While there have been major developments in the capabiltiies of all major tunnel excavation methods in recent years, it is TBMs that have led the drive for more efficient tunnelling, at least of long and/or large diameter bores. They have also improved the security of tunnelling in urban environments where ground control is a particular concern. TBMs hold the promise of an ‘industrial’ process for constructing a tunnel even though, in most cases, the process is still cyclical rather than continuous.

Size matters
The maximum diameter records for TBMs, both planned and actually constructed, have been broken regularly in recent years, albeit by small increments. Herrenknecht leads the way in most cases with the Fourth Elbe Tunnel (14.65m), Madrid M-30 (15.20m), Shanghai (15.43m), and now the Sparvo Tunnel in Italy (15.62m), but also Robbins for hard-rock in the Niagara Tunnel hydropower drives with the TBM ‘Big Becky’ (14.4m diameter), that recently broke through on its final drive, and NFM (14.87m o.d.) for the Groene Hart high-speed rail tunnel in The Netherlands a few years ago.

Hitachi Zosen’s largest diameter TBM to date, at 14.14m, is the slurry-shield machine employed by a JV of Kumagai Gumi, Hazama and JDC for the Trans-Tokyo Bay Highway project on the Kawasaki Tunnel Ukishima North Phase One. Previously the largest EPB TBM to have worked in Japan was of 13.6m diameter, having recently completed excavation of the Central Circular Shinagawa Ohi highway tunnel.

Despite suspicions amongst many that the increasing size of TBMs was just a matter of ‘one-up-manship’, the larger diameters can be easily justified if the capacity is required, as it often is for busy road tunnels. The usual philosophy is that construction of one large tunnel bore is more economical than two smaller ones although, as ever, site conditions might dictate otherwise. Now the Alaskan Way Viaduct Replacement Project is due to use the largest diameter TBM to be built yet, at 16.5-17.0m, following studies of the twin-bore alternative as well. Selection criteria over side-by-side twin bores included lower purchase cost, a smaller operating team, a narrower settlement corridor and escape arrangements.

It follows that the manufacturer has to have the facilities to manufacture and transport such monster TBMs, usually involving cavernous workshops, which have often been in short supply, and carefully planned logistics to reach the tunnel launch site.

Robbins president Lok Home confirms that logistics for site delivery can be a factor limiting TBM size, as can the availability of large machine tools to machine the large components of a TBM. ‘Despite these challenges diameters of up to 20m are feasible today, with larger diameters possible in the next 10-20 years.”

Italian contractor and manufacturer Seli entered in a five-year co-operation agreement with Japanese manufacturer Kawasaki in April to develop earth-pressure-balance TBMs further, and their overall market. Seli president Remo Grandori also believes that TBMs of up to 20m diameter are mechanically feasible today. Although some proponents of drill-and-blast have questioned the desirability of such large machines, particularly in mixed ground, Grandori says, “TBMs for large diameters are always better than conventional methods for tunnels in soil and/or weak rock. In hard, but not abrasive rocks like limestones, the convenience of large diameter TBMs depends on the length of the tunnel, lining, and design requirements. For hard and abrasive rock formations like granitic rock, normally conventional excavation is preferable for large diameter tunnels.”

And shape
It most cases the use of TBMs is synonymous with achieving circular bores, but non-circular bores are possible with similar machines as has been the case in Japan for many years. Such special designs are still in active use there to meet special requirements. Risa Hirano of Hitachi Zosen points out that the spoil excavated by a special-shape ‘TBM’ can be less than that of a single, circular TBM (as shown in figure 1). Such special TBMs can also be used to avoid the numerous underground obstacles (see below), where a circular TBM would not fit. It is usual in Japan to only use TBMs under public space and not private space, particularly under structures, further restricting easement.

It is also worth noting that circular TBMs have been used to carry out the bulk of excavation in larger, non-circular sections such as the hard-rock cavern in New York’s East Side Access Project under Manhattan.

Long drives
It is generally accepted, even by proponents of drill and blast (D&B), that TBMs have a great advantage over D&B when the project has long tunnel drives – variously described as over 2500-3000m depending on other conditions. If the project consists of one heading the bias is also towards a TBM as the D&B process is more cyclical. In general the faster speed of TBM progress outweighs the longer set-up time for the TBM once the drive, including any pull-throughs, is over the stated length.

According to Lok Home there are also other major criteria to be considered. “The project schedule must be able to accept the longer TBM delivery period, compared to drill and blast equipment.”

Substantial savings can be made by reducing or eliminating set-up times so as this part of the process can be more competitive with D&B, increasing the theoretical advantage of TBMs. If the topography allows, it is preferable to launch the TBM from a fairly level surface straight through a portal into the drive, such as through a mountain. If launch and/or reception shafts are necessary, this will increase the time required for set up due to the restricted space available. Robbins, for example, has made efforts to reduce set-up times with its Onsite First Time Assembly (OFTA) procedure as used in Mexico City and Andhra Pradesh.

