In the overcoring process, strain changes measured during stress relief are used to calculate the in situ stresses. During analysis, multiple measurements of the axial strain are usually averaged to provide a single valued estimate of the axial strain consistent with elastic theory. In practice, however, axial strains are observed to be influenced by drilling effects during overcoring so that they vary with position around the borehole circumference in a sinusoidal pattern. This paper presents the results of axial strains measured at six locations around the circumference of a pilot hole during routine overcoring and presents an improved strategy to compensate for drilling induced axial strains.
A strategy based on subtracting a component of the average axial strains from relevant gauges has been used with the ANZI cell for more than two decades but recent numerical modelling and field measurements have led to a significant improvement in this strategy. This improved strategy is based on subtracting the variable component of the axial strains from the measured strains and a corresponding component from the 45° and 135° gauges and recalculating the in situ stresses so that only axial strains associated with stress relief are used to calculate the in situ stresses.
The overcoring method of stress relief is widely used to measure in situ stresses in rock. Strain changes caused by stress relief during overcoring are used to calculate the in situ stresses typically using elastic theory and the assumption that the measured strains are due solely to changes in the stress; drilling related effects are not recognised. In the analysis based on elastic theory and idealised material behaviour, axial strains are single valued and independent of position around the circumference of the borehole. Multiple measurements of axial strain available from most types of strain based overcoring instruments are averaged to provide a single valued estimate of the axial strain. Any variations in axial strain are typically assumed to be a result of measurement error or variability in rock behaviour.
