The review of tunnel excavation methods in this and our May issue holds no claim to be comprehensive. However, we hope to have covered major issues and have highlighted recent trends in the possibilities in using TBMs, drill and blast and roadheaders, and in what circumstances they compete.
Large-section, or sequential excavation open-face methods have not been covered in this review since they can use a wide variety of excavation means such as conventional hydraulic excavators and hydraulic hammers as well as the drill and blast and roadheaders already mentioned. Such operations require integrated and careful attention to temporary as well as permanent support measures and philosophies such as NATM and Analysis of Controlled Deformation in Rocks and Soils (ADECO-RS).
Discover B2B Marketing That Performs
Combine business intelligence and editorial excellence to reach engaged professionals across 36 leading media platforms.
The advances in the development and capabilties of TBMs has made this method the ‘yardstick’ by which other methods are now measured. The promise of a factory-like means by which tunnels can be constructed has been very attractive, especially with the increase in instrumented functions.
Hard rock
Since Dick Robbin’s development of disc cutters for TBMs broke the drill and blast monopoly for hard-rock tunnelling, a major focus has been on the achievements made by hard-rock TBMs, but that is far from the whole story. Whether through familiarity, the length of drives required, rapid set-ups or lower investment, drill and blast still retains a high proportion of the market in all areas.
It is widely accepted, at least in Scandinavia, that Atlas Copco and Sandvik between them still supply around 80 per cent of the world market in hard-rock tunnelling, although on long drives the general preference must be for TBMs (see figure 1) for overall economy.
Urban challenges
With the booming market in metro systems and other urban underground excavations, the interaction between existing structures and the population on the one hand, and tunnelling operations on the other, have been increasingly important. The introduction of EPBMs, and then soil conditioning systems, has simplified tunnelling in softer ground and expanded tunnelling possibilities in urban areas and previously unsuitable ground.
US Tariffs are shifting - will you react or anticipate?
Don’t let policy changes catch you off guard. Stay proactive with real-time data and expert analysis.
By GlobalDataAsymmetrical excavations for stations and other underground space are generally unsuitable for TBMs, although the hard-rock cavern in New Work’s East Side Access project, with other methods, is a notable exception. In most circumstances, open box construction if surface features allow or sequential excavation is more typical.
A concern in many urban locations is that of vibration. Roadheaders can benefit from such concerns over drill and blast, although locations familiar with drill and blast, such as the WTC host, Helsinki, see no problems with drill and blast for this reason. Tunnellers in other areas familiar with roadheaders, such as Australia, swear by them. Some cities have banned drill and blast due to vibration concerns. This situation has been combated by manufacturers’ development of multiple detonators and more adjustable explosive charges to reduce blasting shock.
The big ones
Other major trends in the use of TBMs has been steadily increasing maximum diameters achieved and the maximum possible length of drives.
TBM manufacturers are confident of manufacturing TBMs of 20-m diameter, or perhaps more, and drives of over 25km long have already been completed. Such maxima are generally beyond the economic capabilities of other methods, even if technically feasible, although some doubt has been expressed about the desirability, in ground control terms, of very large diameter TBMs working. It remains whether the client and their consultants want excavations of these maxima. Studies have shown that large single bores are preferable in construction terms to two smaller bores.
Drill and blast, roadheaders and hydraulic excavators can be used with multiple units for large section excavations in suitable ground types, but in softer or saturated ground, substantial ground pretreatment would be required, raising cost.
Accuracy
Manufacturers of TBMs claim that the improved accuracy now possible with drill and blast and roadheaders has not affected their market, but it must have made the existing markets those techniques more secure. There is a greater demand for smooth profiles, not only to avoid overbreak and damage to the support qualities of the ground itself, but also to reduce lining costs and provide better surfaces for membrane linings.
Computerised drilling guidance, with drill pattern design input, has allowed peripheral holes to be drilled more accurately. More rigid drill booms have also reduced hole deviation. This, together with improvements in blasting agents, as mentioned above, produce a cleaner break at the designed profile.
Modern guidance systems can now be applied to roadheaders, whereas until recently the movement and vibration of the machines made accurate guidance very difficult. The high-power, heavy roadheaders made for tunnelling can generate large vibrations from slow cutter speeds, and the weight might cause them to sink in softer ground. Now the new generation of lighter yet powerful roadheaders like the Sandvik MT520 can cut harder rock to an accurate profile although not with practicality in the hardest rock.
Figure 1, graph showing generalised break even point between use of TBMs compared to drill and blast (by The Robbins Company)
