Sampling bias of discontinuity orientation caused by linear sampling technique

Abstract

Orientation data of discontinuities are of paramount importance for rock slope stability studies because they control the possibility of unstable conditions or excessive deformation. Most orientation data are collected using linear sampling techniques, such as borehole fracture mapping and the detailed scanline method (outcrop mapping). However, data acquired by such linear sampling techniques are subject to bias, owing to the orientation of the sampling line. Even if a weighting factor is applied to orientation data to reduce this problem, the bias will not be significantly reduced when certain sampling orientations are involved. If the linear sampling orientation nearly parallels the discontinuity orientation, most of these parallel discontinuities will be excluded from the survey results. This phenomenon can cause serious misinterpretation of discontinuity orientation data because critical information is omitted. In the case study, orientation data collected using the borehole fracture mapping method (vertical) were compared to those based on orientation data from the detailed scanline method (horizontal scanline). Differences in results for the two procedures revealed a concern that a representative orientation of discontinuities was not accomplished. Equal area, polar stereo nets were used to determine the distribution of dip angles and to compare the data distribution for the borehole method with that for the outcrop scanline.