## 3 Early Warning Index System
### Acquisition Methods of the Index Data
The index data are obtained by field investigation. \[R_{22}=R_{22}^{1}/R_{22}^{2}*100\%, \tag{6}\]  
where \(R_{22}^{1}\) is the number of unqualified inspections and installations, and \(R_{22}^{2}\) is the total number of installations. The rate of mechanical maintenance failure (\(R_{23}\)) is an index with which to measure the maintenance of construction equipment. The better the mechanical maintenance, the lower the \(R_{23}\) value, and the lower the construction safety risk. The index data are obtained by field investigation. \[R_{23}=R_{23}^{1}/R_{23}^{2}*100\%, \tag{7}\]  
where \(R_{23}^{1}\) is the number of failed maintenance inspections, and \(R_{23}^{2}\) is the total number of inspections. Concrete and steel are the most important materials for subway station construction. The qualified rate of concrete (\(R_{31}\)) and the qualified rate of steel (\(R_{32}\)), respectively, indicate the quality of these two materials. The lower the values of \(R_{31}\) and \(R_{32}\), the lower the construction safety risk. The data are all obtained from field tests. \[R_{31}=R_{31}^{1}/R_{31}^{2}*100\%, \tag{8}\]  
\[R_{32}=R_{32}^{1}/R_{32}^{2}*100\%, \tag{9}\]  
where \(R_{31}^{1}\) represents the volume of qualified concrete in the field test, \(R_{31}^{2}\) represents the total volume of concrete in the field test, \(R_{32}^{1}\) represents the weight of qualified steel in the field test, and \(R_{32}^{2}\) represents the total weight of steel in the field test. The unit of \(R_{31}^{1}\) and \(R_{31}^{2}\) is m\({}^{3}\), and the unit of \(R_{32}^{1}\) and \(R_{32}^{2}\) is tons (1 ton = 103 kg). The rate of material supply (\(R_{33}\)) indicates the material supply at the construction site. If the construction materials are not supplied in time, the construction can easily be interrupted unexpectedly, thus increasing the construction safety risk. The index data are obtained by consulting the construction log. \[R_{33}=R_{33}^{1}/R_{33}^{2}*100\%, \tag{10}\]  
where \(R_{33}^{1}\) is the number of instances of timely construction material supply, and \(R_{33}^{2}\) is the total number of instances of construction material supply. The rate of material stacking error (\(R_{34}\)) indicates the stacking situation of construction materials at the site. The incorrect stacking of construction materials can easily lead to collapse, so the potential safety hazard is substantial. The lower the value of \(R_{34}\), the lower the construction safety risk. The index data are obtained by field investigation. \[R_{12}=R_{12}^{1}/R_{12}^{2}*100\%, \tag{2}\]0where \(R_{34}^{1}\) is the number of inspections for which the inspection fails to meet the standard, and \(R_{34}^{2}\) is the total number of inspections. The maximum deformation of the foundation pit (\(R_{41}\)) indicates the stability of the foundation pit supporting structure. The lower the value of \(R_{41}\), the more stable the foundation pit supporting structure, and the lower the construction safety risk. The index data are obtained by consulting the monitoring data of the construction site. The unit of \(R_{41}\) is mm. The 24-h maximum rainfall (\(R_{42}\)) represents the rainfall intensity. Because subway station projects are completely exposed to the natural environment, rainfall can easily lead to safety accidents such as landslides, waterlogging, and electric shock. The lower the value of \(R_{42}\), the lower the construction safety risk. The index data are obtained by consulting the construction log or local meteorological data. The unit of \(R_{42}\) is mm. Poor geological conditions (\(R_{43}\)) refer to the adverse effects of the soil type, rock weathering, and the groundwater level on the construction site. The lower the value of \(R_{43}\), the more favorable the geological conditions for construction, and the lower the construction safety risk. The index data are obtained by consulting the previous geological survey data. If the geological conditions are too complicated, expert interviews can be used to obtain the index data.