DETERMINATION OF SLIP SURFACES IN FRACTURE ZONE LANDSLIDES USING ORIENTED BOREHOLE CORE SAMPLES

Abstract

In situ and laboratory observation, geophysical measurement and digital imaging analysis of
oriented borehole core samples are performed to determine slip surfaces in two large fracture zone landslides
in Shikoku, Japan. The following data are obtained from each oriented borehole: (1) a depth distribution of
rock quality designation, magnetic susceptibilities, the Equotip hardness values and wet unit weight of core
samples, (2) geometric orientation of geologic discontinuities (cracks, joints, faults, etc.), and (3) a depth
distribution of numerical color values from digital imaging of borehole core samples. As a result, the rock
quality designation, the Equotip hardness value and unit weight, and the orientation of cracks and joints
showed a clear change near the slip surfaces respectively, but the digital color values clearly varied only in
one of the landslides and no change of magnetic susceptibility of core samples was found at both sites. The
results demonstrate that it is capable of locating the slip surface in a fracture zone landslide by using the
above-mentioned data in combination.