The Earth Moved: Civil 3D, Subsidence & Milkshakes

In recent classes I've taught, several people have asked me about how to deal with the effects of ground subsidence in Civil 3D.  This is not a question I've encountered before, as it is largely negligible in my homestate of Wisconsin. In California, however, it is not uncommon for swaths of land to drop a foot or more over the course of a long life cycle project, wrecking havoc on the earthwork quantities. How can you get good corridor data when your surface target is actually a moving target?

Setting aside the shocking environmental causes of subsidence in California's central valley (weep for the groundwater, children, and enjoy your almonds while you can) I will focus here on how to deal with it in the software. However, I can't help but throw in one of the scariest scenes from modern cinema involving milkshakes and one of the primary causes of subsidence...
Anyhoo... right, where was I? Subsidence and Civil 3D. 

You absolutely MUST use Data Shortcuts (or Vault data references). Even if you are just the proverbial "one man shop" and never intend to share data with anyone, the use of data shortcuts is critical for keeping file size manageable.

The first drawing in the Civil 3D workflow is usually the existing ground drawing containing survey data. To deal with subsidence, I recommend always using at least two surfaces. The _Existing Surface is the surface created as a data shortcut and will end up being the "workhorse" surface for existing ground profiles, corridor target surface and quantities.
The definition of _Existing Surface consists only of other surfaces pasted into it.  This allows for the original data (called Flown Data 10.5.2006 in my example) to be the base but additional surfaces can be added over it as needed. It is important to note that when you use the Paste Surface command, the order in which you paste the surfaces makes a difference on the result. Newer surfaces should take over where they overlap old data. This order can be changed in the Surface Properties Definition tab, if needed. 
So why do I recommend this instead of just swapping out point groups? Creating new, independent surfaces as new data comes in ensures that the obsolete data is completely obliterated in the areas to which it applies. You may also wish to keep a record of the old surface for comparison purposes. 

If and when the surface changes, everyone will get the update via the data shortcuts.

Is anyone else hungry for a milkshake? How 'bout a nice tall glass of saltwater intrusion?

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