Abstract
Pre-excavation field stress state is critical in underground excavation modelling, design and planning, as well as rock failure prevention in mining and geotechnical engineering. It is often essential to understand and reliably estimate the ground stresses in three dimensions in a domain containing the excavation. 3D stress measurement is technically challenging and usually costly and time-consuming. Currently two methods, overcoring and hydraulic testing of pre-existing fractures (HTPF), are in use. The former provides stresses in a very small volume of rock mass and requires complete stress relief at the measurement location, which is a very expensive process. The latter relies on a large number of tests on existing fractures with varying dips and strikes, a time-consuming process with limited application. This paper presents an alternative method using differential-direction drilling (D3 method). By this method, the complete 3D pre-excavation stresses can be estimated through back analysis of borehole deformation in three non-parallel boreholes. No overcoring is required and diametrical borehole deformation is calculated based on the borehole size at zero stress. Verification using simulated data has proven that the developed method is valid and the solution is reliable. This method is expected to provide a more representative stress regime and more meaningful information for excavation design, ground stability analysis and support. For practical application, the drilling effects on borehole size in non-stressed and stressed conditions are being examined at present through laboratory experiments on drilling and borehole measurements. The purpose is to establish relationships among coring bit, stress and borehole size.