Assessing slope protection methods for weak rock slopes in Southwestern Taiwan

Failures of weak-rock slopes adjacent to roadsides in southwestern Taiwan most often occurred during or immediately after heavy rainfall. Field survey of weak rock slopes along the national South-2 Freeway conducted in this study showed that the slope protection methods employed in the study areas mainly included vegetation (82.0%), prestressed rock anchors with vegetation (6.8%), grille beam (5.9%), rock anchors with grille beam (3.6%). The highest failure rate occurred in the slopes that were protected by the vegetation method. The most frequently encountered weak rock formation along the South-2 Freeway is the alternating sandstone–shale formation (36.3%), followed in sequence by sandstone (24.4%), conglomerates (21.9%) and mudstone (17.4%). The field survey also found that the mudstone slopes present the highest failure rate among all rock types, and the most commonly encountered modes of failure were surface erosion and shallow slides. Factors affecting slope failure include inadequate drainage of storm water runoff, disparate rock types and vegetation on slope surfaces, slope angles and heights. This paper presents results of the field survey of the rate of failures of weak rock slopes in Southwestern Taiwan and examines the attributes of slope failures and the effectiveness of commonly used slope protection methods in the region. Requirements or essential features of an effective slope protection method are then presented along with the preliminary results of its field implementation.