Analysis of damaging seismic event on 24 December 2011 in the Pilar Norte sector of El Teniente mine

Abstract

Numerous tunnels of the extraction and undercut levels of the Pilar Norte sector experienced violent damage on 24 December 2011. A seismic event with a moment magnitude of 2.4 was recorded by the mine-wide seismic system at the same time. It is difficult to explain the observed damage with peak particle velocities/peak ground velocities calculated for the basic source parameters of this event (location and magnitude). Seismic data recorded in Pilar Norte in 2011 indicate that failure in the sources is driven by sub-vertical compressional stress. The majority of seismic events located around the undercut level have crush-type source mechanisms (significant implosive component, pancake-shaped deviatoric parts with sub-vertical P-axis) evidencing vertical convergence of the excavations. There are no clear indications of active planar geological structures in the seismic data. The large seismic event that occurred on 24 December 2011 during the blasting sequence and its waveforms are complex. The source mechanism estimated from the low frequency part of the waveforms is of a crush-type. It was hypothesised that the source of this event represents a cascading damage of tunnels. The hypothesis was tested by means of comparing the modelled waveforms of spatially distributed episodes of tunnel damage with the recorded waveforms. A reasonable match was obtained for the scenario including an initial 20 mm convergence of the extraction and undercut drives around the southern Pilar Norte undercut front at a rate of 0.6-0.8 m/s and subsequent spread of smaller (3-6 mm) convergence along the eastern Pilar Norte undercut front and eastern Sub 6 cave abutment at a rate of 0.2-0.4 m/s. There was also significant shearing (ride) deformation along the southern and eastern Pilar Norte cave front. The hypothesis of cascading damage of tunnels also agrees with the underground observations (more than 90% of damage was attributed to the side walls of tunnels) and predictions of sub-vertical orientation of maximum principal stress around the tunnels of the extraction level according to a numerical stress model.

Evaluation of rockburst energy capacity for the design of rock support systems for different tunnel geometries at El Teniente copper mine

“…From 1995 to 2007 there were several seismic events from 0.8 up to 3.2 magnitude, albeit mostly with slight to medium damage. However, a few seismic events were accompanied by significant damage due to rockbursts, for example the events in 2005 in Esmeralda, RENO, and DR (Alviña, 2008;Potvin, Jarufe, and Wesseloo, 2010b;Araneda and Sougarret, 2008) and Pilar Norte in 2011 (Malovichko, Cuello, and Rojas, 2018). In DR rockburst events occurred in 2004, 2005 and recently in 2020 (Figure 1).…”

Major hazards associated with cave mining: are they manageable?

“…This hazard-management strategy serves to change the rock mass characteristics and stress conditions, to remove people from the high-risk development areas, and to contain the ejection of rock due to the rockbursts with magnitude less than two based on Richter scale. Similarly, current practices involve rapid or even a single-blast drawbell opening which, under certain conditions, could cause a sudden stress change leading to violent failure of the rock mass (rockburst) surrounding the drawbell (Constanzo et al 2013;Malovichko et al 2018; Rojas & Balboa 2017) ( Figure 7). The strategy to manage rockburst due to a single-blast drawbell opening is to hydrofracture the pillar between the undercut and extraction levels before the drawbell opening to reduce the stress concentrations in that area, and to use dynamic ground support ( Figure 6).…”

SEG Newsletter 119 (October)

“…Current practice uses rapid or even single-blast events to form a drawbell opening, which under certain conditions can cause sharp stress changes that may lead to failure of the rock mass surrounding the drawbell (Fig. 7; Rojas and Balboa, 2017;Malovichko et al, 2018). A strategy presently used by some companies to manage rock bursts due to single-blast drawbell opening is to hydrofracture the pillar between the undercut and extraction levels (Fig.…”