Cacciuttolo2014

Geosynthetic Mining Solutions at Copper Tailings Dams: A Review of the State of Practice in Chile.

Carlos Cacciuttolo, Engineering Council, Chile.

Jack Caldwell, Robertson GeoConsultants, Canada.

Scott Bernard, CICH AG, Chile.

Outline Introduction

Use of Geosynthetics in Tailings Dams Key Issues

Seepage Control in Tailings Dams

Geosynthetic Types used in Tailings Dams

Constructability Issues on Tailings Dams

Applications

New Trends

Conclusions

Acknowledgments

Introduction

Introduction Some copper-mining operations, which use flotation processes, generate a total of approximately 100,000 mtpd of tailings that need to be stored in a cost-effective, safe and environmentally friendly manner. This means tailing storage facilities (TSFs), which in some cases have a 100 m dam height that need to be designed for:

Optimal containment;

Preventing seepage from the impoundment through the dam and adjacent areas, for environmental protection and dam safety;

Controlling piping issues;

Providing filter and drainage capacity; and

Optimizing the physical and hydrologic stability of the TSF for the mining lifetime (operation and closure), taking into account high seismic activity and possible extreme floods.


Advantages

No clay layers: geosynthetics offer an opportunity to minimize the use of costly clay filters for grading the materials in the dam.

Simplicity: geosynthetics simplify the construction process so that the overall construction period is reduced.

Effective long-term performance: geosynthetics offer better performance against acid-rock drainage (ARD) generationunlike impervious core dam materials (such as clay), whose structural properties may change due to the effects of oxidization and leaching caused by

sulfide-rich mine tailings.

Disadvantages

Prone to damage: geosynthetics are more susceptible to mechanical damage caused by by settling of the support surface, differential deformations, or puncturing of the geomembrane.

Geosynthetic loading: in order to design adequate geosynthetics systems, it is necessary to consider water loading, excessive tailings loadings, or eventual loads of snow on

geomembranes placed on the upstream slope of the dam.

Sensitivity to climate conditions: extreme temperature changes cause geosynthetic thermal

contraction. Geosynthetics can be damaged by UV rays if they are permanently exposed to

them. The effect of wind on geosynthetics can also be critical.


Source: Mafra et al., 2008.

The deposition of slurry tailings from the dam crest causes the development of beaches which typically

have the shape of a fan. The covering by the tailings

acts as a seal that eliminates or at least attenuates

seepage from eventual perforations or tears generated

during its useful life (Barrera and Lagas, 2012).

In the case of tailings sand dams, slimes (cyclone overflow) are discharged into the impoundment. These

materials are very fine (total tailings fine fraction) and

retain a great deal of water once deposited. This

results in higher water losses trapped in interstitial

slimes, with a very low permeability in the order of K ~

10-6 cm/s (Barrera, 1998).

To prevent seepage through the dam foundation, a cut-off trench and grout-curtain control system is

installed along the upstream toe of the dam, which

allows for the interception of the seepage. A

geotextilegeomembrane liner is placed at the upstream face of the cut-off trench.

Geosynthetic Types used in Tailings Dams

Geotextile and Geomembranes Types used in Tailings Dams

Properties of Material Test Method Unit of Measure Required Value Required Value

Weight per Area Unit ASTM D5261 g/m2(oz/yd2) 335 (10) 180 (6)

Apparent Opening Size, Sieve No. ASTM D4751 Mm 0.15 0.21

Grab Tensile Strength ASTM D4632 N 1,110 600

Grab Elongation ASTM D4632 % 50 50

Puncture Strength ASTM D4833 N 665 350

Trapezoidal Tear ASTM D4533 N 445 240

Permittivity ASTM D4491 1/sec 1.2 1.4

Flow Rate ASTM D4491 l/min/m2 3,100 4,500

UV Resistance (after 500 hours) ASTM D4355 % 70 70

Geotextiles

Table: Nonwoven, Needle-Punched Geotextile Properties

Geomembranes

Table: HDPE Smooth-Texture, Black-Color Geomembrane Properties


Properties of Material Test Method Unit of Measure Required Value

Thickness ASTM D5199 mm (mil) 1.5 (60)

Density ASTM D1505 g/cm3 0.94

Strength at Yield ASTM D6693 N/mm 22

Strength at Break ASTM D6693 N/mm 40

Elongation at Yield ASTM D6693 % 12

Elongation at Break ASTM D6693 % 700

Tear Resistance ASTM D1004 N 186

Puncture Resistance ASTM D4833 N 480

Oxidative Induction Time ASTM D3895 Min 100

Carbon Black Content ASTM D1603 % 2.0 3.0

Geomembrane Design Matrix

Table: Geomembrane Liner Design Matrix (Lupo and Morrison, 2005)


Foundation

Conditions () Liner

Bedding Soil ()

Overliner Material

()

Effective Normal Stress (MPa) ()

< 0.5 0.5< < 1.2 > 1.2

Firm or high stiffness

Coarse grained Coarse grained Fine

grained

2.0 mm HDPE

1.5 mm HDPE

2.0 mm HDPE

2.0 mm HDPE

2.5 mm HDPE

2.5 mm HDPE

Fine grained Coarse grained Fine

grained

1.5 mm HDPE

1.0 mm HDPE

1.5 mm HDPE

1.5 mm HDPE

2.0 mm HDPE

2.0 mm HDPE

Soft or low stiffness

Coarse grained Coarse grained Fine

grained

2.0 mm LLDPE

1.5 mm LLDPE

2.0 mm LLDPE

2.0 mm LLDPE

2.5 mm LLDPE

2.5 mm LLDPE

Fine grained Coarse grained Fine

grained

2.0 mm LLDPE

1.5 mm LLDPE

2.0 mm LLDPE

2.0 mm LLDPE

2.5 mm LLDPE

2.5 mm LLDPE

Constructability Issues on Tailings Dams

Constructability Issues on Tailings Dams Climate and Topography Conditions

Source: Belfi, 2013; Incolur, 2013 webpage.

