Site flatten - top down

TXPE

I am attempting to show/create an excavated hole in the ground.  The existing ground surface is in the 500-530' elev range the hole will be dug to a depth of 400' elev.  The excavation boundary at the surface is constrained to a defined boundary.  Therefore, the excavation needs to be inside this line.  Wall slope is 70 degrees.

I draw the area to represent the limits of excavation at the surface, select that area, then do the Calculate Flattened Site Plan Grid option.  The resultant grid surface is drawn such that GM apparently sets the boundary area at 400' and works it way back up (bottom up).  I need it to work top down as shown by the green line in the screenshot.

I've tried the various options and none will seem to do what I want.  Any recommendations?

sphillips

GM flattens the area feature you select to the target elevation you set. Therefore you need to start at the bottom of the excavation and work up to the existing ground. I don't think you can get the excavation to match the exact footprint but you can certainly get it to fit within it.

Potential workflow:

1. Calculate the offset required relative to the excavation boundary for the bottom i.e. approx -36.4' 
2. Use the Buffer tool to offset the excavation boundary by that value
3. Edit the vertices of this new area feature to set them at 400'
4. Use the Flatten site plan to create the slope up to your maximum elevation or higher i.e. 530'
5. If the terrain strays out of the excavation limits then select the boundary and use it to clip the terrain using the Cropping tab of the layer

If you have a questions give us a yell. Good luck.

TXPE

Thanks @sphillips, I had created my own steps to get this down the way I needed.  I ended up doing this several times back in February, but just now wrote down the steps as outlined below.  Because I've slept some since then, the below steps could be missing something.

Create proposed/design excavation to match uneven surface elevations.

1. Draw the limits of excavation ( I use area, not lines) - which would be at the ground surface (500'-530' in example).
2. Determine what the bottom elevation surface will be.  It can be uneven. Choose the lowest elevation.  In this example, that is 400'.  So, elevation difference between bottom and top is 100-130'.
3. What I'm going to do next, is lower the surface elevation (change after elevation value in layer options) by a value greater than the largest difference (130'). In this scenario, I chose 150'.  So, select Options for the surface layer and change after elevation Offset to -150.
4. For my proposed excavation, I want 70 degree walls. This equates to a H:V ratio of 1:2.747. Now, I need to create an area on this lowered surface that "parallels" the area at the top/normal surface.  This is done by selecting the limits of excavation area and creating a buffer area that is a fixed distance of (150/2.747=) -54.61ft.
5. Select the inset buffer area.  Right click and choose Analysis/Measurement -> Apply Elevations to Selected Features.
6. Now, need to undo the change in elevation done in step 3.  Change offset back to 0.
7. Select the inset buffer area. Right click to choose Advanced Creation Feature Options -> Calculate Flattened Site Plan Grid.  Choose Flatten to Area Heights and leave Area Heights Relative to Ground unchecked.  Enter 2.747 for Allowed Slope and make sure benching values are 0.
8. This will generate a new surface showing side walls and bottom surface only.  But, the bottom surface is still at 150' depth.
9. We need to create a bounding area around this new surface.  Right click on surface layer in control center and select Layer -> BBOX/Coverages.  Verify correct layer is checked. In the next pop up box, choose No to create a polygonal area. This will create a new area in User Created Features layer.  Because the edges of this new surface are likely erratic, I select this bounding area and create another buffer area inset 3-8'.  If you don't do this, you will end up with gaps in your data along the edge and will screw up the surface.  You might need to play with different inset values.  You are trying to make sure this inset area doesn't leave any "air" gaps.  I then select this inset area and use it to crop the surface layer created in #7.
10. I already had a flat surface grid created at 400' elevation that extends beyond the limits of excavation area.  This will affectively be used to "chop" the bottom off of the surface created in #7(& #9).  This is done by making sure both layers are on (#7 and this 400' surface). Choose Analysis -> Combine/Compare Terrain Layers.  Choose Maximum Elevation in Operation drop down box.  Select both layers.  I select Default Shared Shader (not required).  Choose Sample Spacing you desire.  Then go to Export Bounds and choose Crop to Selected Area. Click OK and you will now have a single surface grid that represents the proposed/design excavation.
11. You can then go on to incorporate this surface into the site wide surface layer.