The following is an article by Darren Pemberton, ATC Williams which appeared in the Mine Closure 2024 supplement.
Capping and rehabilitation of coal tailings storages
The mining industry has long acknowledged the closure, capping and rehabilitation of coal tailings deposits as being
problematic.
Characteristics of coal tailings and soft tailings deposits
Tailings produced by coal handling and preparation plants (CHPP) in Australia typically exhibit low soil particle densities and high clay and silt fines content. These types of materials have poor consolidation properties. In addition, coal tailings are often contained by impoundments with sub-optimal geometries which result in high rates of rise and limited potential for evaporative drying. Rates of rise exceeding 10 m/annum are not uncommon for in-pit tailings storages.
These two factors combine to form tailings deposits with low settled densities, and hence low shear strength characteristics (Figure 1).
In worst case examples, the tailings can exhibit the properties of a liquid. It can take many years (>10) for the top surface of soft tailings to dry and consolidate to a state that will support direct access by earthmoving equipment to facilitate safe placement of fill materials for decommissioning and rehabilitation purposes.
Planning for closure during design and operation will save time and money
Closure planning for coal tailings storage facilities should commence at project inception and consider development of tailings management techniques that enhance the opportunity for safe future placement of capping fill materials.
When it comes to capping and closure of the facility, significant time and cost can be saved through the implementation of carefully considered tailings deposition practices that facilitate development of a deep surface crust of tailings, increase density and minimise water table development.
There can be significant differences in coal tailings (or fine rejects) properties. These differences occur both within a single mine and from site to site as a result of seam differences, mining methods, and processing methods.
With such a diverse range of properties, a tailings deposition management solution that works for one site, will not necessarily work for another. A unique site-specific management solution will consider the particular tailings characteristics as well as other site constraints.
A safe capping design relies on a thorough understanding of the deposited tailings
The capping techniques that are employed and the time in which they can be conducted post-deposition are mostly dependent upon the shear strength profile achieved prior to capping placement, and the pore pressure in the tailings.
The estimation of the shear strength profile is arguably the most critical factor when considering the design of a capping system for a soft tailings deposit. It is essential to obtain the most accurate strength data possible through a thorough investigation to develop a safe and stable capping design.
There are a number of different methods for measuring the undrained shear strength of soft coal tailings deposits, such as shear vane testing, variable energy dynamic cone penetrometer (DCP) testing and cone penetrometer testing with pore
pressure measurement (CPTu) as shown in Figure 2.
CPTu testing is generally considered the best method for obtaining data with a high level of accuracy. The CPTu has the key advantage of providing reliable and repeatable data to significant depths and provides a direct measure of pore pressure data.
Every site is unique and has its own unique attributes, and as such there is not a ‘one size fits all’ approach that can be applied to the design of a testing regime.
Investigation techniques may require a combination of different methods, and it is the responsibility of the engineering practitioner to determine the most suitable method of investigation for a particular site.
A safe capping operation must be designed to mitigate the key failure modes
When analysing the stability of capping layers over soft tailings, three specific design cases need to be considered:
- Bearing capacity – Bearing capacity analyses are undertaken to assess localised (bearing) failures due to the addition of construction equipment load (typically wheel loads) onto the tailings surface. This failure mode occurs when equipment wheels or tracks of the operating machinery “punch through” the capping layer, sinking into the underlying tailings as shown in Figure 3. This failure mode typically occurs as a result of inadequate crust (near surface strength) in the tailings deposit.
- Local stability – This is the analysis of a progressed “bearing capacity” failure mode, involving localised “slip-circle” failures due to construction plant load, in which the machine “punches through” the capping layer, sinking into the tailings and causing some heaving of the immediate surrounding tailings to occur (Figure 4). This failure mode typically occurs as a result of inadequate strength typically in the upper 5 m of the tailings deposit.
- Global stability – For placement of subsequently thicker capping layers, the critical failure mechanism generally becomes global failures, in which an extensive area of new capping material may subside, causing heaving of a large section of the tailings surface some distance from the leading edge of the capping layer being placed.
Global failures are generally not directly related to the construction plant load, rather the “driving” weight of the capping fill layers. They are particularly critical to the capping design when the tailings shear strength profile does not increase significantly with depth. This allows for deep-seated (i.e. greater than 5 m in depth) circular failure surfaces to develop, as is
shown in Diagram 1.
Capping methods for closure development
The primary objective of the decommissioning and rehabilitation program is to establish a final landform that is free-draining and that will remain stable over the longterm. To achieve a stable final landform, capping layers need to be placed over the tailings in a safe and controlled manner.
Capping methods utilised for a particular tailings storage facility will depend upon several factors, including:
- existing shear strength and pore pressure profiles in the deposited tailings
- existing tailings beach slope
- proposed final land use
- the time frame in which the storage is to be capped.
