It’s hard enough to build a complex, 12-story industrial structure. Try putting one into a deep, concrete-lined hole. Twice. That’s the challenge facing Abe Torres and his colleagues at PC Construction. When the job is finished they will have helped solve a problem more than 200 years old.
PC Construction (PC) is working on a project at the Washington, D.C. Water Authority (DC Water) Blue Plains Advanced Wastewater Treatment plant. The work is part of an effort to prevent release of untreated effluent that can result from heavy rain. Like many older cities in the U.S., Washington’s sewer system collects sanitary sewage and stormwater into common pipes that carry water to a treatment plant. When heavy rains occur, the runoff exceeds the system’s treatment capacity. These events, called combined sewer overflows, result in the release of untreated effluent into area rivers.
DC Water is constructing a series of massive tunnels to collect and store sanitary sewage and storm runoff. The tunnels terminate at the Blue Plains Plant. There, the joint venture of PC and CDM Smith has a design-build contract on the tunnel dewatering pump station (TDPS) and related facilities that will lift the water from the tunnels and deliver it to the treatment systems.
The tunnel that arrives at the Blue Plains plant is 23 feet (7 m) in diameter and roughly 170 feet (52 m) below the surface. It connects to a pair of vertical circular concrete shafts 132 feet (40 m) in diameter. Within these shafts, PC is constructing the massive piping, pumps and equipment that make up the tunnel dewatering pump station (TDPS).
Torres describes the project as placing a 12-story industrial facility with seven levels of steel, mechanical and electrical components into a deep, round hole. The design and construction of such a large, complex facility is a prime opportunity to leverage building information modeling (BIM). By combining BIM with spatial technologies in the field, PC is achieving cutting-edge efficiency and significant cost savings.
3D Modeling Meets the Physical World
When Torres arrived on the project, another firm had already completed construction on the two shafts. The shafts’ concrete liners included beam pockets and keyways to support the new structures. PC needed to verify the dimensions and locations of these and other elements and compare them to the 3D model of the concrete structure that they would build into the shaft. 3D scanning would be a fast, cost-effective approach.
Using a Trimble TX5 scanner, PC crews scanned the first shaft and produced 16 individual datasets. Torres used Trimble RealWorks® software to process the data and create a single point cloud. He could merge the scanning data with the 3D design model of the pump station. Next, engineers determined a best fit to place the structure in the shaft. “We had to turn it a little bit, not much, and it came together pretty good,” Torres said. “With the exact dimensions in hand, we figured out that we could prefabricate our beams and steel. Crews can do much of the assembly outside and then slide it into the hole and put it in place.”
The second shaft came with more challenges. Scanning revealed that the concrete liner was out of plumb in places and that the design would need some adjustments. “Our rebar was going to be too long and we needed to redesign it,” Torres explained. “Had we ordered rebar according to the original design, there was no way it would fit. Doing the scan and checking it against the model saved us a bunch of money.”
Checking the models and feeding up-to-date information to construction crews is a continuous process. Matt Harraka, a virtual design construction manager for CDM Smith, said that engineers and designers use a broad range of software tools to develop structural, architectural, MEP (mechanical, electrical and plumbing) and grading designs. Harraka brings the designs together in large model viewer systems such as Navisworks or Trimble Business Center software. PC uses Trimble Connect to manage and share design and construction documents with field engineers and subcontractors. As design changes occur, Torres uses Tekla software to manage the model and check for fit or clashes before placing updated files onto the Trimble Connect server. Then the data goes to handheld field controllers for layout.
On the site, survey crews PC crews use Trimble Access software for all the surveying work and control. Engineers and tradesmen use Trimble Field Link software and total stations for layout inside the shaft. They all can use visual layout techniques, in which the robotic total station automatically places a laser dot at the precise location of a fixture or attachment point.
Keeping and using an accurate model is an essential part of project efficiency and quality control. Any design changes can be checked against existing conditions before going to the field or fabricator. The extensive checking helps to prevent errors and costly rework. Cloud-based file sharing ensures that everyone is using the same correct and up-to-date plans. Everything in the pump station—from concrete structures down to ductwork, conduits and individual anchor bolts—goes through the modeling and review processes. “If it’s not in the model, it’s not going to be laid out in the field,” Torres said.
Preventing Problems
The work at Blue Plains to date has gone well. “Using the model is one of the best things that could happen to us,” Torres said. “We modelled all the concrete inside the structure, all the anchor bolts and steel pipe supports as well.”
The ability to measure and plan in detail delivers big benefits. One example is the installation of a large header pipe that will deliver water from the tunnel to the pumping equipment. Fabricated in sections, the steel header is 13 feet (4 m) in diameter and runs the width of the shaft. PC needed to rent a special crane to handle the pipe. “We had just two days to put everything together,” Torres said. “We needed to put the pipe on its supports at the correct elevation, so we had to shoot inverts on the pipe supports. We worked with zero tolerance. The pipe is so big and heavy that had we needed to move it then it would probably break all the anchor bolts, which would be a disastrous setback. Everything was thoroughly planned, checked and re-checked.” The rigorous measurement and checking enabled the installation to move smoothly and according to plan.
PC Construction Chief Engineer Ron Ellison said that’s not an unusual result. “It makes it so much better to be able to see something in 3D in the office before it gets to the field. That’s our biggest benefit: to catch and eliminate mistakes in the office before they get to the field.”