From Civilization V Wiki
Once a civilization has learned the appropriate technology, its workers can construct improvements. Improvements can grant access to a specific resource, improve a tile's output, or be constructed as a special power by Great People
Where Improvements Can Be Constructed
Improvements can be built only in appropriate locations. (Farms may not be built on Ice, for example, and Mines can't be built atop Cattle resources.) The Worker Action Panel will only display improvements that your civilization has the technology for and that are appropriate for the tile the active worker occupies.
Generally, farms can be constructed in any tile that doesn't contain a resource. If the tile does contain a resource, only the appropriate improvement can be constructed.
Duration to Construct
Each improvement type takes a certain amount of time. The times listed below are for standard games; it will take longer to construct improvements in marathon games, and shorter in games started in later eras.
- How Much Time is Left?
Hover your cursor over a worker to see how much time is remaining on the current construction job.
Leaving and Resuming an Improvement Project
If you leave a project in the middle and then resume the same project later on, the time already spent will be subtracted from the amount of time it takes to complete the project.
If you change projects, however, all progress from the previous project will be lost.
List of Worker Improvements
- Fishing Boats
- Lumber Mill
- Offshore Platform
- Oil Well
- Trading Post