Soft Tailings Stabilization:

The Combined Experience of

Syncrud Ltd. and Wismut GmbH

by

Gord McKenna, Alex Jakubick and Dianne Trach

Summarized for UMREG2013

IAEA Workshop hosted by

DIAMO s.p., Czech Republic

What are soft tailings?

Soft tailings are the segregated fines (also known as sludge, slimes, fines, fine tailings, paste, thickened tailings, and some non-segregating tailings) produced during tailings discharge in the tailings settling pond/impoundment. They are closely related to dredge spoils, land reclamation (as in reclamation of land from the sea), and various co-disposal products of sand and fines from a variety of industrial activities. Because soft tailings usually contain residues from the ore and from the milling process, they must usually be contained in perpetuity and become part of the mine closure landscape.

Around the world, billions of cubic metres of soft tailings have been produced, and various technologies have been employed for their successful capping and reclamation. However, soft tailings reclamation costs can be high, expertise and techniques highly regional, and many reclamation efforts are done in isolation. Often the high cost of post-closure soft tailings stabilization comes as a nasty surprise.

Soft tailings have special geotechnical properties (very low densities and very low strengths) that are outside of “normal” geotechnical experience. Consolidation of soft tailings is a major consideration, for several reasons including storage volume requirements, volume reduction, contaminant water release rates, subsidence, and shear strength. Shear strength is an important issue for containment dyke design, breach/flowout considerations, ability to cap, overturning and displacement, trafficability of mining equipment for reclamation, and end land use.

What is the relevance of soft tailings to prevention and remediation? Purpose of this presentation.

• Soft tailings are tailings with low shear strength; they present a challenge to stabilize, cap and reclaim. The surface of the soft tailings has a poor trafficability or insufficient bearing capacity to support the earth moving equipment and trucks needed for remediation. They require special capping and reclamation methods. The cost of these special methods is usually high – often 20 to 100 times the cost of typical mine reclamation (per unit area). Costs to stabilize and reclaim soft tailings are usually tens and up to hundreds of millions of dollars for typical mines – often similar in magnitude to the entire costs of tailings disposal during production.

• Although there are thousands of soft tailings sites worldwide, soft tailings stabilization and reclamation projects are done as one-off projects and there is little published guidance available to practitioners. Sharing experience will provide practitioners with a better understanding of the mechanics and performance, a greater range of options, and solutions that provide better environmental performance at a more economical cost.

Generation of soft tailings

Soft tailings are produced by segregation of tailings slurries during deposition. Coarser grained materials settle

out first, forming a beach, and progressively finer grained sediments are deposited farther down the beach.

Interlayering of finer and coarser layers is common. Often there is a sandy beach and a fines-rich “slimes zone”

separated by an intermediate (often interlayered) zone. Some tailings, such as oil sand tailings, have firm sandy

beaches, and fluid fine tailings zone at the toe, with little progressive fining down the beach and no intermediate

zone. Non-segregating deposits, under ideal conditions, form a uniformly soft to firm deposit at all locations, but

in practice will have a very soft zone at the distal end and most of the area of the deposit will require soft tailings

stabilization techniques.

Dried surface of »IAA Helmsdorf« at Oberrothenbach, 1987

Tailings discharge into TSF Helmsdorf by SDAG Wismut

Issues and areas for improvement at the stage of Generation of tailings

• An understanding of the fundamentals of segregation of slurries – both due to scale effects and process variability • Understanding issues surrounding beaching including variation away from the discharge, and being able to forecast and anticipate the deposit characteristics rather than using trial and error • Applications and limitations of subaerial discharge and deposition • Depositional methods that take into account the need for stabilization of reclamation one day • Better record keeping of the history of deposition

Size of tailings ponds

It is interesting to note that a great many metal mine tailings ponds are in the 150ha size.

Two of the larger sites are Wismut in Germany, which has six tailings ponds totalling about 600ha created by its

uranium mining operations.

Syncrude Canada creates several thousand hectares of soft tailings from its oil sands mining operations.

