Selection of excavation method is one of the most complicated and vital decisions in tunnel construction planning. It cannot even be considered in isolation since there are also complex relationships with other functions and factors such as structure – support introduced and natural – groundwater, surface settlement, safety, and useful tunnel life.

Developments over recent years in tunnel excavation methods have greatly increased the capabilities of tunnelling overall, but they have rarely simplified choice, depending on project conditions.

While there are enough considerations to fill a small library, modern developments have been focussed on important factors such as:
1) Tunnel sectional area and shape;
2) Tunnel length in a single drive;
3) Progress rates;
4) Accuracy of alignment and profile;
5) Disturbance of the surface, such as settlement and vibration;
6) Handling a variety of ground conditions;
7) Presence of groundwater;
8) Project cost control;
9) Operational safety.

The major methods that have been tackling these requirements are:
1) Drill-and-blast;
2) TBMs;
3) Roadheaders;
4) Open-face conventional mining.

In this two-part review T&TI is concentrating on the first three, since efficient conventional mining is mainly a matter of good support procedures, with excavation carried out by normal earthmoving techniques. Some more specialised plant models have been developed for tunnelling, based on standard models, to cope with restricted space for manoeuvring and the harsher environment, but their use has been limited. The broad range of possibilities within conventional excavation includes hydraulic excavators and hammers.

The second part will cover some of the factors involved in choosing the TBM (fullface mechanised excavation) method, and the choice of TBM type, and try to draw some conclusions. This month we cover drill and blast and roadheaders.

Bigger and better?
Some technical developments have intensified competitive claims between promoters of the methods involved. For example, now could be described as the age of the monster TBM as manufacturers, and clients, compete to ‘go bigger’ more often, whereas project developers were formerly satisfied with multiple tubes of more standard size to accommodate needs. The chief users of the largest diameter TBMs are multi-lane highway projects, especially with accommodation for escape routes, but they have also been used for water projects.

Fans of drill and blast and conventional excavation have questioned the desirability of a large-diameter closed-face system in mixed ground, especially when ‘something goes wrong’ and face intervention is needed, citing the advantages of being able to see what is going on. Away from the face the comparatively systematic support procedures of TBMs in erecting the tunnel structure have to be compared with the usually more tailor-made (and thus more human-error-prone) methods of drill and blast and other open-face methods.

Ground control is the main concern when considering interaction with the surface, but excavation vibration, mainly in urban areas, is increasingly a ‘battleground’ between the main excavation methods. Naturally it is also of public concern although its affects can often be exaggerated.

Continuous mechanical excavation, as with TBMs or roadheaders, should produce relatively low vibration – arguably much less than the public perception of how they may be affected, unless a rock tunnel is particularly shallow. The vibrations generated by blasting can be large, but explosives manufacturers and consultants have been making major efforts to reduce vibration measured at the surface.

There is a tendency for all three main types of tunnel excavation equipment to be equipped with additional functions, chiefly for additional support needs or probing, especially for rock bolting, grouting and spraying concrete.

Other than a simple reference laser beam, more complex laser guidance used to be the reserve of TBMs, but now blast-drill jumbos and roadheaders can be equipped with systems that not only ensure accurate alignment of excavation progress but incorporate pre-programmed profiles that assist operators in avoiding over- or underbreak. Blast hole rig systems include pre-designed drill patterns that can, if required, be employed as the basis for practically fully automatic operation.

Both drill-rig and roadheader profile controls may also incorporate geological information, which is likely to be more useful for drill-rig operators to predict drilling performance and blast requirements. While all main excavation methods benefit from consistency in geology, both longitudinally and across the face, advances in the use of earthpressure- balance TBMs (use of additives, better control systems and increased tolerance of higher groundwater pressures) have made this method particularly tolerant of mixed ground, though not infallible.

Flexible shape
With the exception of certain designs produced for conditions in Japan, reportedly prompted by the high costs of surface disruption, land values and the reaming head produced by Aker Wirth, TBMs are intolerant of changed section shape, meaning that junctions, stations, cross-passages, etc have to be excavated separately. Drill and blast and roadheaders are relatively adaptable to changed sections and ancillary work, although the bigger the equipment, the less this is so. There is a natural balance between flexibility and potential productivity in method choice.

Cost vs. distance
It’s a well-established principle that the financial investment in tunnel excavation plant must suit the length of the tunnels to be driven. Thus, although TBMs are generally the more expensive, the project progress achievable makes the investment worthwhile if there are time restrictions on project completion. It is generally accepted that drill and blast cannot compete with TBMs for drives longer than 3km.

The relative competitive positions of drill and blast and roadheaders are closer, although the latter can offer an almost continuous process whereas drill and blast is cyclical by nature.