## 4 IFC Data Model for Supporting Safety Risk Identification in Deep Excavation
### IFC Expression of Four Information Elements with Necessary Extension  
#### 4.3.1 IFC Expression of the Design Information (I-1)  
Figure 4 shows the main structural components in deep excavation, which are classified into different categories and systems (e.g., enclosure, wai purlin, and machinery parts, support (the temporary structure), dewatering, and monitoring systems) according to their typical engineering characteristics. In the following context, the enclosure, the wai purlin, and the supporting system are emphasized to show the IFC expression of the design information (I-1), focusing on its primary and secondary components. 1. IFC expression of the structure component information (I-1-1)  
According to IFC standard, attributes of each object entity and assigned values are expressed through a specific property set. Extension based on property set is used to describe the primary components in deep excavation. Primary components can be described by entities defined in the architecture domain of IFC4 to ensure the compatibility and interoperability of the extended IFC model [30]. New property sets involving construction parameters for safety risk identification are incorporated into these objects as an extension. In this research, for example, the geometric representations of IfcBeam, IfcColumn, IfcPile, and IfcWall described in IFC4 are respectively adopted to present the primary components of the inner support and purlin, the column and diagonal bracing, the pile and waterproof curtain, and the underground continuous wall in deep excavation. Moreover, extended property sets (i.e., Pset_SoldierPile (describing the pile and waterproof curtain), Pset_DiaphramWall (describing the underground continuous wall), and Pset_InnerSupport (describing the inner support)) are created based on practical component characteristics. Figure 4: Main structure components in deep excavation. Figure 5 shows the extension of the pile entity (IfcPile) as an example. Relationship entities, such as IfcRelDefinedByProperties or IfcRelDefinedByType can be utilized to establish the relationships between IfcPile and its associated property sets (including the existing sets and the extended Pset_SoldierPile). In correspondence with construction parameters related to risk factors, the properties contained in the extended property set Pset_SoldierPile are shown in Table 3.  
Secondary components can be expressed by instantiating the IfcPoxy entity objects for good compatibility and ease of operation. For example, in deep excavation, soil nails and anchor bolts can be expressed using user-defined model interfaces provided by the IFC [31]. 2. IFC expression of the product type information (I-1-2)  
Product type information (I-1-2) can be expressed as shown in Figure 6, with corresponding product type entities extended. These extended entities can be related to structure components using relationship entity IfcRelDefinesByType. 3. IFC expression of the spatial structure information (I-1-3)  
The IFC4 standard divides the space of building engineering into four spatial levels (i.e., IfcSite, IfcBuilding, IfcBuildingStorey, and IfcSpace) from the top down. Considering the above space division method and the special engineering characteristics of deep excavation projects, this study divides the deep excavation space into three levels: IfcSite, IfcExcavationPartitionZone, and IfcSupportingLayer (Figure 7). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Entity** & **Property Set** & **Contained Properties** & **Data Type** \\ \hline \multirow{4}{*}{IfcPile} & & Entering-rock condition & IfcBoolean \\  & & Depth ratio & IfcReal \\  & & Occlusion condition & IfcBoolean \\  & & Diameter & IfcReal \\  & & Water-sealing measure & IfcBoolean \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Property set extension of IfcPile  
Figure 5: IfcPile with increasing property set. 4.