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[FROM THE ARCHIVE] Danger in store

  • Written by  Chandra Durve and Dr Edward Ferrett
  • Published in Opinions
[FROM THE ARCHIVE] Danger in store
21 Mar
2019
In October 1985, consultant mining engineer Chandra Durve and Dr Edward Ferrett, then head of the Faculty of Technology at Cornwall Technical College wrote an article for Geographical about tailings dams and some of the dangers they pose. The article, reproduced below, is especially prescient given the string of tailings dam disasters in recent years, most recently the devastating collapse of the Brumadinho dam in Brazil 

This is an archive story, published in the October 1985 edition of Geographical magazine. All facts, figures and statistics were accurate at the time of original publication

In July 1985 a dam used by a mining company high in the mountains of Northern Italy collapsed. Thousands of tonnes of mud and water cascaded down the mountains, the village of Stava was engulfed, and a horrific death toll resulted. It is too early at this stage to speculate on the possible causes of failure, since little information concerning the construction and maintenance of this dam is at present available.

However, important questions arise associated with such dams used by the mining industry: why are they used? Are there alternative methods of waste disposal? Is it possible to prevent failure?

As the global demand for metals and other minerals has increased, mining companies have resorted to mining ores of lower and lower grades. For many metals 90 per cent of each tonne of ore mined consists of waste, which has to be discharged after the extraction of the valuable minerals. Most extraction processes use large quantities of water. The ore is crushed and then ground to liberate the minerals; the waste material consists of fine sand in the form of a slurry and is known in the industry as tailings. It has been estimated that 1,500 million tonnes of tailings are produced each year, and the land disposal area required for this material increases by about 45 square kilometres each year.

mineMining for metals is a waste-intensive process. For many metals 90 per cent of each tonne of ore mined consists of waste which has to be discharged

Historically, tailings have been discharged in a variety of ways. Some are uncontrolled but other methods include depositing tailings under a large body of water, or in underground caverns. Both these methods have their limitations.

In the Phillipines, for example, 75.3 million tonnes of tailings were discharged into one river over a 15-year period. The environmental and ecological impact of this method of disposal, particularly around river estuaries, has largely been responsible for a decline in the practice. Lake disposal has never been very common, since the tailings can form a mat on the lake floor, again causing ecological problems. Disposal of waste to sea has been restricted to mines located in coastal regions, and is relatively uncommon. Apart from the environmental considerations, the principal objection to underwater disposal schemes is economic.

As metal prices increase and the technology of metal recovery improves, material which may once have been considered waste can become usable as ore. Several mining ventures in recent years have involved re-working the waste from earlier mining operations and such ventures can become uneconomic if the waste material has to be dredged from under water.

shutterstock 526285300Iron ore must be crushed with huge volumes of water in order to extract the valuable metal

Underground disposal has been used since the turn of the century. The most common method involves the use of the coarser product of the tailings as a supporting medium in worked-out areas of the mine. This can often rob the tailings of material which would aid its consolidation upon discharge. Schemes involving the disposal of all the tailings underground in subterranean cavities have been proposed but rejected, due to concern over effects on local groundwater.

The problems associated with underwater and underground disposal and the need for large quantities of water for the mineral extraction process has resulted in the increased use of tailings dam storage. It is estimated that today more than 90 per cent of tailings are stored in this manner. There are several types of tailings dam constructions but, in general, the topography around a mine site determines the type of dam construction. The valley dam, for instance, is formed by a dam wall across the narrowest point of a valley, with the tailings being deposited behind the wall. When a valley contains an existing stream, this is normally protected by the construction of a culvert or some form of diversion scheme, which is designed not only for maximum annual flow, but also for the one-thousand-year flood.

Earth dams which retain lakes of tailings, waste material from mines, are a potential danger worldwide. The danger is growing as companies mine lower grade ore, so producing more tailings

In an area of flat terrain, lagoon type dams are often used. These consist of a circular or square dam wall, which encompasses the deposition area. Side hill dams are used in areas of undulating terrain, and involve the construction of the dam wall against the side of a hill.

The construction of a tailings dam differs significantly from a water reservoir dam. The main difference being that the former will reach its full designed height, in some cases more than 100 metres, in a series of stages during the life of the dam. There are two principal methods of dam-wall construction - the ups tream and the downstream method. In the upstream method, a smaller starter dyke is placed at the extreme downstream point of the disposal site and the centreline of the wall is built towards the tailings pond.

diagram

In the downstream method, the centreline wall crest is shifted downstream and away from the tailings pond. An upstream dam uses far less wall material and occupies less land than a downstream dam, but can be less stable particularly if seismic activity is a feature of the locality. The primary function of a tailings dam is to allow the solid particles in the tailings slurry to settle and form a stable solid mass, and to store water temporarily. Most of the water which discharges into the tailings dam is normally recycled back to the processing plant.

However, in many cases, some water is allowed to flow through the dam wall in a controlled manner to aid the consolidation of the tailings slurry. This inevitably involves the construction of well-designed drains. Since the tailings material is fine, filters are required to ensure the protection of drains from blockage. A blockage of the drain system can cause a build-up of water pressure within the dam wall. If this pressure is not relieved, then some form of wall failure can occur. This type of failure is perhaps more probable than any other, although it is seldom catastrophic, since it is normally localized and remedial action can be taken.

A blockage of the drain system can cause a build-up of water pressure within the dam wall. If this pressure is not relieved, then some form of wall failure can occur

The recycled water is usually removed by means of decant towers, which skim the clear water from the tailings pond. This facility is particularly important, and has to be designed to cater for sudden downpours of rain. In most dams an additional spillway or channel is constructed. This allows the water level in the dam to be maintained at all times and avoids the possibility of water flowing over the dam wall, thus causing a wall breach.

waterUnlike a traditional water dam such as the one above, which feature an impervious concrete wall, tailings dams are constructed from the waste product itself

Most potential failures can be prevented by measuring variations in the water flow through the drainage system and by the use of instruments (piezometers) which monitor the water pressures. Piezometers consist essentially of small electrical or hydraulic devices which can be positioned at specific locations within the dam wall and the tailings pond.

Piezometer measurements will determine the level of the water table (phreatic surface) within the wall, a crucial factor in providing vital information on the stability of the designed wall, and the state of the drains within it.

In 1977, we commenced a six-year programme of study into the design and monitoring of tailings dams. The work was carried out at Camborne School of Mines, Cornwall, with another research engineer. Dr R. Coulton, and a firm of consultant engineers, Watermeyer, Piesold, Legge and Uhlmann. Part of the work involved the accurate measurement of the phreatic surface and the design of an effective electronic measuring system using a microprocessor. The system consisted of 72 ceramic piezometer cylinders which linked through a scanning device to three electrical transducers. The electrical signal from the transducers was fed to a microprocessor and thence to a computer. The computer supplied a complete picture of the water pressures within the dam and the position of the phreatic surface in both a digital and graphical form. A complete scan of a laboratory dam containing 72 piezometers could be taken within a matter of minutes.

It is this type of continuous monitoring that could provide early warnings of potential failures, and this instrumentation system is currently being assessed for commercial development.

It is worth noting that disasters of the type which overtook Stava are extremely rare, and that substantial advances in dam stability have been made in recent years. Tailings dams provide an economic and sensible method of mine waste disposal. They reduce the effects of necessary mining operations on the environment and are perfectly safe, with one proviso: that they are welldesigned and monitored.

This is an archive story, published in the October 1985 edition of Geographical magazine.
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