Digging deep - shaft sinking, design and construction
Michael Forrest talks to Alan Auld about the history of shaft sinking.
Later this month, the 3rd International Conference on Shaft Design and Construction 2012 will take place at the London offices of the Institute of Materials, Minerals and Mining. The conference will illustrate the advances in shaft design and construction that will be required to meet the challenges of lower grade and deeper mines around the world, but it is not only mining that relies on underground workings. Nuclear waste disposal and rail, road and utility tunnels all require extensive shaft development.
In a lecture in June 2011, Keith Marshall, Global Practice Leader of Underground Mining for Rio Tinto, stressed the importance of the underground mines in future mineral recovery. To access these deep mines at an extraction rate commensurate with today’s open pits, larger and deeper shafts will be required and the rate of shaft sinking will have to advance from the rates per month currently achieved.
Shaft sinking is not new. In Agricola’s book De Re Metallica, published in the mid 1500s, advice on how to follow a mineralised vein underground by the excavation of a shaft is illustrated. The tools, method of use and depth are described, while woodcut illustrations show how a shaft would penetrate the earth and link with tunnels or levels to recover mineral ores. Of particular note are the methods of lifting the ore with a rope and winder, and the setting of timber to support the walls of the shaft.
Alan Auld, Chairman and Managing Director of Alan Auld Group Ltd explains, ‘The principal of shaft sinking has not changed although there have been exponential increases in the speed, size and depth of shaft sinking and the range of engineering uses’. As Agricola reminds us, the initial purpose was to follow vein-type mineralisation from the surface to as deep as practicable. In those times, typical pick-and-shovel methods were used to dig vertically, removing waste in a bucket or kibble that was wound to the surface. In hard rock environments, drilling and blasting was used. As demand for metals and coal increased dramatically in the Industrial Revolution, so did the depth at which these metals were found. In hard competent rock in which there was minimal water inflow, shafts were sunk using drill and blast methods. However, as mining developed, particularly in European coalfields, shafts became larger and deeper often in sedimentary horizons that were unstable or waterlogged. These required advances in technology to sustain shaft sinking.
To overcome water ingress, shafts required lining. The first method used wood, often caulked with bitumen, constructed in the manner of a barrel or tub and fixed to the walls of the shaft. Known as tubbing, these linings were time-consuming and expensive to manufacture and fit, as were brick linings used to support unstable shaft walls. Where these methods failed the resource could not be worked, usually being flooded at the water table. A breakthrough came at the end of the 1800s when freezing the ground during construction was first successfully used in the construction of the Zwartberg shafts in the Belgian Campine coalfi eld. Other coalfields used di erent methods. For example, in the Ruhr coalfield in Germany, non-attached steel and concrete linings took the place of tubbing in water-bearing and non-stable ground – a system that was perfected and exported around the world. In the Netherlands, a system of sliding steel linings was developed whereby the lining was separated from the ground by a layer of bitumen that overcame the problem of the waterbearing overburden.
Shafts are not only related to mining operations. Vertical shafts are used in tunnels as ventilation shafts, and more recently in developing deep underground nuclear waste disposal sites. Auld explains, ‘Companies often led the way developing new technologies, and while civil engineering and mining shafts had common objectives their use of technologies were divergent’.
Many of these technologies were illustrated at the first International Symposium on Shaft Sinking and Tunnelling held under the auspices of The Institution of Mining Engineers in London in 1959. Here it was announced that the UK National Coal Board had completed 35 new shafts. In 1989, a second international conference entitled Shaft Engineering – this time organised by the Institution of Mining and Metallurgy – continued to illustrate progress. This included advances made during the development of the Selby coalfield in East Yorkshire that began in 1977 with the Winstow shafts (390 metres) and completed with the shafts in North Selby (1,044 metres). A technical highlight was the Gascoigne Wood surface drift that combined ground freezing temporary support with a spheroidal graphite cast iron (SGCI) tunnel lining. Freezing was achieved by drilling and inserting tubular pipes around the nascent drift through which a refrigerated liquid was circulated. To ensure a complete impermeable frozen ground condition, spacing and depth of the freeze tubes required careful calculation. At Winstow, for example, some five kilometres of refrigerant pipe was inserted at a mean spacing of 1.39 metres. Today, advances in freezing technology include directional drilling of the freezing holes, mobile freezing plants and ultrasound for measuring the thickness of the frozen zone. In the Gardanne colliery in the Centre-Midi coalfield of France, the largest diameter shaft was sunk in the 1980s with a diameter of 10 metres.
In lining technology, SGCI, combined concrete and steel and unreinforced concrete are the main methods. For shaft construction poured concrete is preferred over pre-cast, which is commonly used in civil engineering. In Europe, the non-connected sliding shaft is the preferred method. These methods have proved to be sufficiently robust to withstand the test of time as shafts may be in use for 100 years, and can cope with earth movements caused by mining.
Elsewhere in the world large diameter drills and raise-borers are widely used. Of particular note is the cutting of ventilation shafts both in mines and civil engineering. Here hard metal cutters similar to those used in tunnel boring machines break the rock for removal. Blind shaft boring machines are used from the surface, while those pulled up from underground via a pilot drill hole are raise-borers. Blind shaft boring machines require the waste to be winched from the shaft, while in raise boring the waste falls to the lowest level of the pilot drill hole and is removed using an ore transport system from underground, or backfilled into mine voids.
This month’s conference will illustrate the advances in shaft design and construction, with 38 presentations covering salt and potash, coal and metal mines and civil engineering applications in underground city railways, water and other utility shafts, and in nuclear waste disposal. Technical aspects of shaft sinking will include rock mechanics for deep shafts, shaft materials, and ground freezing. Papers on shaft closures and infilling will complete the programme.
3rd International Conference on Shaft Design and Construction 2012