Join Eric Chappell for an in-depth discussion in this video Understanding phased grading, part of AutoCAD Civil 3D: Site Design.
- [Trainer] In this chapter, we're getting ready to perform a grading design for a small parking lot. Before we do that, let's do some planning and talk about how we're going to complete the task in stages. The first thing we need to do is talk through the design. What are we trying to accomplish with this grading design? By the way, grading is one of the most difficult topics to teach and to learn with design software, because no two grading projects are alike. There is no linear process where you can say start at step one, complete x number of steps to get to the end.
Grading is not a linear process, there are lots of different skills involved and you're going to apply them in different ways in a different order each time. So one of the first things you'll want to do is just simply talk through what you're trying to accomplish with the grading. For example, with this parking lot, some things that I know that you may not know yet, is that we're going to place a pond on the left side of the parking lot. So that means we need drainage to go toward the pond, that'll make it easier to get the drainage actually into the pond.
So what we'd like to do is slope in this direction, toward this corner. This is going to be the low point of our parking lot. Here's where we'll discharge the drainage that we collect from the parking lot, convey it over and dump it out into the pond. So that's one thing we know, we know that we want the slope to go in this direction. We also can see some things that are pretty obvious. We've got a road out front that we know we're going to have to tie to and that means the elevations where these curb returns end are going to have to match the design of this road out front, or if this happens to be an existing road, it would need to match existing elevations.
We also know we have a short driveway here, so the slope of this driveway or the elevation difference from the existing road to the parking lot can't be so great that this driveway becomes to steep for cars to travel safely. So we've got to constraint there, and that could even be a design regulation constraint. We may look at the local regulations in this area and it may say that the maximum slope allowed in a parking area is 8% or 6% or something of that nature.
And we may be driven by those types of requirements. Something else we're going to want to do is we want drainage to always be away from our building. This is our building, this purple outline so we don't ever want water flowing across the parking lot and running into the side of the building where it's likely going to get in between the building and the pavement and get down into the foundation. We definitely always want drainage to flow away from the building for some distance, and we're going to say that's going to be 10 feet in this case.
We always want to slope away from the building a minimum distance of 10 feet. And something else we know because of our regulations, is that we're going to have curbs around this parking lot. So any rain that falls in the paved area is going to get captured and contained within the curbed area. So that means, even water that falls over in this area of the parking lot is going to work its way around and get either out through the driveway or work its way around toward our low point.
So, these are the kinds of discussions that you may have with yourself or with your design team before you actually set your first elevation or your first slope anywhere on the project. So now that we have an idea what we want to do, we want to try to make the best use possible of the tools that we have in the software. So, in other words, we're just not going to jump right in and start setting elevations on this feature line because we don't know a whole lot about what those elevations should be yet, and we need to do some calculations.
So, I could start calculating from this known existing elevation, some slope up this line and point by point calculate each elevation by hand, working my way around the parking lot. If I'm a pretty good engineer, I can probably do that in maybe 15 or 20 minutes, but I really don't want to do that, I want to leverage the software and have the software do that for me. So what if, instead of calculating those elevations one by one, what if we created a quick temporary surface that sloped over top of the parking lot at the slope that we want, say 2% or 3%.
And then we know we can create surfaces from grading objects, so what if we created a surface out of that and then used the elevations from surface command to simply project that feature line up to that surface? In doing so, we allow the software to calculate all those little elevations for us. Which, when we're talking about a handful of elevations on this feature line doesn't seem like a huge deal, but a more complex parking lot, where you've got 100's of data points, you're absolutely going to get huge advantages from doing that.
Then once we have the elevations assigned to the perimeter, maybe we can do some other feature lines, tie in the grading to existing ground, start working on some detail on the interior. Perhaps we want to add some landscaped islands with curb on the interior, so we start adding more and more detail as we work. So now that we've done a bit of planning, we can begin executing the different phases to complete this design.
- What is site design?
- Creating feature lines
- Breaking, joining, trimming, and extending feature lines
- Adding and removing elevation points
- Raising and lowering feature lines
- Creating grading objects
- Building a grading scheme
- Creating curbs and parking lot surfaces
- Labeling a grading design
- Creating a storm design
- Adding inlets and pipes to a storm design