1. Introduction
Backfill is the term for material used to fill voids (empty stopes) created by mining activity. “The reasons for putting backfill underground range from providing regional support to disposal of a waste product. The fill serves many functions, although it is generally considered in terms of its support capabilities. Other than its own body weight, backfill is a passive support system that has to be compressed before exerting a restraining force. Backfill has little effect on the stress distribution in the surrounding strata. It can, however, have a considerable effect on the strength of a rock structure, even if it only prevents the rock from unraveling. This allows the rock to continue support even though fractured. To maximize support, the fill should be placed as soon as possible to take advantage of wall closure (and before sloughing has progressed)."Singh and Hedley
Function
The original function of backfill in hard rock mines was to support wall rocks and pillars and provide a working surface for continuing mining. This was first accomplished with rock fill and then with hydraulic fill. If cement were added to a hydraulic backfill (30:1), the backfill provided better support for pillars and wall rocks. If enriched at the top of a pour (10:1), the backfill provided a smooth and hard surface that facilitated removal of broken ore and reduced dilution from the fill.
Backfill also afforded the opportunity for more selective mining and greater recovery, including recovery of pillars. Other functions of backfill are the prevention of subsidence and better control over ventilation flow through the mine workings. Cemented hydraulic fill (CHF) or paste backfill may be used to stabilize caved areas in the mine. Backfill is also considered an essential tool to help preserve the structural integrity of the mine workings, taken as a whole and to help avoid major rock bursts in highly stressed ground (refer to Chapter 2 - Rock Mechanics).
Application
“Fill preparation and the placement system should be both simple and efficient, and special attention must be given to the aspects of quality and quality control.” F. Hasani and J. Archibald (Mine Backfill 1998)
The application of a backfill system may be classified into two systems.
• Cyclic Filling
• Delayed Filling
In cyclic backfilling systems, the fill is placed in successive lifts as in a cut and fill mining sequence. Fill in each operation cycle acts as a platform for mining equipment or mining may occur below, beside, or through the backfill.
With delayed backfill, an entire stope is filled in one pass. The fill must not only be capable of existing as a free-standing wall, but the wall must be rigid enough to withstand the effects of blasting and pulling an adjacent stope so that dilution from sloughing is minimal.
Selection
The backfilling method used is often dependent on the mining system adopted.
The following types of backfill employed in hard rock mines are dealt with in the text of this chapter.
• Rock fill
• Cemented Rock Fill (CRF)
• Hydraulic fill
• CHF (normal and high density)
• Concrete fill
• Paste fill
• Ice fill (permafrost regions)
2. Rules of Thumb
General
• The cost of backfilling will be near 20% of the total underground operating cost. Source: Bob Rappolt.
• The capital cost of a paste fill plant installation is approximately twice the cost of a conventional hydraulic fill plant of the same capacity. Source: Barrett, Fuller, and Miller
• If a mine backfills all production stopes to avoid significant delays in ore production, the daily capacity of the backfill system should be should be at least 1.25 times the average daily mining rate (expressed in terms of volume). Source: Robert Currie
• The typical requirement for backfill is approximately 50% of the tonnage mined. It is theoretically about 60%, but all stopes are not completely filled and tertiary stopes may not be filled at all. Source: Ross Gowan
• It is common to measure the strength of cemented backfill as if it were concrete (i.e. 28 days), probably because this time coincides with the planned stope turn-around cycle. Here it should be noted that while concrete obtains over 80% of its long- term strength at 28 days, cemented fill might only obtain 50%. In other words, a structural fill may have almost twice the strength at 90 days as it had at 28 days. Source: Jack de la Vergne
Hydraulic Fill
• Because the density of hydraulic fill when placed is only about half that of ore, unless half the tailings can be recovered to meet gradation requirements, a supplementary or substitute source of fill material is required. Source: E. G. Thomas
Cemented Rock Fill
• A 6% binder will give almost the same CRF strength in 14 days that a 5% binder will give in 28 days. This rule is useful to know when a faster stope turn-around time becomes necessary.
