A paste backfill system seldom operates at a fixed operating point and the system always needs to accommodate considerable variability in materials depending on what ore is being mined and the performance of the process plant. It is therefore important to design a flexible backfill system that accounts for varying backfill rheology while ensuring that fill strengths are not compromised.

This is especially crucial for pumped systems where exceeding the pump discharge pressure will lead to emergency flushing and dumping of paste in the pipeline. Conversely if the pressure gradients are too low it could lead to slack flow and transients at the boreholes and accelerated pipeline wear.

Defining the Operating Range

It is important to define the operating range, depending on the material rheology, for the various fill points and to identify stopes that have a limited operating range that make the system vulnerable to sudden changes in flow properties of the backfill material produced. The wider the operating range is, the more robust the system is to process upsets and natural variability. Allowing for a wide operating range or changing piping routes and configurations in such a way as to achieve wider operating ranges, supports the control of the backfill quality and variability during filling.

A narrow operating range provides very little flexibility and margin to operate between exceeding the pump pressures, pumping the entire pipeline, and slack flow issues.

Wide Operating Range

The figure below shows an example of a wide operating range. The system can be controlled towards the higher discharge pressures, which will provide the best binder consumption, but give room to dilute the backfill if needed in case the discharge pressures rise too high, without causing slack flow in the system.

Narrow Operating Range

The figure below shows a narrow operating range, constrained by both discharge pressure and a long initial horizontal section. This system will be difficult to control and slack flow in the borehole will likely be a common occurrence (if not normal operation) after the operators have exceeded the pump discharge limit once too often and had to interrupt a pour.

Increasing the Operating Range

Operating ranges can be increased by re-routing pipes. Another way to avoid slack-flow operations and increase the operating range without exceeding the pump discharge pressure would be to introduce choking sections within the system.

The figure below presents such an example where the reticulation system was re-routed by dipping below surface earlier in the previous borehole to shorten the horizontal section and lengthen underground piping thus avoiding slack-flow and increasing the operating range.

Controlling Backfill Strength

In addition to accommodating varying rheology, the backfill strength is a key parameter that needs to be controlled. To allow for this the backfill mix recipe parameters need to be carefully measured and regulated. For example, if water is added to reduce rheology, the binder addition needs to compensate to ensure that target strengths are still achieved.


As a paste backfill systems seldom operates at fixed operating points, it is essential to design systems in such a way that large operating ranges can be achieved. This provides for a flexible backfill system that can account for varying backfill rheology while ensuring that backfill strengths are not compromised.

About The Author
Bruno Salvoldi
BSc Eng (Civil), MSc Eng (Civil), Pr Eng

Bruno is a Senior Engineer in Paterson & Cooke’s Cape Town office and heads up the Backfill Department. He joined P&C in 2011 and has worked on slurry and backfill systems, from concept though to detailed design and implementation, and is a registered Professional Engineer in South Africa.

Bruno has a Master’s degree in Civil Engineering, specialising in concrete technology and cement chemistry. He also specialises in slurry pumping systems, mine backfill, pipe stress analysis and support systems design.