Advanced Lasers & Instruments

Site Observations

Galbraith joined a contractor on a project that was high-end in its usage of machine control. The project was a commercial parking lot and access road. It included significant amounts of material placement, concrete paving and curb and gutter construction. In addition to using machine control for the subgrade construction, the project used the technology for several finished grading applications. This movement toward finished grading interests me the most—it’s currently an area that is being pursued by contractors with some experience in using machine control equipment.

On this job, an outside surveyor set the control for the project and provided a calibration file to the contractor. These values were compared and analyzed and then moved into construction. The construction company developed its own 3D models for the machinery by building different models for different tasks. In other words, they created separate surfaces for each level of material: a sub-base surface, a base surface and a top course surface. Then points were developed and provided to the field along with the model data.

This job used a Gomaco GT3600 curb and gutter machine with a Leica TCRP 1201 robotic total station referencing the back of the machine and a Leica GPS unit referencing the front of the machine. The Gomaco machine can also be configured with dual robots front and back or a single robot in a tangent section. The GPS is an alignment reference and the slope and yaw are controlled from the robot in back.

One of the things Galbraith noticed early on was that when the concrete was being laid, an overabundance of trimming of base material seemed to be occurring. Trimming is a function that prepares the bed for the placement of the concrete. It seemed that there were differences between the 3D model of the materials and the point data that was being used. These differences resulted in excessive trimming, which caused anomalies in the paver (although none were critical).

The trimming of the material in advance of the concrete placement isn’t unheard of and often is in the 0.1' range, but on this site Galbraith observed trimming in the 0.3' range. This caused the trimmer to bind at times and release excess material under the machine, whereupon the tractor ran over it causing small elevational jumps. In fact, it appeared that the rear track was running over the excavated material, throwing error into the mix and complications into the work. As the rear track came up on the trim material, the hydraulics had to adjust. The trim material started off as a small pile of dirt and rock and progressively became a bigger pile of dirt and rocks.

The project also involved a lot of curb and gutter placement—and it was all done without any stringline. As a construction surveyor, Galbraith estimates that his company can lay 1,500 linear feet (LF) of stringline per day. At a cost of $1,200/day for a field crew to lay the line, this adds up quickly. The ability to eliminate this task is a considerable cost savings for the contractor. As the work proceeded, the metrics collected indicated that they were able to place 300 LF of curb and gutter in 35 minutes—and it was all on a curve! That adds up to 600 LF per hour or nearly a mile per day.

When asked if there were any potential downsides for a project that had this level of fine grading with no stringline, Galbraith responded affirmatively. “On a job like this, with the curvilinear geometries involved, the lack of strings speeds up the process and cuts out the survey costs; however, a potential downside is that the operator needs to rely solely on the technology. There aren’t any visual references that the physical presence of the string in your line of site provides,” he said.

Essentially, in this situation, operators must refer solely to the onboard screen without a string visually indicating that a tangent section is coming up or that a curve is ahead.

To avoid this problem, most of us in the data prep line of work produce not only a 3D file of the surface(s) but a horizontal file for geometries as well. These stringlines display on the onboard screen, so the operator has a visual reference to what is coming up. In fact, many devices allow the operator to “follow” a stringline with the blade, so this is a handy adjunct deliverable. (As you may have figured out by now, that is where the title of my column “Virtual Stringline” comes from.)

The curb and gutter placement on this project was done without any stringline. A Gomaco GT3600 curb and gutter machine was referenced with a Leica TCRP 1201 robotic total station in the back and a Leica GPS unit in the front.

Quality Concerns

The biggest complaints Galbraith heard on the project dealt with the quality of the 3D models. Some of the operators commented that they don’t always use the onboard displays because their experience allows them to eyeball the site and make adjustments manually. This type of comment is interesting because it shows that, even when surrounded by technology, the experience of a skilled worker can still provide a distinct advantage on the jobsite.

I asked Galbraith why the models seemed to be causing issues and he said, “There was a disconnect between the models and between the points files that were generated. It appeared that these data files were prepared independently.” He continued, “The need for better coordination of effort needs to occur. If all of the models and stakeout data came from a central source, they would all have a common reference and much of the mismatching would be eliminated.”

I also asked Galbraith why he thought the material was incorrectly laid down. “In my opinion, disconnects occurred between the models, the auxiliary files provided and operator activity and interpretation,” he said. “The models appeared to have been assembled in a piecemeal fashion and poor quality control occurred prior to going into construction. It appeared that the subgrade file and the curb file were never checked against each other, yielding excessive trimming.”

For this project, the company built the 3D model and the survey consultant made the point file separately. The resulting outputs were not checked against each other. This meant that the survey stakeout person had to make on-the-fly judgments in the field. While the equipment was burning gas, I’m sure there wasn’t much sympathy for a surveyor who might have wanted to suggest, “I need time to check all of this data.”

Although machine control technology is advancing nicely, Galbraith observed that we are beginning to see a need for industry-wide procedures. I agree. We have all been spending time learning these technologies, building 3D models and troubleshooting both hardware and software. But setting up standard workflow procedures has taken a back seat to learning the emerging technologies.

To sum it up, Galbraith said, “Machine control is like GPS was to a surveyor 10 years ago. If a surveyor doesn’t have GPS equipment and services today, they are likely struggling to keep up. I see this being true for machine control and contractors in the future. Machine control or automated machine guidance is here to stay. The real issue is that we need detailed procedures for operations, QA/QC, etc. Until we have a proper dataflow from design into the field, costly inefficiencies will occur.”

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