I have a existing surface. I want to create a proposal surface so I can know the volume between boths. This proposal surface is represent for a disposal area that can be filled up to a elevation. I have a bottom of P-surface as a feature line at EL 1608, and I want top of P-surface to EL 1613.
Could some one please show a direction of how to create this P-surface.
See attached for png and Civil 3D file.
Thanks much in advance!
I usually have to do this in the other direction, starting from a contour elevation and projecting downward, trying to figure out where the bottom of an excavation should be in order to create a slope back up to the start line. Looking at your sketch it appears that part of your featureline follows the 1608 contour, then it slopes up to match the existing grade at the access road.
Start by creating a grading object from the portion of the FL at 1608, projecting up to 1613 at your maximum specified slope. As your starting line is jagged the resulting grading is probably going to be messy, and might cause the drawing to become unstable. (save before doing this) The extents of the grading object defines the limit of where your top of fill line can be.
Draw a new FL slightly inside of this area at 1613, all the way around. Where you don't have a grading at the access road you can approximate where you think it should be for now. Make sure that this FL is on a new site so that there are no interferences with any of the previous work. Erase the first grading object and create a new one, projecting outward and down to the surface to confirm that it fits within your boundary. Now you can adjust or add PI's to tweak things.
At this point you can choose to use the grading object with a consitent slope but the toe probably won't land on your limit line, or you can delete the grading and just use the new top FL and your original bottom FL as surface breaklines.
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When it comes to create a grading from a feature line, and in the special case of a contour line that is described by anomalies, I usually do a smoothening to a direction that suits the occasion, before creating the grading, as it is not practically applicable during construction to use it (the f l) as it is drawn. (Smoothening = inserting arcs, experiment with radius, between the vertices and finaly drape it on the surface to acquire elevations). Then, there shouldn't be any problem grading to a desired elevation with a certain slope, as soon as the fl's elevations do not exceed the ceiling elevation.
I agree, I would that a guesstimate and hone in on it