A test of different stress measurement methods in two orthogonal bore holes in Äspö Hard Rock Laboratory (HRL), Sweden

Abstract

Within the scope of works to provide the necessary rock mechanics support for site investigations for a radioactive waste repository, the Swedish Nuclear Fuel and Waste Management Co (SKB) has studied some of the available equipment for in situ stress measurements in deep boreholes. A project with the objective of comparing three different types of equipment for in situ stress measurement under similar conditions has been carried out. The tests were performed in orthogonal boreholes at the Aspo Hard Rock Laboratory (HRL), Oskarshamn, Sweden. The two most common methods to measure in situ stress are the overcoring and hydraulic fracturing methods. Among the three tests, two are carried out using the overcoring method, the third uses the hydraulic fracturing method. The overcoring equipment's are the AECL's Deep Doorstopper Gauge System (DDGS) and SwedPower's Triaxial Strain Cell (Borre Probe). The measured results could be verified against known conditions at the Aspo HRL. The results from the three in situ stress measurement methods illustrate the complexity of determining the in situ stresses in a rock mass. To understand the difference in results and answer the associated questions, it was necessary to undertake deeper investigations involving laboratory tests and theoretical calculations.