Borehole Orientation Optimization in 3D Rock Stress Estimation with D3 Method

ABSTRACT: The three-dimensional stress field existing in the ground prior to excavation plays a significant role in developing underground infrastructures and ensuring safety. Both the orientation and magnitude of these stresses are often required for planning and design of excavations, as well as ground stabilization and failure prevention. However, most measurement methods are limited to two dimensions within a plane. 3D stress measurement is technically challenging, time-consuming and thus, costly. A method by differential-direction drilling - the D3 method, has been developed for estimating the 3D rock stresses as an alternative to the overcoring method. With this method, no need of overcoring is required and three sets of the measured borehole deformation in three non-parallel planes are combined to solve for the complete 3D stresses. The challenge of back-analysis of the 3D stresses through 2D measurement data has been overcome. The relative borehole orientations which may affect the reliability of results in field application of the D3 method are investigated here. It is discovered that the space angle between each two drilled boreholes affects the results significantly and the accuracy of the solution can be improved by increasing the space angle interval. For practical application, the recommended space angle interval of the three measurement planes should be larger than 60°. As the space angle gets close to 90°, that is when perpendicular to each other, the back-analyzed stresses agree well with the real solution. This work provides some guidance in planning borehole drilling directions for use of D3 method to reliably estimate the complete 3D field stresses in mining and petroleum industry.