Stress measurements in two pillars have been performed at Noranda's Gasp6 mine in order to evaluate the load they support so optimal ore recovery could be achieved by reducing their cross section. The paper describes the advantages of the stress measurement and' calculation methods used for the project. The modified doorstopper technique was found to be well suited to obtain accurate results with a minimum of unverified hypotheses at a relatively low cost. A total of ten measurements in two pillars were done in less than three days and led to results that are compared to stresses obtained from a 3D numerical analysis using the MAP3D boundary element code.


Located in Murdochville on the Gasl? peninsula in Eastern Quebec, Noranda's Mines Gasp6 started underground operations in 1951 and has since been one of Noranda's leaders in mining and metallurgical operations. During the fifties and sixties, underground mining was done using the room and pillar method (Figure 1) which created a complex array of openings that have been remarkably stable over the years.

As mining of the various ore bodies discovered over the years is coming to an end, the possibility of recovering parts of the huge pillars left in place during the early stages of mining is being investigated.

As part of this investigation, a series of stress measurements have been performed near the base of two rectangular pillars of approximately 13 by 23 m in width and length and up to 37 m high (Figures 2 and 3) to determine how stressed they were and how much of their cross section could be removed without risking global pillar or back collapse. The main interest was to measure the vertical stress component rapidly, with a good accuracy and at low cost without any assumption regarding the state of stress in the pillars.

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