Surveying Utility Pipe Alignment
4 minute read
Underground water pipes tend to leak as they get old. Whether that leaking is due to damage, deterioration or both, repairs can be costly. Replacement boosts those costs dramatically higher. Often, utilities and municipalities turn to techniques such as sliplining––in which engineers install a new pipe inside the damaged one––to save money.
As you might imagine, sliplining requires extremely accurate information about the existing pipe. That’s why, when officials at a California utility found extensive damage in a 54-inch (137-centimeter) diameter steel pipe, they turned to Towill, Inc. And the technicians at Towill, well, they turned to Trimble technology.
The challenge: deliver precise data on nearly 1,800 feet (550 meters) of an active, pressurized, 90-year-old waterline.
This was no simple surveying project. In addition to determining the horizontal and vertical alignment of the old pipe, Towill needed to provide data on the pipe’s ovality––how much it was deformed from its original circular shape. There’s only one way to get such data, and that is to go inside the pipe. To further complicate matters, Towill’s surveyors had just eight days to get the job done, since they were dealing with an active water main.
To handle the work, the Towill team decided to combine the 3D scanning capabilities of their Trimble SX10 scanning total station with total station measurements from their Trimble S7 total station. They would control both instruments using Trimble Access software running on a Trimble Tablet.
- Trimble® SX10 Scanning Total Station
- Trimble S7 Total Station
- Trimble Access™ Software
- Trimble Business Center Software
- Trimble RealWorks Software
The work was challenging, especially given the dank, low-light conditions inside the pipe.
“It was difficult work from the very beginning,” said Towill geospatial specialist J.R. Gregory.
The instruments performed exceptionally well. The Towill technicians accessed the pipe from three different locations, using the Trimble S7 to conduct an open traverse through the pipe and to measure multiple angle sets at each setup. They followed with the SX10, which provided an independent check on the traverse and also produced scanning data on control targets. Using traverse control stations roughly 150-to-200 feet (45-to-60 meters) apart, the team finished 130 scanning setups at intervals of 15 feet (5 meters) in the agreed upon eight days. When the surveying was done, they sealed the pipe, and turned on the water. There was no going back.
Gregory used Trimble Business Center (TBC) software to process the S7 and SX10 data into coordinates on the control points. He processed the SX10 scans and converted them into a project in Trimble RealWorks (TRW). The data revealed multiple horizontal and vertical deflections not shown on earlier plans or as-built drawings. All were highly accurate and reliable.
“The results were well within our project tolerances,” Gregory said. “The redundant traverse provided an additional QA/QC benefit of the work.”
Gregory used TRW to register the scanner data into a single point cloud. He created a best-fit horizontal and vertical alignment along the center of the pipe.
Then the technicians used TRW to create a design cylinder for the slipline and automatically compared it to the point cloud.
This analysis enabled Towill to recommend the optimal size for the slipline and to identify areas of potential problems. In addition to the technical reports, Towill exported the results in AutoCAD Civil3D and LAS formats, enabling the team to visualize and share data with the utility’s engineers. The survey more than paid for itself. The utility used the results to install the largest possible liner, allowing for maximum capacity without major excavation, saving millions in potential costs.
“The survey and analysis enabled engineers to adjust for problems before they occurred. Our work greatly reduced changes in the field that the client might have encountered if forced to use less accurate and less comprehensive data,” said John T. May, project manager.