Technology of Dams and Tailings Disposal

The first studies on dam design and performance of its structures began in 1850, with civil engineer William John Macquorn Rankine, of the University of Glasgow. Since then, techniques have evolved, but some basic principles remain valid.

Currently, there are just over 56,000 dams in the world. The largest concentration is in China (23,842), followed by the United States (9,260), India (5,100), Japan (3,112) and Brazil, which has more than 1,400 dams.

Did you know?

The first record of a dam is in 2900 BC. Located in Kosheish, on the Nile River, the structure was used to supply water to the region

Oldest dams in the world

year 130 Proserpina – Spain
year 400 Ichibanike – Japan Sumiyoshiike – Japan

Tallest dams in the world

335m Rogun – Tajikistan
315m Bakhtiyari – Iran
305m Jinping 1 – China

What is a dam?

Dams are structures built across valleys and used primarily for water accumulation, although they are also used for deposition of other materials, such as tailings from industrial processes. Such structures can be made of land or concrete, and both their design and construction must follow Engineering and Geotechnical techniques.

  • Water reserves
  • Electric energy generation
  • Flood control
  • Sediment containment
    Erosion control
  • Tailings
    containment

Types of dams

There are several types of dam, which vary according to their purpose and the conformation of the terrain where they are located.

Arch

Arc dams are usually built in narrow valleys and are curved in shape, allowing pressure on the embankment to be transferred to the abutments. This type of construction requires large excavations to reach sound rock and ensure adequate support. Loads on the foundation are greater on this type of dam, both on the bottom and on the abutments. Geology is a very important factor for stability in this type of dam. An example is that there may be no discontinuity of the rock at the abutments.

Gravity in concrete

Concrete gravity dams are usually built in places where there is space restriction or difficulty in landfilling (lack of access or lack of soil and rockfill). They require competent sinking and consist of a solid or cast concrete wall. This wall resists, by its own weight, to the horizontal thrust of the water, transmitting the pressures to the floor of the foundation. However, the stability of this type of dam is critical, by sliding and tipping.

Concrete counterfoil

The body of this type of dam is formed by an impermeable slab of concrete, supported by supporting slabs called buttresses. Due to the small area of the base, the pressure is reduced, but still a greater treatment of the foundation (rods and injection of cement) and greater geological control are required. One of the positive aspects of this type of construction is the greater economy in terms of concrete. Example, Dam of the Itaipu-Brasil Hydroelectric Power Plant.

Concrete Face Rocking

This type of dam is a landfill built with fragments of rock and gravel and compacted in layers with vibrating rollers. It is usually built on sound rock foundation, but can also be made on altered rock and other resistant materials. The biggest problem with these dams is the accommodation of the rock formation with the weight and the saturation, that can crack the face. Examples: Segredinho, Machadinho and Itá-Brasil.

Earth

This is the most common and also the oldest type of dam (a dam like this was built in Ceylon as early as 504 BC) normally built to store water. It can be supported on various types of foundations, ranging from compact rock to unconsolidated materials, and can be divided into two types: homogeneous or zoned. The homogeneous is when a single kind of material is used - with the exception of the protection of the slopes, which requires an impermeable material. Slopes need to be smooth for adequate stability. The zoned type has a central impermeable core, surrounded by areas of more permeable materials to support the core. These materials can be sand, gravel, rock fragment or a mixture of the three.

Tailings dam

What is it?

Tailings dams are structures built with earth, rockfill, tailings and even concrete, to store tailings from some industrial processes. In the case of mining, these tailings result from the ore processing, which is when separation of the gross product into concentrate (rich material, with economic value) and tailings (material without market demand) occurs.

How does it work?

The dam is like a barrier. There, the tailings generated in the ore processing are disposed in a controlled, planned and safe manner. The tailings are transported and arranged in the form of pulp, that is, a liquid fraction with suspended solids.

Mine Ore Waste rocks Pile of waste rocks Concentration plant Product Tailings
(solids and water)
Tailings dam Water catchment

Tailings

The iron ore tailings are formed by ultrafine particles containing Iron, Alumina, Phosphorus and Silica, and are classified according to their grain size. The type of tailings directly influences the characteristics and type of dam.

Granular Tailings

It consists of fine to medium sands, not plastic. It presents high permeability, shear strength, and low compressibility.

The use of the tailings in the construction of the dams is the most widespread method, due to the lower cost, material availability and constructive ease. In this case, the dams behave as dry stacks, which are structures constructed by the transport and deposition of soil in aqueous medium.

Fine tailings

Called slime, it consists of silts and clays. It presents high plasticity and compressibility, with difficult sedimentation.

