Underground drill and blast excavations are common to both civil and mining projects. In civils applications, they are used to excavate tunnels; in mining they are used to create declines and adits.

Contractors and personnel who move from one sector to the other often find difficulties adjusting to the requirements of each. This article aims to highlight the different practices that exist in both sectors, as well as their underlying logic.

ORGANISATION OF WORKS

Typically In tunnelling, single- or twin-tunnels are encountered, and usually, the distance required to travel from one portal to the other is very long, especially given the limited equipment that may be available. Therefore, work headings are limited to one or two.

To minimise costs per metre of tunnel, the works must be completed as soon as possible, and this can be achieved by minimising the time each heading stays idle. Therefore, the organisation of work at each heading is based on this principle. However, to facilitate continuous activity means machines and personnel are underutilised as the equipment and personnel required for the next activity are waiting on stand-by for the current activity to be completed. This is why as soon as the face is charged, the area is secured, and blasting takes place. Relevant health and safety risks are easily controlled because, unlike in mining, access to the heading is via a unitary portal, and ventilation of the heading does not affect the other heading.

In mining, where there are many cross-cuts to check, blasting should take place at the end of the shift, as underground openings are more complex, the distance to evacuate the affected area is long, there are exploration holes intersecting the headings, and the ventilation is shared between headings. That is another reason why mining involves multiple headings.

Planning the work ensures that there is always a surplus of headings available to work on. In mining, the organisation of work is based on the principle that resources should never be idle. In this way, one can have more metres excavated with the same resources and the cost per metre of the tunnel is kept as low as possible.

The efficient utilisation of resources, along with the requirement to achieve the monthly plan, requires agile planning – usually performed in the control room.

CONVERGENCE CONTROL

NATM (New Austrian Tunnelling Method) is used in tunnelling; drill and blast in mining. In this author’s opinion, the difference between the two methods is the importance of convergence.

Typically, tunnelling involves large cross-sections and concrete linings. But the lining cannot be constructed if the convergence is not stabilised by rock support. The supported rock after convergence should be outside the concrete lining. The total convergence determines the reinforcement of the lining. The philosophy of NATM is to forecast the convergence and adjust the support measures to control it. This requires extensive monitoring of the convergence, usually with optical targets and total stations.

In mining, the size of excavation is usually relatively small and there is no lining. Convergence is not monitored, as minor convergence does not affect the flow of equipment traffic. If additional support is required, it can be highlighted by cracks in the shotcrete or by the bent plates of rock bolts.

SUPPORT REQUIREMENTS

Support requirements for both tunnelling and mining can be addressed by the same design described in ‘Using the Q-system, rock mass classification and support design”, May 2015, NGI’.

Typically, tunnelling involves longer spans, higher excavation support ratios (ESR) and lower stress reduction factors (SRF). To briefly explain the ESR, an adit that will be used for a couple of months by skilled miners will require less support than a tunnel to be used for decades by the public.

Road and rail tunnels have final concrete linings. To avoid ground water ingress, a membrane is placed between the rock and the lining. This membrane should be protected from puncture by sharp rock edges and extruding rock bolts. This is why rock surfaces are sprayed with shotcrete to smooth irregularities and cover extruding rock-bolt tails, even if not necessary for the purposes of rock support. Having become accustomed to working for many years exclusively under shotcrete, tunnelling personnel may find it stressful to work under rock that is supported only by bolts.

Roof collapses, although never desirable, can happen in both tunnelling and mining. Collapse rehabilitation is very costly and seriously effects time schedules. In tunnelling, collapse rehabilitation is the only way to complete a project. In mining, if a collapse hampers rehabilitation efforts, the alignment of the decline can be changed to avoid the collapsed area, or a drift can be abandoned if the cost of rehabilitation is higher than the value of the ore to be recovered.

OVERBREAK CONTROL

Many tunnels have concrete linings of specific thickness. Therefore, after completion of excavation and support, neither rock nor shotcrete should intrude into the lining area. Any overbreak must be filled with concrete during lining, which can make overbreak a very expensive phenomenon. The need to control undercuts and overbreak require very accurate drilling of contour holes, both as regards hole position and direction/inclination.

