Failures of tailings dams are one of the most hazardous processes that may occur and can cause extensive damage to life, property and health. The mining industry has experienced several significant dam failures in recent history: Merriespruit 1994 (South Africa), Omai 1994 (Guyana), Aznalcollar 1998 (Spain), Baia Mare 2000 (Rumania), Aitik 2000 (Sweden), Bento Rodrigues 2015 (Brazil). Historical records of tailing dam failure are of about 20 events per decade, showing a tendency to shift from developed countries to developing countries.
To maintain the impoundments standing is one of the most challenging tasks in mine waste management. Failures can originate from overloads, anomalous behavior of the material used to build the dam (normally tailings), or from problems with the drainage mechanisms, which result in an increase of pore water pressure, and therefore a loss of resistance. Causes of failure can be attributable to poor dam management that includes inappropriate dam construction procedures, improper maintenance of drainage structures and inadequate long- term monitoring. Obviously, periods of unusual rain may increase the number and severity of the events. Prevention and protection are therefore crucial, and thus real time monitoring and improvement of dam construction are key tools for the safety management of the embankments.
For decades, the auscultation carried out in tailings dams consisted in periodically visiting selected spots along the barrage, where boreholes had been drilled, and performing manual measurements with a variety of instruments, mostly consisting in water level meters, piezometers in open wells, or inclinometers. The recent increase in failure of dams may be attributed to the combined effect of rapid dyke construction along with poor maintenance and monitoring.
The stability of a tailings dam can be controlled by measuring key parameters of the material used to build it and its surroundings. Pore water pressure and embankment deformation in tailings dams are the most important physical characteristics to be monitored. Manual systems of data acquisition provide few data over time, which is a disadvantage since if the dam is losing resistance rapidly they may not avert a crisis. An automatic sensor network makes it possible to get data in a matter of minutes. The measured parameters are directly related to soil resistance, and by measuring them automatically and continuously, information on the safety factor of the dam, as well as the expected deformations is available in real time.
Most of the tailings dams have water level meters and inclinometers installed on boreholes to enable periodic controls. Piezometric sensors provide information of the pore water pressure, whereas the measurement of the ground displacement in depth is automated by an in-place inclinometer which permits to measure the horizontal displacement on depth. Other sensors that may be of interest are waste management gauges to measure the water level in the dam or settlement cells to monitor settlement processes of the walls and surroundings. Meteorological stations to control rain may be critic considering new climatic scenarios.
The design of a consistent monitoring strategy is therefore crucial. The following are some of it main advantages:
- It improves knowledge of the most critical potential failure mechanism, which is the basis for a correct risk analysis.
- It helps detecting situations that are likely to trigger a failure of the dam (e.g.: water level exceeding a certain limit; acceleration of displacements, etc.), and applying corrective measures.
- It serves as a tool for designing the dam’s growth.
- It provides a sound basis for the establishment and implementation of the proper response in case of a failure involving human hazards.
In recent years, the installation of automated in-situ monitoring systems has increased. However, installing traditional wired systems or optic cable to obtain data from sensors permanently installed in the dams is often a challenge or economically non affordable. Some difficulties are the big extension of the tailings dams (which represents long distances of cables) the lack of power availability (massive solar installations are required) and the limited communication infrastructure (which means that data have to be collected in-situ).
LS-G6, the wireless low power monitoring solution from Worldsensing, consists of a data acquisition system which collects the data from the sensors and sends it wirelessly to the gateway (central station). At the same time the gateway uploads the data in the internet or in an intranet. The long range capabilities of the radio communications between the dataloggers (placed next to the sensors) and the gateway make this system a powerful solution for monitoring of spread areas, such as mines with many different sensors.
The long range communication between the gateway and the dataloggers allows for a distance of up to 15 km between the gateway and the dataloggers in an optimal situation (Line Of Sight). In the context of a mine, this means that, by placing the gateway in the mine's central office, almost any point of the mine where a sensor is placed can be reached. In addition LS-G6 dataloggers use embedded batteries that make the system autonomous up to 10 years, which enhance maintenance cycles of the monitoring system.
The data collected are compared to previously defined thresholds in order to achieve an indicator of the dam stability. In this way, the network is also used as a low power warning system for the embankment stability.
Our LS-G6 monitoring system has been installed successfully on several mining sites to provide a real time control of the main mine’s tailings dam. In addition many other sensors have been integrated in further phases to control mine hydrogeology and stability of the open pit process area and access roads.
As a summary, Wireless Sensor Networks on mining sites are increasingly viewed as a reliable and robust solution. The advantages of the long range wireless are without question: efficiency, safety, robustness, flexibility, easy to upscale the network, easy-to-use and cost effective (compared to cable or manual collection).
LS-G6 offers a robust system that, thanks to the alarms defined over the sensors data, becomes a very efficient preventive tool; warnings can be issued to stakeholders or workers in the mine increasing the security of the whole process and the people involved.