Tunnel vision

Jon Lawson

Martijn-K Mannot-Russell explores the use of mechanised tunnel machines in the mining industry

Mining and tunnelling have as much in common as they have differences. Historically, one associates mining predominantly with harder rock formations with the exceptions of potash, softer coal seams and extensively weathered shear/fracture zones. The realm of the tunnellers was mainly at relatively shallower depths (10 to 100m) through a variety of geological formations such as chalk and clays; although a mention must be made for the alpine brethren excavating drives with up to 3.5km overburden in squeezing rock conditions.

First uses of the tunnel boring machine (TBM) methods in mines are mentioned in several papers indicating use of TBMs in mines in the USA in the 1950s and 1960s; with use of a TBM in the UK coal mines in the 1980s at the Selby coalfield to link several coal mines. Advantages were the production rates and excavation method without the use of explosives. The cons were the limitations with respect to driving small radius curves, high relative cost of the machines, at that time and challenges with use of refurbished machines designed for more specific ground formations.

In the USA in the early 1990s TBMs were used at several mines with great success. The TBM development further progressed with companies designing and manufacturing machines in the UK, Germany, France, USA, Canada and Japan. From the USA came the development of the hard rock TBM with cutter discs. Similar machines were developed and manufactured in Germany and Japan, later in South Korea and China. The Japanese manufacturers tended to make TBMs for a specific project due to urban constraints and the bubble economy; whereas the Europeans and North Americans tended to focus on a more solid long-term view.  There are many examples of TBMs being reused, either by refurbishment and/or slight modifications with some machines still operational 30 years on. Currently machines are also manufactured in China and Australia. Many manufacturers have the bulk of the heavy fabrication undertaken in countries with competitively priced labour rates and with the key complex units manufactured at their country of origin. Typically the TBMs are fully assembled in the factory for an inspection by the client, while some manufacturers are providing the options for the first-time assembly on site, with the advantage of reducing the procurement programme.

The development of TBMs also saw the use of bentonite slurry as a face support and muck spoil transport media, laser guidance systems coupled with segment ring selection programmes, electronic cutter wear systems, data management systems, communication and processing of the sensor data from all equipment/motors. As monitoring and sensor technology developed, belt weighing, CCTV and remote access via internet are now standard features.

How fast is fast? Advance rates in hard rock TBMs can be up to 30m/day, 430m/week and excess of 1,000m/month have been achieved. One record I am aware of was in Australia of 70m in 1x12hour shift, hardrock TBM in sand stone – usually after such records the following shift cleans up!

Recent applications of TBMs in mining include projects in Indonesia, South America and the USA. 

Shaft or decline?

The development of TBMs and conveyor technologies has combined to form a competitive option for mine development both underground and from surface. Conveyors are able to operate around curves and TBMs can be designed to negotiate tighter radius curves. According to my records, the tightest radius negotiated by a hard rock TBM is 20m – the design was for a U turn. As a general rule of thumb for tunnels in rock shorter than 1,500m, drill & blast is the more economical option.

So how much does a TBM cost? Last time I was looking the average cost for TBM in the 6.5m diameter size range, configured for mixed ground conditions was approximately US$8.5 million.

Recent studies have shown that an economic alternative to a 1,200m deep shaft is an 8km decline at an inclination of 25 degrees. The pre-feasibility study indicated that considerable CAPEX savings of up to 30% plus a reduction in execution time of such a project by a time of 25% was possible compared to the traditional shaft sinking methods.

The advantage with a decline is that the services and transportation infrastructure is installed as the excavation proceeds. Service conduits, pipes and power cables utilised in the decline construction can later be used by the mine. In zones of challenging ground conditions pre-cast concrete lining can be installed. The dismantling of the TBM can be further facilitated by being specifically designed for easy disassembly.

With the focus these days on cost reduction and pressure to obtain a faster return on one’s investment the use of TBMs should be revisited, alternatively the shaft sinkers have to tighten their belts and up the pace. “What about mechanised shaft sinking”, I hear some ask - well that in itself is worthy of a separate article. 

TBM manufacturers currently still operating worldwide; in alphabetical order are: CRTE; Herrenknecht; Hitachi-Zosen; IHI; Kawasaki; Iseki; NFM; Robbins; Shanghai Tunnel Engineering & Construction; and Terratec.

For more information, visit www.engineerlive.com/ime

Martijn-K Mannot-Russell is with BASF Construction Chemicals Europe.

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