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How Earthwork Cut and fill Calculation works in Civil 3D

Preamble #

This post reflects my learning journey on Civil 3D, especially about the earthwork quantity estimation feature. Instead of focusing on the operation– there are already a lot of excellent resources available online, this post will focus on the concepts and designs behind Civil 3D, so that we can better understand the applicability and the limitations of it, and more importantly, how one can adapt it to the modern urban site design.

What is Civil 3D? #

Civil 3D is a civil infrastructure engineering software produced by Autodesk. It covers the design of urban and rural road, earthwork design and quantity estimation as well as stormwater utility design.

It is a familiar tool for many, because of its association with AutoCAD due to Halo Effect. People trust Civil 3D just because it’s a family member of the Autodesk product lines. But whether such a trust is warranted or not, is of course, subjected to debate.

The concept of Civil 3D in relation of AutoCAD #

The interface of Civil 3D looks extremely similar to AutoCAD, that’s because Civil 3D is vertically integrated with AutoCAD. Judging from the application architecture, one can even go as far as saying that Civil 3D is developed as a plugin to the AutoCAD software.

The benefits of tight integration with AutoCAD software is that greater code reuse can be achieved, and code reuse reduces development time and reduces the recurrence of similar bugs across different product lines. Furthermore, the features that are implemented in AutoCAD can be directly ported over to Civil 3D with no extra effort. There is less cognitive overload for the users who are coming from the background of AutoCAD, as the commands that they have memorized and the experience they gain via working with AutoCAD can immediately be put to good use.

The architecture design of Civil 3D #

Civil 3D inherits AutoCAD functions, which makes it an extremely powerful software. You can do almost everything with Civil 3D. Just take a look at the ribbon interface with its hundreds of visible icons and the tab panels on the left hand side, and this impression will dawn on you. Better still, a lot of settings are shown only on demand (ie: they don’t show in the default settings) when you click on the relevant entities. This is a clever design as it reduces the cognitive overload on the users part, and makes the whole software still manageable despite ballooning complexity.

Image 1: Civil 3D User Interface

If you are like me, who is better with verbal than spatial reasoning, the all versatile and omnipresent command line is always there, at the bottom of the drawing panel, waiting to rescue you from your ineptness to find the correct little wiggly icon to click. This little tiny command line is akin to the omniscience Google search box, it’s a gateway that allows you to summon the whole knowledge of AutoCAD/Civil 3D at your fingertips.

And, AutoCAD/ Civil 3D learning materials are freely available online; on youtube, on the official websites, on user forums, blog posts… everywhere!  A good, user-friendly software is defined as much by the Principle of Least Surprise ( that is, the software should do what you expect to do, despite that you have no telepathic ability) as the Principle of Self Learning ( the ability for new users to learn new things on their own). Having vast resources online where users can simply search their way through is often the most overlooked part of a good software product design; Autodesk gets this right and this helps their domination in the AEC industry.

Finally, if you still think that Civil 3D is missing a crucial function, you can always write your own plugin to complement Civil 3D. That’s why I said, it’s a versatile tool!

The concept of Civil 3D for earthwork quantity calculation #

Everything about earthwork quantity calculation in Civil 3D revolves around the concept of Surface. There are two types of Surfaces supported by Civil 3D, namely TIN Surface, and Grid Surface.

For TIN Surface, Civil 3D will overlay one surface on top of another, and then generate the difference polyhedron, and then summing up all the polyhedron volume. Since such a procedure is repetitive and voluminous, it is most suited for computers to carry out. We don’t recommend you do it manually,  we prepare an example anyway, should you want to try.

Image 2: TIN Surface in Civil 3D for OGL and As Built
Image 3: TIN Surface in Civil 3D with Volume

For Grid Surface, the procedure is even simpler, you just have to divide the site into rectangular grids and read off the difference in elevation between the two Surfaces and times the Area. Job Done. You can go home to have dinner.

Image 4: Grid Surface in Civil 3D
Image 5: Grid Surface in Civil 3D with Volume details

The application of Civil 3D to real life site development design #

So, doing earthwork quantity calculation in Civil 3D is super easy, as long as you can get the Surface right, right?

Right. But the key question is, how can you get the Surface right in real life site development design? In other words, how painless is it to use it in an actual project?

As with the case of cars, software often looks GREAT in demos, webinars and whitepapers, but they fall apart when you start to use them. The idea of TIN Surface is nothing short of astounding in Civil 3D; we also use them too in MiTS, and I can assure you, it’s the best data structure to represent complicated terrains because it is efficient in terms of memory storage and we have the powerful processing algorithms to manipulate it. If you want to do Surface comparison, convert them to TIN with networks of points of triangles, and you have lots of computer packages to choose from. You can’t go wrong with that!

The only problem is that in real life not all sites can be neatly converted into a TIN Surface.

For virgin, pre-development sites, you can easily convert them into a TIN Surface where the whole landscape doesn’t contain abrupt drops. But for human altered landscapes, they are different: they are often defined by a large piece of flat or near flat multi-edge polygons, and there are often drops in between those polygons. So the mathematical techniques that work so well on the TIN Surfaces can no longer work on human altered landscapes, not without some workarounds anyway.

Image 6: MiTS 3D view with streamline

Civil 3D’s way to handle this is simple and direct… It just ignores real life and insists on only handling TIN with triangles and points. So it’s up to the designers to convert each landscape feature into relevant TIN Surfaces and then do the Surface comparison one by one.

