Assessment of the potential for static liquefaction of a tailings dam using laboratory and field testing

Since the catastrophic failure of the Merriespruit gold tailings dam in South Africa in 1994, increasing attention has been paid to evaluating the potential for a similar failure to occur at other tailings storage sites in the country. There is currently a great deal of re-mining of old tailings disposal facilities occurring in South Africa’s Witwatersrand mining district and the residue from these operations is being deposited into far fewer, very large tailings disposal facilities, resulting in what are termed “mega-sites”. One particular tailings disposal facility is discussed in this paper, which describes the results of a series of laboratory and field tests carried out to evaluate the potential for static liquefaction of the stored tailings. The tailings storage facility, which is over 50 m in height, had shown some signs of distress, prompting these studies. A series of piezocone tests indicated that in places the tailings was extremely soft and compressible, with an undrained strength that was probably less than the shear stresses existing in-situ. Laboratory triaxial shear tests on both isotropically and anisotropically consolidated high quality undisturbed specimens recovered from the shell of the dam were used to establish the in-situ stress conditions that could result in static liquefaction of the shell. The triaxial data was complemented by both saturated and unsaturated oedometer testing on undisturbed specimens, the results of which indicate some potential for sudden collapse of the unsaturated tailings upon wetting. The implications of these findings are interpreted together with the other laboratory and field data to provide a comprehensive evaluation of the state of the tailings storage facility.