This paper deals with the effect of freezing and thawing of soil behavior on the progressive failures of slope along the A-Li-San highway in central Taiwan Subsurface investigations through in situ boring tests and soil sampling, clay mineralogy determinated by X-ray diffraction, soil mechanical properties variation measuring under different temperature of freezing and thawing processes in laboratory and slope stability analysis by using PCSTABL II program are critical and necessary for this study. The results of inclinometer monitoring show that the potential sliding zone should be 9-q2m & 21-~24m in depth on the downslope of highway. This was identified by conducting slope stability analysis and boring hole tests in the field. The factor of safety (F.S) drops from 1.12 to 0.83 when the soil strength parameters (C&dp) being used at different conditions of freezing and thawing effect tests. It was found that the slope being monitored at high altitute of 1800m in elevation should be in potential unstabilization. This was verified by the effect of soil freezing and thawing fluctuation during the seasonal change of warm spring and cold winter in the studied area.
A-Li-San highway is a major traffic line connecting the southern and central parts of Taiwan through the mountainous area on the island. Due to the complex geological structure-uneven rainfall and landslides along the highway, especially in the zones abound with faults and folds, frequently occurred during typhoon season. 7 faults and 3 synclines/anticlines were found in this region. The cleavages and joints were abundant in the geological formations. The slope failures mostly occurred between elevation 1600–1900m with slope aspect of south, southeast and southwest. Tsai (1996) indicated that slopes having steepness of 30–45 ° and azimuth of 135–180 ° tended to collapse most frequently and make up 64.7% of the total slope failure sites.
