Shaft sinkersMost underground mines use shafts for ore/man hoisting and ventilation. Conventional shaft sinking and raiseboring remain the two principal methods of shaft excavation, and the companies involved tend to be specialists in this field. There are some overlaps with the civil-construction sector, although its approach in terms of cost and timeframe can differ. 

Lawrence Newnham, executive director for RUC Cementation Mining Contractors, tells Mining Magazine: “Shaft-sinking companies are specialists that are generally specific to underground mining. There has, however, been a shift with ownership of the specialist shaft-sinking companies being taken over by large civil firms, yet seldom do the sinking companies do much work in the civil industry. 

“Raiseboring is primarily a mining-related activity, with a few lucrative civil projects done to improve balance sheets. Some raise-drilling contractors do specialise in the civil market in non-mining countries.

Master Drilling technical director Koos Jordaan says: “There are many applications for raiseboring technology suited for construction projects in mining and the energy sector. Raiseboring is very capital-intensive and extremely high risk in terms of loss of equipment, and under difficult insurance terms. For this reason, most raiseboring companies or divisions are organisations with very strict operating procedures. Most of the time, raiseboring work that forms part of a sinking or civil-engineering project is contracted out to specialised raiseboring services companies due to the varying requirements on different projects and specialised risks involved.” 

Throughout the industry there is increasing focus on greater operational control, better reliability, reduced risk and cost control. The use of computer-aided control systems and of larger boring machines are two examples of current trends. Shaft-sinking methodology is moving towards automation to minimise personnel in the shaft. Another development is the use of raiseboring for hoisting shafts as the available technology enables far greater accuracy.

Safety has improved and modern drilling methods reduce the labour requirements while still achieving reasonable advance rates, yet the production improvements are sometimes being diminished by overly strict interpretation of working practices.

Equipment selectionSome raiseboring companies manufacture their own raiseborers in-house or choose to operate a mixed fleet of purchased machines from the major OEMs, such as Wirth and Robbins, along with their own machines. There are advantages to both methods. Key raiseboring players making their own equipment include JS Redpath, Stu Blattner, RUC Cementation Mining Contractors and Master Drilling, amongst others. 

Mr Jordaan at Master Drilling says: “Using our own raiseborers allows us to offer more competitive contract prices due to the lower equipment cost, as well as a more reliable service due to more rugged and simplified designs. It also allows us to meet clients’ customised construction project requirements on short notice.” 

Availability of equpiment is important. Master Drilling, as an example, can manufacture any suite of equipment to a required diameter and depth with regard to rig and down-the-hole equipment within five months from the date required. Mr Jordaan explains: “We closely monitor quality-related cost, availability and utilisation for incremental innovation to increase reliability and throughput. Equipment is standardised and therefore reduces stockholding costs. We do acknowledge the quality of other OEMs and also consider purchases on a value-added basis.

Regional variationThe means of applying raiseboring varies between different geological domains, but, overall, it is generally much the same, with some personal preferences in terms of technique. Shaft-sinking, by contrast, is applied much more on a regional style where players refer to the Canadian, South African or Australian method. 

Mr Newnham at RUC Cementation comments: “Given the globalisation of our company, having independent businesses in Africa, Australasia and North America, we tend to adopt what’s best for the region and draw from the experience of the specialist entity.”

 Mr Jordaan at Master Drilling adds: “There is a significant difference in operation and approach to the work between different geologies and the application of excavations, which could be within the same region or a few metres apart. To deliver on schedule, budget and specification without transferring risk to a client project or the operator’s own existence, requires a significant exposure to various conditions and applications to form skills, standard operating procedures and equipment suited for the successful delivery of such projects. 

“Case-by-case projects without prior experience are extremely risky. For this reason we had been contracted on various occasions to complete projects failed by clients’ own operations or less experienced contractors. This

includes the completion of a 3.1m-diameter raise in Chile within four months that we were contracted to after one year of unsuccessful operations by another contractor.”

Master Drilling operates and manages over 80 raiseboring rigs at any one time on various types of contract and in different locations, maintaining an annual growth rate of 20%. The company states: “We continue to pursue different projects where we hope to contribute value to our clients’ requirements and assure a healthy, growing organisation. We hope to grow our global presence steadily through successful projects and are capable of execution of works in any global location, with established businesses in South America and Africa and projects worldwide.”

Other alternativesRecent news of Herrenknecht’s Shaft Boring System and its selection by Rio Tinto as a partner in its ‘Mine of the Future’ Programme, has refocused attention on blind boring/drilling technology. Among the established raiseboring players there is some scepticism, given that some of the concepts have been around for some time, albeit in less advanced forms. Some view the technology as essentially a niche market application, but with an important role in soft ground conditions that are waterlogged. There are also some concerns about achievable accuracy versus other methods.

