AUTOCAD CIVIL 3D - GRADING

Heads Up Grading for Parking Lots

In the good ol' days, before data collectors were in wide spread use, we used to get our existing topo on paper. We'd have to throw it over a big ol' digitizing board and trace it with a puck. This was referred to as digitizing which was a very time consuming task. Later, we were able to have the paper scanned, insert the image into AutoCAD, scale it up, and trace it on screen. This was referred to as "heads up digitizing" which was a lot more efficient and faster.

I've coined this term "heads up grading" for those who give up the paper, scale, and calculator way of grading to use the grading tools in Civil 3D instead. The example presented in this post will show how quickly and efficiently one can arrive at a grading solution using this method.  If there are any fundamental concepts in this post that you do not understand, please refer to Mastering Civil 3D by Richard Graham & Louisa Holland.

The illustration below shows a parking lot. The purpose of this exercise is to grade the parking lot which feeds onto a proposed road. The linework for the parking lot is being represented by a feature line in a site named "Parking". There are no elevations applied. The proposed road is represented by a corridor.

Create Grading Setup

1. Add the back of curb feature line as a break line to a proposed surface named "Parking Temp". Associate a surface style that shows contours. Configure a mid-ordinate distance of 1 (don't argue with me).  
2. Add surface spot elevation labels. As you grade the parking lot, these will automatically update.
3. Extract the back of curb feature line from the corridor and apply a style with the highest priority for split point resolution; maintain its dynamic link with the corridor. Since the parking lot back of curb feature line meets this corridor back of curb feature line, the match grade elevations will always be maintained even after the corridor is updated.

   

4. Plan your overall grading scheme. For this scenario, we'll divert the flow northwest to a detention pond.

Result:

Grade from Corridor

Obviously the grade has already been set at the road. Using Elevation Editor, we'll grade from the road to the end of the east curb return using a 6% grade. For the west curb return, use a grade of 4.5%. You can try different values. The surface, contours and labels automatically update as you enter the values into the elevation editor. Or you can manually enter an elevation for that point.

Focusing on the east (high) side of the parking lot, we'll grade the next three segments on the feature line.

1. Grade down at 2% to the beginning of the curb return.
2. Grade up at 1% for the next two segments until the southeast corner of the parking lot is reached. 

This is the elevation we will hold for this parking lot corner.

In the Interim

Let's try out the scenario of sheet flowing at 2% diagonally across the entire parking lot. This will require a feature line intersecting this corner of the parking lot and positioned to project a grading object across the parking lot.

Position this feature line to be perpendicular to a line drawn from this point of the parking lot to the opposite point which will be the low point. Place the feature line in a site named "Interim" and set its elevation to 98.99.

To project a -2% slope in the northeast direction from this feature line, open up the Grading Creation Tools and configure a grading group:

1. Set the site to "Interim".
2. Create a grading group named "Interim".
3. Turn on the Automatic Surface feature & configure a no display style.

Using a "by distance" grading criteria, we can create a grading object that projects for a distance of 300' and a grade of -2%.

Next we'll use the "Interim" surface to assign elevations to the back of curb feature line.

Results:

At the north turn out, the back of curb will be wrapped up by raising the northwest elevation to 95.35.

What's Happening Inside?

To model the inside of the parking lot, a grading group will be applied.  Open the Grading Creation Tools:

1. Set the site to "Parking".
2. Create a grading group called "Parking Final".
3. Configure grading group to generate a surface using a contour style.
4. Set the style for the "Parking Temp" surface to a no display style.

Three types of criteria will be generated to model the top, face, and gutter pan.

The beauty of using grading groups is that if you regrade the top of curb, each grading object is regenerated using its criteria based on the top of curb feature line elevations. If the gutter criterion is only required on one side of the parking lot, you can supply a start and end point for it. The pavement can be filled in with an infill.

Results:

Finished!

This took minutes. The contours are generated and the grading annotation is done. Perhaps a minute or two could be taken to arrange the spot elevation labels. But if the grading needs to be revised, once the feature line is edited, all the grading, surfaces, and annotation will update. And, incidentally, the curb in the corridor at the entrance can be wiped down by adding sections at the entrance and decreasing the curb height dimension in Section Editor.

Parking lot grading can get a lot more complicated than this. Combinations of these grading tools can be used to arrive at a solution. Just formulate a plan of action before you start. Just remember as you are heads up grading:

• Keep your interim grading groups in separate sites.
• Give your sites, grading groups, & surfaces relevant names.
• Use spot elevation labels; these will update instantly and it's easier to identify elevations than using the elevation editor.
• Use grading groups for curb & gutter. Feature lines created by stepped offset do not automatically update.
• Be aware of your feature line style priority for split point resolution.
• Remember, corridors can work with site grading too! Extract feature lines for site grading reference and define those entrances.