In mountainous regions, rockfall is a typical geological disaster which might bring immense casualties and economic losses, but also endanger the safety of civil engineering construction. Many tunnels are being built in the southwest of China, thus a comprehensive assessment for rockfall risk is needed. For this purpose, in this paper, based on normal cloud model theory, we created a multi-index evaluation model for the rockfall risk assessment. Then, according to previous research and specific geological conditions, potential tunnel dangers are classified into four ranks, and some geological factors are considered as the principal factors. In order to fully express the opinions of experts, the qualitative indices were quantified by continuous value scale. Moreover, the value of each index is determined by expert scoring. In view of different evaluation units, we used the normalization method to make geological indices dimensionless. And three numerical characteristics (Ex, En, and He) were calculated by the cloud generator algorithm with MATLAB. In this study, we assigned the weight of indices by simple dependent function to avoid the influence of subjective. Finally, by means of a normal cloud generator, we determined the integrated certainty grades. To ensure the accuracy of the normal cloud model method, it was tested in rockfall cases in Jiefangcun tunnel. And the results obtained by the cloud model method are in good agreement with the practical situation. Moreover, the results are better than those of the AHP-FUZZY and artificial neural networks methods after comparison. The cloud model-based method realizes a multi-criteria assessment of the rockfall risk in tunnel portal section and provides a practical guide on safe tunnel construction for similar projects.
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