Hydrofracturing in situ stress measurements of itabirite in the Brazilian Ferriferous Quadrangle and their limitations

Abstract

This article presents a first attempt to carry out in situ stress measurements (magnitudes and orientations) of the itabirite in the Brazilian Ferriferous Quadrangle by hydraulic fracture testing at a depth of 399 m. Previous studies on this rock have considered estimated values of the k index (ratio between the horizontal effective stress Sh, generally controlled by tectonism, and the vertical effective stress Sv, generally controlled by gravity), and it is not a common practice to carry out in situ stress tests in this region and rock type to support geotechnical analysis. The area of study was located at a depth of 500 m in a pit because the determination of the in situ stress distribution is very important to assess the stability of the open-pit mine. The planning, execution and results of the tests are presented. The studied rock mass exhibits different geological structures, such as banding and foliation, which resulted in difficulties with performing the tests, and only 12.5% of the tests were successful. The results can help interpret the strains and stresses induced by mining activities in slopes in the Brazilian Ferriferous Quadrangle and their impacts on the surrounding structures. For a better determination of the regional in situ stresses in the rock of the Brazilian Ferriferous Quadrangle, hydraulic tests on pre-existing fractures are recommended.