Another approach, facilitated by the accuracy of modern directional monitoring and guidance systems, is to eliminate shafts with planned vertical curves. Such approaches were pioneered for microtunnelling and other small-bore systems, following the example of directional drilling/guided boring, but now the principle has begun to be applied to larger and larger diameters, employing special steel structures to achieve the initial thrust reaction necessary such as with the Herrenknecht TBMs in St Petersburg driving metro connections at steep grades.

Hitachi Zosen cites the example of the so-called URUP method employed by a JV of contractor Obayashi and Jinno for an undersea gas pipeline conduit for Chubu Gas. It employed a Hitachi Zosen EPBM with a 2.13m cutterhead for a 1.1km-long drive. No shafts are necessary in this procedure, the first drive of which was completed a few months ago.

Once the set-up has been made, the maximum length of a single drive is generally well over most current needs. Remo Grandori, president of Italian engineers Seli, reports, “The maximum length of tunnel we have excavated from one portal is 25km, and we could have continued for a few kilometres more. Normally the limiting or critical factors, in maximum length, are ventilation and transport systems for spoil and personnel. The tunnel diameter therefore has an influence since, in smaller tunnels, it is difficult to fit in a suitable ventilation system and transport system. In general the ideal diameters to bore long distance are between five and six metres.”

“For greater lengths than 25km, or with smaller tunnels, intermediate shafts can help with ventilation and/or mucking out.”

Similarly Home reports, “We have tunnels currently under construction that are over 25km in length, double this distance could certainly be feasibile.”

These statements concentrate on technical capabilities whereas the limiting factor is usually running cost. Home says, “After 10,000m, the costs per metre goes up as amortisation of the basic machine is no longer the driving cost factor. Other factors come into play at this point, such as labour-related costs to get to the machine (mainly muck haulage over a long distance and adequate ventilation). Long distance tunnels also mean more risk because less geological knowledge is typically available over the entire length.”

Hard and soft
Several experts have remarked that excavation choice ‘all starts with the ground conditions’. While TBMs, in general, have become better at tackling a wide range of ground conditions, mixed ground still presents the most difficult challenges. Mixed ground, particularly with hard inclusions, could cause havoc with excess wear and other damage to cutters if they are not designed for it, thus causing delays and extra running costs even with backload changing possible.

Expert Lok Home of Robbins claims, ‘A mixed face isn’t really problematic for disc cutters. Today’s high quality designs are capable of excavation in a wide range of conditions, so the percentage of rock in the tunnel face is not problematic.

Grandori commented, “Large-diameter cutters with stronger bearings and discs can handle mixed faces and hard-rock intrusions without problems. Of course the TBM in these conditions must be operated properly.”

At the extremes, Home, for example, claims, “There is no strong, hard rock that a TBM cannot excavate.” He adds, “In unstable squeezing ground TBMs need design improvements and integration of NATM technology to be competitive.”

Performance
The main advantage of TBMs is speed, once set up, so what can be expected? Remo Grandori of Seli says, “In (ideal) rock the record advance rates we have performed for excavation and precast lining are 2500m in one month (30 days) and 110m in one day. In soft/EPB TBM operation the records we achieved are in the range of 22 rings (33m) per day. A rock TBM should normally perform better than 20m/day to ‘overkill’ drill and blast.”

Precision
Highly sophisticated, laser-based, directional monitoring and guidance systems are now widely available for all types of TBM. Not only are these used to ensure precise direction in all planes, including around curves, but also to optimise installation of pre-cast segmental support rings. Their integration into the overall TBM control system aids ground control.

Even with all this sophisticated instrumentation and guidance systems, the ground properties can still make guidance difficult, especially if very mixed or soft. Risa Hirano of Hitachi Zosen says, “It is popular in Japan to excavate tight curves with small/middle-size TBMs. Contractors have to do grouting [to make soil solid] on the external side of the curve when going round it in very soft ground [see figure 2]. If they do not grout the external side of the curve the TBM will go straight, not making the turn.”

Good neighbours
There are many factors affecting whether a tunnel excavation method is suitable for urban environments including, primarily, settlement control, as well as noise/vibration, spoil transport, supplies delivery, services and road diversions, and rights of way.

TBMs offer many advantages under cities, at least in theory, helping to increase the amount of tunnelling under densly populated areas, particularly in difficult ground, but ground control is always of particular concern, especially to avoid differential settlement that may damage adjacent and overlying structures. TBMs offer the prospect of more accurate control of excavation rates, which is now beginning to be linked with the geotechnical instrumented monitoring of structures. Such control can also be of benefit in non-urban situations, particularly where the ground structure is understood to be potentially unstable. Developments in the use of EPBMs, especially the use of additives for soil conditioning, have greatly improved ground control measures and the range of ground that can be tackled more simply. That is not to say that EPBMs do not still need skilled operation and application expertise.

Other ground support and monitoring systems can be used in conjunction with TBMs, even though a TBM can be an integrated system itself. “Face support, ground conditioning, backfill grouting and monitoring…all these technologies are improved year by year by the industry and today technology is well advanced,” say’s Seli’s Remo Grandori.

