When it comes to creating benches in our grading designs, a lot of us panic. It seems like a complex endeavor to create multiple tiers with drainage slopes at set intervals, right? Not to worry – it’s really a pretty simple process, once you know the right tools to use.
Benching is the idea of cutting a slope into stepped tiers, to provide stability and reduce speed of water runoff, which helps prevent erosion of a hillside. It’s also an important safety concept along roads that are on steep embankments, because it can keep vehicles to limited slip/fall distances. It’s better to slide 5′ down a hill and stop, than to roll a few hundred feet down to the bottom of a valley!
In Civil 3D, all we need to do is use the Feature Line > Create Feature Line from Corridor tool to extract the edge of a road from a designed corridor model. From there, we can make use of some simple tools to build a benched Surface that meets all our design needs very quickly.
Create Feature Line From Corridor – Pick Road Edge
Next, we can use the Feature Line > Create Feature Line from Stepped Offset tool (below) to create offset lines that aren’t just offset by a horizontal distance, but by elevations and slopes as well.
Stepped Offsets Tool
At the command line, specify the distance of your bench (I’ll use 25′ for this example) and the side you want to offset to. The system prompts you to specify an elevation/grade/slope/difference for the new feature line. This will provide the vertical change (slope) between benches. I’m going to use a set drop of 10′ for this example. (Note, you want to input the value as -10 for a drop value). Then, offset the new line for a 1′ batter with our 10′ drop. You’ll want to adjust the values to your specific design need of course, but the process stays the same, no matter the number used. Now, you can just repeat the Stepped Offset command process with the same values, to get the benching you need in just a few seconds, like so:
Stepped Offsets in Plan View
Now all you need to do is add those benched feature lines to your surface as “Breaklines” and we’re done! Here’s what the final Surface looks like:
Benched Road Design
See? That wasn’t hard at all! Just a few clicks of the mouse and you have the graded benching you need for a safe, and secure, steep slope design of your roadway.
– James Coppinger