Soft tailings stabilization options

Water capping

Mechanical capping (with and without geofabric and or geogrid), with our without interim cover

Subaerial hydraulic capping with tailings sand or slurried sands or gravels

Allowing natural dessication and crusting

Crust management (active measures to promote dessication and crusting)

Freeze-thaw dewatering (either in-situ, freezing in thin lifts, or spray freezing)

Subaqueous capping by bottom dumping barges or raining in sand from spray nozzles

Chemical amendment (flocculants, coagulants, cements, pH control)

Accelerated consolidation: Vertical drains (band drains (wicks), sand drains)

Pumping and /or underdrains

Surcharging

Alternative tailings technologies such as: Belt filtration; Cone thickener / paste; Thickened tailings (centrifuges or

cyclones)

Backfilling underground workings

Issues and areas for improvement at the stage of Decision making and planning

• The need for a more rational and consistent approach to soft tailings stabilization, involving a combination of lab testing and analytical work, assessment of alternatives, field testing and trials, and use of the observational approach during full-scale remediation • Better communication with and involvement of stakeholders in decision making

Issues and areas for improvements: Influence of regulatory agencies and other stakeholders

• Options for dry capping — putting the deposit into productive use • Options for wet capping – with a focus on acceptance and administration and long-term management • Clearer definition of intended end land uses • Expanded timing for soft tailings remediation to lessen costs and risks • A better understanding of the public participation in costs

Effect of Water and Clay Chemistry

Another important factor is whether the clays are dispersed or flocculated – a function of the water chemistry and the clay mineralogy (chemistry). Highly dispersed clays are slow to settle and remain in a fluid-like state much longer. While still important, this distinction plays less of a role once the soft tailings reach a soil-like state. The degree of flocculation or dispersion can be controlled by chemical amendments and are often strongly pH and salinity dependent. Alas, chemical amendment of tailings during operation is often not practical due to potential negative effects of recycling this water through the mill. Although important, understanding of the role of water chemistry on soft tailings behaviour is usually overlooked[3]. Small changes in clay content, the fineness of the milling process, depositional methods, and water chemistry can have dramatic effects of the soft tailing behaviour. Some of these can be controlled in the design of the milling and tailings systems, but the tailings remediation is generally thought of as capping whatever comes out of the processing plant.

Issues and areas for improvements: Characterization

• A better understanding of the role of structure, macro-permeability, and shearing during placement on the consolidation properties of soft tailings • Better definitions of the uses and limitation of vane shear strength measurement techniques for soft tailings. A piezovane might be a worthwhile development.. • Use of geophysics (gamma ray, time domain reflectometry (TDR) etc to better define in situ density and clay content of soft tailings • Better assurance that pore-pressure dissipation tests run during cone penetration testing give accurate far-field pore-water pressures. • Better methods of installing piezometers in soft tailings so that the elevation of the piezometer remains constant and the results reliable. • Commercially available very-low capacity cone penetrometer (a “soft tailings” penetrometer) • Better understanding of the effects of container shape of laboratory consolidation measurements • Better assurance that the consolidation properties from the consolidometer and columns can be reliably applied to field prediction. • More published case histories of characterization programs.

Stabilization of the fine tailings zone

In a moderate and humid climate zones, such as NE Germany the fine tailings are usually covered by the

supernatant water of the tailings pond.

Before deciding to pump off the supernatant water one has to consider the following:

To be able to discharge the contaminated tailings pond water a water treatment plant must be in place;

Removal of the water cover increases the radon exhalation from the denuded tailings surface thus

increasing the radiation dose to the remedial workers and general public.

The pore pressure in the denuded fine tailings may cause uncontrolled slides in the tailings.

So, how to go about construction of an interim cover and stabilization of the fine tailings in the central parts

of the impoundment?

Stabilization and covering of the tailings impoundments

For stabilization and covering of tailings a multi-layer system was used constructed in two construction

phases.

1. Interim cover

An interim cover was constructed (usually out of mine waste rock) on the coarse tailings of the beach zone

and intermediate tailings zone, (Fig 8). In these zones, the load of the interim cover (1-2 m) sufficed to

expel sufficient water out of the tailings pores to achieve a shear strength sufficient for surface trafficking.

In addition to an interim cover, the consolidation of fine tailings (in the centre of the impoundment) required

geotechnical enhancement measures such as use of geo-grids and wick drains to make the tailings

surface accessible.

2. Final cover

After sufficient consolidation of the tailings construction of the final cover on the interim cover that served

within the multilayer cover as the drainage layer.

Landscaping of the surface of the remediated tailings impoundment.

Stabilization of the beach and intermediate tailings zones

The water free beach zone of the tailings is stabilized by construction of an interim cover on the tailings

surface that serves several goals:

Minimizes radon exhalation, dusting and infiltration of precipitation.

The geotechnical purpose of the cover is to initiate dewatering of the pores in the tailings by putting a load

on the coarse tailings. As the pore water is being squeezed out the shear strength of the tailings is

increasing.

The interim cover drains the infiltration and squeezed out contaminated pore water into the supernatant

pond in the (usually) centre of the impoundment.

