When Kenaidan Contracting Ltd., of Mississauga, Ontario, Canada, and its subcontractors were faced with the task of expanding the local water treatment plant from a capacity of 347 million liters per day to 500 million liters per day, they knew the size of the area and the complexity of the structures would present a few challenges. The project involved building a walled-off excavation measuring roughly 50 by 55 meters in plan. The perimeter of the excavation was a polygon with 10 sides located near the existing water treatment plant. The depth of the excavation ranged from 5 meters to 21 meters.
A subcontractor to Kenaidan drilled 1-meter-diameter caissons side by side around the excavation and filled them with concrete. In every second caisson, the subcontractor placed a steel H-pile. When the caissons were all placed, the subcontractor excavated the hole. As the excavation went down, anchors were drilled through the wall into the soil to tie back the secant wall formed by the big caissons. “The project was going 100 miles per hour, and we had this big hole with no access to the caisson walls to measure them,” says Dorin Nita, building information modeling (BIM) manager at Kenaidan.
Kenaidan had figured on a tolerance of 7 centimeters on the secant wall. Interior concrete formwork and rebar had to fit tightly against the wall; any mistake in caisson placement would cause a problem. “We use 3D models on the job,” Nita says. “The challenge for us was to secure accurate and representative as-built information that covers the entire area in question.”
Getting the Real PictureNita and his team considered their options and consulted with Greg Kettle, survey and GIS sales representative for Geoshack Canada. “At first, we thought we would scan the caisson walls using a reflectorless robotic Topcon GPT9000,” Nita says. “Then I talked to Greg, and we decided that the reflectorless station was too slow.”
Using conventional surveying equipment to obtain the necessary detail would have taken at least two weeks, and the excavation and other ongoing activities would make it difficult for a person in the hole to survey the walls. Instead, Kettle recommended a Topcon Positioning Systems Imaging Station (IS), a robotic total station with dual integrated imaging cameras and stream video capabilities. The system automatically takes a series of images defined by the work area and then returns to capture scans precisely aligned with those images. “The Topcon IS has faster performance with laser scanning,” Nita says. “And the built-in digital camera helps you correlate as-built information with a real picture.”
Nita rented the equipment and returned to the jobsite. Eight hours later, he had all the data he needed to create the as-built model. “From just three setups, I shot all of the walls,” Nita says. “On the instrument, we set a grid-scanning pattern to 75 millimeters horizontal and 1 meter vertical, and we produced over 150,000 points in one day.”
Making AdjustmentsThe instrument generated several point cloud files, which were transferred to a computer for further analysis. Nita used Topcon ImageMaster software to interpret the results and filter down the information. “Using the Topcon application, we were able to see the pictures in the background of the point cloud and eliminate any points that were in question,” Nita says.
Next, Nita transferred the point clouds to Autodesk AutoCAD and Navisworks Manage, where he matched them up with his 3D design model for the structure, which he created in Autodesk Revit. That information was used to identify points of conflict between the as-builts of the caisson walls and the future concrete structure. The entire process, including data analysis, conversion of the drawing to a PDF, and video editing, took about eight hours. “The caissons were not perfect, but they were close,” Nita says. “We saw very few areas of interference. By having the entire point cloud, I could rotate the model and see how it defines the caisson. You can see all of the deflections in the caissons.”
To resolve the interferences, Kenaidan adjusted the location of the interior walls and, in two locations, chipped away concrete from the caissons to accommodate the interior structure. The company’s fast response enabled construction on the project to continue without a hitch.