Backfill – Key Properties

When designing an underground mine, one of the key decisions is whether to backfill the voids left by mining. The backfilling decision is generally influenced by a range of factors which commonly include:

• Environmental constraints
• Surface disposal limitations
• Management of surface subsidence
• Promotion of mine stability
• Reducing the volume of waste hauled to surface
• Enabling a more complete extraction of the ore body
• Recovery of remnant ore pillars
• Minimising waste-rock dilution
• Installation of an engineered media in the mining environment to help protect or shield the workforce from poor ground or seismic conditions

As with any engineering design the first step is to define the problem and the parameters which are critical to a successful implementation. Once the problem is understood, a range of solutions can be identified. In the case of backfill, not all methods may be suitable and multiple approaches may need to be considered within the mine.

Assuming backfill is identified as a possible solution the first step is to identify sources of filler material. In many mines this might be development waste or wet tailings which would otherwise be destined for a Tailings Storage Facility (TSF). In other cases, filler material may be imported from waste stockpiles, old TSFs, from off site or in some cases, from a quarry established specifically for the purposes of backfilling. Clearly there is an economic consideration for each source of filler.

P&C’s standard approach is to determine the Life of Mine material balance with respect to the voids which require filling. This will immediately rule in or out a range of backfill options based on supply and demand.

The mining method may dictate what fill types are applicable. For example, the Avoca mining method cannot be undertaken using a hydraulic fill, and uncemented rock fill cannot be placed in open stopes which will be exposed vertically unless rib pillars or other means of confinement are applied to prevent the waste from cascading into the void and diluting the ore.

Consideration must also be given to the financial position of the operation. Options such as paste backfill tend to be capital-heavy at the outset of mining whereas Cemented Rock Fill (CRF) systems tend to have lower capital cost and higher operating costs. Quality must be considered hand in hand with the economics: pastefill is a high quality, homogenous product whereas CRF can be highly variable depending on the production methodology selected.

The following sections provide a high-level overview of the more common backfilling techniques.

Rock Fill

• Description: Placement of waste rock into the mining void. Generally undertaken using trucks and gravity, but sometimes placed or pushed up using LHDs
• Size fraction: Development or run-of-mine waste (sometimes imported waste)
• Delivery method: Load Haul Dump (LHD) loaders, haul truck or conveyor
• Exposure: Suitable as a working platform or for secondary stopes. Cannot be exposed vertically or undercut
• Considerations: Headroom for tipping trucks and the development of tip heads in some cases. Ejector trucks may also be considered
• Quality: Little quality control, highly variable particle size
• Relative cost: Low
• Excellent for: Filling voids which will not be exposed during future mining

Cemented Rock Fill

• Description: Cementitious slurry applied to waste rock. Systems vary but slurry is added to the waste rock, is mixed, and then placed into the stope using a truck or LHD
• Size fraction: Ideally crushed and screened rock, development, or run-of-mine waste (sometimes imported waste). Particle size optimisation required to maximise packing density
• Delivery method: LHD or haul truck. Cementitious slurry via agitator truck or slickline from surface
• Exposure: Possible to achieve strengths >4 MPa depending on mix. Vertical exposure
• Considerations: Mixing or dosing bays required. Headroom for tipping trucks and the development of tip heads in some cases. Ejector trucks sometimes considered
• Quality: Size manipulation, mixing systems and deposition method greatly affects the final product quality
• Relative cost: Low to High. Trade-off between capital and operating costs
• Excellent for: Filling voids which will be exposed during future mining

Hydraulic fill

• Description: Hydraulic disposal of tails into stope, with or without Solids will settle and require topping up to full on several occasions. Fill fence designed to allow water to escape
• Size fraction: Refer to Figure 1; coarser than paste fill. Often requires hydrocyclone (or similar) to modify Particle Size Distribution (PSD)
• Delivery method: Reticulated from surface. Contained behind fill fence
• Exposure: Generally vertical exposure or as a working platform. Cement required for vertical exposure
• Considerations: The backfill dewaters during placement so increases demand on mine dewatering system. Requires topping up as backfill dewaters. Water cannot be allowed to accumulate in the stope as this poses a risk of failure or inundation to the mining operation
• Quality: Good quality control possible, susceptible to binder segregation during dewatering
• Relative cost: Low cost for a hydraulically delivered product but cement cost is high where required
• Excellent for: Establishing working platforms and vertical exposure where mine water is not considered a problem

Paste fill/ Cemented aggregate fill

• Description: Binder added to whole-stream tailings, with or without aggregate and reticulated into the mine void
• Size fraction: Refer to Figure 1; rule of thumb is minimum 15% passing 20 µm
• Delivery method: Reticulated from surface. Contained behind fill fence
• Exposure: Vertical or horizontal. Strengths vary by design
• Considerations: Engineered barricades required to manage placement. Cannot be placed without binder. Generally, requires a cap of hard material to enable equipment movement on surface
• Quality: High quality control, homogenous product
• Relative cost: Capital cost generally higher but operating costs lower than for hydraulic fill. Costs must be traded off vs backfill quality
• Excellent for: Stabilising voids and backfilling workings and filling voids which will be exposed during future mining exposures

Figure 1 Idealised PSDs for Paste, Sand and Hydraulic Fill[1]

A range of backfilling options have been presented, and within each of these are a range of permutations to suit individual mines. The choice of backfill type should be driven by the mine requirements and most importantly, should be designed to reflect the life of mine material balance and to enable the quality control necessary to achieve the expected strength results.

[1] Sivakugan, N., Veenstra, R. & Naguleswaran, N. 2015. Underground Mine Backfilling in Australia Using Paste Fills and Hydraulic Fills. Int. J. Geosynth. And Ground Eng. 1:18