Identification and Mitigation of Error in the Terzaghi Bias Correction for Inhomogeneous Material Discontinuities

Abstract

Use of the scanline mapping technique in geometric surveys of rock discontinuities can often lead to a bias, in that discontinuities are not always observed when they are at small angles to the scanline. Terzaghi introduced the concept of a blind zone to explain this bias, and developed a widely used procedure to correct for it. Unfortunately, little is known about errors that may occur when the Terzaghi procedure is used outside the blind zone. This paper presents a detailed derivation to show that such errors arise with this application of the Terzaghi procedure. This error was evaluated using simulated orientation data and a case study of the 2008 Wenchuan earthquake (Sichuan, China). The results of these tests yield the optimal values of grid size and sample density for reducing the error.