Penstock decant system for large tailings dams

SRK Johannesburg has again become involved in the design of ring dyke tailings dams to accommodate a fairly large deposition rate. The availability of sufficient land is becoming a problem and is increasing the final heights of the tailings dams. The tailings dams therefore have to be designed to use the available space effectively and this is resulting in tailings dams covering an area of approximately 700 to 800ha and with heights of between 80 and 120 meters at the final penstock inlet system.

Designing these decant structures to last for the operating life of the tailings facility, which can vary from 50 to 75 years, needs to be carefully considered; the intake structures need to be constructed in phases as the height of the tailings to be confined by the dam rises. Typically, the project includes three phases that vary from 40 to 50 meters in height. The design of these structures must take into account the down drag forces that will be applied to the structure as the tailings consolidate, the founding conditions on which the structure base rests, seismic activity in the area, wind loading, and possible aviation activities. Another area of concern to be considered during the design of the entire decanting system is the jointing of the penstock outlet pipeline to the intake structure as differential settlements could produce problems in the future.

The design of these structures has resulted in penstock intake towers constructed with reinforced concrete with an external diameter of some 5.5 meters and a wall thickness of 1 meter for the first 50m lift of the tower. Thereafter, the wall thickness reduces to 750mm, and finally 500mm for the last 40 meters. The foundation base for this tower structure is approximately 10 meters by 10 meters in size with a depth of between 5.5 to 6 meters.

Danie Venter: dventer@srk.co.za



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