Many techniques of stress measurement or stress monitoring are based on the principle of gluing a strain-measurement device on the wall of a pilot hole using an epoxy resin. The curing time needed for this epoxy glue to achieve full hardening is rarely put into questions. Here, we present an in situ study of the impact of curing time on the quality of overcoring stress measurements using CSIRO Hi cells. The tests were conducted in an argillite rock at a temperature of 12°C. We show that the conventional curing time (16 hours) is clearly insufficient in this context and leads to anomalous response in strain readings during both overcoring and biaxial tests, thus hindering stress determination. We claim that in low-temperature near-surface environments, much longer curing times may be needed in order to ensure good quality of stress measurements.
Knowledge of the in situ state of stress is of utmost importance for assessing the safety and stability of underground openings and geo-engineering structures, ranging from old dams to geological waste repositories. Yet, although much technical progress has been made over the past decades, measuring in situ stress in rock masses remains a challenging task (see e.g. Amadéi & Stephanson, 1997, Hakala, 2006). One of the most widely used methods for rock stress determination is the overcoring method. It consists in measuring the strains that develop at the wall of a small diameter borehole (pilot hole) when this one is relieved from the surrounding in situ stress field by overcoring. Assuming a rock constitutive law (usually linear elasticity), the in situ stresses may be determined from the measured strains. This inversion requires the values of the rock elastic parameterswhich are usually determined from biaxial testing on the retrieved overcore.