Source: Joel Rheault
• As the fly ash content of a CRF slurry is increased above 50%, the strength of the backfill drops rapidly and the curing time increases dramatically. A binder consisting of 35% fly ash and 65% cement is deemed to be the optimal mix. Source: Joel Rheault
• The size of water flush for a CRF slurry line should be 4,000 US gallons. Source: George Greer
• The optimum W/C ratio for a CRF slurry is 0.8:1, but in practice, the water content may have to be reduced when the rock is wet due to ice and snow content of quarried rock or ground water seepage into the fill raise. Source: Finland Tech
• The actual strength of CRF placed in a mine will be approximately 2/3 the laboratory value that is obtained from standard 6 inch diameter concrete test cylinders, but will be about 90% of the value obtained from 12 inch diameter cylinders. Source: Thiann Yu
Paste Fill
• Only about 60% of mill tailings can be used for paste fill over the life of a mine because of the volume increase, which occurs as a result of breaking and comminuting the ore. Source: David Landriault
• Experience to date at the Golden Giant mine indicates that only 46% of the tailings produced can be used for paste fill. Source: Jim Paynter
• Very precise control of pulp density is required for gravity flow of paste fill. A small (1-2%) increase in pulp density can more than double pipeline pressures (and resistance to flow). Source: David Landriault
• 40% of paste fill distribution piping may be salvaged for re-use. Source: BM&S Corporation
3. Tricks of the Trade
• In general, when contemplating backfill design, the following aspects should be investigated.
− Geology of ore deposit, dimensions of ore body, dip, ore grade.
− Physical and mechanical properties of ore and host rock-mass.
− Environmental requirements.
− Fill material resources.
− Mining method, production capacity and operation schedule.
− Fill strength requirement analysis.
− Determination of fill composition based on strength analysis and available material.
− Determinations of quantity of fill constituents.
− Fill preparation system and facilities.
− Fill placement system and related equipment.
− Overall economic analysis.
− Location of the stope openings relative to surface facilities.
Source: F. Hasani and J. Archibald (Mine Backfill 1998)
• The way to get a really high-density sand fill (80%+) is to use carefully selected alluvial sand and add some pure silt to obtain optimum gradation. Use only Portland cement as the binder (no fly ash or slag cement). No flocculating agent or other modifier is added. After adequate mixing, and “priming” the line with a water flush only (no compressed air), feed the mixture down an uncased borehole drilled 6 inches in diameter and makes no more than 5 gallons per minute of ground water. A larger diameter borehole is a detriment because it permits segregation. If it is done correctly, the backfill comes out of the fill line at the consistency of toothpaste and there is zero water bleed (decant). When the lateral transport underground is a considerable distance, we have to reduce the solids content to 76% to maintain an adequate gravity head. Source: Crean Hill Mine
• The coarse particles within cemented rock fill tend to migrate away from the impact area(s). This segregation forms weak layers in the stope that may cause stability problems when the adjacent blocks are later mined. Proper orientation of the fill delivery systems into the stope (raises, etc.) is vital for producing effective rock fill. Source: S. Peterson, J. Szymanski, and S. Planeta
• Free-fall of paste or hydraulic fill in a vertical pipe column is unacceptable, but the principal of free-fall can be employed to advantage under certain circumstances. If ground conditions are suitable, using an unlined borehole as the main vertical delivery pipeline is possible. In this case the high wear rates and high impact pressures do not present a problem. Source: A.J.C. Patterson, R. Cooke, and D. Gericke
• When a cemented fill cures in the stope, the binder cements absorb almost an equivalent mass of water in the hydration reaction. For a paste fill, this removal produces an unsaturated fill with potentially advantageous negative pore pressure. Source: Barrett, Fuller, and Miller
• Installation of rupture devices at key points in a paste fill distribution network will protect against over-pressure caused by line blockage. A rupture device consists of a machined pipe section with housing and downspout to direct the flow of material when failed. Source: BM&S Corporation
• Pressure sensors installed at key elevations are of great assistance in evaluating impact of process changes and determining the location of a blockage in a paste fill pipeline network. Either diaphragm-style or strain gage sensors may be employed. Source: BM&S Corporation
• A paste fill pour should be preceded with a water flush and followed with a water flush followed by a water/compressed air flush. Source: BM&S Corporation
• The quality control for the paste mixture should include amperage measurement on the mixer drive motor. The amperage draw is proportional to the viscosity of the mixture. Source: Fred Brackebush
• A stope raise is normally required when employing sublevel retreat. This can be avoided by
placing Styrofoam blocks suspended with wire ropes against the ore wall when backfilling with cemented rock fill. For paste fill, we drill a hole in the fill near the wall with a production rig soon after placement and back ream it to 24 inches diameter. This procedure is even less expensive than the Styrofoam. Source: Jacques Perron
4. Types of Backfill
Rock Fill
Originally, backfill consisted of waste rock from development and hand picked from broken ore. Some of the larger mines in the USA quarried rock and dropped it down fill raises to the mine workings. Filling with rock alone is seldom practiced today except for filling tertiary stopes.
