Abstract: As urban rail transit construction increases year by year, conflicts between new tunnels and existed bridges become increasingly obvious. Aiming at the risk assessment problem of shield tunnel crossing the existing bridge construction, and based on the system idea, we introduce the C-V-T model to classify the engineering risk into consequence severity, which is called C; the vulnerability of the existing bridge system, which is called V; and the threat of the new tunnel system, which is referred to as T, so as to construct a three-system risk assessment process of disaster-causing, disaster-bearing and post-disaster. By introducing the two-factor catastrophe theory to establish a three-level index system, we obtain the catastrophe progression of the existing bridge to express its vulnerability. In order to improve the accuracy of T-value, we use the interaction matrix to measure the cross influence between the indicators of the new tunnel, and then give the indicator-system weight method. Finally, introducing the complex mapping relationship and transformation situation between the quantitative indicators and qualitative concepts to express multi-dimensional cloud and forward generator optimization algorithm objectively. Additionally, we clarify the consequence severity grading criteria, by which defining the direct economic loss, the surrounding environmental loss and human casualty, give the coupling method of the three system risk assessment results, and thus build a comprehensive assessment model for the construction risk of shield crossing existing bridges. We apply the proposed model to the risk assessment of Yixin Bridge Crossing Project of Kunming Metro Line 5, which shows that the evaluation results match with the actual situation of the project. The construction monitoring data also verifies the rationality, standardization and applicability of this model.

Key words: tunnel engineering, risk assessment, multidimensional cloud, shield crossing, mutation theory, interaction matrix