Grading - Thinking Outside the Box, Part 1 (CAD Clinic: Civil 3D Tutorial)

When it comes to creating a finish grade surface, we all struggle with finding a way to accomplish it easily. Let's face it, there's definitely no "easy button" for this. Many of us grew up on the Land Desktop grading wizard, and it worked in certain situations. Now we have the Civil 3D version, the AECC Grading object. Again it's somewhat useful, as long as you don't push it too far. Then, of course, there are feature lines, which seem to be very promising, flexible, and stable.

This article describes some outside the box ideas for grading because I think there are some very powerful tools in Civil 3D that get passed over when it's time to create that proposed parking lot or multi-bay pond.

We all know what we have in the Grading pulldown menu, and for better or for worse, we've all tried our best to get the job done with those tools. Now let's re-think what we have in the Corridor pulldown menu and how those tools could be used elsewhere.

So, what I want to do first is compare the grading object to a corridor object. They both start with a 3D, linear design layout object; grading objects use a feature line, and a corridor starts with an alignment and profile. Keep in mind that alignments and profiles are more flexible design tools, with options like spirals and vertical curves. Once you have a starting line to project a grading scenario from, the grading object offers just four options. You can grade at a slope using horizontal distance, a vertical change, a set elevation, or to a surface. That's all. On the other hand a corridor offers those four options, plus a multitude more.

Let me elaborate on just one typical grading scenario: grading to a surface (a.k.a. daylight). With the grading object you can grade to a surface with a defined cut and fill slope. Again, that's all. With a corridor assembly you have daylight subassemblies, generic links, and actual subassembly parts like curbs and retaining walls. The daylighting subassemblies have minimum and maximum width versions, along with benching, multi-intercept, multiple surface options, pin to a set distance, and even rounding. Rounding, by the way, is a blending of the proposed slope into the existing surface. (And, yes, then the proposed contours have the little radius at the end where it ties into the existing contour!)

Rounding option in a daylight subassembly. (Click image for a larger version)

Now add in the fact that you can use the targeting options in a corridor to attach the grading subassemblies to polylines, feature lines, or other alignment/profiles to control widths and elevations, and you can start modeling some pretty interesting designs (or we can all keep modeling rectangular ponds with rounded corners).

The most important part of this comparison might be the Corridor Section Editor that you can use to edit the corridor on the fly where ever you need to. You could change the default slopes, curb heights, sidewalks, and more section by section if you wanted to. Now, of course you can answer No to the grading object question — Apply to entire length? — and attempt to do a slope transition. I got that to work the way I wanted it to — once! But if I can use the Corridor Section Editor to do that for me, how much easier could it get?

Now there are some places where corridor grading doesn't grade right, like with intersecting slopes on an inside corner. It will not miter it for you, so you have to use other subassemblies or manually add a few feature lines to grade that area. The good news is if the corridor can't solve the grading scenario you designed, it just shows you that it can't on screen, and you can decide what to do next. As opposed to filling out one of those error reports that you get when the grading object has trouble figuring something out, before paying a visit to your Windows desktop, of course.

Okay, so now that I have got the gears in your mind turning outside the box in regards to some of the options you have when designing that new parking lot, I urge you to try a few things on your own. In next two parts of this tutorial, I will discuss a couple of scenarios using these theories. Part 2 will be about grading a pond with subassemblies, and Part 3 will show how you could apply this theory to create a parking lot surface with 100% dynamic curb islands that update instantly when you redesign the main slopes of the main parking area. Yes, it's easier than you think: Create an alignment and sample a proposed surface, then base the corridor off the sampled profile — more about that in part 3.

Parking lot surface with dynamic retaining wall and 2’ benched grading (just to show what you could do). Please note: No grading objects were harmed in this example — because there aren’t any! (Click image for a larger version)

<> Caption: Parking lot surface with dynamic retaining wall and 2' benched grading (just to show what you could do). Please note: No grading objects were harmed in this example — because there aren't any!

When you look at the possibilities, subassemblies and corridors really blow away the grading object when it comes to real grading options. If only we could just attach an assembly to a feature line, life would be even easier.