On the accurate strain measurements for the crack initiation determination

Due to the huge demand for natural resources and minerals at a global scale, mining depths have progressively increased over the past decades to 1,000 m and deeper. However, despite many successes, deep mining operations are now facing new challenges never experienced before, including rock spalling and unwanted slabbing failures. This phenomenon is characterised as a sudden explosion-like fracture, which can affect the long-term viability and stability of deep underground mining. According to the literature, the most indicative predictor of the spalling strength at laboratory scale is the determination of the crack initiation point, which is defined as the onset of stress-induced damage in low-porosity rocks after the closure of pre-existing cracks. Hence, many methods have been developed to identify this critical design parameter, based mainly on the measurement of vertical, lateral or volumetric strains. That is, an accurate measurement of strain is deemed critical in determining the onset of the crack initiation threshold in the study of rock failure. Nevertheless, it remains difficult to determine the actual sample deformation in many geotechnical test apparatuses (i.e. multi-stage triaxial, Hoek cell, true triaxial, etc.), in which the measured deformation by linear variable differential transformers (LVDTs) is the cumulative deformation of the load frame itself, the loading platens, and the sample. As a result, relying on these deformation measurements can lead to erroneous estimation of the material’s strain behaviour. This work presents a qualitative study on how to measure the actual sample deformation in a multi-functional true triaxial testing apparatus recently commissioned at the Geotechnical Engineering Centre (GEC) within the School of Civil Engineering at The University of Queensland (Brisbane, Australia).