Baekrokdam rock mass at the crest of Hallasan in Jeju island, Korea is composed of two volcanic rock types: Baekrokdam trachytbasalt at the eastern region and Hallasan trachyte at the western region. On-going rockfall and subsequent collapse covered at the western region of Baekrokdam are closely linked to the weathering of Hallasan trachyte. Hallasan trachyte samples of different weathering grades had been collected and the polarizing microscopic observation, electron microscopic observation, Xray diffraction analysis, chemical analysis, and engineering property tests had been conducted. Formation of secondary minerals, especially clay minerals, by chemical weathering has not been identified, but the change of chemical weathering index with respect to the degree of weathering indicated that leaching of chemical composition had been occurred. Changes in physical and mechanical rock properties due to weathering has also been examined. Variations of mineralogical-petrographical characteristics of Hallasan trachyte with respect to the degree of weathering were investigated to analyze the weathering mechanisms. Physical and mechanical characteristics of Hallasan trachyte have been deteriorated as the weathering has been progressed. Also physical weathering degree index(PWD) based on the porosity change is higher than chemical weathering degree index(CWD) obtained from LOI variation. This fact illustrates that the physical weathering dominates the chemical weathering in Hallasan trachyte. Artificial weathering test of freezing-thawing revealed that the process of crack initiation and propagation ultimately accelerate the deterioration of the mechanical characteristics of Hallasan trachyte.
Weathering process affected by climate condition, geological structure, and mineral composition and texture of rock can change physical and mechanical property of rock, and decrease geotechnical stability. However, most studies of volcanic rock of Jeju island are concentrated to stratigraphical and geological studies, and petrologic studies to discover geochemical composition of minerals and its change. In this study, geological characteristics of volcanic rock of the Baekrokdam were investigated via in-situ geological survey, and its geologic distribution, structure and weathering property were evaluated. Three main investigations were performed;
mineralogicalpetrographical property analysis was performed to evaluate the change of mineral composition and its quantitative analysis of the content depended on weathering phase,
chemical property analysis was performed for the quantitative measurement of molecules presented in the rock and its change of chemical composition depended on weathering was investigated, and
engineering property analysis was performed to discover the geotechnical characteristics of volcanic rock of the Baekrokdam, which is crater lake on Hallasan, depended on weathering magnitude via mechanical property test and its weathering pattern was evaluated. From the information of 1448 mineralogical-petrographical, chemical and engineering characteristic of Jeju Island, changing of physical characteristic and weathering mechanism of the rock of Jeju Island were evaluated via the weathering speciation and origin of trachyte of Hallasan which is major rock mass and reflect violent topographic change owing to the weathering process around Hallasan summit, and its engineering property analysis.