Using Existing Cross-Slope for New Outer Lane

Introduction

During Hagerman and Company’s recent launch event in Nashville for the new 2009 products, two gentlemen brought up a question that, on the surface, seemed complex given the number of words necessary to get the idea into my thick skull. After the light bulb turned on and upon brief reflection, I answered “yes,” that their task could be done. Of course, with AutoCAD® Civil 3D®, the answer is always “yes” and sometimes I throw that answer out there prematurely, not knowing the exact set of steps necessary to complete a given task but there is always a way, and often several ways. In some cases, I have even said “yes” the instant after I hear the ubiquitous opening “Can Civil 3D…?”

Their question had to do with adding a lane of traffic to an existing road using the (probably variable) cross-slope of the existing lane of traffic. With apologies to my contractor friends, we all know that slapping the standard 2 percent cross-slope on a typical section on a set of plans does not necessarily mean it will be built that way and when lanes are added later, we certainly don’t want there to be a grade break running between two lanes of traffic going the same way. My reflection at the time the question came up focused on a new capability within Civil 3D 2009: creating “best-fit” alignments and profiles based on existing topology.

Process – Step 1

In my mind, the first step would be to identify an element on the existing road from which to base our design and the perfect candidate for that is the existing centerline. The existing edge of pavement is too random and cannot “describe” the road with its ragged geometry. In our example file, we have some centerline shots taken along the existing road crown and we will use that information to create our existing centerline.



We will now use the command “Create Alignment by Layout” to name our alignment and assign an object style, labels and a beginning station. Once this has been accomplished, we will choose the option for creating a tangent section called “Fixed Line – Best Fit.”

The next dialog box allows us to choose the entities we will use to identify our alignment. In this case we will select Civil 3D Points.

The next step is to identify the actual points. In this case I can manually select each point as I run along my intended alignment or I can identify them by number or group. Once the points have been chosen, regardless of the method, a panorama window pops up with the regression data used for best-fit calculations. I can choose to exclude any of the identified points or add more weight to a particular point. Once any changes have been made here, simply hit the “Save” button at the upper right of the panorama and you are in business. The same procedure can be used for curves within alignments.

Process – Step 2

Once the alignment has been identified, we can now establish the existing cross-slope of the road. We will do this using a series of Civil 3D Feature Lines placed at a certain interval along the alignment from the centerline to some point in the direction of the cross-slope we are trying to measure. The length of these Feature Lines should be long enough to get a measure of the full lane width but short enough so as not to get past the edge of pavement and onto the road shoulder or ditch, throwing off the cross-slope measurement. In my example, I have created a series of polylines 11.5 feet in length at a 20 foot interval along the alignment.

We will now use the “Create Feature Lines from Objects” utility to turn these flat polylines into Feature Lines and assign elevations at their vertices based on the existing surface. Simply begin the tool, select all the polylines, and set the command options as follows:

The “Assign elevations” check box will allow us to set elevations on our Feature Lines by using the Existing Ground surface.

You now have a series of Feature Lines drawn along the alignment, each one measuring the existing cross-slope at its station.

Process – Step 3

Now we need to establish where the proposed edge of pavement will be horizontally. In my example, we will simply offset the existing centerline alignment 24 feet to establish our intended edge of pavement. Even though the existing edge of pavement is not a static width away from the centerline, we will establish our new edge at a static width and build a new lane of traffic with variable width.

We can now use the “Extend” command to push the cross-slope Feature Lines edges out toward the new edge of pavement. This will allow us to set elevations at the new edge of pavement using the existing elevations and cross-slopes we have established.

Process – Step 4

We can now use the proposed edge of pavement to establish a profile setting elevations for our new lane of traffic. We will use the “Create Alignment from Polyline” command to turn our proposed edge of pavement into an alignment.

Now use it to cut an existing ground profile.

Now we can set up our proposed elevations at the endpoints of our Feature Lines at the new edge of pavement. The easiest way to accomplish this is to create a preliminary (dummy, proxy) surface using those cross-slope Feature Lines, then create the proposed profile “by layout” using the “Draw Tangents” option and the “Profile Station and Elevation from Plan” transparent command.

Clean Up

To take the next steps, you may consider using this proposed profile as a guide for another proposed profile done with Best-Fit entities. If you used a series of curves and tangents to mimic the segmented “tangent only” profile, you will end up with a result only as good as the geometric match between the two.

Also, when it comes time to build a corridor for your model, use an assembly controlled by this proposed edge of pavement that projects a variable width and variable sloped pavement toward the existing edge of pavement and a curb or shoulder in the opposite direction.