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.