Let's face it... the Stage Storage Tool in Civil 3D needs a complete overhaul. The only time it works for me without having to do a bunch of extra work, is on a cleanly graded design surface. Seems that most of my work is incorporating existing data which is never good enough for the Stage Storage Tool. So I ran across some links in explaining alternative methods that I need to try... saving these so I don't forget and you're welcome to look into as well. Hopefully, they provide great results that can create less work in making the software do what I want so I can focus on the design...
ALTERNATIVE STAGE STORAGE TECHNIQUE – CIVIL 3D TABLES
STAGE STORAGE ANALYSIS OF STOCKPILES/DAM WALLS (BETWEEN TWO IRREGULAR SURFACES)
