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 Ferrifer- ous Quadrangle by hydraulic fracture testing at a depth

of 399 m. Previous studies on this rock have consid- ered estimated values of the k index (ratio between the

horizontal effective stress Sh, generally controlled by

tectonism, and the vertical effective stress Sv, gener- ally 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 distri- bution 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 perform- ing 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 determina- tion of the regional in situ stresses in the rock of the

Brazilian Ferriferous Quadrangle, hydraulic tests on pre-existing fractures are recommended.