Review of overcoring testing to measure in situ stress for tunnel projects in Sydney

The magnitude and direction of in situ stress is important to the design and performance of an underground project. A number of techniques are available to estimate in situ stress in rock, with borehole deformation strain cells (a form of overcoring) and hydraulic fracturing being the most common used in investigative boreholes in civil projects in Australia. An assessment of recent stress measurements obtained from the overcoring technique for tunnels in Sydney indicates lower results for the magnitude of the horizontal stress, when compared to available published data on the Sydney horizontal stress field by hydraulic fracturing. Overcoring stress testing involves drilling a pilot hole in a borehole and measuring strains while overcoring. The calculation of the in situ stress further requires laboratory testing of the overcored rock sample to measure Young’s modulus and Poisson’s ratio. An iterative calculation is undertaken to determine the in situ stress in the rock. The intrinsic aspects of the overcoring testing method are discussed to assist in interpreting the stress field for tunnelling.