Creating Geological Surfaces from Probed Depth Values:
Probed depths or borehole logs provide us with information on the subsurface geological layers for the site. When importing and using this data in Civil 3D there are a couple of issues that need to be looked at.
Issue 1: The surfaces created from the depth values in Civil 3D are of limited use. What we really need are the elevation values at each of the probe locations. We need to convert the depth values into elevations.
Issue 2: Due to differing surface data resolutions (typically more points in the existing ground surface) the bottom of peat surface may not appear to accurately represent the geological layer – it may not ‘follow’ the lie of the land. See screen grab below. (You could in some cases see your subsurface extend above the existing ground in section). We need to create a surface that uses the probed depths and also ‘follows’ the existing ground in the areas in between where we do not have any probes.
There is workaround that has previously been posted on a number of blogs and forums that solves both of these issues. Here it is described as applied to a windfarm project - with a little bit of explanation of what is going on in the background from a Civil 3D point of view.
- Create a TIN surface from your probed peat depths – call it Probed Peat Depth.
- Create a volume surface using the Existing Ground surface and the Probed Peat Depth surface – call it Peat Volume. The order in which you add the surfaces is important (base – Probed Peat Depths, Comparison – Existing Ground). You now have a volume surface that has depth values that are equal to the elevation for the bottom of peat.
- Create a new TIN surface and call it Surface from Peat Volume. Paste in the Peat Volume surface. Pasting a volume surface into a TIN surface creates a surface with elevations equal to the depth values of the volume surface– we now have a TIN surface representing the bottom of peat.
The surface created in step 3 uses the probed peat depth values and follows the existing ground in between probes. Note – this is not a true representation of the sub-surface geology but it is a good base to start with.