제주도 한라산조면암의 풍화미캐니즘과 물리적 특성변화

Abstract

Baekrokdam rock mass at the crest of Hallasan 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 of Baekrokdam wall rock are closely linked to the weathering of trachyte covered at the western region of Baekrokdam. Hallasan trachyte samples of different weathering grades had been collected and the polarizing microscopic observation, electron microscopic observation, X-ray 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 trachyte with respect to the degree of weathering were investigated to analyze the weathering mechanism of Baekrokdam trachyte. Physico-mechanical characteristics of Hallasan trachyte have been deteriorated according to the weathering grades. 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 over the chemical weathering in Hallasan trachyte. Artificial weathering test of freezing-thawing reveals the process of crack initiation and propagation which ultimately accelerate the declination of the mechanical characteristics of Hallasan trachyte.