Evolution of the in-situ stress state in an upstream constructed tailings dam

Abstract

In recent years, there have been several large tailings dam failures around the world that have resulted in significant negative environmental and social consequences. These failures are clearly a cause for concern and have led to a major review of the current design and operational practices of tailings dams. The outcomes of the independent panel investigations into the causes of these failures have highlighted several areas of uncertainty, mostly involving undrained shearing and the mobilisation of undrained shear strengths.

Within the context of undrained shearing, a specific area of uncertainty is the in-situ stress state. Although it is known that the in-situ stress state theoretically influences the undrained shear response for sandy soils, there are very few reported attempts to measure the in-situ stress state in literature. The objective of this study was therefore to design, construct, verify and implement a remote monitoring system capable of measuring the in-situ stress state in an upstream constructed tailings dam.

Such a system was developed, and it comprised several commercially available sensors, as well as some sensors constructed at the University of Pretoria. To measure the total earth pressures, two Earth Pressure Cells (EPCs) were used. These were installed in tailings dams such that the vertical and horizontal stresses could be measured. Tensiometers were used to measure the pore pressure, and a moisture probe was used to record the soil moisture content. Several other components such as dataloggers, batteries, and solar panels were included to ensure the completeness of the system.

A rigorous verification process followed the development of the system which comprised testing of the individual sensors, as well as long-term testing of the entire system. Once the system was verified, it was installed on three active tailings dams.

Data from 27 months of monitoring of the in-situ stresses in an upstream constructed gold tailings dam are presented. Although there was some variability during the early monitoring periods, the in-situ stresses stabilised and remained within a fairly narrow band after a few months. It was found that the in-situ stress state is not isotropic. Rather, the stress state, described using the geostatic stress ratio, was found to vary between 0.4 and 0.6 for the gold tailings dam monitored. Shorter-term data obtained from an iron ore and platinum tailings site showed similar stress states.

To place these findings within a behavioural framework, a series of oedometer and triaxial compression tests were conducted on the gold tailings. It was found that gold tailings exhibited a unique Normal Consolidation Line (NCL) and unique Critical State Line (CSL) for remoulded samples prepared using the Moist Tamping technique. In terms of the undrained shearing response, it was found that the gold tailings is influenced by both the density and stress anisotropy. It was specifically found that these parameters influence the yield undrained shear strength and Brittleness Index. This is important as both of these parameters are used directly in design and safety evaluations in some international guidelines. Relatively small changes in both void ratio and stress anisotropy resulted in meaningful changes in yield undrained shear strength and brittleness index.

In terms of the performance of the EPCs, it was found that these sensors appear susceptible to temperature influences that cannot be corrected using the thermal correction factors provided by the manufacturer. In this study, a number of laboratory and field experiments were conducted where EPCs under constant load were subjected to large variations in temperature. This thermal influence was found to vary between 0.3 and 6.5 kPa/°C.

In summary, this study presented experimental data showing the large influence the in-situ stress state can have on the undrained shearing behaviour of loose gold tailings. In particular, the influence on the yield undrained shear strength and Brittleness Index were highlighted. A remote monitoring system, capable of monitoring the in-situ stress state in upstream constructed tailings dams was presented. The system is cost-effective and simple to install. The system installed on the gold site remains fully functional 27 months after installation and will likely continue providing reliable data for many years to come.