Let’s take an example,  if you have two platforms side by side, one is a rectangle residential plot of land, another is an irregular pond which is adjacent to it, but is 3m deep compared to the residential area. To compute the cutfill, you will have to create one Surface for each platform, and then create a TIN Volume Surface to compare between the platform and the Original Ground Level, in order to calculate the cut and fill. Repeat this process for all of the platforms and you are done!

Image 7: MiTS 3D view – Platforms and irregular pond platform

Which means that if you have a large site project with a lot of uneven platforms, or platforms with drops, or platforms with slopes attached to it, you will spend all day creating Surface and its cousin, TIN Volume Surface. For those who are interested, you can refer to a step by step guide on how to do it. You can tell, by just glancing the guide that it’s a tedious and error-prone process.

The volume results for the Surface are not properly aggregated, so you still need a spreadsheet to keep track of all the surfaces and summing all the volumes up manually. And please remember to keep the volume sign consistent ( ie: you define the base and comparison surface in Civil 3D consistently), it’s no fun to find out that the required imported volume is  a lot more than the expected, only when you are at the site, just because you suffered a lapse and accidentally switched the sign for just one Surface earlier on.

The MiTS Concept to Cut and Fill Calculation in a real life urban development setting. #

MiTS, as mentioned above, also uses TIN Surface, but only when it’s appropriate for the job, that is to say, we use TIN Surface for pre development sites or anything that is definable by surveyor points. For post development sites, we support the concept of Platform naturally.  Which is to say that we support you– our users– in drawing the shape of your platform arbitrarily, no need to restrict your imagination to triangles and a group of triangles. You can throw at MiTS quadrilaterals, pentagons, hexagons and it’ll handle just fine. For elevation levels of the platforms, they need not be flat or slanting, we just give you the tools to model it easily, by keying in a number, or defining 3 points.

Not only that, the concept of platform supports retaining wall AKA drop naturally. For two platforms with level differences, just key in their respective elevations and you are good to go. The software simply knows how to convert them to appropriate data structure and process the cutfill efficiently.

Oh, you can key in all the platforms at one go, and the results will be aggregated according to boundaries or other criteria, and you won’t have the fear of making the simple stupid volume sign mistakes anymore. You can go home and have dinners with peace of mind.

Image 8: MiTS 3D view with dropped platforms
Image 9: MiTS Earthwork Volume detail report

I would not want to say that our approach is superior to Civil 3D’s one, after all, user friendliness– just like beauty– is in the eyes of the beholders. For those of you who are more familiar with Civil 3D’s workflow, good to you, you have my approval to continue using it! After all, our results do converge and in engineering software, nothing is more important than the accuracy of the results. The ease of operation is really secondary. After all, you can get sued or land in jail for providing inaccurate design, but karoshi is just a statistic that we all grow numb to.

The interoperability limitation of MiTS and Civil 3D #

However, the discordance between MiTS and Civil 3D does create a headache when we want to convert MiTS’s Platform to Civil 3D’s Surface via our MCIntegrator plugin. They are not always the same thing! Asking us to fit Platform to Surface is akin to fitting a round peg in a square hole; you can do that as long as you are willing to slice a portion off the peg.

So in our MCIntegrator, when we Civil Sync MiTS Platform to Civil 3D Surface, we are doing the same thing, we transform the data irrevocably so that the Platform can ‘fit’ to a TIN Surface.

Image 10: Platform Sync parameter
Image 11: Successful Platform Sync from MiTS into Civil 3D

Which is why the Civil Sync for Platform is a one way street (ie, you can’t go from Civil 3D to MiTS), unlike the Civil Sync operations for other types of entities. This has to do with the fact that the Platform in MiTS has been broken up into a patch of triangles in Civil 3D, despite maintaining their polygonal profiles.

Image 12: Platform sync border vs Platform triangle profile in Civil 3D

So you can still synchronize MiTS Platform to Civil 3D Surface for the purpose of BIM integration or just to confirm that MiTS earthwork quantity volume is correct. But remember, Civil 3D Surface is not a MiTS Platform, it’s not the real thing.

The concept of Platform sounds cool, can I get Civil 3D to implement it also? #

Yes you can! If you are the Civil 3D developers and you have the willpower to do it, that is.

Otherwise, if you are a normal user, I would say…. good luck. For all we know, Autodesk does maintain a user forum whereby you can post your feedback and queries, but that’s the limit Autodesk is willing to go to support you. It doesn’t really matter if you are an individual user, a big corporate user with 1000 license seats, or just a pirate user.  Better still, most of the helpful answers are not even coming from Autodesk employees, they are coming from unpaid volunteers, which explains why sometimes you get subpar answers or may not get any answers at all.

This is not to discourage you from using the forum. We do have some success stories also; if your questions/feature requests are sufficiently well-defined and generate sufficiently many upvotes, Autodesk will finally implement them. After many years, perhaps.

To be frank, Autodesk is not an example of “listening to users”, so you better help yourself. Either you can buy over Autodesk Inc. and demand change from within, or you can buy MiTS software and to create Platforms in MiTS, and then sync to Surface in Civil 3D.

I would say that the second option is a lot cheaper. I think you will agree with me. And oh, we do provide phone and email support, and we promise you we won’t ask unpaid volunteers to help you at user forums.

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