Creating a TIN of a Planar Surface

[superchicken15]

I'm new to CAD and I am probably missing something very simple, I can't seem to create a planar surface that shows up in the toolspace as a TIN.

Some background, I have a TIN of a pile of dirt that I created from a point cloud (it shows up in the toolspace under the surface section). I also have a plane that intersects that dirt pile which indicates ground level. I want to be able to calculate the volume of the dirt pile by creating a volume TIN with the ground plane and pile TIN, but the ground plane does not show up in the toolspace under the surface section so I can't use it to create a volume TIN. I created the plane with the planar button in the ribbon and manually lined it up with the pile.

How can I convert this plane that is clearly in my project but not in the list of surfaces in the toolspace into a TIN to use in a volume TIN? Am I going about this the correct way?

I'm running Civil 3d 2013 and you'll probably have to explain things like I'm 5 because I'm not familiar with the interface yet. Any help is appreciated 

[mathewkol · Accepted solution]

You need a polyline or feature line at your plane elevation. Then add it to a new surface as a breakline.

[superchicken15]

Thanks mathewk, using a pline around the plane did the trick! 

[antoniovinci]

Another way is to draw a classic "Coon" plane surface (command _EDGESURF), then exploding it, and finally adding the resulting quad 3Dfaces to the surface definition: here's in white colour.