Alan Sharp

Site Modeling Tips and Tricks

Blog Post created by Alan Sharp on Nov 27, 2018

Today's Tip started out as a question from George Henry Schneider.


He was asking about how to model scenarios where e.g. a Layer or more of the subgrade Site Improvement beneath e.g. a Parking Area extends a distance below the curb and Gutter Section which also has a Site Improvement on which it sits.


In the example that I created, I had a Parking Area Material Stack that includes the following


4" Asphalt

6" Aggregate Base

3" Sand


Beneath the Curb which is itself 6" deep at the interface with the edge of Asphalt, we had the following material stack


4" Aggregate Base (this is 4/6 of the Aggregate Base beneath the Parking Area)


In addition the design calls for an additional 3" sand layer for a distance of 6" beneath the Curb section, extending from the subgrade beneath the parking area.


Beneath the Sidewalk we have


6" Concrete

2" Sand


So Something like this


So the steps to model this that I took were as follows


1) Create the "Earthen Select" Materials in the Materials And Site Improvement Manager for the following



Aggregate Base I

Aggregate Base II




2) Create the Subgrade Site Improvements as follows


Subgrade: Parking Area (Site Improvement I)

Asphalt Layer 4"

Aggregate Base I Layer 2"

Aggregate Base Layer II 4"

Sand Layer 3"


Subgrade: Curb and Parking Area (Site Improvement 2)

Aggregate Base II Layer 4" 

Sand Layer 3"


Subgrade: Curb Area (Site Improvement 3)

Aggregate Base II Layer 4"


Subgrade: Sidewalk Area (Site Improvement 4)

Concrete Layer 6"

Sand Layer 2"


Note that the 6" Aggregate Base Layer under the Parking Lot has been split into two Aggregate Base Layers - one 4" thick and the other 2" thick, and the materials used are Aggregate Base I and Aggregate Base II. This is so that we can match up the layers beneath the Curb Section correctly. The dashed green line in the picture above.


In the Design Model for Finished Grade we have the following lines for the main features as follows


Edge of Pavement

Flow Line

Top Face of Curb

Top Back of Curb




Because the application of site improvements will not work beneath a finished grade surface that tracks the Edge of Pavement, Flow Line, Top Face of Curb, Top Back of Curb shape, I recommend offsetting the Edge of Pavement String and Top Back Of Curb strings to the bottom of curb and then using those offset lines along with Edge of Pavement, Top Back of Curb and Sidewalk to make the Finished Grade model. That way when we apply site improvements below the curb area of the surface it will offset below correctly.


If you want a Top of Curb Surface then you can take the Edge of Pavement, Flow Line, Top Face of Curb and the Top Back of Curb strings and make a surface called Top of Curb. Note that when you do this - in the example shown in the video you will also get the Pavement surface of the parking lot in this Top of Curb Surface. To stop that from forming, I then applied a Null Site Improvement to the Top Of Curb Surface in the area of the parking pavement to remove those triangles from the Top of Curb Surface.


Lastly, we need to create the offset line for the 6" offset (where we need to extend the Aggregate Base and Sand Layers beneath the Curb section. I then draped that offset line on the Finished Grade Surface (the one that runs around the base of the curb section).


Now we can apply the Site Improvements to the Finished Grade surface as shown in the diagram.


Lastly we can create the Subgrade Surfaces for the Bottom of material layers that we need for the Volumes or machine control / site positioning applications using the Create Subgrade Surface command.


Check what you created using Surface Slicer and 3D Views etc. to make sure you didn't make any mistakes and you are done. The video shows all of these steps.



I enclose the TBC v5 Project so that you can take a look at it in your own time, let me know if you have any further questions


Happy Modeling