In this paper we briefly review the rock mechanics works carried out to analyse the stability of an old room and pillar hematite mine. Field work was initially carried out to recover a good number of discontinuity data, also a number of rock samples for lab testing were collected. UCS and triaxial tests were performed and rock masses were characterized in order to estimate rock, pillar and rock mass properties. Room stability and pillar strength and stability studies were carried out in all the relevant areas of the mine to find out a general good level of stability. Only in a location of the mine incipient instability problems were detected. Simplified stabilization methods were proposed, analysed and implemented, including the construction of a timber crib and the perimeter cabling of not-so-stable pillars in order to avoid rock-fall and progressive failure and to slightly increase pillar strength, as demonstrated by means of numerical approaches.
The studied case involves an old underground room and pillar mine where a hematite rich bed of Cambrian age is exploited. The mine follows the stratum dip of around 15° and rooms are located around the main gallery. The ore rock mass present bedding and other four joint sets- which are perpendicular two by two (Fig. 1). Once largely fractured, the rock suffered a near to metamorphism process that reassembled the rock mass and re-glued faults and joints, producing a good quality rock mass (RMR=70).
An exhaustive laboratory program which involved UCS and triaxial strength tests with strain measurement was planned to characterize the rocks that exist in the mine (hematite ore and dolomite on the walls) obtaining the most relevant parameters of strength and deformability as well as other important index properties (density, joint friction angle…). All these parameters of the intact rock are presented in Table 1.
Using the above mentioned RMR, the characterization of the intact rock performed in the laboratory and the software RocLab (Rocscience, 2011), a characterization of the rock masses in terms of strength and deformability was also performed. But the emphasis should be placed in the pillar scale, once the failure criterion was obtained for both the rock (upper limit) and the rock mass (lower limit), one can interpolate between these limits and obtain the failure criterion for the pillars.
