Located in North Aurora, Illinois, the Lafarge-Conco plant is a room-and-pillar mine that is in active production of aggregates, taken from the Galena- Platteville formations. To better understand how stresses are distributed among the pillars over periods of mining production, we collected data to create tomographic images of the interior of the pillar using seismic data collected in November 2012 and March 2013. Seismic tomographic images showed changes in seismic velocity between the two surveys, which is interpreted as change in stress. The southeast corner pillar showed a significant stress increase, which could indicate a possible area of very high stresses within the pillar. Understanding how stress and moisture distribution changes within pillars during excavation is valuable to mine design and to the maintenance of mine safety.

1. Introduction

Characterizing rock properties and responses is beneficial for the development and planning of mines for engineering, economic, and safety reasons. Studies have shown that pillar failures led to 36% of fatalities and 12% of injury related work days lost, from 1996–2005 [1], and 15% lost operation days [2]. Understanding how stresses are distributed across pillars during production in a room and pillar mine can aid in better design of operations and proper maintenance of mines. Pillar failure can occur from too much loading of a pillar and from discontinuities in the internal structure of the pillar, such as large fractures. Moist conditions have also been shown to increase the risk of ground fall [2]. The goal of this study was to image the interior stress distribution of a pillar in a room and pillar mine, in North Aurora, Illinois, using seismic travel time tomography. Previous work has imaged stress and damage in pillars of mines after production has been completed [3]. In this project, we analyzed a pillar's stress distribution over time while production was actively occurring from November 2012 to March 2013. During that time frame, mining operations took a brief hiatus from early December to mid-February, because demand for aggregates decreases in winter [4]. Seismic travel-time tomographic was used to image the stress distribution of the pillar in a non-destructive way. Using non-destructive techniques gives a better understanding of the actual in-situ stresses caused by the removal of neighboring supporting material during mining operations.

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