Overcoring is a method to measure the in-situ stress of rocks for geotechnical investigation. The radial displacement is measured and converted to stress magnitudes along with other properties such as Young's modulus and Poisson's ratio derived from cored samples.
Overcoring is done in a pilot hole fitted with a strain gauge, beginning at the borehole diameter. Radial displacement is recorded by the strain gauge and used for estimating stress for various geological conditions.
Trenchlesspedia Explains Overcoring
Overcoring measures the undisturbed stress levels and involves drilling a large diameter borehole (60 – 150 mm). At the end of the large borehole, a small pilot hole (300 to 500 mm in length) is drilled as concentric to the larger hole as possible.
The larger hole is again resumed relieving stresses and strains partially or totally within the cylinder of rock. These changes in strain and dimension are recorded along with the rocks modulus and Poisson's ratio. Strain gauges can be used to measure the changes and can be mechanical or electrical resistance strain gauges.
They are used to measure the in-situ stress based on stress release around the borehole after overcoring. The recovered overcore can be taken for laboratory testing to determine other physical characteristics of the rock.