground: “Inclusions of boulders in soft ground are very dangerous for cutter durability, due to the shocks they produce. In such cases it is important to increase inspection frequency to avoid the breakage of even one disc, which can cause a series of breakages. In a very short time it could lead to a catastrophe.”

Of course, discussions on the correct cutters for different rock types imply full knowledge of the ground conditions. For Maurice Sturrs of UK supplier TBM Cutters this is often the major problem: “You have to have good information on the geology,” he points out. “The usual impact type boreholes leave a lot to be desired.”

The attitude of some contractors can exacerbate the situation. “There is a reluctance to spend money before the contract gets going,” says Sturrs. “Contractors may even put up with something they know won’t work properly, until money starts coming in.” As with all other equipment, especially in the arduous duties of tunnelling, the cheapest on price is unlikely to be the cheapest in overall cost. On long tunnel drives the accumulated costs of cutters may well be higher than the cost of the TBM assembly itself, so price becomes more important in these cases.

On specific suitability Sturrs says, “Roller cutters are OK if the rock is hard enough, but clay cutters are also needed in mixed ground, soft ground and clay. There is no one cutter that will do everything, so mixed ground, within the face or along the drive, can be a problem.”

A tendency for steel disc cutters to stop rotating and form flat spots can be a big problem in mixed ground where soft muck accumulation prevents free movement. “Where a disc cutter can ‘flat-spot’,” says Sturrs, “a roller cutter will still rotate, but there is a big sacrifice in penetration. Everything is a compromise. If rock abrasiveness is a big factor, a roller cutter will still flat-spot. Normally though, the tungsten carbide inserts will act like the spokes of a wheel, keeping the cutters rotating.”

Compromise is arguably a bigger theme when considering closed-face TBMs, since the greater progress possible through more secure ground control has to be balanced against other factors. Maurice Sturrs commented, “Cutters can work more effectively on closed faces but it comes at a cost. There can be a variation factor of four times in cutter life, and so costs can be very high.” Again the attitude and cashflow of contractors can be an important factor. “Contractors don’t have a lot of money at the beginning of a contract, but will improve replacement procedures as they go on.”

Mounting importance

Since they have to carry high thrust and side loads, at high temperatures (up to around 180?C), whilst allowing rotary movement of the cutter ring, the bearings and their mounting design and manufacture are of high importance. Different designs of bearing, and seals to prevent contaminants from the arduous environment, have been adopted with varying claims to success.

The Black Diamond Cutters’ design for cutters of 365mm-483mm diameter incorporates a single multi-rowed tapered bearing and a heavy-duty dual seal. The assembly procedure is said to negate many problems associated with preloading of bearings and calibration of bearing spacers. The result is said to be a dramatic reduction in man-hours required for cutter assembly and disassembly.

Means of change

It must be possible to change cutters, when required or planned, quickly, commensurate with safety (principally in terms of access and handling) in order to minimise downtime. For these reasons back-loading mountings are often preferred, allowing cutters to be changed from behind the cutterhead rather than between the face and the front of the TBM, where additional ground support may be required.

Project delays introduced by cutter changes and additional ground control measures will be highly significant to overall costs, whatever the price of the cutters themselves. Cutter reliability is important since unplanned changes will be likely to cost more than planned ones, especially with EPB machines where cutterhead access can be an issue.

“In the hard rock of the Guadarrama project [in Spain],” reports Stein Narvestad of Split-Ring, “boring time was 42% of the TBM availability, and 20% was cutter change time. In hard rock tunnels cutter change time is usually more than 10%.”

“You can, of course, calculate machine availability with cutter change time excluded,” says Alastair Biggart, “but if you don’t set aside enough time to change cutters then you will reduce availability due to cutter breakage and the unexpected need to go into the face and change cutters. You may also, as happened on the Storebaelt project [Denmark], get to a stage where the cutterhead itself is damaged, and hence produce a very large amount of downtime with a serious impact on availability.”

Changing procedures can be a lot more difficult with closed faces, such as with EPB machines tackling some harder rock. “In EPB tunnelling you are likely to need the use of compressed air interventions if you are below the water table,” says Biggart. “Some TBM manufacturers have muck doors on the cutterhead, which arguably improve safety for personnel, but they cannot hold out water so don’t avoid the use of compressed air. Depending on the project geology it may be possible to use the ‘safe haven’ principle, and change cutters in cohesive areas. Safe havens can even be created in advance by grouting. This would save tunnelling time.”

Vittorio Guglielmetti confirms that cutter change time can be very important: “In my experience, the loss of time for changing cutters can cause real disruption to work cycles with dramatic lowering of machine performances.”

Urban sensitivity

Geodata’s experience with urban TBM tunnelling is particularly significant. “Working in sensitive areas with an EPB machine and cutterhead with disc cutters, as in Porto [Portugal] for example, the problem of changing time is of paramount importance,” he says. “On the Porto Metro it was necessary to enter the excavation chamber under air pressure every day to control and change disc cutters. That meant losing more or less a shift per day in cutterhead maintenance, due to the long time for pressurisation and depressurisation. However, it is still better to enter the face every day to check, rather than be surprised when unexpected cutter wear occurs.”

“The experience from Porto shows that it is possible, and useful, to enter the pressure chamber under compressed air to change cutters when the pressure is relatively low. In the case of higher air pressure being necessary, grouting may be required.”

A logical development of cutter replacement procedures is to replace only the cutting disc itself. This is the idea behind the Split-Ring concept, made possible by constructing the cutter ring in two halves.

The main advantage is that the cutter ring, as the main wear element of the whole cutter, can be changed quickly without disturbing the main body of the cutter and its mounting, being usually serviceable for much longer. The lighter masses involved (the heaviest part weighs 12.5kg compared to around 150kg for a full cutter) mean that the cutter ring can be replaced by one man, without lifting gear, in around 5-10 minutes.

A higher price is a factor since the cutters are claimed to be built stronger so that they can remain on the TBM for longer, and because the cutter ring must be made of high-quality steel so that they return form stability after splitting. The higher price is said to be compensated by reduced transportation and workshop costs.

Data and improvements

Assessment of disc cutter performance in the expected ground conditions is crucial for correct estimation of project costs in any rock TBM work. Major TBM manufacturers will be able to carry out this work themselves, but independent testing can also be important for project engineers and for developing new designs.

In addition to cutting performance data for specific rock types, there is a need for accumulated information on data such as wear rates and relative costs from actual projects. Such information is likely to be collected by major TBM manufacturers and contractors, but with commercial sensitivity that makes it unlikely to be available to the wider community in the short term.

It appears that product improvements are limited, with more progress possible in correct application. Alastair Biggart believes that there is limited scope for improved metallurgy, but suggests other possible improvements: “Use of unidirectional cutterheads improves muck gathering, and optimises the tool layout. Cutter monitoring should also be improved. The main advance in this will be when the disc wear can be measured remotely, and whether or not the disc is turning can also be measured remotely.”


17″ disk cutter, as used on the Porto Metro Disk cutters are refurbished at a Herrenknecht cutter shop A Robbins TBM equipped with 19″ disk cutters Geodata’s record for part of the Line C extension of the Porto Metro shows broad relationships between rock types, cutter replacement and cutterhead thrust.

(Blue = average number of discs replaced per ring; Pink = cutterhead thrust force; Green = In-sGeodata’s record for part of the Line C extension of the Porto Metro