Before I start on the above post, just to remind that a hotfix for Civil 3D 2009 have been released to resolve section display and volume calculation issues. Hotfix and installation instructions can be found here.
Now to the main topic. We were recently asked to compute earthworks volume quantities of different geological materials for a 5km highway scheme.
Ground investigation data in spreadsheet were provided, import them in to Civil 3D using points, create a surface and cut sections to see how the materials look- no problem.
However due to the distance between borehole locations, the data is not sufficient to correctly model the surface of the underlying material. On the section shown below, the unsuitable material layer in some chainages are above the existing ground.
This will obviously generate erroneous volume quantities. I thought of two ways to do this. First, is to edit the surface by adding points along the sample lines with the correct levels either by adding COGO points or surface points.
Another way is to edit the section, copy and paste the section data to a spreadsheet and import them back. We preferred to use this option as we have these instances on a couple of sections. Here's how we did it using Civil 3D 2008.
Change all the material's section lines to static. Right click a sample line> Sample Line Group Properties.
In the Sections tab, change the material's update mode to Static and click OK.
This should enable the section line grips.
Grip edit the section.
After editing the section, right click the section line> Edit section. Right click the table, click Copy All.
Paste the copied table on a spreadsheet. Continue editing the section lines for the other chainages and adding section data to the spreadsheet as you go along.
On the spreadsheet, delete the Section Gradient In and Out columns and the header row.
Replace the point number column with the chainage number where the section was taken from and remove the unit "m" from the values using the replace command in Excel. Save as a comma delimited (.csv) file format.
Import the file to Civil 3D. Points menu> Create Points - Alignments> Import from file.
Note that the file type it will look for is .txt. Type *.csv in the File name field to see the file.
When asked for a file format, choose '2' - Chainage, Offset, Level.
Choose ‘2’ – Comma for delimiter type and hit Enter for the next two prompts.
Select the alignment and the points should appear along the section sample line.
Create a new surface and use all the project points (existing and added). I’ll leave the volume calcs to you. 
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