Drill and blast is a time-tested method of excavation. The basic principle of cutting a hole, filling it with explosive, plugging and detonating, has remained steadfast as the equipment, the materials and the projects have become ever more sophisticated and ambitious. None more so than the drill rig. A replacement for the hammer and rod that was used to cut holes into rock faces in the early years of deep mining, the drill rig is now a complex, intelligent and agile machine that can manipulate the excavation cycle, reacting quickly to the changing environment to speed advance rates, minimise draws and ultimately reduce cost.

The agility of the drill rig is pitched against the brute strength of the hard rock TBM in the intensifying battle of methods. However, the low cost of equipment and the manufacturers’ unrelenting drive for improvement is helping drill and blast hold its position as the dominant technique for hard rock tunnelling.

Drill and blast, often referred to as conventional tunnelling, is able to deal with complex shapes and much closer proximities to building than alternative methods, "The tunnel does not need to be round," says Wilhelm Papst of Sandvik’s mechanical cutting division. "There are many underground possibilities. Highway, railway and metro tunnels at the traditional end of the market, and less conventional are storage facilities, shopping centres, even ice hockey arenas, of all kinds of shapes."

Papst explains that five years ago the technology was not able to handle these complex jobs, but developments in both explosives and data and computing systems have transformed the playing field.

This flexibility and its lower power consumption helps keep a steady demand for the method, "it requires less energy to blast rock than it does to cut it," says Papst.

The continuing research and development of drill rigs has focused on improving a few key factors: increasing the speed and reliability of hydraulic drills for increased productivity and penetration; improving accuracy through rig structural changes such as more rigid booms; improving accuracy through electronic guidance and drill-pattern systems, leading to less overbreak; and developing instrumentation for the drilling process, which can be used for more automatic control, and input to blast design.

Automation
For this conventional tunnelling method to develop and secure the most complex tunnelling projects the equipment needed to become less reliant on the operator.

The blast patterns required to maximise the draw from a single blast and minimise the impact on existing infrastructure has called for laser guided drilling methods, accurate data gathering and sophisticated software to make sense of it all.

"The level of automation has improved significantly," says Papst.

iSure (Intelligent Sandvik Underground Rock Excavation) software is a new tool for managing tunnel or underground excavation projects. With it, the drilling and blasting design takes place in the blast plane.

This way, parameterisation of the drill holes, and burden calculation can be used for optimising the locations of the holes. The explosives used in different parts of the pattern are also specified as the degree of charge and the relative strength of the explosive are used for calculation purposes during the design process.

Based on this information, the total consumption of the explosives per round, and for example charge detonating at specific time (per specific delay), can be illustrated.

Another of the features being introduced with modern drill rig software is the ability to include detonators and group or surface delay detonators in the design process.

The software enables real time monitoring of momentary situation as the design advances. If selected, information is available on the real delay times with or without the extra surface delay; the number of detonators initiating at the specified delay time, and the amount of explosives initiating simultaneously.

As the excavation advances, the designer can revise the vibration measurement results and go back to the drilling and blasting pattern design to trace the cause of increased vibrations and make modifications as required.

In addition to the real time momentary illustration, the design process is made easier for the user by blast simulation. Simultaneous detonators on a specified delay time can be highlighted, while the already initiated delays are displayed faintly.

The spice of life
The customising of the drill rig to suit the job will bring the best results in performance and project delivery. The comparatively low cost of drill rigs means the project will usually have greater freedom to order a bespoke jumbo.

"Put simply, for each of the different models there are a lot of packages and options tailored for the job. There is a range of computerised jumbos, particularly designed for tunnel construction. Then there is a range of jumbos that are not computerised, a more conventional setup – and there’s a bigger number of these."

"We have a range of jumbos starting from one boom jumbos, which are particularly used in mining and small tunnels, to two or even three booms with coverage of more than 200sqm. Our DT line is targeted at the drilling industry and our DD line is aimed at miners."

Across all lines the development driver has been increasing drilling accuracy, "mainly due to the cost of excavating the tunnel," says Papst. "There’s always a minimum shape of the tunnel that is defined by the client, over excavation will drive up the cost of tunnelling. In many cases, a tunnel needs to be concrete lined.

So if you’re not accurate, and you over excavate there will be costs associated with filling that over excavated space with concrete, which is expensive. It’s a double whammy if you don’t do it correctly."

Large cost savings are possible by achieving a smooth profile by drill and blast, chiefly through rig data controls.

An example project called for a 770m-long rock cavern top heading with planned sectional dimension of 10m x 20m (approximately 153m2), thus requiring an excavation of 118,000m3 of solids. With minor additional drilling costs, some EUR 720,000 (USD 10.5M) in savings were achieved in the extra concrete that would have been used to fill overbreaks.

Savings were also made from reduced sprayed concrete use amounting to EUR 103,000 (USD 150k), and also in mucking out and explosives, further totaling around EUR 973k (USD 1.41M). The work was also completed nearly a month earlier than otherwise.

While rigs without the sophisticated computer systems still lead the market, the trend is towards great automation. Europe has lead the way and computerised jumbos are prevalent on drill and blast job site. But developing countries are considerably behind and may be some way from using the latest technology: "The sale of computerised jumbos will increase," says Papst, "But a lot of tunnels are still done with hand held drills – in many developing countries mechanised drilling is not used yet. Therefore, mechanised jumbos with fewer features will replace hand held operations initially.

Afterwards, moving into the future, more technologically advanced solutions will be introduced." The temptation to produce lower quality drill rigs for the developing market has been avoided.

Papst explains, "Even though it might seem that the market cannot justify all the safety features from a business point of view, we cannot sacrifice the safety of the operators by trying to cut corners."

He says that when designing the equipment "safety is our philosophy, we need to meet the strictest standards. It might sound unnecessary for the global market to apply a standard enforced in just one country or even just one state, but in the end, safety must take the lead."

Papst adds, "We often have the same contractors operating all over the world, and they want to have the same equipment wherever they are operating."

When to drill and blast
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, whether in general or related to specific project conditions.

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. Some would even prefer two, two-boom rigs.

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.

Although still at early stages of introduction, measurement-while-drilling (MWD) results can provide data to determine the most appropriate hole 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 to 2m in fair rock and 2m 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 average advance rates in various rock conditions are seen in Table 1, above.