The evolution of mine design concepts, toward maximum safety conditions, has been requiring the use of powerful computer simulation tools, in order to predict rock mass response around slopes, open pits and underground excavations in rock. Recently, ornamental marble quarries in Portugal are evolving to underground exploitations, in order to reduce related costs and environmental impacts, usually associated with low recoveries, typical in open pit excavations. In this paper, this kind of underground quarries are reviewed according to detailed mechanical characterization of the rock mass needed for these projects, and concerning most common geotechnical problems, aiming the best compromises between safety and economy. The Boundary Element Method was applied to predict displacements, stresses and safety factors around excavations surfaces, upon different simulation scenarios. The results are analyzed and the significance for this computer simulation tool is discussed, in order to predict the rock mass response around underground excavations.
In Portugal, non-metallic mineral resources, rocks (including dimension and industrial stones) and industrial minerals, constitute a sector of large economic relevance due to the significant volumes produced and to the number of workers involved (more than 3000). Considering the production of ornamental stones all over the world, Portugal ranks parallel with Brazil in 6th place (IGM, 2000). Portugal also imports unprocessed stones from other countries for upgrading its value. The present production of dimension stones is approximately 1.200.000 tons per year, from which about 40% is exported. Considering the total exportation per year, usually, 71% are processed products, 17% slabs and only 12% blocks. Marbles, 1imestones and granites are the dominant materials, responsible for 96% of the total production (Bernardo, 2001). This is a very dynamic industrial activity, which has progressively been modernized in order to meet the more advanced exploitation and processing technologies, thus contributing for higher levels of quality, production figures and for increased competitiveness in the markets. Due to their strength, toughness, hardness, and resistance to environmental damage, these stones find application, mainly, in building, construction, monuments and curbing. Marbles are mainly produced in the anticline of Estremoz - Borba - Vila Viçosa (Fig. 1). In this area over 130 quarries and 40 processing units are operating, representing approximately 55% of the total production of dimension stones in Portugal. The pink type, named "Rosa", is the most valued, but other ornamental types, with a variety of colors ranging from greenish and grayish to black and white, are also produced. Although large reserves of ornamental stones exist in Portugal, several problems (like: increasing depths in many quarries, serious environmental impacts created by most surface operations - where waste dumps are included) forced the developing of new exploitation methods, in order to increase reserves and maintain production levels common in open pit exploitation sites. The most relevant example is marble exploitation, the existence of large marble reserves at depths reaching 400 meters, led IGM (the Portuguese Geological and Mining Institute) to start a new project for the development of know-how on the extraction of marbles by underground methods.