Geomechanical characterization and analysis of the influence of durability tests on metabasalts

Abstract

Laboratory tests are essential to provide knowledge on rock matrix properties, to predict field behaviour and to plan and execute improved engineering design. The paper presents the results of several laboratory tests performed on samples of a basic rock from N4 mine, located at Carajás Complex, on sound and cycled (artificially weathered) rock material. Carajás complex is located in Carajás sierra, Amazon region, North of Brazil, and is currently known as the biggest mineral province of the world. This region has great geological and economical importance and, on this context, studies related to rock mechanics are essential to evaluate open pit stability on long term periods. This study comprises a mineralogical, physical and geomechanical characterization of a metamorphised basalt rock, throughout a comprehensive laboratory test campaign encompassing the following tests: thin section description (including micropetrographic and weathering indices determination), physical properties, uniaxial compressive strength, point load strength, P wave propagation velocity, Schmidt hammer and durability test (accelerated cycling). Results from other works in the literature are presented to provide a basis for comparison. Petrographic analyses allowed classifying the rock as a hydrothermalized metabasalts. Micropetrographic index has shown that rock presents low weathering, with a higher quantity of sound minerals. Porosity, dry and saturated specific weight and water absorption capacity were calculated for samples with and without cycling and the results show that the number of water – salt cycles were not sufficient to cause a significant change on these physical indices. Wave propagation velocities were determined and the results are similar to the ones found in literature for similar rocks. Schmidt hammer were used to determine uniaxial compressive strength (UCS) trough Miller abacus. Another strength tests performed on rock samples were point load test, and the results were also used to correlate with UCS and other studies results. All these results were later compared with uniaxial compressive strength tests data and the values were similar. Furthermore, data from UCS tests were also used to calculate elasticity (Young) modulus and the material has shown a highly elastic behaviour.