Grade On!
Computer-aided design (CAD) software AutoCAD® Civil 3D®, enriches the design process with all its sophisticated tools. We are able to design faster and more accurately than our mentors did mundanely 20 years ago. They would spend countless hours hand-grading and drafting. I’m not saying it’s a bad thing, just outdated. And with hand-grading, mistakes are easily made.
Of course, even with automation you must understand and know what information to give the software and be knowledgeable enough to know what Civil 3D gives you in return. Just as with any form of technology, Civil 3D continues to grow and advance, and as a Civil 3D user you must always try and continue your professional development by obtaining new knowledge and experiences.
Grading in land development transforms the land’s existing surface to make it compatible with the intended land use. The three most basic grading tools in Civil 3D are Grading by Points, Breaklines, and Contours. Civil 3D has created more in-depth grading tools to help with the grading process. In this article I will run through some of the tools I found very helpful.
In land development you always start out matching the existing terrain. Take a simple feature line along the outside edge of your project. Assigning existing elevations and Inserting intermediate grade break points to your feature line ensures you get a nice constant surface.
This is the first step to any and all grading for me. I feel it is one of the most important because it gives you a clear boundary of your grading. I know my limits and I can see how I need to proceed with my grading.
The first process I want to cover is the “STEPPED OFFSET” command. This command is good for grading at an exact and constant distance from your original feature line. You are able to select a feature line, polyline, or 3D polyline and specify what side and elevation difference you need.
Right-click on the feature line and select STEPPED OFFSET. Civil 3D will ask you a series of commands. The first is an offset distance and then the specific side on which to offset. The next command it will ask is to specify elevation difference; your choices are Elevation Difference or Grade Slope Elevation Variable. You will need to decide what is best for your grading. For grading curbs I like to use Elevation Difference.
Voilá! You have a perfectly sloped feature line adjacent to your original feature line at an exact distance.
The next process involves the Grading Creation Tools. Let’s say you want a constant slope at 2 percent from your current feature line, but at different distances. Now comes the fun part, fellow CADsters! We make a secondary grading surface. What?! Yes!
We start with the Grading Creation Tools. This toolbar has many different tools you can use for surface creation but for this article we’re only covering one: The Distance @Grade.
The prompts ask you to select a feature line, I’m selecting the original one I created. Pick a side to grade to and also pick apply to the entire length. Specify distance. Choose a length longer than the maximum distance you want to grade to, then pick a grade.
Create a new feature line where you grade to and once again assign it elevations to the temporary surface, and add it to your surface. You now have a perfect graded site.
Another way to grade is with corridors. The first thing you need to do is set up an alignment and a ground profile. The Alignment is the curb line, so when we make our subassembly it will grade a specific distance from the curb.
You will now need create an Assembly. From the home ribbon, click Assembly, then Create Assembly. Your assembly can be as simple or complex as you want. You also need to decide if your Assembly will contain subassemblies on each side or on one side, etc. Assemblies can be mirrored and edited other ways—you just need to know what you need for your grading. Make sure you remember what you name, because you’ll need that later.
In the next step we’ll need the Civil Imperial Sub-Assemblies under the tool palette, then Generic tab, then LinkWidthAndSlope. LinkWidthAndSlope’s general purpose is to add a surface link by specifying its width and slope.
Figure 1: Civil Imperial Subassemblies menu
From here you will be able to adjust Side, Width, Slope, etc.
Figure 2: LinkWidthAndSlope options
With all these steps complete we can now create a Corridor. Corridors are great because they get built using information from alignments, profiles, surfaces, and assemblies. From the home ribbon, click on Corridors and then Create Corridors.
Figure 3: Create Corridor options
In this dialog box you select the Alignment name you created the corridor on, the Ground Profile you drew, and the Assembly you created.
Figure 4: Editing options
From here there are several columns for you to edit. You are able to change your Alignment, Profile, and Assembly, as well as the beginning and ending stations. I like to start and end with all the stations of my alignment, then when I see my surface that’s when I start changing the stationing. The Frequency is something you will want to change on a case-by-case basis. Then click OK. Just as with anything in Civil 3D, once it’s created you can always go back and edit it if you need to. In the Toolspace under Corridors, right-click and edit what you need.
Now we get to create a Surface from the Corridor. As the Corridor changes, the surface updates automatically. From the Toolspace, right click on the Corridor and select Properties. This is the next and final dialog box.
Figure 5: Change surface style and layer
Here click the Create a Corridor Surface icon. You are able to change the Surface Style and Layer. The Data Type should be Links and The Specify Code should be set to TOP, then click the ADD button. Click OK and Rebuild the corridor. I like to use this method because when you adjust the Alignment, your surface automatically updates.
The more of these methods you use the easier they get. You’ll learn what works in different situations, and as an Civil 3D user you’ll learn how to manipulate the surfaces to do what you want them to do. There are numerous ways to design, and only you can determine what works best for you.
Grading is very important when it comes to land development projects. It determines a dirt balance and should be cost-effective, which reflects how much money you save (or NOT save) your client. Grading is the foundation of every project and understanding the fundamentals and complexity of grading and grading in Civil 3D will help you better understand the project and help you get better on future projects. I hope this little bit on knowledge helps out. GRADE ON!
Gabrielle DeAnda is a Civil CAD Operator in the Land Development Department at Kimley-Horn Associates in Fort Worth, Texas, is part of KHA’s Regional CAD Team and has 8 years experience in Civil 3D.
