Developing the management criteria for the Alishan Highway based on the data of typhoon-induced heavy rainfall and slope failures along the highway

In this study, the famous Alishan Highway (Tai-18) is taken as an investigating case study. A new model is developed to assess the possibility of the slope failure along Tai-18 based on the rainfall data corresponding to stable/failed slopes during typhoons and heavy rainfalls from 1996 to 2010. In the analysis, the maximum rainfall intensity (I-mm/hr), effective rainfall (R-mm), and effective rainfall duration (T-hr) are basic parameters. The relations of I-R and I-T for the stable/unstable slopes at different altitude highway sections are investigated. Finally, a probability-based rainfall criterion of the stable/unstable slopes is proposed to manage the Tai-18 Highway in the typhoon-induced heavy rainfall. The new probability-based rainfall criteria of failure slope under heavy rainfall and typhoon at high altitude section, medium altitude section, and low altitude of Alishan highway are concluded as follows: R_min, I_min, and T_min are the rainfall threshold to cause slope failure. In the high altitude section, they are 286.5 mm, 28.5 mm/hr, and 13 hr. In the medium altitude section, they are 256 mm, 22 mm/hr, and 12 hr. In the low altitude section, 306.5 mm, 38 mm/hr, and 12 hr. Meanwhile, R_max, I_max, and T_max are the rainfall upper bound of the stable slope. In high altitude section, they are 773.5 mm, 53.5 mm/hr, and 71 hr. In the medium altitude section, they are 828.5 mm, 56.5 mm/hr, and 72 hr. In the low altitude section, they are 592.5 mm, 70.5 mm/hr, and 71 hr. In addition, the R_mid, I_mid, and T_mid are the rainfall data, which the probability of slope failure P(F) is the same as that of stable slope P(NF). In the new highway management criteria, when the rainfall data of R, I, or T recorded at each representative rainfall station at each highway section ≥ R_min, I_min or T_min, the authority must carries out "warning of slope failure" to the people and car on the highway. If the rainfall data of R, I, or T ≥ R_mid, I_mid or T_mid, cars and people can use the highway only with authority permission. When the rainfall data of R, I, or T ≥ R_max, I_max or T_max, there will be forbidden to enter the highway.