The current view is that such technology is not an immediate threat or substitute for raiseboring, mainly due to the lower cost and higher production rates when excavating shafts in hard rock where bottom access is available using raiseboring. Some issues that may have to be overcome with the new system will be how to cool the working area and equipment, and addressing problems such as major underground aquifers and vibrations.

Mr Newnham at RUC Cementation says: “The so-called ‘new mechanical excavation system’ is not new, and I was responsible for delivering shafts using that methodology 20 years ago. The V-Mole shaft-boring machine, for example, has completed a large number of shafts in the German coal mines, South African hard-rock mines and a project that I managed in Australia at Broken Hill in 1990. 

“The system was limited due to the need for a waste pass, pre-drilled in the centre, yet had been used in blind drilling as well. I believe the system being developed at the moment will draw on some of those experiences and, with sufficient R+D funding, should improve on our earlier experiences.” 

Mr Jordaan at Master Drilling states: “Blind shaft-drilling concepts are very exciting, but an extremely expensive, calculated risk endeavour, given that many of the concepts had been around since the 1970s. Questions should be asked about their suitability and reasons why they could not be sustained as concepts and business services. We are actively researching such concepts and possible applications to strategically establish a service as an alternative to drill-and-blast sinking practices. 

“The extent of new concept risk on such large-scale projects makes it extremely difficult to define a suitable project and willing client. We hope that more clients such as Rio Tinto, in collaboration with large contractors such as Herren-knecht, can provide opportunities and technological advances. I think the challenge will be in proving that these operations are sustainable and cost-competitive.”

Conventional sinking or raiseboring?Both conventional methods for shaft sinking and raiseboring have their place in the excavation of shafts, whether for ventilation, production or other uses. Deciding on the method to use involves cost and time considerations, the rock type and structure, safety issues, the availability of bottom access, and the capabilities of the equipment. 

A spokesman at Murray & Roberts Cementation in South Africa says: “Raiseboring is a mechanical operation relying on very few personnel where sinking is a highly-paid, labour-intensive system of constructing a hole in the ground. Both means of constructing a hole have their place in the system –  raiseboring has limitations while sinking has a greater risk of personnel injury. The decision as to the shaft-sinking methodology used is very much dependent on the competency of the ground and if there is bottom access. 

“It is also important to remember that shafts can now be raisebored up to a 6.1m diameter with depths of up to 1km. Decisions are always on a case-by-case basis, and factors such as ground conditions and support requirements are taken into account, as well as whether the shaft will be used for ventilation or hoisting.” 

Cost is an obvious issue. Raiseboring on a per-day basis is more costly, but shaft sinking is much slower, often taking months or, in very large projects, even years to complete. 

Mr Jordaan at Master Drilling says: “I would expect the return on investment from raiseboring to be higher, but with a larger capital investment involved than in shaft sinking. Raiseboring is a much more productive method given that bottom access is available. Perhaps more important is that is creates a safer working environment for personnel where no access is required within the shaft.”

Decision making should also consider the thrust and torque capacity of the equipment, the stability of the shaft once excavated, the verticality, and the presence of bottom shaft access to begin excavations. 

Currently, Master Drilling has the capacity to excavate 8.5m-diameter shafts in softer rock and 6m-diameter shafts in hard rock up to 1,300m deep, but on larger-diameter shafts the determining factors are how long the shaft wall can stand unsupported for different geologies and the verticality of the shaft. 

The company states: “Regarding these limitations, we have invested in services where verticality of the centreline of the bored shaft can be within a radius of 250mm from the shaft’s theoretical centreline to an unlimited depth.” 

This is achieved by a combination of using rotary steerable stabilisers and regular directionality conformation using true north-seeking gyro surveys, as well as improved down-the-hole equipment design. 

The company is also experimenting with remote support equipment that will allow shaft support even while reaming on shotcrete, mesh and bolting applications.

A spokesman at Murray & Roberts Cementation comments on the choice between the two methods: “Raiseboring is always the cheapest and safest option. It is, however, dependent on the stability of the ground and whether there is bottom access. 

“Murray & Roberts Cementation considers it essential to undertake a Stacey & McKraken geotechnical risk assessment to determine whether the ground conditions are suitable for raiseboring activities and whether any form of support will be required. Where there is no bottom access, as in the case of a greenfields project, the shaft will have to be blind sunk.” 

If there is bottom access, Murray & Roberts outlines three available options: the shaft can be raisebored to its final diameter; the pilot hole can be raisebored and then the shaft sliped to its final diameter; or the pilot hole can be raisebored and then V-Moled to its final diameter. 

The firm adds: “Usually, raiseboring is the most cost-effective, with slipe and line being the next cheapest option; however, on deeper shafts, V-Moling is cheaper than the slipe-and-line option. Raiseboring remains the safest and quickest, with V-Moling as the next safest methodology.”

