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Civil 3D corridors: what are they and how to use them?

If you work with Civil 3D on engineering and construction projects, you’ll probably need to master Civil 3D corridors. This article gives you insights into exactly what they are and how to use them. Ready? Let’s dive in!

Civil 3D corridors for BIM Level 2

By now, you already know that Civil 3D has extensive capabilities over AutoCAD. One of them is the ability to create corridors. But what are corridors, exactly? To put it simply, a corridor is the 3D intelligent representation of a road within the Civil 3D model. Although it can also be used for railway design, the truth is that Civil 3D’s core strength is roads and highways. And let’s be honest, the majority of civil engineering projects involve some sort of road since most projects or sites need to be accessed by road. This makes Civil 3D the perfect software package for most civil engineering work. What’s even better, Civil 3D produces fully compliant BIM Level 2 models.

civil 3d corridors what are they
Corridor creation command on Civil 3D

Alignments – the foundation of every Civil 3D corridor

You’ve probably heard that ‘you cannot start a house from the roof’. The same way, you cannot just create a corridor without some key features. You need to start by creating an alignment. Think of a Civil 3D alignment like the skeleton of your corridor. It represents a 2D path or string which will be followed in 2D by the corridor. The alignment may be made up of straight lines – called ‘tangents’ in Civil 3D –, curves and transitions.

Once you’ve got your topography in place you can proceed to create an alignment. Motorways and A roads in the UK should be designed with transition curves, according to the Design Manual for Roads and Bridges. This means, if you are designing a Civil 3D corridor for a motorway or a link road, your alignment will necessarily include straight lines, curves and transitions, as shown in the image below. On the other hand, if you are designing an urban road or access track, you may not need transitions – also called ‘spirals’. In those instances, your alignment may only have straight lines and curves. When it comes to designing horizontal alignments, the main considerations are safety, visibility and site constraints like land parcels, buildings, etc.

civil 3D alignment for corridors with straights, curves and transitions
Civil 3D alignment with straights (red), curves (blue) and transitions (green)

Topography and design vertical profiles

The next step is to create a vertical profile for your corridor. This is a 2D element, but this time located in the vertical plane, rather than the horizontal one. Typically, you will need to develop two vertical profiles – one from the topographical survey and one design profile for your road. The design profile will generally have straight gradients and vertical parabolic curves. When creating a profile in Civil 3D, the main design considerations are safety, adequate drainage and earthworks balance.

civil 3d profile for corridors
Civil 3D survey profile (green) and design profile with gradients (red) and parabolic curves (blue)

Typical cross-sections – or assemblies for Civil 3D corridors

Once we’ve got the alignment and profile in place, it’s time to look at the typical cross-section – which is called ‘assembly’ in Civil 3D. An assembly is formed of one or more sub-assemblies. Examples of sub-assemblies could be asphalt layers, kerbs, footway, verge, median, hardshoulder, etc. Civil 3D has built-in catalogues with most of the sub-assemblies you’ll ever need. Especially if you install the UK & Ireland country kit from Autodesk, you’ll have direct access to traditional English and Irish road elements. As you can see in the image below, with just one click you can obtain a full assembly for an urban road with two lanes, verges and earthworks – a Civil 3D draftsman dream!

assembly typical cross section
Civil 3D urban road assembly from UK Country Kit

Put it all together and you’ll have a Civil 3D corridor

After having modelled alignment, profile and assembly, we are ready to create our Civil 3D corridor. The way Civil 3D works – explained simply – is that it will take the assembly (typical cross-section) and will ‘extrude’ it along the horizontal alignment, following the levels in the vertical profile. I say ‘extrude’ because it’s not really an extrusion. What happens behind the scenes is that Civil 3D places a ‘section’ (assembly) at certain intervals, and connects the same points by interpolation between those sections. A corridor is, therefore, the sum of many cross-sections in a 3D environment, all linked together.

You can set different frequency parameters for the interval at which Civil 3D places those sections when creating a corridor. For most projects, you can use 10 or 20 metres intervals for straight sections. For curves, you should reduce that value, maybe to 5 or 10 metres. If you have curves with small radii you may want to reduce even further to 1 metre or less. The smaller the frequency – that is, the more sections Civil 3D adds to the corridor – the more time it will take to build and re-build the corridor. So, bear that in mind and don’t use very small frequencies, especially if you don’t have a particularly high-performance BIM workstation.

civil 3d corridor in 3d view
Civil 3D corridor in 3D realistic view


Then, finally, you select your alignment, profile and assembly. In a matter of a few clicks, you will automatically have a Civil 3D corridor! This includes all the elements in 3D, like pavement layers and earthworks. From here, you can create surfaces and cross-sections to calculate material quantities and volumes. And you can share your Civil 3D corridor on your Common Data Environment with other teams and stakeholders using Data Shortcuts. But we will leave those for another post!

What’s your experience with Civil 3D? Have you used it on your projects? Let us know if you’d like us to cover any particular aspects of it by commenting below and stay tuned to our blog for more on Civil 3D!

This Post Has 4 Comments

  1. Colin Bell

    Hi thanks for taking the time to write this, nice and concise.
    I’d be interested in hearing your views on attaching and exporting data to/from corridor models.

    1. Darren Bond

      Hi Colin, thanks for taking the time to read this article. Could you please make your question a bit more specific, as to what type of data you’d be looking to export/import from corridors? Are we talking about setting out data for example, like coordinates, elevation and chainage along different string lines, or more like metadata associated with pavement layers for example? Many thanks again. If you could mention a specific example that would be even better.

  2. Usman Yusuf

    Good day,
    1.Pls how do I get back at my point when I mistakingly pick a wrong point while creating my profile and designing my road?
    2. How do I create a median at Centre, while creating assembly for a dual carriage way road?

    1. Darren Bond

      Hi Usman,

      Thank you for the great questions!

      1. Generally you can press “Ctrl+Z” to go back one step, then pick the right point. Otherwise, you can finish designing your profile, then you can go back to the wrong point (PVI) and then just drag it to the right position. You can always continue adjusting the points as needed, so don’t worry if you don’t get it right first time.

      2. The easiest and most efficient way is to use one of the median sub-assemblies built into the specific country kits. For example, you can download and install the UK country kit here: Once installed, just open the sub-assemblies palette and search a median sub-assembly, and just drag and drop to your main assembly.

      I hope this helps.

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