Seismic re-entry analysis
Do you know what your seismic re-entry controls are based on? And how many damaging seismic events have you historically captured with these controls? Not sure, then continue reading.
There is an increase in seismic hazard after blasting/firing and large events in a stressed rock mass. This increase is due to the fracturing of the rock mass as the stress redistributes after these significant changes. Experts call these stress-redistribution seismic events aftershocks. Aftershocks can vary in size, distribution, and duration.
The occurrence of seismic events after these changes follow a theoretical law; the Modified Omori Law, or in short, the MOL. Geotechnical engineers widely use the MOL in mining to describe the event count in the hours after blasts/firings and large events.
In my experience seismic re-entry controls are one of the least understood control measures employed by mines. This is in most part because engineers, although understanding the theoretical part of the process, does not consider the effectiveness of the control measures.
Effective control measures ensure that hazards are managed in line with the company’s risk policies. These control measures should focus on ensuring the safety of workers while attempting to limit the impact on the production cycle as much as possible. This concept can be best explained by considering a chart from Woodward (2015).
From Woodward (2015)
The aftershock events are shown by the MOL model (blue-red), while the seismic hazard during this period (based on certain assumptions) is shown by the blue curve. In this case, the hazard is expressed as the annual probability of having an event of magnitude 2.0 or greater. Geotechnical engineers can, based on the company’s risk policies, determine the most efficient exclusion times.
There are several other methods which engineers can use for determining seismic re-entry times. Each method has different pros and cons and limitations, based on the typical rock mass response. Therefore, it is advised that engineers should consider all methods available when determining the most relevant seismic re-entry controls for each individual company.
One way of determining which method is most efficient is for engineers to do a back analysis of all methods based on historical mine data. Tierney and Morkel (2017) is an example study of such a back-analysis process. They evaluate the effectiveness of re-entry methods by determining how many events above a certain magnitude is captured within a certain distance from a blast/firing. The curves produced by them (see the chart below) can clearly help engineers determine effective re-entry times. To read this chart, determine the percentage of events which should be captured (under company policy) and read across to determine which method will give you the shortest average re-entry time (black dotted lines).
Are your seismic re-entry protocols as effective as they could be? Have these seismic re-entry protocols prevented exposure of personnel to large seismic events in the past? Is the exposure of personnel to the seismic risk in line with your company’s guidelines? These are important questions to consider.
IGM Geotechnical has more than a decade worth of experience in applying seismic analysis techniques to geotechnical problems. If you feel unsure about your seismic re-entry needs, or just want to know more, please do not hesitate to get in touch. We are here for your “seismicity for engineering” needs.