Cemented Rock Fill
CRF originally consisted of spraying cement slurry or CHF on top of stopes filled with waste rock (Geco and Mount Isa). The method was based on a civil engineering procedure known as Prepakt® that was already employed in the construction of concrete dams and bridge piers. Today, a cement slurry is added to the waste rock before (or as) the stope is filled. In most cases, rock is quarried on surface and dropped to the mining horizon through a fill raise. Trucks or conveyors are used for
lateral transport underground.
The advantages of CRF include a high strength to cement content ratio and provision of a stiff fill that contributes to regional ground support. CRF is still selected for some new mines and many operators prefer this system. A variation now employed at Mount Isa shows promise. CHF replaces the cement slurry. The improved gradation of the resulting mixture is believed to be responsible for obtaining high strength with very little cement binder.
Hydraulic Fill
The first hydraulic fills consisted of a portion of the mill (concentrator) tailings that would otherwise have been deposited on surface. The mill tailings were cycloned to remove fines (slime fraction) so that the contained water would decant. This fill was transported underground as slurry, hence the term “hydraulic fill.” Initially, hydraulic fill was sent underground at approximately 55% solids, since this is the typical underflow from a thickener and the pulp density normally used for tailings lines.
When the grind from the mill was too fine for decanting in the stopes, alluvial sand was employed instead of tailings. This type of hydraulic fill is often called sand fill. Particles of alluvial sand are naturally rounded enabling a higher solids content to be pumped than hydraulic fill made from cycloned tailings. (The cyclone is discussed in Chapter 26 – Mineral Processing.) Many mines still employ non-cemented hydraulic fill, particularly for filling tertiary stopes.
Cemented Hydraulic Fill
Portland Cement (binder) added to hydraulic fill provides strength. Later, it was found economical to replace a portion of the cement with fly ash (pozzolan) and occasionally a portion was replaced with ground slag, lime, or anhydrite. Normal and high-density CHF is employed at many hard rock mines worldwide.
If cement is added to a hydraulic backfill at a ratio of 30:1, the backfill provides better support for pillars and wall rocks. If enriched at the top of a pour (10:1), the backfill provides a smooth and hard surface that facilitates removal of broken ore.
The addition of cement reduces dilution from the fill. It also affords the opportunity for more selective mining and greater recovery, including recovery of pillars. One of the main problems with hydraulic fill and CHF is the requirement to bleed (decant) excess water from the filled stope.
The dirty decanted water, along with flush water, picks up slimes and transports them to the mine sumps. The decant from CHF may contain particles of fresh cement (not yet hydrated), which has been blamed for causing hang-ups in ore passes. For these reasons, miners have directed attention towards producing a hydraulic fill with less contained water. As a result of these efforts, many mines were able to increase the solids content to 65 -75% and more (hence the term “high-density fill”).
Widespread research has been directed at completely eliminating the bleed water from high-density cemented hydraulic fill. Most of the investigations involve applying two additives that react with each other to produce a hygroscopic silica or silicate. One of the results is a promising system that is now commercially available. The requirement for stiff backfill in South African mines resulted in the development of Fillset® by Fosroc (Pty) Limited.
This two-component additive allows backfills with free water present to be placed in stopes with negligible resulting run-off or drainage of free water. The first component (which is supplied in a powder form and known as Fillcem) is added at the backfill plant along with the powdered binder. The second component (Fillgel) is in a liquid form (sodium silicate) and is injected into the backfill piping at the stope being backfilled. In the case of classified tailing fills, the use of Fillset® eliminates the need for in-stope backfill drainage systems and lessens the load on the mine settling/dewatering system. In the case of total tailing fills, the use of
Fillset® allows the backfill to be transferred to the stopes in a high-density slurry form rather than a paste form, without the requirement for large binder addition rates. The Fillset® system does have the disadvantage of requiring the Fillgel to be transported underground and added at the stope as it is backfilled.
