Shafts and Raises

Shafts and Raises

The geological investigation prior to shaft sinking or raise construction should provide detailed information relating to the character of the ground conditions. The hydrogeological conditions, that is, position of the water table, hydrostatic pressures, especially artesian pressures, location of inflow and its quantification, as well as chemical composition of the water, are obviously of paramount importance. Indeed, groundwater inflow from deep aquifers presents a major hazard in shaft sinking operations since hydrostatic pressure increases with depth. The investigation also should provide detailed information relating to the character of the rocks involved, noting, where appropriate, their fracture index, strength, porosity and permeability. The data obtained should enable the best method of construction and the design of the lining to be made. In addition, the data should indicate where any ground stability and groundwater control measures are needed. Hence, a drillhole should be cored along the full length of the centre-line of a shaft and appropriate down-hole tests conducted.

According to Auld (1992), the best shape of a shaft lining to resist loads imposed by rock deformation and hydrostatic pressure, particularly one that is deep and has a large cross-sectional area, is circular since the induced stresses in the lining are all compressional. The circular shape minimizes the effect of inward radial closure exhibited by rock masses under high overburden stresses. In fact, no pressure is applied to the lining of a shaft in strong competent rock masses as inward deformation is minimal, being compatible with the elastic...