# Multifactor Uncertainty Analysis of Construction Risk for Deep Foundation Pits
## 4 Project Case Analysis  
### Project Overview  
Employing the risk evaluation method for deep foundation pit construction proposed in this study, taking the deep foundation pit of a city subway as the evaluation object and considering the complex factors influencing the related risk, a risk evaluation is carried out for deep foundation pit construction. Figure 3a presents the distribution of the deep foundation pit, which is irregular in shape (i.e., L-shaped). The excavation depth of the foundation pit is 9.25 m-15.3 m (i.e., >5 m; it is a deep foundation pit), and the area is about 1500 m\({}^{2}\).  
A comprehensive survey method was adopted, combining geological surveys with surveying and mapping, drilling, geophysical prospecting, in situ testing, and hydrogeological tests and indoor testing of soil, rock, and water samples. Investigations prior to excavation are relatively comprehensive, and monitoring points are arranged within the influence range of the excavation site of the foundation pit excavation (i.e., 1-4 times the excavation depth of the deep foundation pit) for real-time monitoring. Technical risk in engineering construction is low. There is a round gravel layer in the deep groove of the foundation pit, which has strong water permeability, and the project faces adverse geological conditions. The surrounding environment is complex, and environmental risks are worth considering. The normal groundwater level of the foundation pit is approximately 10 m underground. Before excavation, a safety appraisal of the surrounding buildings (A and B in Figure 3a)  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Grade** & **1** & **2** & **3** & **4** & **5** \\ \hline \(P\) & Impossible & Rare & Occasionally & Possible & Frequent \\ \(C\) & Negligible & Need consider & Serious & Very serious & Catastrophic \\ Grade quantification & (\(-0.5\), \(1.5\)) & [\(0.5\), \(2.5\)] & [\(1.5\), \(3.5\)] & [\(2.5\), \(4.5\)] & [\(3.5\), \(5.5\)] \\ \hline \hline \end{tabular}
\end{table}
Table 5: Quantification unification of \(B^{*}\) and the risk grade of the indices \(P\) and \(C\).  
was carried out, and the results show that the design risk of the engineering construction is low.  
Figure 3b depicts the profile of the supporting structure of the deep foundation pit. The main body of the foundation pit adopts an 800-mm-thick diaphragm wall, and three internal supports are used for foundation pit support (reinforced concrete supports, and the foundation pit is locally equipped with plate supports). To reduce the impact of foundation pit construction on the surrounding buildings, buildings A and B, which are close to the foundation pit, are equipped with isolation piles (the distance between the diaphragm wall and the isolation piles is 1.6 m, and they are connected by pull beams). The design of the supporting structure meets the safety requirements. Risk factors such as pit bottom heave and surface subsidence are factors to which the construction is prone, and construction safety risks urgently need to be considered. The overall construction scheme and the technology employed in the project are relatively mature, and the construction risk is low. No harmful gas that could jeopardise the project was found, and there are few extreme bad weather events in Nanning; therefore, accidental risk is not considered in this scheme. Therefore, on the basis of the survey results obtained for this project, a total of 26 risk factors from R1 to R6 are selected as the evaluation indices, and the Delphi method was used to invite relevant experts (i.e., five valid questionnaires were selected) to evaluate the importance of the indices for occurrence possibility and loss in light of the actual situation.