ALMEIDA, J. W.; ALMEIDA, José Waldy de.
Resumen:
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.