Civil 3D cut and fill: how does it work?

Civil 3D cut and fill: how does it work?

Civil 3D’s cut and fill analysis tool represents a very useful resource for many engineering and construction projects. Read on to find out how it works.

Civil 3D’s cut and fill tool, loved by contractors

As we have previously discussed, Civil 3D has many advantages compared to AutoCAD when used on civil engineering projects. One of them is the ability to create Civil 3D corridors. These come in very handy when designing linear infrastructures, such as highways and railways. Another key difference is a tool to create Civil 3D gradings. These are helpful on a variety of applications, including site designs and design of parcels.

On top of these, one of the most popular tools amongst contractors is the Civil 3D cut and fill analysis. Estimating the earthworks volume often becomes one of the most challenging parts of any construction project. Traditionally, this estimate was a manual process, using cross-sections from topographical surveys. This method was very time-consuming and often led to substantial errors due to the human factor.

Civil 3D cut and fill volumes plan

Creating a volume surface in Civil 3D to obtain cut and fill quantities

Nowadays, thanks to the advancement in technologies and capable software like Civil 3D, this type of process has become far more efficient. To obtain an accurate cut and fill analysis, the first step is to become familiar with the Civil 3D surface analysis tool. In fact, the Civil 3D cut and fill tool works very much as a surface elevation analysis. The only difference is that the analysed surface is not a standard TIN surface, but rather, a volume TIN surface.

A volume TIN surface represents a volume contained between two surfaces. Therefore, before we can create a cut and fill volume in Civil 3D, we need to create two comparison surfaces. Generally, these two TIN surfaces will represent the existing ground, or EG, and the finished ground, or FG. The former is created from a traditional topographical survey or a point cloud from drones or laser scans. The FG is normally obtained directly from the design.

Once we create the EG and FG surfaces, we can proceed to create a volume TIN surface. On this surface, we will select the EG as ‘base surface’ and the FG as ‘comparison surface’. After this, we will have a volume surface in Civil 3D containing accurate cut and fill volumes. The software obtains these volumes by comparing the volume contained between the triangles of the EG and those of the FG. We then just need to open the volume dashboard and add our newly created volume surface. The volume dashboard will give us the amount of cut and fill in the volume surface, as well as the net volume (the difference between cut and fill) so that we know whether the project will have an excess of material (net cut) or will have to import material (net fill).

volume dashboard in Civil 3D

Cut and fill factors in Civil 3D

It is also possible to apply cut and fill factors. These are commonly provided in the geotechnical reports and ground investigations from the project. The cut and fill factors are really helpful to obtain even more accurate data. This is because, generally, 1 cubic metre of ground shown as cut in the model, will become slightly more after being excavated. A typical ground can have a cut factor of 1.1 or 1.2, meaning that the material will have a 10% or 20% more volume after being excavated, compared to the volume measured in the model. This is helpful to plan and price properly for the transportation of material.

Similarly, the fill factor will tell us how much the material will shrink after being imported, placed and compacted on site. Again, this is of particular interest for an appropriate procurement and planning, since 1 cubic metre of compacted soil in the model will likely need more imported soil, generally around 1.1 or 1.2 cubic metres of material.

Displaying the cut and fill in plans

In most projects, it will also be helpful to show the cut and fill areas of the project on a plan. This is fairly straightforward in Civil 3D. We just need to open the properties of the volume surface and set a style that has ‘elevations’ activated. Then we go to the analysis tab, just as we did for surface analysis, and set the elevation banding required. A standard rule is to use a gradient of red colours for cut and a gradient of greens for fill. This will help the project team understand where there is cut (red) and where there is fill (green).

Civil 3D cut and fill elevations banding

Do you have any questions about Civil 3D cut and fill volumes? Let us know in the comments below and we’ll be happy to help!

 

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