Tailings Beach Slope – An Important Design Parameter for Conventional and Thickened Tailings Disposal System

On the back of Paul Williams’ recount of his journey in the area of tailings beach slope characterisation, we hear from Dr Behnam Pirouz on the more fundamental application of tailings beach slope, including the “grittier” side of the research completed by ATC Williams. The research that Behnam describes formed the basis of his PhD and has embedded him as an ATC Williams’ expert in this area and an industry authority.

Our commitment to Beach Slope Prediction

ATC Williams’ firm view is that accurate estimation of tailings beach slope is critical to the design of Tailings Storage Facilities (TSF).

The tailings beach slope has a direct impact on many factors in the design of both conventional and thickened tailings storage schemes, such as:

  • Tailings storage capacity
  • Decant pond location
  • Storm storage capacity and available freeboard, and most importantly
  • The height and safe configuration of the retaining embankment

Figure 1 provides examples of the importance of understanding beach slope.

Figure 1: Tailings Beach Slope in Central Thickened Tailings (CTD) and Conventional TSF Applications

Understanding beach slope dynamics has allowed ATC Williams to significantly expand the options offered to the mining industry for storing tailings more safely and efficiently. Stacked tailings schemes, such as central thickened discharge (CTD) or down-valley discharge (DVD) systems, have increasing applications as viable alternatives to more conventional systems that the industry might otherwise have adopted.

Behnam emphasises that beach slope prediction to support the design of such systems is an interesting challenge.  He acknowledges Paul Williams in the journey, retelling Paul’s story about the inspiration gained by Eli Robinsky’s seminal papers in the 1970s and recognising the sensitivity of TSF design to beach slope. After all, for a given footprint for a CTD scheme, if a design adopted 2% and achieved only 1%, only half of the planned tonnage could be stored in the TSF.

As a historical footnote, the earliest stacked disposal schemes adopted design slopes of 5-6% but achieved only around 1%. A very serious problem indeed and highlighting the critical nature of beach slope as a design parameter. Even Robinsky found to his dismay that there was no known theory or practical test methodology to enable realistic beach slope prediction. For this reason, the research commitment made by ATC Williams has extended for over 30 years.

One of ATC Williams’ earliest studies was in the mid-1980s at the Bougainville copper mine in Papua New Guinea. The project involved conversion from tailings discharge into the Jhelum River to a purpose-built stacked tailings scheme. As part of this work, we undertook a large-scale stacking trial.

Figure 2 shows the trial setup that was adopted.  According to Paul’s account, despite this pilot being well-considered, it did not necessarily achieve the desired outcome.

Figure 2: Large Scale Stacking Trial at Bougainville Copper Mine

Since then, ATC Williams has sponsored three PhD theses on the subject of beach slope prediction. Several research projects involving full-scale field trials and flow-through flume testing have been completed to develop the critical knowledge base around beach slope.

Figures 3 and 4 show the advancement of field trials over the years, using flumes to simulate full-scale beach slope dynamics. The works shown on these photographs were carried out at the Peak Gold Mine in NSW in 2003 and the Chuquicamata Copper Mine in Chile in 2011.

Figure 3: Peak Gold Mine Beach Slope Flow-through full-scale experimental flume (2003)
Figure 4: Chuquicamata Copper Mine Beach Slope Flow-through full-scale experimental flume

The fieldwork, supporting ongoing studies and analysis, has led to developing in-house theoretical models for beach slope prediction. The company has published more than thirty technical papers (seminar and journal) on beach slope design. This fundamental research and effort have provided ATC Williams with the basis for establishing design parameters that we now apply in our routine TSF design work.

Theory of Beach Slope and Principles for Prediction Models

It is a matter of common observational experience that when a continuous flow of tailings is discharged over a surface, the flow initially spreads itself out in the form of sheets and fans, which provides a bed of fresh tailings.

As time passes and a bed layer develops from newly deposited tailings, a narrow self-formed channel quite suddenly appears within the now stationary laminar sheet, and the flow depth and velocity of the tailings flow increase within this channel. This channel development is demonstrated in Figure 5.

Figure 5: Development of self-formed channels on thickened tailings stack

This phenomenon is the basis for the ‘Equilibrium Channel Slope Model’, developed by Behnam as part of his PhD thesis. The theoretical proposition embraced by this model is that the overall beach slope of a “stack” formed by non-segregating tailings is determined by the limiting equilibrium slope of the self-formed channels.  In other words, each channel evolves to achieve an optimum shape controlled by the minimum energy required to maintain steady state movement of tailings solids within the stream. The bed slope required to maintain this condition is the critical factor.

As Behnam would express it, the ‘Equilibrium Channel Slope Model’ predicts the channel shape and minimum slope required for a self-formed channel to carry tailings slurry in a steady state, total transport condition with no solids deposition or bed erosion.

He emphasises through the study of geomorphology and hydrodynamics of tailings beaches that beach slope is directly proportional to tailings solids concentration (i.e. a function of the science of rheology) and is also inversely proportional to discharge flowrate.

Thoughts on Practical Application

The knowledge gained around tailings beach slope demonstrates that the design of a TSF involves more than an embankment or more than the application of a model for prediction of beach slope.  Practical knowledge and understanding of the depositional characteristics of a tailings stream within a tailings impoundment are critical.

For instance, splitting the total tailings flowrate by spreading discharge via multiple spigots will result in the formation of a steeper beach profile. TSF storage capacity is therefore increased for DVD and CTD schemes.

Two different spigots arrangement systems are commonly used for flow splitting, these being the Linear Discharge System (LDS) and the Central Discharge System (CDS).  Examples of these systems are shown in Figure 6.

Figure 6: Flow Distribution Arrangements for CDS and LDS

An important factor common to both these systems is that uniform distribution and discharge of tailings to fill the storage evenly has optimised the effective tailings storage capacity.

There is a range of other factors to be considered to achieve an effective storage regime. Therefore, any such system can be optimised to achieve a desirable beach slope. The primary objective is to create a safe and efficient system while reducing the overall cost of the project.

ATC Williams acknowledges the expertise of Behnam and his Slurry and Mechanical Engineering team based in Melbourne, built on the grassroots work of Paul Williams. We acknowledge close associates to this journey, including Keith Seddon and Sadegh Javadi.

Through a long period of research works, ATC Williams has developed an internationally recognised in-house theoretical model for accurate prediction of beach slope for any given tailings properties and spigot arrangement. This expertise, along with practical knowledge of tailings deposition, is effectively applied by our design teams across the business to create tailor-made, safe and efficient, cost-effective tailings storage schemes.

Innovation Tailings