Available from the Australian Centre for Geomechanics is the Handbook on Mine Fill (2005). They sent me a copy and I have just spent a while reading it. Here are my impressions.

As with all publications from the Australian Center for Geomechanics, the printing and formatting is superb. The main sections deal with these types of backfill, as defined by Yves Potvin in the Introduction. (As always I edit for context and focus.)

  • Hydraulic fill is free-draining, mill tailings transported through pipes and boreholes at a percentage solids that results in turbulent flow. The percent solids may vary from 50 to 70 percent. Bulkheads contain the hydraulic fill within the stopes and allow drainage of excess water. Sometime binders such as cement, flyash, or crushed slag is added to cement the fill.
  • Paste fill is also mine tailings with a lower water content than hydraulic fill: 78 to 87 percent soils. “Real” paste produces no bleed water.
  • Rock fill is waste rock, quarried rock, or aggregate. A hydraulic component (cement slurry or cemented tailings) may be combined with the bulk material to produce a cemented rock fill mass.

Chapter 2 takes us through a detailed examination of the nature and properties of these three backfill materials. The authors point out, for example, that comminution generally increases the volume of mined rock by at least a factor of 1.8; there is no way you can dispose of all your tailings as backfill in your underground workings.

Chapter 3 is a review of the geomechanics of mine fill. Nicely done. I enjoyed this dramatic description: “Liquefaction may occur when the pore water in a saturated and loose granular fill is suddenly pressurized due to shearing or shock or vibration to the extent that the intergranular contact stresses are reduced to zero and the fill mass starts moving like a thick fluid or paste. Liquefaction can occur when the fill is fine grained, open structured, saturated, uncemented, and subject to a sudden shock. The sudden loading may be induced by blasting near the fill, or by the fall of a large block of rock from the crown of the stope. Sudden shock loading could also be generated by the fill that escapes when a bulkhead fails. When the fill in the access drive is mobilized and pushed out, the saturated fill in the stope develops piping or funnel flow conditions. Funnel flow occurs in a non-uniform manner and imparts shear loading repeatedly to the fill mass, which eventually brings about the liquefaction of the fill mass. Fill escapes through access drives in spurts and transfers shock loading to the remaining saturated fill mass.”

A. G. Grice in Chapter 4 picks up on the topic of the fluid mechanics of mine fill. Here are: (1) a comprehensive overview of the fluid mechanics of hydraulic fill, paste rheology, and paste flow in pipes; (2) a discussion of the design of reticulations systems; (3) a brief overview of drainage through hydraulic fill; and (4) a summary of how to test hydraulic fill and past tailings so as to design the required reticulations systems and bulkheads.

The issue of drainage through hydraulic fill is covered in much more detail in a paper Water management in hydraulic fill operations by M. Helinski and A.G. Grice (the same) presented in Montreal at the 2007 Mine Backfill symposium. The authors have kindly supplied me with a copy. Here is their abstract followed by my summary:

In the authors’ experience inappropriate water management in hydraulic fill masses has been the root cause of most barricade failures. Inappropriate water management can lead to high water pressures, reduced effective stresses and increased barricade stresses. In addition the high hydraulic conductivity and non-plastic nature of most hydraulic fills makes susceptibility to piping failure a high possibility in many hydraulic backfills where a static water head is available. In order to avert both of these failure mechanisms it is essential to control the elevation of the phreatic surface within the fill mass. This is usually achieved through campaigned filling and resting periods. The duration of filling and resting periods are often estimated using some form of stope drainage simulator. Due to the random nature of many characteristics that influence the fill mass hydraulic conductivity, simulation of hydraulic fill drainage can be difficult. The high consequences of inaccurate phreatic surface estimates combined with the high variability in drainage properties often results in operators taking a conservative approach to this modeling. This conservative approach can lead to significant delays in the filling cycle which can have a major impact on the productivity of many hydraulic fill operations and can often results in an unnecessary movement towards capital intensive paste fill systems. This paper presents a water management technique which combines a simplified drainage model with in-situ monitoring to provide a rational approach to water management.

The authors modeled flow of water from a mass of backfill using MODFLOW. Being a flownet junkie, I make bold to reproduce their results.

On the basis of their flow nets, they decided that if you put piezometers into the backfill somewhere above the opening and outflow point you can reasonably accurately monitor the overall situation and hence decide on when to put in more backfill without risking trouble. In the case of backfill trouble comes in two ways:

  1. The fast flow of water down through the backfill to the bulkhead may induce piping of the backfill; the whole mass of backfill may inrush to the mine workings.
  2. The great head of water in the backfill can put too much pressure on the barricades, and they may fail.

The solution, or at least the way to avoid these failures, is to place the backfill in a series of successive sessions. Allow the water pressures from one backfill placement to dissipate before you place the next. Piezometers in the backfill enable you to monitor water pressures. And if you understand the flownet in the backfill, hence the water pressure distribution, you can use the results of the piezometers to tell you when you can add more backfill. .

Back to the Handbook on Mine Fill. Writing of hydraulic fill, A. G. Grice notes these advantages and limitations of hydraulic fill:

  • The risk of inrush and its consequences can be higher in uncemented hydraulic fill compared to cemented hydraulic fill and paste fill.
  • Fill placement rate is constrained by the drainage rate—provision must be made for resting periods and establishment of unsaturated conditions.
  • Surface processing plant is relatively simple: hydraulic fill can be produced cheaply, but it is all too easy to produced hydraulic fill that is out of specification.
  • Inadequate collection of draining water results in poor roadway conditions, damage to vehicles, and may impact ventilation systems.

The chapter on paste fill provides these vignettes of the history of paste fill:

  • Considerable effort was applied by South African gold mines in the late 1980s to implement paste fill as an alternative to tailings hydraulic fill. This was partially in response to the problem of low recovery of suitable tailings (<10%) resulting from a fine grind (>75% finer than 75 µm). research indicated that addition of coarser material to the paste reduced pipeline friction losses. The major problems associated with paste backfill in South Africa include: extreme mining depths; long horizontal distances to pump; and high operating costs.
  • The Lucky Friday mine in Idaho developed a tailings paste fill system in the early 1980s to help manage very high ground stresses. Tailings were dewatered via a combination of cyclones, thickener, and filters. Cement was added to the mix. After filling a stope, a new underhand cut could be mined beneath the new layer of fill starting as soon as five days after completion of filling.
  • In Canada it was not until 1994 that the first INCO paste plant was constructed at Garson mine.
  • In Australia, the first paste fill system was at the Elura mine. The system was not successful and was converted to an hydraulic fill system. Mount Isa now has plant that can deliver either high-density uncemented hydraulic or cemented paste fill.

The remaining chapters of the Handbook include a fine survey of rock fill in mine fill, other fill types and practices including co-disposal with municipal solid waste by Mike Gowan (where did he do that?), filling of open pits (required by law in California), and lots on cemented aggregates. But these topics deserve their own piece and we will return to them another day.