Correcting indirect strain measurements in laboratory uniaxial compressive testing at various scales

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ORIGINAL PAPER

Correcting indirect strain measurements in laboratory uniaxial compressive testing at various scales Leandro R. Alejano 1

&

Javier Arzúa 1,2 & Xián Estévez-Ventosa 1 & Johannes Suikkanen 3

Received: 25 April 2019 / Accepted: 20 May 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract Various devices can directly or indirectly measure strain and record the deformational response of rock materials in uniaxial compressive strength tests. Strain gauges glued to specimens directly measure electrical resistance, while displacement sensors, typically linear variable differential transformers (LVDTs), indirectly measure linear displacements. However, no guidance is available regarding how to position the LVDTs to accurately measure axial or radial strain. A common practice is to measure axial strain with displacement sensors on the basis of the reduction in span between steel platens in relation to specimen height. If the deformational response for axial strain measured using strain gauges and displacement sensors is compared, however, it may be that neither maximum axial strain nor elastic constants are the same. This paper describes an approach to correcting platen-toplaten displacement measurements based on energy calculations at different scales in Olkiluoto gneissic rock specimens, with control specimens tested by combining strain gauge and displacement sensor measurements. It is suggested that the approach may be adaptable to other rocks. Keywords Rock mechanics . Rock testing . Uniaxial compressive strength tests . Strain measurements . Deformability . Elastic Modulus

Introduction Obtaining the Young’s elastic modulus and the uniaxial compressive strength (UCS) of a rock is crucial for a number of analyses related to rock behavior (Fattahi and Moradi 2018; Pepe et al. 2018). The original version of the method proposed to determine the UCS and deformability of rock materials (Bieniawski and Bernede 1979; also included in ISRM 2007) refers to a number of systems to measure strain for rock specimens, including electrical resistance strain gauges, compressometers, optical devices, and displacement sensors, specifically, linear variable differential transformers or transducers (LVDTs). However, that method does not clearly state

* Leandro R. Alejano [email protected] 1

Department of Natural Resources and Environment Engineering, University of Vigo, Vigo, Spain

2

Department of Metallurgical and Mining Engineering, Universidad Católica del Norte, Antofagasta, Chile

3

Department of Rock Mechanics, POSIVA Oy, Helsinki, Finland

how LVDTs should be positioned for proper measurement. Some laboratories use LVDTs to directly measure axial strain on the basis of the reduction in the span between steel platens in relation to specimen height. An updated method for the complete stress–strain curve (SSC) for intact rock in uniaxial compression (Fairhurst and Hudson 1999; also included in ISRM 2007) recommends measuring axial deformation with extensometers making contact with the specime