Geotechnical design considerations for mine shafts

The design, development and operation of a shaft system is a critically important element in mining, and needs specialised technical input.

Experience has shown that the consequences of damage to shaft systems caused by geotechnical factors are serious and costly.

Control of ground conditions is a key factor in the design and sinking of shafts, as these conditions may vary considerably throughout the length of a shaft and require specific technologies to ensure safety and functionality.

Close to the surface, weathered rock and residual soils are common; these materials generally are too weak to carry the high foundation loads exerted by large headgear structures, winder houses and other surface infrastructure. SRK has designed concrete piles to carry surface loads exerted by the shaft headgear. In swelling soils, an innovative double pile system has been used to carry loads and also to prevent soil pressure from damaging the infrastructure.

Ground water often affects upper areas of the shaft and must be controlled by grouting to prevent water from entering the shaft, or by drainage systems built into the lining structure.

Natural breaks in the rock, such as joints and faults, affect the behaviour of the rock mass at shallow and intermediate depths (down to 1,000m below surface). Dolerite sills encountered near surface pose their own problems; they tend to break up into small blocks. Often, around 1000m below surface, the stresses that exist naturally in the ground become large enough to begin to induce additional fractures in the rock mass.

Simple support systems – like rock bolts or split sets, wire mesh and shotcrete – are adequate to protect the workforce involved in sinking the shaft under these conditions. A monolithic concrete lining, usually 300-500mm thick, gives permanent support and carries the fastening systems for buntons, pipework and electrical cable bundles. It also seals the rock and prevents weathering and deterioration over the planned operating life.

Certain weak geotechnical zones exhibit time dependant behaviour and can deform excessively (squeezing) when the shaft experiences stress changes during its life. This squeezing can cause severe failure of the lining. SRK has designed and implemented a support system which can accommodate large deformations in the shaft barrel and maintain serviceability of the shaft.

SRK has developed a systematic design process during the sinking of several shafts, which incorporates experience gained from rehabilitating existing shafts. The design process comprises gathering data from borehole cores and mapping of exposed rock; analysing this data to assess the rock mass response to excavations; assessing the possible modes of failure; analysing excavation and support designs, through to monitoring the implementation of the design.

William Joughin: wjoughin@srk.co.za


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