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Shafts – providing access to the underground world

Shafts are used for transporting men and materials and for conveying the mineral product to the surface. They also serve as entry and exit points for the mine air, which is essential for all activities below ground. Shafts therefore have to provide reliable service for many years and must be capable of withstanding the stresses generated by mining operations.

View from the inset looking out into shaft no. 10 at Prosper V colliery showing the sinking platform and cactus grab

European shaft sinking activities and developments in shaft construction technology over the last fifty years can be traced via a series of major projects for the extraction of raw materials and also for the preparation of final disposal sites for radioactive waste. Shaft sinking has also featured prominently in the civil engineering industry, including the construction of ventilation shafts for Alpine tunnel construction projects and the building of underground infrastructure for research centres - which while usually set at shallow depth frequently involve the excavation of larger cross-sections than those required for the civil engineering and mining industries.

During this period shaft construction technology has primarily been influenced by the mechanisation of the main working cycles of rock cutting, debris loading and clearance, and support setting. Most of the shafts were sunk by the conventional method of drilling and firing and the vast majority were vertical structures originally designed to serve as surface mine shafts or staple pits. In Europe very few were sunk using shaft boring machines, while even inclined shafts were generally only used in areas with non water-bearing overburden. 

Of course superlatives feature prominently in the shaft sinking industry. Finished diameters of six to eight metres have become the norm and while ventilation boreholes tend to be smaller in size mine shafts designed for large-capacity conveyances have an even larger profile. Germany has the deepest mine shafts in Europe, with the north shaft at Saar colliery currently holding the record with a bottom landing 1,712 metres below the mine surface. The first eight-metre shaft was sunk in 1984 at Maltby colliery in the United Kingdom. In Germany work began on the first eight-metre shaft in 1971 at Ost colliery (the Lerche shaft) and this was followed in 1986 with the 8.1 metre-diameter Radbod 6 shaft and then with the Göttelborn 4 shaft, which had a finished cross-section of 8.3 metres. High-performance machines and equipment now permit average shaft sinking rates of of three metres a day, with peak performances of as much as four metres. Whitemoor no. 2 shaft in North Yorkshire, which in 1982 was sunk to a depth of 131 metres in a mere 31 days, held the European shaft sinking record for many years.

Mining considerations usually determine the shaft collaring point, while mine ventilation and/or transport and conveying requirements define the cross-section dimensions. If a colliery is to be cost effective it is vital that the main surface shafts are sunk at just the right location.

Local geology (stable or unstable ground) and the associated presence of ground water have always determined the choice of shaft sinking technique and this continues to be the case. Modern mine shafts are now almost exclusively lined with concrete of strength category B25 to B45 with structural reinforcement. Regardless of any possible static requirement the shaft lining is usually designed for a wall thickness of 30 to 50 centimetres purely for constructional reasons. The lining is often installed in sections using sliding or interchangeable formwork with ring heights of around four metres and leaving a small annular clearance to allow the shaft column freedom to move.

Source:

Steinkohle, the staff magazine of RAG Aktiengsellschaft, 11/2008