Question about modify Topo surface?

[omar.amohamed]

How to cut the topo surface in the attached picture?

That is a circular tank that looks like a cone in elevation view.

I want to remove the topo surface that highlighted with a red mark only without removing what exists at bottom of the tank.

[kh_jea]

Have you tried using Building Pad?

If you're trying to create a void in the topo, this isn't possible. Only vertical cuts created with a Building pad (or split toposurface). 

[omar.amohamed]

Yeah, I am trying create a void in toposurface cause the pad only create vertical cut.

Could you explain how to achieve that by split toposurface?

[kh_jea]

You can split a toposurface, but you will never be able to create a "void" (like a tunnel) under a toposurface.

https://knowledge.autodesk.com/support/revit-lt/learn-explore/caas/CloudHelp/cloudhelp/2019/ENU/Revi...

[barthbradley]

Looks like a Sloped Building Pad would do the trick.