In mining, the cost of overbreak is mainly limited to the cost of hauling the excess spoil to the surface. Therefore, controlling overbreak in mining is desirable but not as critical as in tunnelling. Contractors and jumbo operators experienced in mining often have difficulty adjusting to tunnelling and unintentionally cause excessive cost.

Another factor that affects overbreak is the pull length of the blast. The larger the blast length, the faster the advance rate of the excavation and therefore, the lower the cost per metre. However, long pull-lengths often cause overbreak due to falling wedges, or due to the poor rock mass that may exist between the face and the supported area.

In tunnelling, the cost of concrete to fill any overbreak is taken into account, therefore, tunnellers should limit the blast length to control the overbreak, according to the rock mass quality.

In mining, the overbreak tends to be not very significant, therefore blast length is made as long as possible. The only considerations are the ability to charge long blast holes and the risk of unsupported ground collapsing.

EQUIPMENT SELECTION

Typically, the cross section of tunnels is larger than the cross section of declines in mining. Therefore, equipment used for tunnelling tends to be bigger than equipment used for mining. In mining, the size of adits is dictated by the size and shape of the ore body, to reduce dilution and increase ore recovery.

In this author’s opinion, there is also a difference in mindset when it comes to equipment selection. Tunnelling typically uses the largest available equipment that can fit in the tunnel: the larger the machine, the more powerful and productive it can be. Therefore, with larger equipment, faster completion of the tunnel is possible, minimising costs and increasing profits.

In some cases, this author has noticed that mining projects use the smallest machine that can do the job, in the belief that it reduces both the mine’s CAPEX and its installed electrical power. But this can be a shortsighted approach because it can reduce productivity, increase the cost per ton of ore, and reduce the profitability of the mine.

The installation of rock bolts using jumbos or bolters is another difference between the two sectors, and causes much controversy in mining. The excavation cycle broadly comprises face drilling, charging, blasting, ventilation and mucking, scaling, shotcreting (if required) and bolting. The use of a bolter could be safer and could require less manpower. However, after the completion of bolting, the bolter should leave the face to allow the jumbo to enter. If excavation support is carried out using a jumbo, this interchange of equipment is not needed, and allows each excavation cycle time to be reduced by up to half an hour.

Additionally, in tunnelling, where headings are limited, there will be large underutilisation of the jumbo and bolter. For this reason, rock-bolt installation in tunnelling is carried out by jumbo. In mining, considering the multiple headings available for excavation, the impact on equipment use and advance rates is minimal. In mines that are adequately mechanised to allow one operator per machine, the bolter is often the best option to reduce mining cost.

Another difference between mining and tunnelling is the system used for mucking after blasting.

In tunnelling, mucking is from the heading to the dumping area outside the tunnel. The reason is that the large cross-section of the tunnel allows loading of trucks at the heading and that the hauling distance is small, so a limited number of trucks can do the job effectively. The loader is loading the trucks at the heading and there is usually an adequate number of trucks, which means that the loader is not waiting for trucks to arrive. In this case, the selection of loader is not the largest that can fit in the tunnel, but rather small enough to load the large-size trucks at the face.

In mining, the mucking is from the face to the remuck bay. This is because there is not enough space at the face to load trucks with the loader, and the distance to the dumping area is long. Mucking is achieved using the biggest loader that can fit into the adit, to a re-muck bay, where the intersection has adequate height for the loading process. After mucking is completed, the face is available for the subsequent activity, and at the same time, the muck is gradually loaded onto trucks and hauled to the surface.

CONCLUSION

The core knowledge of drilling and blasting underground is common to both mining and tunnelling. However, there are significant differences that can determine the efficiency achieved in either field. This article has aimed to share the author’s experience and knowledge, and could be of use to underground professionals in both civil and mining sectors. As always, readers are advised to use their judgment to ascertain whether the above is suitable for their purposes.