How to Simplify Lot Grading in Civil 3D15 Feb, 2021 By: Cadalyst Staff
The Feature Lines tool — which is standard in Autodesk’s civil infrastructure design software — is all you need to create remarkably detailed grading designs.
Grading a subdivision lot with yard breaks and side swales can be a struggle. The concerns are numerous: Are your pads the highest point? Do you have sufficient slope to guide water away from your structures? Have you added high points around septic fields?
All these tasks can seem daunting when you’re working in Civil 3D. We’ve watched people struggle with grading objects, direct surface edits, and even third-party grading add-ins to attempt to handle lot grading. One of the most common processes in the civil/site design world can, ironically, become one of the most difficult and time-consuming.
The good news is that it doesn’t need to be that way. Lot grading can be done in just a few minutes, with great accuracy, and without requiring expensive add-ins or learning new software. In this article, we’re going to take you through a full lot grading condition using one simple Civil 3D tool — which you probably already know how to use! You just need to apply a bit of imagination for it to handle all your lot grading needs.
What tool will we be using? The simple Feature Line tool. That’s right, we’re going to use nothing but Feature Lines to handle a lot grading process. So, let’s start with a basic lot layout in 2D, like the one pictured below. All the linework is done with polylines, with no elevations (well, technically “0” is an elevation, but you get what I’m saying).
We’re going to begin by changing the 2D polylines to Feature Lines, using the FEATURE LINES > CREATE FEATURE LINES FROM OBJECTS tool on the Home ribbon.
We’ll select all the lot lines and use the dialog that comes up to erase the underlying polylines and replace them with Feature Lines. We’ll also use the “Assign Elevations” option so we can tie these lot lines to the Existing Surface (EG) in the next step.
Clicking “OK” brings up the Assign Elevations dialog box, where you can set all vertices (points) on the feature line to specific elevations or read the elevations from a Surface. We’re going to read from our existing surface and “Insert Intermediate Grade Break Points.” This places elevation data along the feature lines at each point where they cross the underlying TIN lines of the surface.
Looking in the Object Viewer, you can see that the property lines are now draped onto the surface:
By clicking on each feature line and using the ELEVATION EDITOR on the ribbon bar, we can locate and identify the high points on each feature line, so that we can set the pad height to be above the highest property line point in the next step.
We can repeat the process of creating Feature Lines from Objects and apply a set elevation (197 in this example) to each of the building pads, like so:
Looking again in Object Viewer, we can see that the building pads are now the highest points on our lots:
We can use the same process as we did for the pads to generate septic fields, catch basins, driveways, etc. Next, we can use the FEATURE LINE > CREATE FEATURE LINE FROM STEPPED OFFSET command to offset the front yard to get a set slope/grade up to a yard break.
We’re going to offset the line 25 ft, at a grade of 12% from the front yard line. This gives us the result below:
Note that each vertex (blue grip point) elevation is calculated at a 12% grade from the bottom green line in the image above. We can continue this process to create mid-yard breaks, etc., as needed. We can also go back to the ELEVATION EDITOR to manually override to any specific elevations that we need.
Lastly, we can use the FEATURE LINE > CREATE FEATURE LINE tool on the Home ribbon to sketch in items such as side yard swales, culverts, or drainage ditches where needed.
When we use the OSNAP tools and snap to a Feature Line, the elevation of the point we snap to is automatically provided to the Feature Line tool, so we can snap to the midpoint of the yard, for example, and sketch out a drainage swale for rear yard run-off using the grade/slope/elevation control options in the Create Feature Line command.
Looking in Object Viewer, you can see that the Feature Lines are sloping to the design settings that we’ve set (a bit exaggerated in this example for visibility) and that we have the outlines of a fully developed grading surface.
From here, all we need to do is build a new surface and add all our designed Feature Lines to that surface as “Breaklines” to get a finished grading with detailed contours.
That’s all there is to it! Feature Lines — a standard Civil 3D tool — is all you need to create remarkably detailed grading designs. This approach works for lots, as discussed here, as well as parking lots, steps, roads, curb return conditions, or just about anything else you can think of. Take some time to get comfortable with the proper use of Feature Lines and what they’re capable of; it may be the best investment of time you’ll ever make in your civil design career.
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