ABSTRACT:

The slopes in the study area had suffered from frequent collapses. To investigate the relationship between slope instability and the engineering parameters of the rock masses at the boundary of slate formation and Tananao schist formation, interpretation of the spatial distribution of collapses from multi-temporal remote sensing images, investigation of slope instability mechanism, surveying and geological mapping, and evaluation of engineering parameters of rock masses have been performed and reported in this article. The outcomes indicate that the Q values of the rock masses at the boundary of slate and schist are lower than those of the surrounding rocks, resulting in areas prone to rock slides, debris slides, and rock fall. The adaptive structures, such as axial faults, pre-existing shear, and dense fractures near the axis of a mesoscale fold, cause rock fragmentations, which are often the source of rock debris and rock fall.

INTRODUCTION

Lithology and geological structure determine the engineering characteristics of a rock mass, and the exposed position, scale, and structural influence range of rock mass will affect slope stability. Metamorphic rocks contain complex structures and often present anisotropic fabrics, therefore, the understanding of factors affecting slope stability is lower than that of sedimentary rocks (Stead & Wolter 2015). More case studies are needed to explore the rock mass engineering properties of all kinds of metamorphic rock.

This study collected remote-sensing images and UAV-produced orthophotos from 1952 to 2021 and produced digital terrain models (DTM). The spatial distribution of slope collapses was interpreted in multi-period images, and types of collapse as well as rock mass classification were verified and obtained through in-situ investigation. Eventually, the influencing factors of slope collapse in metamorphic rocks are discussed thoroughly.

STUDY AREA

The study area is in the eastern section of the Southern Cross-Island Highway (No.20 highway) in southeast Taiwan, from Wulu to Lidao in Haiduan Township, Taitung County. The strata include the Pilushan Formation (part of the slate belt, Eocene) and the Kaolin Schist (part of Tananao Schist complex, Late Paleozoic - Mesozoic), and the study area is between the slate and schist belts. The lithology of the Pilushan Formation is dominated by slate, intercalated with metasandstone. The lithology of the Kaoling Schist is mainly quartz-mica schist, mixed with intercalated quartz schist, chlorite schist, and marble (Central Geological Survey 2013).

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