Tunneling VS Mining

Introduction

Underground drill and blast excavations can be found in both Civil and Mining. In Civil they are called tunnels and in Mining they are called declines or adits.

Contractors and personnel that move from one sector to the other, often find difficulties adjusting to the requirements of each sector. The purpose of this article is to highlight the different practices that exist in tunneling and in mining, as well as their underlying reasons.

1.      Organization of the works

In tunneling, we encounter single tunnel or twin tunnels and most of the times the distance to travel from one portal to the other is very long to utilize the same resources. Therefore, the work headings are limited to one or two.

To minimize the cost per linear meter of tunnel, the tunnel must be completed as soon as possible, and this can be achieved by eliminating the time that each heading stays idle. To achieve that, the equipment and personnel required for the next activity is waiting stand-by for the current activity to be completed. The organization of the work is based in the principle that in tunneling the heading should never be idle. The machines and personnel are underutilized, to have continuous activity in each heading.

For the same reason, as soon as the face is charged, the area is secured, and blasting takes place. Relevant health and safety risks are easily controlled because access to the heading is single and the ventilation of the heading doesn’t affect the other heading.

 In mining, blast 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 hole intersecting the headings and the ventilation is shared between headings. That is one additional reason that in mining there are multiple headings.

The planning of the works ensures that there are always surplus of headings available to work and the organization of work is based on the principle that in mining the resources should never be idle. In this way one can have more meters excavated with the same resources and the cost per meter of the tunnel is kept as low as possible.

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

 1.      Convergence control

Typically, in tunneling we use NATM (New Austrian Tunneling Method) and in mining we use Drill and Blast method. To my opinion, the difference between the two methods is the importance of convergence.

In tunneling we have large cross sections and concrete lining. The lining cannot be constructed if the convergence is not stabilized by the rock support. The supported rock after convergence should be outside of 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 convergence. This required extensive monitoring of the convergence, usually with optical targets and total station.

In mining, the size of the excavation is relatively small and there is no lining. The convergence is not monitored, as small convergence doesn’t affect the traffic of the equipment. If additional support is required, one can understand it from the cracks in the shotcrete or the bended plates of the rock bolts.

 2.      Support requirements

The support requirements for both tunneling and mining could be addressed by the same design described in “Using the Q-system, Rock mass classification and support design”, May 2015 from NGI.

Typically, in tunneling we have larger Span, higher Excavation Support Ratio (ESR) and lower Stress Reduction Factor (SRF). To briefly explain the ESR, an adit that will be used for a couple months by skilled miners will require less support that a tunnel that will be used for decades by the public.

 Road and rail tunnels have final concrete lining. To avoid the ground water dripping inside the tunnel, a membrane is placed between the rock and the lining. This membrane should be protected from rapture by sharp rock edges and extruding rock bolts. For that reason, the rock surface is sprayed with shotcrete to smooth irregularities and cover the extruding rock bolt tails, even if that is not needed for the rock support. Having worked for many years exclusively under shotcrete, tunneling personnel find it stressful to work under rock that is supported only with bolts.

 Collapses of the roof, although never desirable, could happen both in tunneling and in mining. Rehabilitation of a collapse is very costly and effects seriously the time schedule.

In tunneling, rehabilitation of a collapse is the only way to complete the project.

In mining, if a collapse is having serious rehabilitation effort, the alignment of the decline could change to avoid the collapsed area, or a drift could be abandoned if the cost of the rehabilitation is higher that the value of the ore to be recovered.

 3.      Overbreak control

Tunnels most of the times have concrete lining of specific thickness. Therefore, after completing the excavation and the support, there should be no rock neither shotcrete extruding into the area of the lining. On the other hand, any overbreak must be filled with concrete during lining, making the overbreak very expensive. This requirement to control undercuts and overbreak requires very accurate drilling for the contour holes, both for the position of the hole and the direction / inclination of the hole.

In mining the cost of the overbreak is mainly limited to the cost for the haulage of the extra rock to the surface. Therefore, in mining controlling the overbreak is desirable but not as critical as in tunneling.

Contractors and jumbo operators experienced in mining, find difficulties adjusting in tunneling and unintentionally cause excessive cost.

 Another factor that effects the overbreak, is the pull length of the blast. The larger is the blast length, the faster is the advance rate of the excavation and therefore the lower the cost per linear meter. However, the long pull length often causes overbreak, because of fall of wedges or poor rock mass that are between the face and the supported area.

Again, in tunneling they consider the cost of the concrete to fill this overbreak; Therefore, in tunneling should limit the blast length to control the overbreak, according to the rock mass quality.

In Mining, that the overbreak is not very significant; therefore, in mining the blast length is the longest possible. The only considerations are the ability to charge the long blast holes and the risk of collapse of the unsupported ground.

 4.      Equipment selection

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

 I find that there is also a difference in the mind set, in the selection of the equipment.

In tunneling should use the largest available equipment that can fit in the tunnel. The larger is a machine, the most powerful and productive it is. Therefore, with the largest equipment one can finish the tunnel faster, minimizing the cost and increasing the profit.

In some cases, I have noticed that in mining would select the smallest machine that can do the job, because it reduces the capital expenses of the mine and the installed electrical power. I find this a wrong mentality because it reduces the productivity, increases the cost per ton of ore and reduces the profitability of the mine.

 The installation of rock bolts using jumbo or bolter is another difference and big controversy in mining. The cycle of an excavation is mucking, scaling, shotcrete (if required), bolting, face drilling, charging and blasting. The use of a bolter could be safer and could require less manpower. However, after the completion of the bolting, the bolter should leave the face and the jumbo should enter at the face. If the support of the excavation is done with the jumbo, this interchange of equipment doesn’t exist, and each excavation cycle time could be reduced by almost half an hour.

Additionally, in tunneling, where the headings are limited, there will be large underutilization of the jumbo and the bolter. For that reason, in tunneling the rock bolt installation is done by the jumbo.

In mining, considering the multiple headings available for excavation, the impact on the utilization and the advance rate is minimal. In the mines that are adequately mechanized to allow one operator per machine, the bolter is the best option to reduce mining cost.

 Another difference is the way of mucking, after the blasting.

In tunneling the 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 adequate number of trucks, so that the loader is not waiting for trucks. In this case, the selection of the 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 re-muck bay.  The reasons are that there is not adequate space to load the trucks with the loader at the face and the distance to the dumping area is large. The mucking is done with the biggest loader that can fit in the adit, to a re-muck bay, where the intersection is having adequate height to do the loading. After the mucking is completed, the face is available for the next activities and at the same time the muck is gradually loaded to the trucks and hauled to the surface.

 Conclusion

The core knowledge to drill and blast underground is common in both mining and tunneling. However, there are significant differences that define how efficient one can be at his work.

With this article I shared my knowledge and I hope it will helpful for underground professionals to be effective in both civil and mining sectors.

Bring it all together and use your judgment whether the above are suitable for your purpose.