Mining operations, both open-pit and underground, like the majority of large public and private enterprises, rely on the continuous operation of their capital-intensive infrastructures to meet their operational or production targets.
However, these can be interrupted by unanticipated seismic shocks in active areas. Mineral processing plants contain multiple pieces of large rotating machines that must run continuously to meet production targets. As an example, grinding mills and large pumps are designed to operate within an envelope that takes into account variations in loading due to changes in feed and flow rates or work index, but their bearings are typically not designed to deal with the peak vertical and horizontal accelerations associated with a major seismic event.
Similarly, large conveyors, skips and personnel hoists are typically not well suited to withstand significant horizontal accelerations when operating at full speed.
Fortunately a technology now exists that, in the event of an earthquake of a magnitude great enough to cause damage to processing or hoisting equipment, can provide operators with enough time to bring equipment to rest before the onset of the seismic event. In this way, the forces due to the accelerations associated with the earthquake do not combine with the normal operating loads to cause irreparable damage to the equipment.
The technology that provides the advice warning of an impending earthquake is the Earthquake Early Warning System (EEWS) developed by Weir-Jones Engineering Consultants Ltd. of Vancouver, B.C. Weir-Jones has been active internationally in the mining, mineral processing and materials handling sectors for nearly 40 years.
The development of an on-site earthquake early warning system commenced in early 2006 when the B.C. Ministry of Transportation was looking for a way to protect motorists on a major highway in Vancouver from being trapped in a tunnel during a significant seismic event.
Engineers at Weir-Jones recognized that the destructive energy of an earthquake is related to large S-waves that are preceded by the smaller-amplitude P-waves arriving on site seconds earlier. An early warning system would detect and characterize the P-wave and issue an alarm, which would prevent vehicles from entering the tunnel and therefore minimize potential losses.
The difficulty of the problem is associated with the detection algorithm, configuration and type of sensors, system reliability and data communication. Weir-Jones was contracted by the ministry at the end of 2008 to design and install the system and by mid-2009 the equipment was installed and had entered the test phase.
EEWS monitors ground motion in real time to detect the presence of P-wave signatures of a remote strong earthquake. The equipment consists of a set of triaxial vibration sensors mounted at various depths in two boreholes located at both ends of the critical facility.
Mechanical vibration is converted into a fluctuating voltage signal that is digitized using 24-bit analogue-to-digital converter. Multi-channel data streams are merged by the data acquisition equipment and sent to the central processing computer. Time synchronization is achieved using GPS technology. The central computer runs the seismic detection and proprietary classification software, which continuously analyzes data streams and triggers only when a set of multiple criteria is met.
These features enhance the reliability of the hardware/software system and guarantee accuracy of the issued alarms. Due to the multiple redundancies built into the system, a failure of individual components has no effect on the overall performance. For example, if a subset of the sensors fails, the software parameters will be adjusted to work with the remaining ones. Only high-reliability components were used to build the system, which has been operating continuously for two years without a failure or a false alarm.
The output of the EEWS is a binary signal reporting the absence or presence of a precursor of the strong ground motion associated with the S-waves. If the early warning system reports a positive state, meaning a target P-wave is detected, the central computer will generate a visual and audio alarm, and will close the electric circuit connected to the external alarm sub-system. This autonomous decision-making takes less than a second.
This external system might be a control closure mechanism that turns off critical processes at the protected facility to minimize potential damage in terms of human lives and/or valuable equipment. Since shutting down the processes is usually associated with high costs, the EEWS is required to be highly reliable and not prone to false alarms. It should be emphasized that even the Japanese earthquake early warning systems, which pioneered these types of applications in the world, have reported false alarms due to various technical reasons.
Design and installation of an on-site seismic early warning system is more art than science since local circumstances are unique and the system architect must take into account a series of parameters that will influence the effectiveness of the entire project.
The ultimate success of EEWS is measured in terms of reducing the risk to human lives and valuable assets due to seismic hazard. The benefits are obvious: it provides an early warning in terms of a few seconds to up to dozens of seconds allowing companies and institutions to stop critical processes; it informs people about the coming destructive ground shaking; it reduces potential damage to vulnerable components and sub-systems and it records the strong vibration for further post-processing and engineering investigations.
In our experience, the cost of the EEWS is a small fraction of the damages that might result from a severe earthquake in a variety of sectors, including mining.
Key features of the EEWS
- Systems can be designed, built and installed for about one-third less than other functionally equivalent units.
- Multiple sensors, robust redundant software and the ability to bury sensors underground eliminate the risk of false alarms, making the system more reliable.
- Systems are completely autonomous, so it takes less than one second to determine if a hazardous earthquake is imminent.
- All components are custom built to meet the precise needs of each client.
- Ease of operation. Basic data are visualized on a computer screen and a relay switch can close and open multiple electric circuits in case of an event.
- Users can log into the system via the Internet to configure the system, perform data transfer and update software/firmware.
- The system’s data format supports common standards for seismological data.
- Channels are synchronized with the GPS time, which gives high accuracy of time stamps.
Iain Weir-Jones Ph.D., P. Eng., is a mining engineer, president of Weir-Jones Engineering Consultants Ltd. and chairman of the Weir-Jones group of companies. Anton Zaicenco, Ph.D., is a seismologist and earthquake engineer at Weir-Jones Engineering Consultants Ltd.
