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Civil 3D gradings and site design

Civil 3D gradings are one of the most useful tools for site design in the engineering industry. From surfaces to cut and fill calculations, Civil 3D gradings can help you save a lot of time and effort. This article will explore what they are and how to use them.

As we mentioned in a previous article, Civil 3D is a powerful software to deliver BIM services. Civil 3D corridors, for example, are great to design linear infrastructures, such as highways and railways. However, they are not so easy to use when it comes to designing features like sites, parking lots or building pads. Although it is still possible to use Civil 3D corridors to design and 3D model those features, it is more efficient to use Civil 3D gradings.

How to get started with Civil 3D gradings

There are several important factors to consider when designing gradings and sites. Firstly, you will need to assess the needs and requirements of your site. It is not the same designing a small parking lot for a family home than a large site for commercial development. After defining the needs, it is important to look at the existing ground conditions and constraints. The available space and surrounding stakeholders will determine how much land you can use. Also, the existing ground levels and constraints will affect the geometry of your site, both in plan and profile. Drainage and flood zones are additional key constraints which are sometimes overlooked, with potentially catastrophic consequences.

Before creating a Civil 3D grading you will need to set up the existing ground – often called ‘EG’ – as a surface. This surface will be your base or datum for volume comparisons. If you are using the UK and Ireland (UKIE) Country Kit from Autodesk, you will find many Civil 3D grading settings ready for you to use. To create a grading, you will need to start with a feature line. In case you are not familiar with these, feature lines are just like ‘clever’ lines. Or to be more precise, they are like intelligent 3D polylines. They are intelligent because you can modify them in different ways. For example, you can set the elevation of a vertex or the gradient between two vertexes with just a simple click.

For simple sites, you may just need to create one feature line. This could represent the edge of a parking lot for example. Using feature lines will ensure you can design the correct cross-fall for your site. This, in turn, will help your site to drain properly to the desired outlets. With traditional 3D modelling, you would then need to model every single line manually to create the grading. But thanks to the Civil 3D grading tool, you can do it pretty much automatically. Once you launch the grading tool, you will be able to select criterion for your grading.

Different criteria for Civil 3D gradings

  • Distance: it allows you to define a set distance to offset the grading from your feature line with a specified grade or slope. You could use this criterion to build a hard shoulder around the parking lot. For example, you could easily build a 1m wide hard shoulder with a 2.5% grade towards the outside for correct drainage.
  • Elevation: it allows you to define a set elevation to target when offsetting the grading from your feature line. This is generally used for cut/fill slopes and allows you to also specify a grade or slope for the offset. This criterion is helpful when you know a specific elevation that you need to meet, for example if you have a proposed building pad next to the parking.
  • Relative elevation: it works like the elevation criteria but, in this case, you input the relative elevation compared to your feature line. For example, this criterion could work very well if you wanted to design a berm around the parking lot which is 1m below the parking edge. Again, it works with a specified slope or grade to offset the feature line until it reaches the desired elevation difference.
  • Surface: this is one of the most used criteria, and for a good reason. Using a surface as criteria will help you target a surface for both elevation and distance. This means that you can specify a grade or slope from the edge of the parking lot, and the grading tool will figure out where your slope intersects the existing ground. With these criteria, you can easily build cut and fill slopes for your Civil 3D grading.

Much more than a simple grading tool

As you may have noticed, the main Civil 3D grading tool works by building offsets to the feature lines, both in distance and elevation. So, you may be thinking, ‘how about the internal area of the parking lot?’ Good question. Of course, the guys from Autodesk also thought about it. Because of that, they have added an extra tool for ‘infill’ areas. Once you have created the external gradings – hard shoulder, berm and cut/fill slopes in our example – you just need to press the infill button and click somewhere inside the grading. Civil 3D will recognise the parking lot automatically and convert it into an infill area. This is important since the Civil 3D grading tool will automatically give you the cut and fill volumes. Of course, you need the infill so that it takes the parking lot volume into account, and not only the gradings outside.

Once finished, you can easily apply the built-in styles and show the cut slopes in red and the fill slopes in green, for example. In just a few clicks, you can model gradings in 3D, obtain the cut and fill analysis, create automatic surfaces, display contours, and much more. In a nutshell, Civil 3D gradings are an essential part of your BIM toolkit if you work with sites and gradings.

Would you like to know more about Civil 3D gradings? Let us know your questions in the comments!



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