Engineer's Surface - Worth The Risk???
As GPS machine control use continues to grow, many Engineering firms are readily giving out digital files. In addition to the standard 2D CAD file, some Engineers also include a 3D surface model. Using these “free” 3D models for machine control has lead to some common issues during construction and is a source of regular client phone calls here at Quantum.
Here are how many common problems with grading surfaces can crop up in the field:
Automatic blade control is choppy, chatters, dives into or jumps above grade.
Grades match where spot elevations are shown on the plans but not in the in-between areas.
Your GPS controller or screen in the machine runs slow or freezes up.
Transition areas, like where sidewalks and green space meet, aren’t right.
Tie-ins to existing grade don’t match or your site is not draining to plan.
Your grading does not match the paving stakes.
Keep in mind that the Engineer did not make a “bad” surface for his needs but it is rarely optimized for machine control.
Below is a screen shot of the grading plan sheet for a project we just completed. Note the spot elevations called out as well as the basement and finish floor elevation.
Below are two screen shots of the 3D model for this project. The model at the top was provided by the project Engineer. The model at the bottom was built by Quantum Land Design, for machine control. Note that the model at the top (the free one) might show how the water will run on the site but clearly does not contain important construction details like the building pad or spot elevations. The driveway on the lower left side is a mess, too.
Quantum's model on the bottom (with the basement) was built from scratch to match the design shown on the paper plans. Note that the building pad (with basement), curb lines, storm water swales and driveway are clearly defined. The model is smooth and will operate flawlessly with automated GPS guidance. Often we provide multiple 3D models for a site - underground utilities, subgrade, over excavation - whatever the contractor needs to be efficient.
To understand why the Engineer’s surface does not work for machine control, let’s look at the how they approach a project. In designing a project, the engineer’s primary concern is drawing a set of plans that defines the construction of a project site. The main focus when drawing these plans is creating a clean, legible and detailed paper document (smooth contours and clear text). This document, with the Engineer’s stamp, will be used for both bidding and building the project. Sometimes these plans are created months (years!!!) before a project is started.
In the process of designing a grading surface, the engineer is focused on water runoff and detailing the main areas of concern (storm water inlets, high and low points, roads and surface slopes). These items and spot elevations are clearly detailed in the plan set. What is not a concern, on paper, are the areas between contours and spot elevations. In the Engineer’s 3D surface these undefined areas often have incorrect slopes and elevations or have data that the machine control system cannot operate automatics on. You can understand the design intent on paper but the model does not properly translate the design to your GPS rover or earthmoving equipment.
When making a design change, it’s always more time efficient for the Engineer to edit text - like a spot elevation or slope - on a the plans versus changing the 3D surface and then changing the text. The Engineer’s 3D surface may have been used to design site drainage or run bulk earthmoving calcs but is often not altered to match minor design changes as the project moves through the design, code review and revision process.
Now that you understand how an Engineer might use a 3D surface, you can see why they do not optimize it for machine control purposes. The new civil engineering software has a function to export a 3D surface compatible with your GPS system. These files are normally exported months after a project was designed and no one confirms they agree with the construction plans, but these files are usually free.
You got a free model but what happens when you take it to the dirt? Before receiving the files, you must sign a release form saying all liability for using the digital files falls on the contractor. So you get the surface, but if it does not match the plans, it’s your problem. The files are usually in the wrong format, so they have to be converted by another software or company - another opportunity for error. Once the file is finally loaded and used on site there will be areas where the blade shakes, dives or doesn’t read grade at all. Maybe you can deal with poor automatics operation or coverage gaps during bulk earthworks. When you close in on finish grade the problems really start to crop up - and your schedule gets tighter. You might be grading out a drainage swale but it just does not look right - it is at grade on either end but there is a low spot in the middle. Then on another part of the job the grade does not transition into the back of curb quite correctly.
You call the surveyor or engineer to figure out what the problem is. The surveyor stakes the problem areas, of course, your surface model does not match the stakes. Your GPS reads elevations at the new stakes correctly but cut/fill to the model does not match. Your system measures into site control perfectly. The problem is in the “free” model. Now what do you do? The engineer will either not correct the file, they did give it to you at no cost, or have time to do it in a few days or weeks. In the meantime you have a project to get done. To get the project back up and running you will have a surveyor stake the site or hire a modeling company to redo the file ASAP for an inflated price.
There IS an easy way to avoid the problems caused by a poor surface model. Hire a professional modeling company, like Quantum Land Design. It’s easy, send Quantum’s experts the plan set and cad files then specify any subgrades or over-excavation areas desired. In 3 days or less, Quantum’s pros will send your files and you can get to work.