Failure modes and countermeasures of large-scale rock slope composed of phyllite in Korea

Phyllite is a low grade metamorphosed rock. Phyllite is mainly distributed in the Ogcheon Supergroup around the Chungju and Boeun areas in Korea. It is mainly composed of platy minerals, and has foliation developed in metamorphic process. The main characteristics of phyllite are anisotropy, fissility and easiness for weathering, and they are considered as major causes for slope failures. Therefore, this case study aims to introduce failure modes of rock slopes composed of phyllite in Korea and its countermeasures. The location of the slopes for the case study belongs to the age unknown Hwanggangri Formation of the Ogcheon Supergroup in Korea In these slopes, well-developed fissility along foliation and weathering were significantly observed. It was inferred that decrease of shear strength of discontinuities occurred due to characteristics of mineral composition and discontinuity in phyllite. Then numbers of slopes were collapsed during excavation. Failure modes of the slopes were plane failures along foliation and wedge failures by intersections of joints, etc and the scales of failure varied from small to large. In this paper, characteristics of rock slopes in phyllite are highlighted in the aspects of geology, geological structure and rock engineering. In addition, the failure modes, optimized geotechnical investigation, analysis and reinforcement methods are presented. We hope that this case study would be a useful reference for stability analysis of rock slopes whose site conditions are similar to those of this study site such as geological structures and geotechnical properties.