Create a Formation Surface for a Platform in Civil 3D

Civil 3D has some powerful tools for earthworks, through the use of surfaces and grading objects. In the next few lines, we will use an example to illustrate a workflow for a common scenario.

In this example, we have a survey for the existing ground (EG) and we want to designa platform. We need to discard the upper layer of topsoil from the existing ground and there is also a minimum depth from the finished platform to underside that we must meet.Our goal is to combine this information to obtain a surface with the levels of the proposed formation, which will be the starting point for the fill to take place.

To obtain that surface, we need to select the minimum level from the Base of Platform and the EG less Topsoil surfaces. If we could take only the cut and isolate it from the fill between those two surfaces, we could get the proposed formation by combining the EG less Topsoil surface with the cut obtained as previously described.

There can be several approaches to tackle this problem, I summarise some of them:

1. Create a volume surface between Base of Platform and EG less Topsoil and Exclude elevations greater than 0 in Surface properties, therefore ending up with a surface with cut only. The problem with this approach is that this option cannot be used in the volume surface directly. You will need to extract the triangles from the volume surface (Surface contextual ribbon -> Extract from Surface -> Extract Objects) and then create a new surface from those triangles.

2. Draw polylines for the intersection of both surfaces (Base of Platform and EG less Topsoil) using theMinimum Distance Between Surfaces command (from the Civil 3D Ribbon, Analyse tab, Ground Data Panel (expanded)) and use the polylines that enclose fill areas as Hide boundaries.

3. Compute the intersection between both surfaces as above and create grading groups to complete the surface.

I am going to follow the third method, which allows accurate control of the slopes using grading objects. The step by step process is as follows:

Create a surface for the existing ground (EG) from the survey data.

Create an EG less Topsoil surface by pasting the EG surface into it and offsetting it vertically by the topsoil depth.

• Create a grading group for the platform, selecting EG less Topsoil as the target surface. You will also need to attach a surface to the grading group.

• Create a Base of Platform surface by offsetting the surface attached to the grading object by the minimum depth below platform.
• Compute the intersection between the Base of Platform and the EG less Topsoil surfaces using the Minimum Distance Between Surfaces command.

• Convert the polyline from the previous step into a feature line and complete the area in cut with another grading group (and an attached surface), targeting the EG surface this time. You will also need to place it in a different site, to avoid interaction with the top of the platform.

• The final step to get the finished proposed formation surface is to merge all of the surfaces together by pasting them into a new surface and applying boundaries accordingly. An additional grading object can be added to tie in the EG less Topsoil and the EG surfaces.

The final Proposed Formation surface looks like below in Civil 3D object viewer in our example.