After reviewing everyone's initial feedback on types of linear structures which should be included in the DHC data model, I have compiled an initial list. Here is the list:

Some specific questions I could use some help on are:
1. Tunnels: Are there different types of tunnels. If so, what are they?
2. Are tunnels a shared structure across multiple utility systems (hot water, steam, chilled water, water, electric, telco, etc) systems?
3. Pipe Casings. Are there different types of pipe casings. If so, what are they?
4.Are Pipe casings a shared structure across multiple types of pipes (hot water, chilled water, steam, water, etc)?
5. What makes a concrete trench box different from a trench? Is a Concrete trench box actually just a type of trench?
6. Are there different types of trenches?
7. What is the difference between a conduit and a duct?
Thanks
Tom DeWitte
Esri, Inc