Civil 3D: Site Grading with Corridors

Have you ever been frustrated that grading with feature lines doesn’t seem to give you the same level of information and detail that corridors do? Let’s be honest, corridors have a lot of benefits and open the door to possibilities, so, why not adapt these linear tools for use on site development grading? Let’s explore how to customize different tools in Civil 3D to put corridors to work!

Feature lines have been promoted as the primary tool for site grading for years, utilizing grading objects to make adding daylights and curbs more user friendly. However, the use of featurelines and grading objects do not show depth of pavement, subbase or topsoil. Sure, there are ways to try and add this detail, but it never seems to be quite the same as the materials used in corridors.

This article will explore a different approach to site grading by customizing subassemblies, utilizing alignments, manipulating corridors, and establishing code set styles to enable the use of cross sections, material quantities, and cut/fill volume calculations in site design. There is an assumed knowledge of some Civil 3D tools, including alignments, offset alignments, connected alignments, profiles, assemblies, and corridors.

Why use Corridors for Site Grading?

Corridors are built from three basic entities, horizontal alignments, vertical profiles, and assemblies. These basic entities most often relate to roadway design, with all the parameters centralized around the horizontal alignment or baseline. Once the baseline has been established, the vertical profile and assembly(s) determine how the grading will be achieved. Wouldn’t it be great to design a parking lot with one baseline instead of trying to control everything from the perimeter? The resultant of using a baseline instead of perimeter feature lines is continual longitudinal and cross slopes. In addition, cross sections can be cut across the site showing dynamic pavement, curb, and sidewalk sections with accurate slope labels. This workflow is best achieved with the creation of custom subassemblies and code set styles.

Simple Subassemblies with Subassembly Composer

If you have never worked with Subassembly Composer, the thought of creating custom subassemblies for your project may seem intimidating, however some of my most simplistic subassemblies are the ones most employed on my projects. I have created custom subassemblies for bituminous pavement, concrete pavement, curb, and sidewalk that are based on the typical details we provide on our projects. Basing these subassemblies on our details allows for more accurate design and breakdown of the materials that will require quantities as the project progresses. The most important setting to note when building your custom subassemblies is to allow target parameters for offset and elevation. It will be critical to site design with corridors.

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In the figure above, a bituminous pavement section has been created in Subassembly Composer. The target offset and target elevations will be applied for P2 in relation to the origin/P1. Once this has been established, allowing for flexibility in design, the remaining sections (e.g., base course, subbase) can be added by depth. Now you have the basic subassemblies to begin.

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Customizing the Code Set Styles

When utilizing custom subassemblies, frequently you will create your own codes to define them. For instance, the shapes may be coded to represent a particular material, or a point may be coded in order to add a specific label in a cross section. In my work we utilize different strengths of concrete pavement, so we code them as heavy duty and low volume to distinguish them in material quantities later in the project design. Once you import your custom subassemblies into Civil 3D, you need to import the custom codes you created and add them to your Code Set Style.

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I have several different Code Set Styles that display my codes differently, depending on the purpose. Have fun and play around with the styles to achieve just the right look for your projects.

Setting the Baselines & Profiles

The baseline, also known as the horizontal alignment, will be the foundation of design. The projects I typically work on are facilities projects, with a perimeter access drive to access various portions of the building, and a parking lot for administrative needs. The baselines I establish on such a project are typically the centerline of the access drive and the main access of the parking facility.

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Once you have determined the best locations for your baselines, the vertical profiles will need to be created. Some calculations will be necessary to determine the optimal elevations of the profile to ensure sidewalk areas surrounding the building continue to meet ADA requirements. No need to worry too much about variations from the baseline, we will utilize offset alignments with profiles and connected alignments with profiles to control differences in slopes from the baseline.

Creating Assemblies

Keep your assemblies simple. The best way to manage your assemblies on a project is to have as few assemblies as possible. It is much easier to maintain five assemblies than twenty-five. Typically, I try to use one assembly for the parking lot and one assembly for the access drive, but this can vary depending on the site layout. Intersections require an assembly for the connected alignment, sometimes referred to as curb returns, and a separate assembly is needed for perimeter curblines or daylights.

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For assemblies, the cross-slope parameter inputs are the typical cross slopes. As stated previously, variations in slope will be controlled by offset alignments and profiles. Curb heights or widths can also be controlled with profiles by utilizing custom dimension control subassemblies as demonstrated by Jeff Bartels here (Applying Transitions to Virtually ANY Civil 3D Subassembly, on YouTube).

