More contractors are taking advantage of stringless curb technology today. At TOPS, we got involved with stringless curb when it was proposed as an alternate application for machine control. In their quest for increased productivity, some of our clients are early adopters which we credit with our first-hand experience in using the technology.
The Concept
The idea is straightforward. Adapt the use of alignments for paving systems to a curb machine and eliminate the string. There isn’t more to do for the transfer of the guidance and the technique seems easy to perform. Unfortunately, we encountered some problems along the way. But lucky for you, we have taken the time to point them out and guide you on this process.
Stumbling Blocks
You need to build an alignment-based project that has the usual elements, horizontal and vertical alignments, and a template. Brands vary but the template can be used to pick the side of the alignment the curb is on, as well as, slope for fill or spill curb.
One of the difficult things to do is to make the alignments as if you’re in the field doing the work. This varies for our clients and it is something you need to coordinate with the curb crew.
In the example here, I bring up two interesting points:
This job is the addition of more parking to an existing lot. When joining to that curb, we need to get accurate shots in order to smoothly pick up the slope of the current to future curb.
The 90-degree corner will necessitate a stop in the alignment. The question for the field is where to start one alignment and stop another.
Alignments
As with any road job, a horizontal and vertical alignment are required. With curb, things need to be different. There will be some figuring with both.
Horizontal
Each curb is a separate road job. As shown earlier, you need to coordinate with the field as to the start and stop points. Experience has led us to have this consistent, but it takes time to coordinate. When you
get it figured out, it will stay standard for the most part.
When working with a closed island with all curves, the line needs to either stop short of the end or go past it and not be on the same path. This example shows the alignment stopping .10 feet from the start. This gives the machine a chance to complete the run without the software problems of an alignment running back on itself.
We will also have the alignment bypass the start by a couple hundredths of a foot when it gets back to the start point to keep the lines from intersecting.
While building the alignments you are also providing a full takeoff of the curb so the field can schedule concrete and plan the pour accurately.
Vertical
This is where things get interesting. We all know that good plans have elevation callouts for the major points of a curb. This example is trying to do that, but this job has sheet graded contours that make things more difficult. We need to pay attention to closed areas that may trap water and make them back-flow into the main slope plane.
Breakover Angles
This is by far the most critical part of designing stringless curb files.
Here are the elevation points as called out in the plans. The curb moves along but there are angle breaks with a 2% delta. When entered into the machine like this they will cause it to abruptly change slope and make a mess. To remedy this, two things must happen.
Vertical curves must be added to the alignment to smooth out the transition in slopes. Our method for figuring the amount of curvature has been derived through experience working with machine control and curb machine vendors. Experience will need to be your teacher here.
After making the curb look right, the new edge of pavement 3D information needs to be incorporated into the model so the subgrade and paving are not affected.
Here is the alignment after the addition of the vertical curves. The transition is now smooth, and the machine will make the slope change gradually so things look right and perform well.
In the images, the difference may seem subtle. In the field, it is scary to see the machine try to do an instant 3- or 4-degree slope change. I’m sure the question will come up in a cart and horse fashion. If you are changing the parking lot surface, should you design the curb first? Most jobs don’t use stringless curb. The ones that do are usually requested after the surface file has been made and the curb contractor wants to use the technology.
The initial file creation is procedural and a process should be followed. This is because there may be 50 alignments for a big site job, and you don’t want to go back and check every line to see if you missed something. There isn’t much rework involved after the curb alignments are returned to the surface, we just want to make sure base depths conform to plan. The following is an outline of the entire process.
Create Curb Template Alignments
Save the file as a new version to keep the surface file alone
Create breaks for machine control
Create a vertical and horizontal alignment from the curb lines
Station the horizontal
Add vertical curves to smooth out the profiles
Create the proper exportable road file. Brands and requirements vary
Create New Surface
Do another save with a new name
Remove the use of the initial curb elevations in the model
Set the vertical profiles as the new curb lines
Offset the lines in three-dimensions to get the lines locations and elevations to gutter or edge of pavement
Adjust the surface in areas that make it smooth and well drained
A bit easier said than done, but experience has really helped us get this operation efficient. The curb files we make for clients can guide stringless curb with confidence. I remember years ago when the head of Gomaco asked me why I thought anyone would use stringless and how I planned on giving the crews confidence to spend tons of money and time with no string to lead the way. Here is what I stated and how a company gains trust.
The curb (or white paving) is derived from the model used to blade the surface. If that looks good, things are okay.
Use a 4-wheeler to do a dry run. Load the road job and drive along the curb, you will see any problems before the pour.
With all these advantages to stringless mentioned, file preparation is not a big chore. I have some sad stories about stringline that caused us problems over the years. A few dry runs of practice and maybe some “air-paving” will get you comfortable and ready to make the move to automating curbing and paving.
