Fracture mapping for geological prognoses. Comparison of fractures from boreholes, tunnel and 3-D blocks.

Paper in proceeding, 2012

One of the reasons for uncertainty in geological prognoses is that geological investigations, such as drillings, only represent a small part of the rock mass, and that they may be directionally biased. This study is based on a unique set of data consisting of geological mapping of a tunnel during construction, fracture mapping from three core-drilled boreholes along the tunnel, and fracture mapping of blocks sawed out from a section of the same tunnel. By comparison of the different data sets it is shown how input data to geological models vary depending on what type of fracture mapping it is based on. The study focusses on orientation analysis of fractures using stereographic projection. The results indicate that there are obvious differences in the models due to sampling scale and dimensionality. Yet, the fracture mapping of tunnel faces and boreholes respectively was found to give similar results even though the scanlines along the boreholes are perpendicular to the tunnel faces. The results also indicates that there is a need to improve the detailed conceptual understanding at the tunnel site of the generation and intensity of fracture sets in order to make a reliable interpretation of fracture intensities and orientations.