Concrete Fill
Cement-rich (1:2 cement to solids ratio) hydraulic fill was once used for mats where poor ground conditions dictated undercut-and-fill mining. Since the major cost component of backfill is the cement, this fill is not economical. To make the mats less expensive, the mats were then made from ready-mix concrete, which has 10-12% cement content for a standard 3,000-psi (20 MPa) mix. In some cases, the pour was completed above the mat with weak ready-mix concrete produced from the same batch plant. A similar procedure is practiced today at mines in Nevada and elsewhere.
Paste Fill
At the Grund mine in Germany, the “paste fill” system was first developed. The ready-mix concrete required for undercut-and-fill mining was replaced with a cemented fill using mill tails that did not require cycloning (“total tails”). The first paste fills contained a coarse aggregate fraction (sink-float product), similar to a regular concrete mix, which permitted transport at very high solids content (± 88%) and resulted in high strengths with respect to the amount of cement. Cement was added at the stope entry. Today, paste fill is used to replace hydraulic fill without benefit of the coarse aggregate fraction and with cement mixed in before transporting underground.
The distinction between paste fill and high-density fill is an item of contention. In general, a highdensity fill has the properties of a fluid while paste fill has the physical properties of a semi-solid. Today, paste fill is found desirable for many mining methods. In fact, paste fill is often the default selection when planning for new mining projects and, in a number of instances, has been installed at older mines to replace or supplement an existing backfill system.
An interesting variation (under research) is to agglomerate and cure a portion of the paste fill. The hardened pellets are then added to the regular mix. The result is a higher strength fill due to the improved gradation of the mixture.
Ice Fill
Ice has long been proposed as backfill in permafrost regions; however, to date, ice has only been used in Norway and the CIS (Russia).
Other Types of Fill
Other sorts of backfill are occasionally employed (e.g. smelter slag), but none have gained wide acceptance to date.
5. Properties
Table 21-1 shows the typical properties of structural backfills.
Table 21-1 Typical Properties of Structural Backfills
*Cemented hydraulic backfills have a wide variety of properties, dependent upon their application, as shown in Table 21-2.
Sources: Sprott, Bawden, Moss, Yu, Farsangi, Reshke, and Moerman
Table 21-2 shows the typical properties of hydraulic, cemented backfills.
Table 21-2 Typical Properties of Hydraulic, Cemented Backfills
6. Case Histories
Tables 21-3 and 21-4 provide case histories for paste fill and CRF.
Table 21-3 Paste Fill Case Histories
* Mine now closed
Table 21-4 Cemented Rock Fill Case Histories
*(C = cement, FA = fly ash)
7. Cooling from Paste Fill
When considering air conditioning for a hot mine, the cooling effect of backfill is taken into account in the heat balance computation. A common misconception is that the heat of hydration of the cement in the fill adds to the heat load of the mine; however, in most cases, this heat is not sufficient to warm the backfill up to the virgin rock temperature. Following is a sample calculation to determine the amount of cooling provided by a typical paste fill mixture.
Example
Winter Summer
Facts:
1. Average delivery temperature of paste fill: 45°F 60°F
2. Average VRT* at mining horizon: 90°F 90°F
3. Temperature rise: 45°F 30°F
4. Specific heat of H2O = 1.00 Btu/lb.°F
5. Specific heat of solids = 0.23 Btu/lb.°F
6. Heat of hydration of cement: 90 cal/gram = 162 Btu/lb.
7. Average cement content of fill: 3.5% by weight
8. Average total solids content of fill: 84% by weight
9. Average water content of fill: 16% by weight
10. Application rate: 7 days per week = 1,600 tpd
Solution:
Total cooling per day (24 hours under summer conditions):
Water = 1,600 × 2,000 × 0.16 × 1.00 × 30 = 15,360,000 Btu/d
Solids = 1,600 × 2,000 × 0.84 × 0.23 × 30 = 18,547,000 Btu/d
---------------------
33,907,000 Btu/d
Less heat of hydration of cement:
Cement = 1,600 × 2,000 × 0.035 × 162 = 18,144,000 Btu/d
Net cooling per day = 15,763,000 Btu/d
Net cooling rate = 15,763,000 / (24 × 60) = 10,950 Btu/min
* VRT = virgin rock temperature