Of course accurate monitoring of settlement is not just a matter of checking on its effects but also the usual cause – over excavation – including the measurement of material removed by hard rock and EPBMs. “The most commonly used systems are measuring devices that calculate the volume of muck removed,” says Home. “These include radar monitoring of the the muck profile, and belt scales. Radar devices ‘fire’ down on the belt over a certain area, measuring the height of the muck. They then calculate the volume of muck being moved out versus the TBM advance rate in order to prevent voids. The radar method is not as useful for slurry machines because useful data cannot be generated when the radar passes through the slurry material.”

But, predicting future performance in the ground is not so easy. Home adds, “As an industry we haven’t advanced very far in terms of good monitoring or predicting ground conditions in advance of the face. The primary and best method is still probe drilling. This and pre-grouting are absolutely recommended and, in most case, necessary. Probing and grouting are becoming integrated into TBM design, which allows for ground consolidation ahead of the machine. New systems such as the McNally Ground Support System utilise steel or wood slats to hold disturbed rock in place, with the result of vastly increased safety.”

In hard ground under built-up areas, where ground control is unlikely to be so much of a problem, TBMs can still have some advantages over the main competition of roadheaders and drill and blast. “Roadheaders have their limits in hard rock,” says Home, “Particularly above 100 MPa UCS, when advance rates tend to drop off dramatically. D&B is restricted in many urban areas because of regulations on the use of explosives. In general TBMs are more cost effective in urban environments,” he claims, “Although it depends somewhat on the individual project conditions.”

Urban environments are often crowded underground as well as on the surface. This can be a particular problem in Japan affecting passage rights.

Hirano points out that there are a lot of obstacles including pipes, other tunnels and piles of various types. “If the government cannot acquire enough cross section for a single, circular TBM avoiding such obstacles, they could use an odd-shaped such as horseshoe section, rectangular or twin-circular.”

Ground control extras
Where sufficient ground support cannot be assured by normal TBM control methods, such as face pressure counterbalance, excavation volume control and designed lining, design engineers may insist on other measures that could necessitate modifications to the TBM to create necessary space for, typically, probe drills, rock-bolting rigs, grouting and sprayed concrete systems.

Opinion seems divided on the need for such measures but obviously a lot depends on the ground conditions anticipated and/or encountered, especially such features as cavities, flowing ground and high groundwater pressures. Even so, such additional equipment may not be enough.

Grandori states, “A TBM should be equipped with probe drilling and grouting facilities but this is not sufficient to overcome all adverse geological conditions. New TBM designs and extended pre-treatment facilities have been developed, and are being continuously developed by TBM manufacturers including Seli.”

Hirano reports that probe drilling is not so popular in Japan, partly because the government and contractors already know well what obstacles are in the ground before starting construction. Also the commonly used small-diameter TBMs do not have enough room for probe drills to be built in the TBM assembly. If unforeseen obstacles are encountered, it may be necessary to disassemble the interior of the smaller diameter TBMs to make room for probe drilling.

As for grouting, contractors in Japan sometimes do separate operations from the surface, including the improvement of ground properties for better TBM control, as mentioned previously.

The cost
However efficient a TBM might be in mechanical terms, the project budget must be able to handle the cost of the TBM selected, but, as stated earlier, the faster progress of TBMs can compensate for this over longer drives in project amortization and cost per metre.

Home says, “The cost of TBM boring per metre must be considered in terms of the faster speed, less ground support necessary, and better physical structure compared to drill and blast; all of which ultimately makes the TBM method more cost effective in longer tunnels.”

Post-project second-hand value or manufacturer buy-back options might also help, but often the machine is written off against the project. This may include the TBM shield, at least, being ‘sacrificed’ in the ground, either sealed off or used as part of support design.

Overall the cost of the TBM(s) is a vital matter for negotiation in the financial as well as the physical success of a project.

Many suppliers are also experts in refurbishing previously used TBMs to reduce costs.

Seli has been a leader in refurbishment. President Remo Grandori reckons that TBMs tend to depreciate 70-80 per cent on the first project. If refurbished for a second project costs for this, he says, vary between 15 and 20 per cent of the cost new, if no modifications are required.

Whilst price can be forecast, it is virtually impossible to predict running costs without reference to particular project conditions. In addition to amortisation, running costs that must be considered include power, lubricants, cutters, service parts, other consumables and manpower costs. However, more generally, cost per metre rise with drive length and can be a limiting factor as stated previously.


Figure 1, comparison of twin ‘binocular’ mechanical excavation with a single, circular TBM bore [Hitachi Zosen] Hitachi Zosen’s largest TBM is a slurry-shield machine for the Trans-Tokyo Bay (TTB) Highway project, Kawasaki Tunnel Ukishima North Phase One. Breakthrough on the Brenner base tunnel pilot bore (10.5km long) by the ATB Consortium using a 6.3m-diameter Aker Wirth double-shield TBM Figure 2, it may be necessary to grout the outside of a curve to provide enough resistance to enable the TBM to follow a curve [Hitachi Zosen] Probe drilling within a Robbins double-shield on the Pula Subbaiah Veligonds water transfer project in Andhra Pradesh, India This Seli double-shield is designed for tough duties in squeezing ground for HCC on the 14km-long Kishanganga hydro project in northern Kashmir