Establishing Offset & Connected Alignments

Frequently, the cross slopes of a project will vary and will not be the same throughout the entire project site. Often, these variations are handled through multiple assemblies, but that can become cumbersome to manage and edit as the project progresses through design. Instead, I like to employ the use of offset alignments with profiles to manage varying slopes. The cross slope on the access drive for my example project changed from a -2% cross slope to a +2% cross slope and back to -2% to account for the existing terrain. The use of offset alignments allowed for one assembly to be used and the changes in the slopes to be established with the offset alignment profiles along the edge of pavement, setting the stations where the transitions would occur.

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The same concept applied the access drive is applied to the parking lot design. Offset alignments with profiles can be established at the breaklines of the design, such as where the drive aisle and parking stall meet. The offset alignments at drive aisle and access drive intersections are required in order to create intersections. For parking lot access design with corridors, I find it best to manually create the intersections with targeting rather than utilizing the intersection tool. Once the offset alignments have been created on the edges of the drive aisles and access drives with their associated offset profiles established, the connected alignments with profiles can be generated.

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Generating the Base Corridor

Now you have all the elements to start creating the base corridor for your project. Yes, there will be more than one corridor required to create a fully functional site grading. Corridors are inherently a linear tool, therefore any curb, sidewalks, or daylights surrounding your parking lot will need to follow a different baseline.

Create a corridor and add your baselines, profiles, and assemblies as usual, avoiding the intersections. Set the targets of your custom subassembly, target the offset to the offset alignment and the elevation to the offset alignment profile. Manually create the intersection by adding the connected alignments as corridor baselines and target your access drive alignments or offset alignments.

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Overall, after creating the base corridor the majority of the site pavement areas should be covered by the corridor.

Creating Curblines, Sidewalks, and Daylights

Once the base corridor has been created, you can start the finer detailing of your grading scheme. For curblines, I create a featureline from the perimeter curbline linework and set it relative to the base corridor surface. Then, if you change your horizontal baseline, the offset alignments, connected alignments, and the featurelines will all change with it! Isn’t it great to let Civil 3D help do the heavy lifting for you? A separate assembly will be needed to set along the curbline, adding any sidewalks or daylighting needed in the assembly. Create the curb corridor using the featureline as your baseline and no profile will be needed. Don’t fret if you need to create a curb depression or driveway entrance, Jeff Bartels gave us the tools in the dimension control mentioned earlier (Applying Transitions to Virtually ANY Civil 3D Subassembly, on YouTube). By using the dimension control, you can easily transition between a full height and depressed curb.

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Depending on the layout of your project, you might be able to utilize a custom sidewalk section and target the offset and elevation of your building. If that is possible for your site design, you have even more coverage with your corridor.

Cross Sections & Materials

Finally, to the part you have been building up to and waiting for, the cross sections! With the base corridor, curbline corridor, and composite surface you will be able to run sample lines and cut cross sections through your access drive and parking lot. You will be able to see the changes in cross slope, add elevation labels as needed and present all the information the contractor should need. Creating typical sections to place on your plans should also be a piece of cake now that all your grading has pavement depths, subbase, curblines, etc.

In addition, cross sections enable the use of volume and material calculations. You could create a volume comparison surface, using the datum surfaces from the corridor for a more accurate analysis, or enable the use of volume calculations with methods such as average end area. Utilizing corridors to create your grading also gives you the ability to provide material calculations. This is where those custom subassemblies really come into play by splitting up different materials such as heavy-duty concrete, low volume concrete, and sidewalk concrete. A material volume table can be provided showing the breakdown of each of these materials, helping to calculate quantities for your project.

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After working with a team of engineers who were previously experienced in highway design, I realized how much grading with featurelines wasn’t providing the best result. Simply trying to control a large parking lot only by the perimeter and the occasional breaklines didn’t make sense. In addition, the use of a grid surface with feature lines and corridors did not suit my needs for design. Therefore, I sought to apply the same methodology of highway design to site grading with the use of baselines and cross sections.

Applying corridors to site grading was an adventure, and I talked to a lot of people along the way while I tried to figure this workflow out. I have put the workflow outlined in this article to the test across several projects and the results have been better than I could have hoped. There are a lot of initial setups required, but the flexibility in design throughout the duration of a project, adjusting one piece of the puzzle and having the program domino that change throughout numerous other entities has made revisions much easier to accomplish.

Shelby L. Smith has a B.S. in Civil Engineering from Penn State University with a minor in Architectural History. She has been working with Autodesk software for approximately 20 years and is an Autodesk Civil 3D Certified Professional. Her background is diverse in civil site design including transit authority headquarters, private & commercial land development, and a brief time in oil & gas. Researching and implementing new methods for modeling is an aspect she enjoys, especially with a passion for grading design. Shelby likes the challenge of creating templates, customizing styles and tools to enable a better workflow in Civil 3D. She has led the effort to implement and create drafting and design standards for her company utilizing Autodesk AutoCAD, Civil 3D, SubAssembly Composer, and InfraWorks software.

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