At a time when you might think the technology should be in maturity, it is still being developed in many respects. This is a remarkable testament in itself to the efforts of drill-andblast engineers in meeting the competition with mechanical excavation in advance rates, as well as increasing environmental concerns. It also shows that a wellmanaged cyclical process can compete with a more continuous one in many circumstances.

Starting point
Jorma Kalliomaki of Sandvik Mining and Construction says that the starting point for the most economical tunnelling method selection has to be the ground or rock condition (see figure 1, right), especially in terms of strength and stability, but also abrasivity. The size of the project, in all dimensions and locations, is also a crucial factor. The diagram indicates the capabilities of and relationships between three popular excavation methods for medium-to-hard rock. Drill and blast has a wide possible application in harder rock in various states of stability. Despite improvements in roadheader capabilities the diagram still shows a major advantage in hard rock.

Use of hydraulic hammers is only really effective in looser, fractured rock, but security considerations about explosives in some countries has prompted their use in preference to blasting. They can also be useful for small sections such as pumping chambers and sub-stations.

Once the most economical method has been chosen, it may be considered that the project schedule, such as related to financial return on investment and cashflow, may require a more productive method, if any, that may be less economical.

Drill and blast can be applied to a wide range of dimensions, depending on the size of the drilling equipment chosen. A limit for mechanised drilling at the lower end is determined by safety considerations when working around machinery for drilling and loading out in a narrow space. Thus the smallest possible mechanised drill and blast tunnel is about 2.5m by 2.5m (although maybe non-mechanised drilling could be used).

The largest section that can be covered by a drill-rig from a single lateral position is about 200m2, although rigs could be used in parallel or in sequential excavation. Whether a large number of booms (up to four are available on mobile rigs) is desirable is a subject of much debate between contractors. Clearly an efficient computerised operational system can make the best use of the maximum number of drills, but some contractors question whether a fourth produces much advantage in most tunnelling.

Rock condition is an important consideration in deciding whether or not to use drill and blast since movement on joint planes, voids, and even changes in rock hardness can seriously affect drilling and blasting efficiency. Modern drilling control systems can be set up to automatically adjust for some unusual boring progress to avoid stuck bits and broken drill steel, pulling back slightly when difficulties are encountered. MWD (measurement-whiledrilling) results can be used to provide data to determine charging.

Another way of coping with poor ground in drill and blast is to reduce the drill round length. In good rock conditions the maximum is about 6m as the longest drill steel are 21ft (6.44m) long. Round lengths are progressively reduced for poorer rock from 5.1-4.6m in good rock, 4.0-2.0m in fair rock and 2.0m or less in poor rock.

Round shortening is mainly a matter of reducing the length of initially exposed ground if the rock is poor, to maintain natural support. However, it follows that the shorter the hole drilled, the less likelihood there is of drilling problems in one hole due to ground instability. Shorter rounds tend to increase cycle times and therefore tunnelling progress. This is also affected by extra time for additional ground support unless, perhaps, it is possible for one drill rig to work on more than one face, depending on the tunnel project layout. This may not be so much of a problem with a TBM unless the ground is blocky.

The effects of ground conditions on drill and blast in various rocks are in Table 2.

Saving by accuracy
It has long been recognised that accuracy could be a major issue for drill and blast, and manufacturers have been working on ways to improve it (see feature, p50). Good blast design, combined with drill patterns, is important to avoid overbreak and other disturbance of the surrounding ground.

With good guidance, profile accuracy should be inherent in TBM tunnelling and, with electronic profile guidance, also be easy to follow with roadheaders. As such support costs should be quite predictable with these methods unless the ground is unexpectedly poor.

Large cost savings are possible by achieving a smooth blast profile chiefly through rig data controls. An example offered by Sandvik was for a 770m-long rock cavern top heading with planned sectional dimension of 10m by 20m (approx. 153m2), thus requiring an excavation of 118,000m3 solid. With a small extra drilling cost, savings were achieved in concrete to fill overbreaks (EUR 720,720 – USD1.05M), and in lower sprayed concrete use (EUR 103,448 – USD 150,129), but also in mucking out and explosives, to total EUR 972,868 (USD 1.41M). The same work was also completed nearly a month early.

Blast improvements
Not all improvements in the drill and blast process are to do with drilling. Leading explosives companies have put a lot of work into blast design and initiation techniques to reduced vibration, as a major example. Vibration is often mentioned as a problem with drill and blast, although it does seem to be a ‘topical concern’ as regards to underground excavation in urban areas. This does, however, ignore the many urban tunnels that have been completed successfully in the past with few problems.

Jorma Kalliomaki cites the example of Helsinki, where the World Tunnel Congress will be held this month, where a city built on hard rock has 500km of tunnels beneath it, all excavated by drill and blast.

The use of bulk explosives for easier handling and charging has also become more refined. The use of explosive materials that are not explosive until charged have improved the safety and security situation.

Preference for safety
You might think that, with the destructive nature of blasting, and the need for miners to work at the face on frequent occasions, that the process is inherently hazardous. But, with ingrained safety conscience among experienced miners, established safe methods of working, plus good training and supervision, there is no reason why safety should not be maintained.

Personal operator and contractor preference can play a big part in the choice of excavation method and equipment. This should not be discounted since familiarity with equipment can greatly increase productivity. On the other hand it should not be a block to taking on new methods and technology, or there would be no progress.

Figure 1, Effect of rock condition on capabilities of three major excavation methods Figure 2, section coverage possible from one position by a Sandvik DT1230i drill-rig. All measurements are in mm. Table 1, selection criteria for drill and blast compared to roadheaders and hammer excavation (Sandvik) Table 2, average advance rates for single- or multiple-face operation of blasthole drilling