Determinação do estado de tensões em maciços portadores de rochas ornamentais.

Abstract

This research proposes a methodology for the determination of the state of stress in ornamental stones rock masses, based on the stress relief of the rock surface. It is adopted as reference of that study a granitic ornamental stones quarry located in Pocinhos County, Paraiba State, where relief joints resulting from high stresses have been observed. For measuring the stresses, three openings in the rock surface are prescribed. Those opening are made by contiguous drill holes, in the directions North-South, East-West, and Northwest- Southeast. The distance between the openings should be more than five meters in a horizontal surface, in order to simplify the interpretation by avoiding interference among the openings. The measurements should be made before and after the drilling of the openings, using a tape extensometer for measuring the displacements, considering a nominal precision of 0,01mm. Because the stresses are not measured directly, a numerical model based on Phase2D software is adopted, where the mechanical properties of the rock mass are prescribed (Young's modulus and Poisson's ratio). In the models, the horizontal stresses are inferred with various orientations, using a plan view model and a cross section one, such that the displacements calculated may be compared with the future measurements in situ. The comparison between the displacements calculated using the numerical models and the displacements measured, in the openings with three different directions, will make possible the determination of the two principal horizontal stress components and their orientations. It must be considered that the horizontal stress components are the most important in this study, because there is no rock weight in surface works, and also considering that in 65% to 70% of such cases, the horizontal stresses exceed the vertical one, and also it is known that stresses up to 20MPa have been reported in shallow works (20 meters depth). In the numeric analysis the cut was modeled considering an excavation of 0,04 m of thickness, 0,50 m of length and 0,50 m of depth, simulating similar dimensions to introduced it in the suggested methodology. They were inferred in the modelling the magnitude of the horizontal maxim stress of 10 MPa and the relationships between maximum horizontal stress and low of 1, 1,5 and 2. The model in plant considered the angular variation of the maximum horizontal stress in the horizontal plan, whose adopted angles were zero (orientation of the perpendicular maximum stress to the opening), 30°, 45° and 60°, in the counterclockwise sense. For both models, he was considered a module of Young of 50 GPa and a coefficient of Poisson 0,3. In the model in traverse cut was verified that the variation of the minimum horizontal stress, staying the constant maxim, it doesn't affect the total displacement. In the model in plant the largest displacement was obtained for a relationship between maximum horizontal stress and low of 2 and for an angle zero among the maximum horizontal tension and the cavity in the horizontal plan; it was verified that there was not modification of the displacement value with the angular change considering the tensions maxim and minimum same and that it happened, for the other relationships, progressive decrease of that value with the angular increase. In the sensibility analysis to the parameters used in the modelling, for which different properties of the massive and magnitudes of the inferred stresses were adopted, only the value of the resulting displacement was modified, staying the tendencies here described. The practical application of the methodology here defined is proposed for future works, aiming at the diagnostic of possible limitations, and is also suggested the application of new techniques based on the borehole stresses relief method, where the measurements may yield more accurate results.