Lawrence Newnham, executive director for RUC Cementation Mining Contract-ors, refers to the issue of accuracy: “The main concern with raisebored shafts has been accuracy constraints. This has been improved by utilising Rotary Vertical Drilling systems with raiseborers or controlled diamond drilling with extremely tight tolerance specifications that are then hammer-drilled to full pilot-hole diameter, before raise drilling. We are starting to see an interest in raisebored hoisting shafts since we addressed the engineering concerns to deal with slight deflections in verticle raises.” 

He adds: “Geotechnical conditions over the full depth of the shaft are equally important in raise-drilled shafts and therefore not all mines can raisebore their hoisting facilities.” 

A typical 1,000-1,500m deep raisebored shaft at 5m in diameter would be able to hoist 2-3Mt/y of material. This would require an inter-level above the 1,000m elevation; therefore becoming a two-stage operation. The advantage is that it works out at half the cost of a conventional strip-and-line shaft, and possibly a third of the cost of a blind-sink shaft.

Raiseboring recordsThe table (p24) shows a series of record raises sunk by Murray & Roberts Group and its main divisions involved in mining – RUC Cementation Australia, Murray & Roberts Cementation and Cementation Canada. The ‘record raise’ depends on how the record is defined.

Until recently, the largest diameter raisebored shaft was in South Africa at Sasol’s Bosjespruit Colliery, which had a final diameter of 7.1m. Another shaft with the same diameter has just been completed at Khutala, with a depth of 89m. The deepest raisebored shaft was successfully completed by Murray & Roberts Cementation in a joint venture with Thyssen Schachtbau at Primsmulde, Germany. This shaft has a diameter of 1.83m, but is 1,260m deep. 

The world record for diameter and depth together was established in South Africa in 1986, having a depth of 1,036m at 6m diameter, during the excavation of the Frank Shaft. The longest incline drilled by Murray & Roberts is at BCL’s Selebi Pikwe Mine in Botswana. The 3.5m-diameter shaft was drilled to a depth of 755m at a 65º incline. 

The deepest shaft that has been V-Moled is at the Oryx Gold Mine in South Africa, with a 6.5m diameter and depth of 972m, while the deepest shaft V-Moled in hard rock has a 7.1m diameter and depth of 755m. The latter was completed by Murray & Roberts Cement-ation in a joint venture with Thyssen Schachtbau for Western Deep

Levels. 

Significantly, the most accurate raisebored hole is in the platinum sector in South Africa. This is at Townlands and has a 3.8m diameter. It was completed with only a 50mm deflection over its 823m depth. 

RUC Cementation finished the Australian record raise-drill hole in January, having a depth of 930m at 6m in diameter. This raise was drilled for Newcrest at its new Cadia East mine in New South Wales.

South Africa-based Master Drilling completed its deepest single pass of 850m at Tara Mines in Ireland (now part of Boliden), and its record raise has a 6.1m diameter in South Africa for Sasol Mining. The company is also contracted to complete two 7.2m-diameter shafts in South Africa for Exxaro, with construction starting in May. These are set to be the largest-diameter raiseboring shafts ever excavated. 

Master Drilling also refers to other types of raising work that are not necessarily overall records, but are just as challenging. These include: some smaller-diameter shafts that were completed with difficult requirements and at depths close to 4,000m below surface in South African mines; working at altitude in the Andes; working in very weak ground formations in Brazil; and remotely supporting shafts up to a depth of 500m in Mexico. 

The firm has also completed remote contracts in Iran. Another project in Ireland saw a 4.5m-diameter shaft, 700m deep, excavated in three months, with up to 1,200t/d of material removed.

Downturn effectIn common with all parts of the mining industry, shaft excavation was affected by the economic crisis of 2008-09. 

Mr Newnham at RUC comments: “We did notice a slowdown in demand for raise drilling over the period of September 2008 to February 2009, yet we were able to retain our trained workforce, and had the opportunity to do extensive maintenance and upgrading of equipment.

“However, the mining development and sinking projects were severely disrupted, and are still trying to recover, with an abundance of new equipment and machinery on the market after early mine closures and contract terminations.” 

But, demand in terms of different project scales varies. Mr Newnham adds: “In my 30 years in the raise-drilling business, I have never seen such a demand for large-diameter raise drilling as we are currently experiencing in the mechanised-mining sector. We expect to be drilling approximately 4,000m of 6m raises and an equal quantity of 5m raises over the next 18 months. Shaft-sinking projects are recovering after the global downturn of last year and we have been awarded a number of 200m shafts to be constructed in Hong Kong.”

Mr Jordaan at Master Drilling says: “Strategically, we have positioned ourselves in different countries with different commodities and clients. This enabled us to focus on gold projects when copper was down, for example. We also seriously worked on our cost and efficiency drivers to bid more competitively. Additionally, the services that we offer are not only limited to large mining companies, but also to smaller clients. More difficult, shorter contracted and less profitable projects form part of our operations, providing a stable base of work when capital projects are put on hold or cancelled.”

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