ABSTRACT:

In central Taiwan, two major terraces were uplifted and distributed around the eastern part of the Peikang High basement. The two Quaternary lateritic terraces are composed of the diluvium deposits formed in the Pleistocene. Microscopically, the erosion and failure of a terrace slope is mainly the destroy of the matrix of fine soil in the voids of the gravel layer. Seepage of water into the gravel slope is the major cause of erosion and failure. However, macroscopically, there are more than one failure modes for those terrace slopes. Based on field investigations and failure cases review, a general hypothetic model was established for the development of the terrace rim slopes. The failure mechanisms of the terrace slopes were analyzed and correlated with control factors such as slope height, slope angle, wet zone thickness, groundwater level, and talus deposit height. The results suggest that the two hypothetic failure modes, i.e., shallow sliding and circular sliding, is reasonable for the terrace slopes. The shallow sliding mode is for a slope with a 1–2m wet zone beneath the slope surface, and the circular sliding mode is for a slope with strong groundwater influence. Besides, the secondary failure such as the debris flow may also occur in the erosion trenches.

INTRODUCTION

Leading by the frontal thrust faults (Tachia-Changhua faults), two major terraces, i.e., the northern Tatu terrace and the southern Pakua terrace, were uplifted in western central Taiwan. Besides, the slope hazards in this area are complicated in failure mechanism which motivate this study. The Tatu and the Pakua terraces composed of the lateriric conglomerate with a 3–5m lateritic soil covered at the top. The lateritic conglomerates, generally called gravel formations, govern the behaviour of the terraces. Their engineering properties are quite different from other soil or rock formations.

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