## 5 Results and Discussion
### Practical Application (Backward Inference)  
Based on the Bayesian network backward inference, the most likely cause of the accident can be identified as the coupled risk accident in the construction of a DFP in the adjacent existing underpass tunnel (i.e., risk-causing diagnosis can be achieved to help better control the risk and prevent the accident from occurring in future construction). The principle of diagnostic termination is that the risk factor leading to the accident is defined, and the next factor to be examined has not yet occurred. The process of backward inference is shown in Figure 16.  
According to the special risk report, on 2 January 2022, the daily settlement rate of rainwater and sewage pipes on the north side of the pit reached 2.3 mm/d, which occurred at time slice T3. In the DBN model of the T3 time slice, the evidence P was set as (F = low) = 1, and the posterior probability distribution of the B level was inferred by the diagnostic function of DBN, as shown in Figure 17a. Based on the posterior probability, there was no FOP (P (B10 = Yes) = 8.00%), HE (P (B11 = Yes) = 7.65%), BID (P (B8 = Yes) = 7.00%) at the accident site. It was not until IRS (P (B4= Yes) = 6.83%) was discovered that the horizontal displacement of the top of the rotary spray pile on the north side of the 64 th axis was 37.6 mm in a low safety state. The reverse diagnosis was performed for the C-level risk factors under IRS by setting the evidence P(F = Low, B9 = No, B10 = No, B11 = No, B4 = Yes) = 1, as indicated in Figure 17a. The first risk factor that should be concerned is the occurrence of LOSC (P (C11 = Low) = 18.6%), and the site investigation found a large area of dip on the north side of the 63-68 th axis. Construction workers have used high-pressure grouting to form the leaf vein-like structure of cement to seal the gaps in the soil. Figure 18a shows the scene when the danger occurs and demonstrates that there are still many water seepage traces on the concrete of the spray anchor. The spray anchor lag, the continuous seepage of the upper layer of stagnant water affect the water stopping effect of the spray anchor. Let P (C11 = low) = 1, no HC (P (C8 = Low) = 10.1%) was found. On 21 February 2022 (T11), the 83-84 th axis support axial force reached 151.7 kN, more than 70% of the design value. Set evidence P (T = severe) = 1, no risk factors B11, B10, B9, FPI (B5), IRS (B4) occurred until the presence of ECTS (P (B7 = YES) = 6.34%) in the site. Set evidence P (F = Low, B9 = No, B10 = No, B11 = No, B4 = No, B5 = No, B7 = Yes) = 1, as shown in Figure 17b. There were no large weak interlayers in the accident site (P (C17 = Low) = 35.1%) and no irregularity in the operation of machinery (P (C19 = Low) = 26.4%). However, the excavation speed of the pit was too fast (P (C18 = Yes) = 4.69%). At this point, the construction team immediately stopped construction and adjusted the excavation plan. It is worth mentioning that at this point, the tunnel lateral was located within a reasonable range. Setting evidence P (C18 = Low, C19 = High) = 1, no DSTS (B6) was noticed to occur during the subsequent inspections. Figure 15: Mutual information of root nodes (risk factors). t=T14(2022/01/18-2022/01/24)  
Figure 16: Three scenarios of backward inference. On 14 March 2022 (T14), the uplift of the center column at the bottom of the 94 th axis of the foundation pit reached 2.14 mm. Based on the previous evaluation logic, the dewatering well between the 92 nd and 94 th axis had a sudden surge (P (B5 = YES) = 6.98%). As shown in Figure 17c, during the inspection, two risk factors (SPF (C12) and HGL (C13)) occurred in the C-level under FPI (B5). The construction team filled in the small diameter stones at the time of the hazard and filled in the dry material concrete and micro expansion concrete. Figure 18b shows the scene at the time of the dangerous situation. It can be seen that there was a large amount of undrained sewage at the bottom of the pit.