Landslide-related maintenance issues around mountain road in Dasha River section of Central Cross Island Highway, Taiwan

The winding Dasha River section of the Central Cross Island Highway in Taiwan is 9.44 km long. More than 20% of its length has been rerouted in the past six decades due to landslide-induced maintenance problems. To investigate the influence of geological conditions on the maintenance of this mountainous road section, this study establishes inventories of highway alignments and nearby landslides based on interpretations of multi-temporal aerial photos and highway maintenance records. Types of landslide and topographic features are verified through field investigation, and the engineering characteristics of rock masses are evaluated using the Q-method semi-quantitatively. The results reveal that three sections with rerouting of road or structural modification are located near the undercut slopes of the Dasha River and the boundaries of strata where water erosion and scouring are active and considerable gullies develop. Various metamorphic strata with intricate folds are formed by orogenic deformation and metamorphism under the influence of weathering, such as water erosion and scouring; these evolve into meanders and steep slopes. Gullies develop along the boundaries of strata with distinct lithology, where the engineering characteristics of rock masses vary markedly, disturbing the stability of nearby slopes and hindering the maintenance of the mountain road for long periods. Rerouting the highway and the use of a tunnel instead of the slope-cut-in halfway road effectively mitigate the influence of landslides near meanders where water erosion and scouring are significant. A gallery tunnel effectively mitigates rock fall. However, a gallery tunnel must be constructed without causing overcut-induced slope instability.