A Method to Correct Indirect Strain Measurements in Laboratory Uniaxial and Triaxial Compressive Strength Tests

Abstract

When carrying out uniaxial, standard triaxial, or true triaxial compressive tests on rock, the strain response of the specimens can be measured directly or indirectly. The most commonly used devices are strain gauges (local direct measurements) and displacement sensors (global indirect measurements). Strain gauges are glued to specimens and directly measure electrical resistance. Displacement sensors, typically linear variable differential transformers (LVDTs), indirectly measure displacements. The former is more precise at its smaller measurement scale, but may not work under high temperatures or when a strain gauge detaches due cracking under stresses close to the peak or in the post-peak portion of the stress–strain curve. It was observed that the strain responses recorded using strain gauges and displacement sensors tend to differ, which is why a corrective approach was proposed to recover stress–strain curves based on indirect measurements. This study describes an approach to correct platen-to-platen displacement measurements based on energy calculations to derive axial strain and to obtain radial strain using modified computations based on the volume fluid change within the Hoek’s cell in triaxial testing. The study concludes that part of the displacement recorded by LVDTs can be associated with displacement in the plate sample contacts, but the component of energy transmitted to the samples can be estimated at different levels of load, so that these indirect measurements can be corrected. For the case of triaxial tests, the volumetric or radial strain can be computed based on the volume change in the Hoek’s cell. This approach is useful for correcting indirect strain measurements. It can also serve as a starting point for the development of corrections for application to data from other test types, such as for true triaxial tests. The study aims to ultimately contribute to a better understanding of strain measurements while conducting compressive tests on rock.