The Art of Modeling Geology on a Linear Route in AutoCAD Civil 3D

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The techniques used to model geology on sites often do not work well for linear routes where the positions of the boreholes are not conducive to generating usable triangulated surfaces. This article will discuss the techniques to employ when working with boreholes positioned along a length of proposed road or rail network.

It will also examine the workflow of using geotechnical profile views to sketch geology interpretation, which can then be incorporated back into the strata surfaces, letting the user visualize and model the subsurface geology. It will step through the process of creating geotechnical profiles, sketching and updating profiles from which 3D feature lines are created; these are then incorporated as breaklines back into the AutoCAD Civil 3D surfaces, producing usable triangulated surfaces, enabling the geology subsurface to be modeled and visualized on linear routes.

The standard techniques for modeling geology need reasonable quality triangulated surfaces from which strategically placed feature lines can be drawn and used as breaklines to manipulate the geology surfaces.

Well-placed borehole locations
Well-placed borehole locations.

However, the strata surfaces automatically generated by the Geotechnical Module on a linear site are often lacking and do not form suitable surfaces to work from, so we need an alternative method to model linear routes.

Typical surfaces from linear borehole locations
Typical surfaces from linear borehole locations.

Review of Modeling Techniques

First let’s review the technique to model geology. It was refined over a number of years and involves creating standard breaklines to force the surfaces to pass through the desired locations. The difficulty is knowing where the 3D line should be drawn and what surfaces they should be added to as breaklines.

Breaklines shaping geology surfaces
Breaklines shaping geology surfaces.

In the above image, feature lines have been used to define the extents of surfaces as well as shaping the height of an area of geology. Each vertex of the feature line will have an X, Y, and Z, with Z, the elevation, often being taken from an already-modeled surface above.

Standard 3D geology modelling workflow
Standard 3D geology modeling workflow.

It is important that the feature lines are created in the right place. This will save a lot of time and effort later in the process.

Feature lines should not cross different geologies
If the geology above or below changes, the feature lines should stop and a new feature line should be created. Otherwise, when the feature line is added as a breakline to a surface, the surface will be extended over the other surface.

Snap to edge of surface
Try not to let your feature line dangle over the end of the surface or stop short. Snapping to the surface edge will make cleaner models.

Don't overhang feature lines
If you do overhang feature lines you need to ensure that any vertices have the correct elevation.

Ensure feature lines have the correct elevation
It's essential to the modeling process that feature lines have the correct elevation, although this can be refined and edited later.

Understand the order of strata
Again, it is important that the order of strata and thus the Civil 3D surfaces are known. It is essential that the feature lines are added as breaklines to the correct surfaces.

This approach may not be suitable for linear sites, as the appropriate surfaces to take the Z values from may well be incomplete or missing. This makes creating the original feature line much harder, so we need to look at other techniques to create and position them.

It’s also important the right type of data is modeled. Too much variation in the data, for instance soil descriptions, will make modeling very difficult and potentially pointless if adjacent boreholes show completely different strata orders. 

Relevant Experience

Geological maps, paper, scanned and/or digital

A quick health warning: The interpretation of the subsurface can be complex. Relevant knowledge is required and should be used when modeling the geology, together with various resources to ascertain information on the underlying geology. These sources may include:

• Geological maps, paper, scanned and/or digital
• Web mapping services
• Photos
• Aerial photography
• Local knowledge
• Geophysics
• Down hole information

Workflow Required for Modeling Geology on Linear Sites

We could look at generating a 3D fence diagram to represent the geology along a linear site, but if you do need a 3D model the following workflow can be used to generate a ribbon of geology for the site.

3D fence diagram
3D fence diagram.


Linear site model
Linear site model.

The main problem with modeling linear sites is the positioning of the feature line, or more precisely getting the elevation of the feature line correct, as the X and Y can be drawn in plan. To help model linear sites we can make use of profile views: the X and Y will be derived from the alignment for the profile view and the Z from the profiles.

The secret is to transform the profiles depicting the strata into 3D feature lines. This can be done using the poorly-named “Create feature line from alignment” command, which is in fact a “Create feature line from profile” command. There are four main steps with a fifth optional step:

Workflow for modelling linear sites
Workflow for modeling linear sites. 

The main difference between this and the standard approach is the use of the profile view to create the feature line. Once the feature lines have been created they can be used as breaklines in the standard strata modeling technique.

Gary Morin originally trained as a civil engineer and has over 30 years of experience working in the production and support of a range of geographic information and CAD software systems. He is the technical director of Keynetix, which he co-founded in 2000 to specialize in geotechnical data management software. He heads up the geotechnical Building Information Modeling (BIM) development and is responsible for the design and support services for a range of products designed to manage geotechnical data in the BIM process, including the HoleBASE SI Extension, the Geotechnical Module from Autodesk, Inc., and the advance HoleBASE SI Extension for AutoCAD Civil 3D software. In recent years Morin has been at the forefront of geotechnical BIM and how it can change working practices to enable a better understanding and integration of geotechnical data into the wider construction process.