Currently, SITEOPS does not have a feature to model an Underground Detention Basin; however, there are a few things that can be done to account for the displaced earth and tie-in point.

1. Calculated the approximate displacement of earth for the underground basin. 
2. In the default properties, locate the Desire Earth Import and set it to the calculated quantity.  This allows SITEOPS to account for the earth displaced from the underground basin.
   
   
   
   
3. Place a Stormwater Tie-in Point and set the Utility Depth to the underground basin tie-in crown.

4. Optional: To keep stormwater pipes from passing within the basin area:

• Draw an Easement => Undefined area to the appropriate size
• Uncheck the Shape Triangles option. This will keep SITEOPS from shaping the TIN... SITEOPS will not break the TIN and it won’t apply any grading information (i.e, surface type, surface depth, slope, etc.)
• Uncheck the Allow Storm Piping option. This will keep pipes from passing through the area.

This is an optimized solution: