Back Analysis for Evaluation of Blast Re-Entry Analysis Techniques

This paper details back analysis conducted at two mines with the purpose of informing exposure management procedures to manage seismic risk. The analysis was intended to help each mine determine which blasts should have an exclusion,what the area of exclusion should be and when re-entry should occur. Short-term seismic responses to blasting were assessed to evaluate the dependence of blast type, location and size on the likelihood of triggering a response and the magnitude of the triggered response. Exclusion procedures should be targeted towards blasts most likely to trigger a seismic response. The area of exclusion is best defined on a case-by-case basis but the spatial distribution of seismic responses to blasting are useful to discover general trends. This was done by evaluating the distance in each axis between the event and the blast. The number of events on each grid point on the XY, YZ and XZ planes is evaluated to allow for a ‘heat-map’ of event location probability. In general, our recommendations for each mine were for a minimum exclusion distance that should be extended if there are known high hazard zones such as active structures, pillars and abutments. Given there are multiple re-entry assessment methods used throughout the industry, the back-analysis investigated six different methods of determining re-entry time. Three methods use a raw or absolute threshold value for re-entry and three others use thresholds relative to the background seismicity. Each re-entry assessment method was tested using pre-set (pre-determined) re-entry, real-time re-entry and a combination of the two (a pre-set minimum re-entry before a real-time review). The results of the back analysis allowed the mines to determine their optimal re-entry procedure. Once the mine has chosen a tolerable level of hazard outside of exclusion, they choose the re-entry technique which will result in the shortest average re-entry time for this level of hazard. The most effective method varies slightly depending on the measure of success used and the typical re-entry time. The management of seismic risk requires a holistic approach and exposure management is simply one component that works together with other control measures.