Mine facilities in Chile are located mainly in the Atacama Desert or the Andes range areas, where the is an adverse climate and extreme topographic conditions.

Construction of TSFs presents unique challenges including demanding construction schedules and difficulties in sourcing specialized contractors.

The geomembrane deployment typically requires grading slopes not higher than 2H:1V.

Under windy conditions the use of sand bags to secure the geomembrane stability is necessary.

Constructability Issues on Tailings Dams Geosynthetics Seam Works

The geosynthetic sheets are unrolled down the slope and cut to the appropriate length to cover the entire inclined slope.

The panels are joined by manual thermofusion weld seaming along the slope,

Panels are joined by manual extrusion weld seaming along the length of the dam.

All welds and liners are inspected using industry-accepted QA/QC controls and test procedures.

Workers, installers, and inspectors must receive high-angle work and safety training, which emphasizes the correct use of anti-puncturing shoes, and elements of personal safety for steep-slope works.

Applications

Applications Geosynthetics at Starter Dams Constructed with Borrow Materials

An impervious core is installed on borrow materials. This consists of a 1.5 mm (60

mill) flexible, HDPE geomembrane

bedding with two anti-puncture layers that

protect it against possible damage by the

construction materials: first a cushion layer

of nonwoven, needle-punched geotextile,

and second a silty sand layer are placed

as a filter core on the borrow material.

Source: Belfi and Incolour, 2013 webpage.

The starter dam is the embankment of the TSF in the early stages, needs to have a continuous impervious barrier along the upstream face of the

dam, as well as along the bottom of the dam (cut-off system), which

waterproofs the riverbeds alluvium and the shoulders colluvium.

Applications

Source: Barrientos, Tailings 2013 webpage.

Geosynthetics at Cycloned Tailings Sand Dams

(1) a new wooden trestle lift of 5 m needs to be installed

along the dam crest and relocated to the tailings sand

pipelines;

(2) a slimes pipeline needs to be relocated; and

a new stage of nonwoven geotextileHDPE geomembrane on the upstream slope is required.

(3) Once the slimes pipeline have been relocated the

geosynthetic installer builds a new anchorage trench along

the tailings sand dam, starting the placing of the 200 g/m2 (6

oz/yd2) anti-puncture, nonwoven, needle-punched geotextile,

and once it is well advanced, placing the 1.5 mm (60 mill)

HDPE geomembrane.

(4) Finally the slimes spigot pipelines are installed.

The cycloned tailings sand dam typically has a crest width of 20 m, to provide: a safe base for the wooden trestles (support of two tailings sand delivery pipelines); an adequate

geomembrane liner anchorage trench/slimes delivery pipeline space; electric lighting lines

space; and prompt access for adequate trafficability of vehicles maintenance/surveys.

When a dam crest of 20 m and a 2H:1V upstream slope is constructed:

Applications Geosynthetics at Rockfill Dams

Zoning within the waste rock dam section will need to be accomplished with the filter criteria to avoid piping (Sherard, and Dunnigan 1985)., and can be achieved by varying

lift thickness and compaction specifications for different areas of the embankment.

An impervious core of geosynthetic materials is installed on the upstream face of the dam, consisting of 2 mm (80 mil) HDPE geomembrane cushioned by one 400 g/m2 (10

oz/yd2) anti-puncture, nonwoven geotextile that protects it against possible damage by

the dam materials

Additionally, a nonwoven geotextile sacrifice layer of 200 g/m2 (6 oz/yd2) is

placed over the HDPE geomembrane to

provide UV ray protection while the TSF is

filling with tailings

New Trends Bituminous Geomembrane (BGM)

New Trends Bituminous Geomembrane (BGM)

A bituminous geomembrane (BGM) liner is used in projects worldwide, and shows the waterproofing versatility of this type of geomembrane in cases of earth dams with gentle slopes, and concrete dams with subvertical slopes.

The joints between the BGM panels ensure an additional advantage over other types of geomembranes. The joints are welded at a high temperature created by a simple gas torch.

This liner has advantages in terms of withstanding wind effects, having no stringent base layer requirement, being useable on other dam types such as rolled compacted concrete (RCC) dams, and decreasing construction times and the quantity of borrow materials required.

Source: Coletanche webpage.

New Trends Smart Geosynthetics

New Trends Smart Geosynthetics

A new emerging smart geosynthetic, fiber optics technology (Dijcker et al., 2011), is used at TSFs to cushion the geomembrane liners, monitor the dams behavior, and provide information.

The new trends in future will be providing geosynthetic solutions that will control and prevent the leakages at TSFs.

Source: Dijcker et al., 2011.

Conclusions Although the geomembrane is not completely waterproof, it is nearly waterproof when compared to other materials and some soils, even clay soils. The main function of geosynthetics on TSFs is to serve as liner layer between different materials to prevent and mitigate eventual leaks that may occur. The covering by the tailings acts as a seal that eliminates or at least attenuates seepage derived from eventual perforations or tears generated during its useful life. Use of geosynthetics at TSFs is growing daily, that support environmentally friendly tailings-management solutions, that control and reduce the tailings seepage, and decrease the borrow pit works for filter dam construction.

Acknowledgments The authors of this paper wish to express their appreciation to the organizers of this important conference for the invitation to participate and be part of it, and also for having the opportunity to share experiences with colleagues from around the world. Carlos Cacciuttolo, Chile. [email protected] Jack Caldwell, Canada. [email protected] Scott Bernard, Chile. [email protected]

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Cacciuttolo2014