Source of information: Aspects of advanced grouting during shaft sinking in South Africa
Fixed cost for shaft sinking operations are extremely high, therefore precautions must be taken to avoid unproductive standing time of the shaft sinking crew. Water in-rushes or unconsolidated grounds sometimes cause stoppages to sinking operations because the sidewalls must be sealed or stabilised. Pregrouting can to a large extent avoid such delays. The paper describes ways and means how to execute pregrouting under varying conditions and draws attention to problems which may be encountered. The author concludes that the cost for preventive grouting operations are small compared with the overall expenses of shaft sinking operations and the cost of unproductive standing time.
Overhead costs for sinking large diameter shafts are known to be upwards of U.S. $250,000 per month, irrespective of meters sunk. Therefore every effort which will allow to speed up shaft-sinking and save unproductive time is economically justified. The overhead costs of shaft sinking are for all practical purposes rise disportionately with standing time. One of the main causes which delays shaft sinking operations is the ingress of water during excavation. The nature and size of water bearing fissures or cavities in the vicinity of the future shaft depend of course largely on the type of rock. Dolomites, which cover wide areas of the Transvaal are known to be highly water bearing formations. However, other rock types, such as shales and lavas, can yield considerable quantities of water, if intersected in the course of sinking operations.
In South Africa, Colliery shafts are usually very shallow. Shafts for gold or base mineral mines are often of considerable depth, sometimes well beyond 1000 m.
Quantities of water which were struck during shaft sinking operations in Southern Africa have varied tremendously but from the author's personal knowledge have been as much as 200,000 litres/hour, resulting in the loss of the shafts in certain cases.