Métodos de reforço para galerias em rochas brandas, mina Ipueira III, Andorinha, Bahia.

Abstract

This research has the objective of studying reinforcing systems to stabilize Level 400 access gallery (ramp) of Ipucira III Mine, which is located in Andorinha County, Bahia State, and is owned by Cia. de Ferro Ligas da Bahia — FERBASA. The gallery intercepts a shear zone formed by weak rocks, characterized by intense tectonic process, together with varied degrees of metamorphism and alteration. The rock mass shows low compressive strength, and low residual strength, with a transitional brittle to ductile behavior. A squeezing rock condition (plastic flow) has been observed in that rock mass (JOHSTON, 1993), as predicted using the empirical correlations proposed by Singh et al. (1992) and Goel et al. (1995). The rock mass in that shear zone is composed by marble and serpentine-marble completely fractured; gabro fractured and altered; and also by serpentine very fractured. Because of the low rock mass quality (O values between 0,01 an 0,1), it was possible to extract only two specimens for compressive testing from rock blocks collected at Level 380. It was not possible to make triaxial tests on the samples because of mismatch between the diameters of the samples and the diameters of the Hoek cell available. Point load tests were made, trying to estimate the uniaxial compressive strength of the rock. The results of point load tests (18 to 36 MPa) were considered higher than expected because the samples showed excessive deformation at points of contact with the pins prior to reaching failure. The expected values of compressive strength arc in the range 5 to 10 MPa, based on the type of rock and the calculated Q values. The vertical load to act upon each post of steel sets was estimated based on empirical equations of Terzaghi (1946), Barton et al (1977), Singh et al (1992), Goel et. al. (1995), and Bhasin e Grimstad (1996), giving values between 25 and 35t. Convergence measurements in the gallery indicated that there is an increase in deformation in the transition between the shear zone and the sound rock. Geological Strength Index — GSI and the mean uniaxial compressive strength (7 MPa), calculated based on Q values, were used as input data for RocLab software to calculate the values of mh, sea. Three stability analyses were run using Phase2D finite elements program, comprising: shotcrete without invert support; shotcrete and invert support with reinforced concrete; and shotcrete plus invert support and cable-bolting. It has been concluded that the system shotcrete together with cable—bolting is the most efficient means to limit the plastic flow around the opening.