# A Systematic Review of the Trends and Advances in IFC Schema Extensions for BIM Interoperability
## 5 Evaluation of IFC Schema Extension Cases
### Analysis of Extension Cases by Product and Process Representations
#### 5.3.2 Detailed Analysis of Extension Approaches in the Infrastructure Domain  
Table 5 presents the cases of extensions in the infrastructure domain, categorized based on their representations and extension approaches. A total of 22 cases utilized entity and property definitions to represent new products, whereas 21 cases defined entities and related properties to represent new processes. No cases relied on property extensions for product representations, although three cases defined properties exclusively for introducing new processes. The following provides detailed analysis results according to the specified criteria:  
1. Product-oriented representations: - Using entity and property extensions. Entity and property extensions were used heavily to represent the major types of structures in the infrastructure domain. In the bridge sector, efforts were marked by [68; 69; 70], who devised approaches to represent the basic types, components, and related properties of bridges. The road sector was also addressed by [78], who delineated entities representing spatial elements and further refined them to depict curvilinear features. Subsequently, ref. [79] developed entities for drainage facilities, followed by [80] who introduced additional entities to capture details of elements in auxiliary facilities. Several studies were directed towards improving the representation of tunnels and their construction methods. Early on, ref. [59] established entities to represent the soil layers and conditions required for shield tunneling as well as entities for caves and caverns. Refs. [60; 61] expanded upon this study by proposing supplementary entities to facilitate multi-scale representations of shield tunnels. Ref. [57] centered their efforts on representing NATM tunnels as well as on introducing new entities of _IfcTunnelElement_ and its related \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Extension** & **Sector** & **Product** & **Number of** & **Process** & **Number of** & **Total** \\
**Approach** & **Sector** & **Oriented** & **Cases** & **Oriented** & **Cases** & **Saces** & **Total** \\ \hline \multirow{3}{*}{Entity + Property} & General & \multirow{3}{*}{[38; 39; 40; 41]} & \multirow{3}{*}{4} & \multirow{3}{*}{[34; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51]} & \multirow{3}{*}{9} & \multirow{3}{*}{13} \\  & building & & & & & \\ \cline{1-1} \cline{5-5}  & MEP & - & - & [56] & 1 & 1 \\ \hline \multirow{3}{*}{Property only} & General & \multirow{3}{*}{-} & \multirow{3}{*}{-} & \multirow{3}{*}{[46; 47; 52; 53; 54]} & \multirow{3}{*}{5} & \multirow{3}{*}{5} \\  & building & & & & & \\ \cline{1-1} \cline{5-5}  & MEP & - & - & [55] & 1 & 1 \\ \hline \hline \multicolumn{2}{c}{Total} & \multicolumn{1}{c}{-} & \multicolumn{1}{c}{4} & \multicolumn{1}{c}{-} & \multicolumn{1}{c}{16} & \multicolumn{1}{c}{20} \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Extension cases by approach type in architecture domain. subtypes and attributes. Similarly, ref. [64] designed entities for TBM tunnel boring machines. Ref. [58] went further to encapsulate various aspects of TBM tunnel elements, such as ground and boring machine models. Ref. [63] suggested new entities, such as _IcfStation_, to be used exclusively for underground metro stations. A string of studies was also conducted for the rail sector. Ref. [84] introduced entities that distinctly outlined tracks and ties. This was later expanded by [87], who continued to innovate this topic by incorporating alignment concepts and thus enabling differentiations between physical and spatial elements. Recently, ref. [86] crafted entities to represent rail track drawing details and distinguish between multiple layers present in rail subgrades. Other studies worked on representing entities for specialized sectors: ref. [92] defined entities for 3D tiles in dams, ref. [18] for quay walls of harbor facilities, and ref. [19] for elements in wastewater treatment plants. A few notable cases focused on representing entities for different types of civil works: ref.