Western Areas Gold Mine. North shaft has experienced large magnitudes of induced mining stress which have caused creep deformational movements in the incompetent Basal tuffs of the Ventersdorp lavas.
Under the present stress regime these weak rocks no longer behave elastically but act plasticaly and are squeezed into excavations where they are exposed. This has resulted in the structural failure of the concrete lining of the shaft and a shortening of the vertical axis in its barrel.
An unacceptable risk in the safe operation of the shaft system was thus created. This paper describes the measures taken to contain and negate the deformations in both the shaft and station excavations. The observations. instrumentation and computer simulations to identify the mechanism of failure are discussed.
The design parameters for refurbishing the effected areas for the safe operation of the shaft for its planned life are discussed.
Western Areas Gold Mine is located on the West Rand approximatly 45 kilometers from Johannesburg.
The North Shaft was commissioned in September 1961. The 8 m diameter concrete lined shaft was sunk to a depth of 1518 m below surface, while a ventilation shaft sited 40 m NE was sunk to 1102 m.
Both shafts are sited in a pillar which takes the form of an ellipse approximatly 480 m on strike and 450 m on dip.
The strike of the stratum is east - west and the strata dips at an average inclination of 16 degrees to the south.
Instability problems have occured in the Main Shaft since 1975. Trouble was first experienced in the ore pass between 36 Level and 38 Levels. (fig. 1) This occured in the "dog leg" above 38 Level which was sited in the tuffaceous rocks at the base of the Ventersdorp Lava. Exploration holes drilled into the ore pass in late 1982 established that the caving, ore pass had come alarmingly close to the shaft (6 m). The ore pass was immediately waste filled and grouted.
During 1982/1983 fracturing and spalling in the concrete lining of the shaft at and just above 38 Level necessitated the installation of extensive support units. Five rings of 15m long. 1000kN, cable tendons were installed in the shaft.
(Figure in full paper)
The level itself was resupported and extensive bunton and shaft steel work was carried out.
In December 1986 further instability problems were encountered. particularly with the 38 Level Tip crosscut area. An intense resupport program using 6 m long 400kN cable tendons was undertaken. An electric substation immediately south of the shaft on 38 Level had to be abandoned due to bending steelwork. sidewall slabbing and footwall heave. (fig. 1)
Observation boreholes drilled through the pillar seperating the south access crosscut and the tip crosscut revealed extensive fracturing of the pillar such that most of its load bearing capacity had been lost (fig. 1). Instrumentation of the excavations immediately south of the shaft showed appreciable movement, with closure of 25 mm occuring over a period of 2.5 months.