The run off and seepage water collected outside of the impoundment is collected and pumped back into

the supernatant tailings pond during construction of the interim cover.

The interim cover layer is to serve as working platform for the dozers and trucks during construction.

A 1-2 m thick layer of mine waste rock proved to be suitable for the purposes of stabilization of the beach

zone.

Crusted champagne CT

Interim covering of the „accesible“ surface of the Helmsdorf

tailings

GordMcKenna testing whether the soft tailings are trafficable

Use the „Observational Method“ in soft tailings remediation

Two extreme approaches to soft tailings stabilization are common:

(1) brute force (trial and error) versus (2) highly analytical / numerical approach.

We recommend an approach somewhere in between with an adequate lab and analytical work, a field

investigation, one or two instrumented field trials, and well-supervised construction. This could be termed “ an

engineering supervised brute-force approach”

The “engineering supervision” part of the business would include the application of the observational method , i.e.

designing for the most likely conditions, having realistic contingencies worked out in advance, and monitoring to

modify the program during construction.

Some projects have successfully used the observational method on a hectare by hectare approach – adjusting

methods based on observed performance for different areas as the program progresses.

A ‘value of additional information’ analysis will help to determine how much additional engineering would be useful.

Bearing capacity / trafficability

kPa 0.1 1 10 100 0.01

Fluid tailings Soft tailings

Water capping

Raining in sand

Hydraulic sand cap (beaching)

Soft ground strategies

“Normal” terrestrial reclamation

2

Sheer strength of the soft tailings

Soft tailings are very weak and the numerical values of the strength are outside the experience of people not

working directly with these unique materials.

The mechanics of these materials straddles the line between soil mechanics (using shear strength) and fluid

mechanics (using viscosity).

Different units are common in different areas of the world. Here are some approximations.

1 kPa = 1 kiloPascal = 1 kN/m2 @ 0.145 psi @ 20.9 psf @ 0.01 tsf @ 0.01 kg/cm2 @ 0.01 bar.

Fluid soft tailings have shear strengths often measured in Pascals.

Rolling out geo-materials Installing

wicks Stabilized surface

5 T trucks bringing capping fill Special backhoe to place fill

This is the approach one uses if capping is left to the end.

Covering of the „accesible“ surface of the tailings deposit:

The „thin cover layer“ approach

Basic concept of interim cover construction used at Wismut for stabilization of intermediate

(mixed coarse and fine tailings layers) and fine tailings

Interim Covering of the Intermediate Tailings Zone

$$$

Issues and areas for improvements: Stabilisation technology and engineering

• More work into adjusting tailings depositional methods to create less costly stabilization and capping is required • A database of worldwide field performance, both empirical and analytical for rates of consolidation and capping technologies • Documentation of more rules of thumb for stabilization and capping • Determination of the mechanics of bearing capacity mechanisms of embankments and machines on soft tailings • A better understanding and prediction of consequence of bearing capacity failure for embankments, as especially for equipment and operators • Greater use of plants for stabilization of soft tailings • Use of underdrains for creating bearing surfaces through suctions (enhanced crust management) • Methods of slope matching for hydraulic capping of soft tailings • A better understanding of the distinct roles of design and prediction

Types of band drains and enhancement of consolidation

Many soft tailings stabilization projects consider the use of vertical drains – the most common form being band

(wick) drains, but sand columns are also used.

Perhaps surprisingly, the decision to use vertical drains is often a highly charged issue, perhaps due to some basic

misconception. Many managers (and engineers) incorrectly believe vertical drains can be used to increase the

total amount of subsidence (settlement) of any tailings deposits.

But vertical drains are only useful to accelerate consolidation, not to change the absolute amount of consolidation.

And vertical drains are only effective if there are excess pore-water pressures in the deposit. About half the sites

for which vertical drains are initially considered have significant excess pore-water pressures.

A systematic approach to the use of vertical drains in soft tailings

Estimate an average permeability (or coefficient of consolidation, cv) and use textbook equations to estimate

the vertical drain spacing for a desired rate of consolidation. Compare to case histories. One needs to choose

the goals of the accelerated drainage (increased tailings storage area, increased water release rates,

strengthening, early subsidence, etc.) and a suitable timeframe. The spacing (and economics) is often very

sensitive to timeframes – longer timeframes are usually much more economical).

Using laboratory or back-analyzed parameters, re-run this analysis using emerging large-strain finite element

computer codes that permit axis-symmetric flow to a vertical drain

If economic feasibility is given at the spacing determined, test the wick with the slurry for clogging, blinding,

and release-water chemistry by means of inserting a small piece of wick horizontally across the base of a

consolidometer (over the bottom drain).

