In situ Stress Testing for Tunnel Design in Sydney – Hydraulic Fracturing and Overcoring

Over the last decade there have been several road and rail tunnels designed within the Sydney Metropolitan area that are now well into construction. For these tunnelling projects, a key design input has been an estimate of the magnitude and orientation of the in situ stress. Designers typically consider the measurement of in situ stress from boreholes located along the project alignment and also from published stress measurements and empirically derived relationships for the Sydney Basin. This is because there is often significant variability in the results obtained from limited stress measurements for a given project and a reasonable interpretation of these results needs to take into account a number of factors such as geological and topographical situation. Measurement technique is also a source of variation and practitioners need to be aware of the limitations and advantages of these to ensure that a stress measurement program has the opportunity to deliver the required project needs. This paper considers in situ measurements made within boreholes in Sydney using the commonly-used hydraulic fracturing method and the more recently used overcoring method by the Sigra IST tool. Results from two infrastructure projects, where these two methods were used, are presented and the possible reasons for differences obtained between the two methods are discussed. The process of overcoring is a focus including the factors that influence the determination of the elastic properties of the rock for the calculation of the in situ major horizontal stress. Numerical modelling is used to examine the potential for brittle microcracking in the porous Sydney sandstones, which can damage an overcored sample and invalidate elastic parameters obtained.