Uncontrolled placement of fill over a tailings dam will likely cause failures and significant displacement of the existing tailings, and this method is no longer favoured.
Methods available for capping of soft tailings deposits in a controlled placement manner include:
- Placed capping layers (conventional capping)
- Placed capping layers with reinforcement
- Radial Telescopic Conveyor Capping
- Beached capping layers and
- Placement of secondary flocculated tailings.
These methods are described below, together with commentary on their relative suitability for application to coal tailings storage facilities.
Placed capping layers (conventional capping)
Where the strength of the tailings permits, it is possible to place an initial, relatively thin layer of fill using lightweight equipment (e.g. low ground pressure dozers, or smaller purpose-built machines).
This layer provides additional confinement and strength gain in the tailings and support for subsequent layers. Placement of this first layer is critical and is illustrated in Diagram 2 and Figure 5.
Placed capping layers with reinforcement
Where the strength of the tailings alone does not permit the placement of thin layers of fill material, reinforcing layers can be used to increase the bearing capacity of the tailings. Reinforcement materials may comprise woven geofabric, geogrids, or various combinations (geocomposites) of these. These materials provide tensile reinforcement in the capping layer and improve the stability, particularly local stability.
The technique for stabilisation of very low strength soil with reinforcement material is illustrated in Diagram 3 and Figure 6. The method relies upon tensioning of the reinforcement material so that stresses can be transferred laterally through the reinforcement material. This will act to support placement equipment and reduce the probability of local failures.
At the same time, the use of ‘fingers’ reduces global loading and helps prevent failure by this mechanism.
This capping technique is expensive, and best suited for small confined areas of a tailings storage facility with very low shear strengths such as decant pond areas.
Radial telescopic conveyor capping
Radial telescopic conveyor capping can provide an alternative to placing an initial confining layer using earthmoving equipment without the need to place machinery or personnel over the tailings surface where the storage is narrow enough to achieve coverage.
This capping approach involves the initial placement of a layer of fill material over the tailings surface using a radial telescopic conveyor. Capping fill material is fed into a hopper which filters oversize particles and places suitable material on a radial telescopic conveyor. The radial telescopic conveyor gradually places the material in an arc formation to the desired layer thickness by rotating from side to side and extending the arm of the telescopic conveyor.
Once a suitable capping layer thickness is achieved in an arc formation out to the limits of the radial telescopic conveyor, the conveyor is moved along the edge of the storage to place the next section of capping material.
This process is repeated until complete coverage of the storage is achieved.
This capping approach is particularly suited to narrow storages where coverage is achieved from the perimeter of the storage without the need to traffic over the tailings surface.
Beached capping layers
The alternative to placing an initial confining layer using earthmoving equipment is to place a layer of granular fill using methods of hydraulic placement, i.e. “beaching”, as illustrated in Diagram 4 and Figure 7.
Slurry formed by a mixture of granular material (e.g. coarse reject, spirals reject sands or fly ash) and water, is discharged across the surface of the tailings. The solids settle out to form a sloping beach, and the water is recovered and recycled. Once a full cover of beached material is achieved, the aim is to construct a final placed capping layer with earthmoving machinery over the top.
The significant advantage of this method is that it provides a safe way for placement of an initial capping layer, without any need to traffic over the tailings until the layer is complete. When used with fly ash, a beached capping layer of significant strength is achieved in a relatively short timeframe to support first capping layer placement using mining equipment.
Placement of secondary flocculated tailings
This technique is similar to the beached capping layer approach, but instead of utilising fly ash, secondary flocculated tailings are hydraulically placed over the existing tailings surface at a controlled rate of filling. A strengthened layer is formed that is capable of supporting the placement of subsequent capping layers using conventional methods.
This method can increase the initial tailings settled density and beach slope, improve water recovery, and enhance tailings shear strength by improving the influence of evaporative drying.
The key to the success of this method is discharging secondary flocculated tailings at a low rate such that the rate of rise of the tailings surface is no more than 1-2 m per year. This maximises the drying and consolidation time and shear strength development for each deposited layer of tailings.
Summary
The right capping technique to be deployed for a particular tailings storage facility will depend upon the in-situ conditions, client requirements for final land use and the desired time frame for rehabilitation.
The key to successful capping closure of a coal tailings deposit is to take a holistic view of the life of facility implications and
consider deposition practices with rehabilitation in mind. Good management of tailings deposition and implementation of water management strategies can help to facilitate capping closure operations by generating tailings with improved shear strength characteristics.
A safe capping design relies on a thorough understanding of the deposited tailings and design of a capping system that mitigates the key capping failure modes. Careful planning, investigation, design and co-ordination of mine site activities by experienced engineers is essential to ensure that soft tailings deposits are capped and rehabilitated in a controlled and safe manner to achieve a final stable landform.