Next install by hand a few vertical drains (two to five perhaps) in the field and monitor release water rates,

subsidence, and change in pore-water pressure radially away from the drains using piezometers. Use the

results to update the model.

Next perform a field trial of 100 to 200 drains at full scale using normal installation techniques. Use two

different spacings – one at that predicted in the analysis and a second at drain spacing of 1.5 to two times

greater. Monitor these tailings for subsidence and pore-water pressure change. (Monitoring beyond surveying

for surface subsidence is a bit of an art). These test areas are typically about 1 hectare in size.

Using the results of the field trial, optimize and go to full scale. Continue observing and monitoring to further

optimize results.

Spacing of vertical drains and economics

Typical vertical drain spacing for shallow drains (less than five metres deep) in soft tailings is one to 1.5 metres. Typical spacings for deeper drains is at least three metres, but more commonly five to eight metres – greater spacings are much less expensive and can be used for soft tailings with higher permeabilities and for situation where longer consolidations times can be accommodated. Screening level economics can be done assuming $1.50 to $2.50 CDN per metre of wick drain. Where densification for volume reduction the main goal (usually only useful for densifying fluid tailings in this regard), costs can be expressed as cents per cubic meter of timely storage space gain.

Issues and areas for improvements: Design and performance of vertical drains

Better communication of the potential roles of wick drains for soft tailings

A commercially available wick-drain finite strain consolidation model

Assessment of the use of vacuum assisted wick drains

• Greater documentation of performance of wick drains in soft tailings along with rules of thumb about spacing

and effectiveness

Construction of the interim cover on tailings

Hydraulic Sand capping

Stabilization of the fine tailings zone:

Sub-aquatic construction of the interim cover

Basically, the fine tailings covered hydraulically or mechanically. With the agreement of the regulator (who

demanded that no (mud-) slides in the fine tailings, a novel method was tested at the Helmsdorf tailings

pond – the mechanical sub-aquatic covering (Fig’s 9, 10, 11)

The method consists of bringing the cover material on a pontoon above the designated tailings pond

segment. The positioning of the pontoon above the designated segment is by means of GPS. The material

is released by opening the chute of the pontoon and letting the material sediment through the supernatant

solution on the tailings.

The advantage of such “gentle raining” of the cover material is that the cover material stays on the surface

of the fine tailings (does not sink into the slime) and by repeating the procedure allows to build up an

interim cover on the surface of the fine tailings prior to removal of the supernatant pond. This approach

prevents the increase of dusting, radon exhalation and dose increase upon removal of the supernatant

water.

The geotechnical disadvantage of sub-aquatic cover construction is that the load applied on the fine

tailings is only about half of the dry load (due to lessening of weight in the water) and consolidation is

accordingly slow. In the particular case of Helmsdorf the consolidation of the fine tailings has been

enhanced by placing into the fine tailings 5 m deep wick drains. The combination of the sub-aquatic cover

and wick drains proved to be sufficient for partial dewatering of the fine tailings, thus for the required

consolidation.

Loading of material for the sub-aquatic construction of the interim cover on the fine

tailings of the TSF Helmsdorf (10)

Cover material release from the boat on a well defined spot in the TSF Helmsdorf (11)

Issues and areas for improvements: Long-term landscape performance

• Better understanding of the shallow groundwater hydrology of internal drainage layers within the covers • Better prediction of long-term subsidence and differential subsidence • Better framework for designing long-term covers that are tolerant to settlement

Issues and areas for improvements: Modelling and analysis

• Coming up with a better split between the efforts that go into empirical and analytical methods • A better understanding of the role of creep in densification of high void ratio slurries and the timing of the availability of strength required for capping • Better understanding, predicting, and handling of lateral variability of deposits – extending current one-dimensional models into two dimensions (including lateral deformations) • An analytical framework for understanding soft tailings bearing capacity for embankments (covers) and equipment • A better understanding of the seemingly overly beneficial effects of geofabrics • Better marrying of fluid mechanics with soil mechanics for the performance and description of soft tailings bearing capacity behaviour

Contouring of TSF and Dam flattening

Issues and areas for improvements: Costs

Value added and cost-benefit strategies associated with tailings management and reclamation

Methods to reduce the high cost of soft tailings stabilization and reclamation

• Better framework for assessing alternatives and overcoming limitations of discounted cash flow analyses

Bring your kids to the „old tailings“