The owner’s concern centered on how a proposed pit lake – the body of water that may form in the mine pit due to natural surface and ground water infilling – would impact pit rock wall stability. (Read more about the hazards pit lakes present and SP’s use of drones for safe water sampling.)

At this site, the proposed pit lake was determined to increase pore pressure within the steep sides of the pit. This might in turn increase the likelihood that a rockslide or landslide could collapse into the pit lake and generate destructive surface waves. To understand the effects of a landslide and resulting waves, the owners of the mine asked WSP to evaluate the impacts using state-of-the-art modeling software and propose mitigating factors to manage the risk.

WSP determined that in the narrow reaches of the proposed pit during closure, there was a potential risk that a landslide-generated wave (or seiche) will form. The generated waves were then determined to potentially inundate and damage infrastructure inside the pit. In some scenarios, the landslide-generated wave was determined by WSP to overflow the pit.

Given the unprecedented magnitude of potential landslide volumes entering such a confined area, predicting potential outcomes is difficult. Building computer-based models able to accurately predict potential outcomes pushed the boundaries of landslide and wave modeling tools currently available. Often these events involve complicated phase mixtures of gasses, liquids and solids, and can be challenging to model in most commercially-available software packages.

WSP’s team simulated potential landslides using the industry-accepted software Dan3D, while wave generation and propagation were modeled using the computational fluid dynamics software Flow-3D.

WSP tested models for a range of possible sizes of pit lakes. A smaller lake would mean less water to create waves – but it might also mean that rock falling into the lake could fall from a greater height, resulting in bigger waves. Larger pits lakes carry their own hazards – including the risk that higher water levels in the pit lake’s walls would make them softer and more prone to collapse, resulting in larger failure masses.

One result of WSP’s work was to determine the upper and lower boundaries for the height of an optimal pit lake.

Now completed, WSP’s study is being used by the mine’s owners to help plan future operations, with one of the objectives being the proactive management of the size and shape of the pit lake.

The problem WSP addressed in this project is one shared by mines around the world, and WSP’s solution can support mine owners in mitigating landslide risks and determining cost-effective closure strategies.

* This work was performed by Golder professionals who joined WSP in an acquisition completed in 2021.