- Marula builds best-practice Tailings Dam Two
Tailings dams are an integral part of a mine. The design and construction of Marula’s Tailings Dam Two is in line with current South African and global best practice.
The new Marula tailings storage facility – called Tailings Dam Two – is an ambitious engineering project. The 77 hectare dam, lined with synthetic geotextile material, high density polyethylene, required the installation of 31 kilometers of drains, and 490 000 cubic meters of starter-wall material. It replaces Marula’s Tailings Dam One, which is reaching the end of its safe operational life. Importantly, it extends Marula’s life-of-mine by 20 years.
The Tailings Dam Two project was first conceptualised in 2007. But the prevailing economic conditions meant that a decision on the layout and design was only made in 2015. The current design was submitted to the Department of Water Affairs in June 2017 and approval given in early 2019.
Tailings Dam Two was designed according to the Global Industry Standard for Tailings Management. The design was completed using SRK Consulting and sent for third-party review at multiple points - at preliminary design phase and again at detailed design phase. The Department of Water and Sanitation indicated appreciation for best practice and compliance.
The dam is 77 hectares, lined with a geomembrane, and liner drains above and below provide drainage and leak detection. The design, at a lower side-slope angle - 1:4 versus 1:3 - improves the dam’s stability and integrity. It is designed to safely decant an unusually large rain event – modelled at having a one in ten thousand chance of happening – aided by the dual penstock outfall pipeline and decant system.
The design considered sliding failure due to the friction angle against the liner. The basin was contoured to cater for this, and textured liner is installed in areas where additional friction is needed. A concrete lined solution trench, with energy dissipation, prevents spills during rain. The dam will be equipped with vibrating wire piezometers, which allow real-time stability monitoring.
Compliance issues were top of mind. The Marula team obtained all required regulatory permissions including the Environmental Impact Assessment, the Water Use Licence and Environmental Management Plan.
Tailings dams, by their nature, pose risks – the largest of which are impoundment failure and groundwater contamination. The dam’s geomembrane lining and draining systems address its groundwater impact. The risk of impoundment failure was addressed in all facets of the dam’s design, including its ability to accommodate heavy rain and a lower side slope to improve stability. Contoured herringbone drains and textured liners in key areas of the basin mitigate potential instability caused by the new liner.
Marula strives to make a positive socio-economic impact through its projects and mining activities. Tailings Dam Two is no exception. It created employment for 84 community members. The project has spent R69.3 million of its budget within the Greater Tubatse Fetakgomo local municipality, of which R61.8 million has already been spent within the directly affected communities. As part of its corporate social investment commitments, the project has also contributed to the construction of a 2.3km community road and funded training to the value of R3 million for community members. Marula integrates environmental management into all aspects of the business with the aim of achieving sustainable world-class environmental performance. This is important as tailings dams have substantial environmental impacts. Marula’s Tailings Dam Two’s geomembrane lining and draining systems address groundwater contamination impacts. But the high wind conditions typical in the Marula area results in dust. The plan is to actively rehabilitate the side-slopes of Tailings Dam Two and irrigate the perimeter to prevent dust. In the medium to long term, other dust mitigation practices will need to be introduced, such as alternate vegetation practices and dust hedges. Once Tailings Dam One is decommissioned, a topsoil generation project is planned to address the dust from the retired dam.