ORIGINAL PAPER Relationship between durability index and uniaxial compressive strength of a gneissic rock at different weathering grades João P. Monticelli 1 & Rogério Ribeiro 2 & Marcos Futai 1 Received: 16 June 2018 /Accepted: 25 September 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract This article presents a study on durability index obtained in the slake durability test (SDT) and the uniaxial compressive strength (UCS) of a gneissic rock from the Nova Venécia Complex, southeastern Brazil. The gneissic rock mass, excavated for a railway tunnel without a lining, has presented problems of instability of blocks due to a long-term exposure to weathering. The charac- terization of the gneissic rock matrix was made tactile and visual and by means of microscopical petrography, combining the tests of physical properties, SDT, and UCS with weathering indexes. The importance of describing the heterogeneity of the gneissic rock to interpret its influence on geotechnical parameters is discussed deeply. The interpretation of the results indicates that the rock studied has one facies with greater susceptibility to weathering, loss of strength, and durability. This study also proposes an equation to access the alterability parameter, recently added in the 2014 update of the rock mass rating (RMR), which may be overvalued to crystalline rocks, as observed in the analysis. Keywords Slake durability index . Gneissic rocks . Weathering grades . Alterability parameter . RMR update Introduction The Monte Seco Tunnel, belonging to the Vitória-Minas Railroad (VMR), responsible for the transportation of ore from the region of Belo Horizonte (BH) to Vitória (VT), southeastern Brazil, was excavated in a gneissic rock of the Nova Venecia Complex (Vieira et al. 2014) (Fig. 1). Given the very favorable stability conditions of the rock at that time (decade of 1950), neither shotcrete nor bolts were used to treat the rock, but in the last years, as reported by the railroad maintenance team, problems of instability of blocks in this and other tunnels of the VMR have been noticed. Although they do not indicate a generalized risk, these problems can cause severe logistic losses in cargo transport. This theme has recently motivated researchers from the GeoInfraUSP of the Department of Geotechnical Engineering of the EPUSP to develop several studies to evaluate the geomechanical behav- ior of the Monte Seco Tunnel (Cacciari et al. 2013; Cacciari 2014; Monticeli et al. 2015, 2016; Cacciari and Futai 2017). The block instability observed in the Monte Seco Tunnel results from several factors associated with the tropical climate that, through the years, caused the degradation of geological- geotechnical parameters of the rock mass, causing blocks to loosen from the tunnel roof and walls, particularly in fractured and weathered zones, making excavation unsafe through the years. In this manner, the detailed study of modifications in the characteristics of heterogeneous rocks, such as gneissic rocks, at different weathering grades, has proved to be funda- mental for reevaluating rock mass behavior. In the present study, information is collected regarding the durability of the gneissic rock matrix, aiming at a better un- derstanding of the intact rock disaggregation in the Monte Seco Tunnel. The effect of the rock heterogeneity, classified according to petrographic facies, is analyzed in terms of the petrographic analysis, physical properties, uniaxial * João P. Monticelli jpmonticeli@gmail.com Rogério Ribeiro rogerioprx@sc.usp.br Marcos Futai futai@usp.br 1 Graduate Program of Civil Engineering, Structural and Geotechnical Engineering Department (PPGEC), Engineering School, University of São Paulo (EPUSP), Av. Prof. Luciano Gualberto, travessa 3, n 380, São Paulo, SP 05508-010, Brazil 2 Engineering School of São Carlos, Department of Geotechnical Engineering, USP, Av. Trabalhador São Carlense, n 400, São Carlos, SP 13566-590, Brazil Bulletin of Engineering Geology and the Environment https://doi.org/10.1007/s10064-019-01625-9