Similar to water containment dams, but built with clayey or rocky soil with a clayey core.

Behavior of tailings

Once disposed in the dams, the tailings undergo several physical transformations over time.

  • Construction phase
    Sedimentation, densification,
    immediate compression, and filtration
  • Upon completion of disposition works
    Densification and filtration
  • Lifespan of tailings dam
    Desiccation and desaturation

Dry stack

The construction of dams by dry stack can be made from three main methods: upstream raising, downstream rising, and center line method. All of them include the construction of a starter dam.

  • Upstream method

    The tailings are deposited hydraulically from the crest of the starter dam, forming a tailings beach. Over time, this material becomes denser and serves as a foundation for future raising dams, which are made with the tailings material itself. The process is repeated until it reaches the expansion elevation.

  • Downstream method

    Subsequent rises to the starter dam are made for the direction of the water stream, until reaching the project elevation. In this case, each rise is structurally independent of the tailings arrangement, which improves the stability of the structure. In addition, it is possible to build the entire rise of the dam with the same starting dam material.

  • Center line method

    System with an arrangement similar to the upstream method. The tailings are discharged from the crest of the starter dam. The construction progresses in a similar way, with rises with successive dams, but maintaining the axis of symmetry of the dam constant. This is the safest method for building tailings dams.

The discharge of the tailings dam is made along the dam crest, by cyclones or series of small pipes, so that there is a uniform formation of the beach. The sedimentation of the particles occurs due to their size and density, that is, the finer and lighter particles are suspended and are transported to the dam center, while the coarser and heavier particles settle rapidly closer to the dam.

Composition of a tailings dam

Cofferdam

Its function is to divert the waters from the river bed, totally or partially, in order to allow the treatment of the foundations in these areas and, possibly, in the areas of flood plains, allowing the dry construction of the earth dikes or concrete structure. The most common cofferdams are those built of earth and blocks of rock. In some cases, the use of metal sheets or impermeable diaphragms is required.

Diversion tunnels

They have the same purpose as the cofferdams, but are built in water courses with steep valleys and, where possible, where there are curves. In many cases, the diversion tunnel is subsequently used as an adduction tunnel to transport water from the reservoir to the engine room.

Freeboard

The freeboard is the vertical distance between the dam crest and the water level of the reservoir. This is intended to safeguard against overflow that can be triggered by the action of waves formed by winds, avoiding damage and erosion in the downstream slope. Determining it is based on predictions of the height and action of the waves.

Crest

At the crest, there must be a drainage system to allow rainwater to flow safely, avoiding erosion and water puddling. If there is no traffic of vehicles on the road, it can be protected only by planting grass. If there is frequent vehicle traffic, the crest shall have a protection, such as the construction of a pavement, or surface wear may occur due to the action of rainwater or even the passing of a few vehicles.

Upstream and
downstream slope

The upstream slope is that part of the massif that is directly in contact with the water of the reservoir, which requires special attention in the Project phase, in the calculation of its stability, besides special care in its maintenance during the phase of operation of the reservoir.

Spillway

Functions as a safety device when the flow of water reaches values that endanger the stability of the dam or serve to prevent the maximum level established for the dam from damaging the agricultural or industrial progeny downstream of the dam. The vector capacity is calculated in order to allow maximum flow (catastrophic flooding), which could occur in the dam section. In popular language, the spillway is known as bleeding

Water intake

It is the set of works that allows the withdrawal, of the reservoir, of the water to be used, to obtain energy or for other purposes. The type of water intake varies according to the type of dam. Thus, in the concrete barrages, the water intake usually consists of a conduit that can cross the barrage mass or be placed in its proximity, while in the earth dams the water intake is built in the reservoir shoulders.

Rip-rap

The rip-rap consists of a dimensioned layer of blocks of stone thrown on a filter of one or more layers, so that it acts as a granulometric transition zone, serving as an obstacle to the escape of the fine materials that make up the mass. The rock to be used must have sufficient hardness to withstand the action of climatic factors. The stones or blocks used in the construction of "rip-rap" should preferably have an elongated shape, avoiding, as much as possible, round-shaped blocks, thus, reducing the chances of landslides.





Alternative to tailings dams

Vale and ITV are already working intensively on the search and development of technologies that make tailings reuse a reality. This is because the tailings deposited in dams have the same – and in some cases, even higher - content than the raw mineral that is mined in other mines.

The recovery and utilization of this material would allow an increase in metallurgical and mass recovery in the processing plants, and contribute to the reduction of the environmental impact and liabilities of the mining companies. Therefore, it would still be possible to avoid the costs and the time spent to obtain the environmental licenses necessary for the expansion or construction of new tailings dams.