Square building in a round hole


    It’s hard enough to build a complex industrial structure 12 stories tall. Putting one into a deep, concrete-lined hole is even more difficult. And then you will need to do it twice. That’s the challenge facing Abe Torres, senior virtual construction engineer, and his colleagues at PC Construction. However, when the job is finished, they will have helped solve a problem more than 200 years old.

    The project is taking place in Washington, D.C., a busy, compact city that is among America’s most popular tourist destinations. The city’s resident population, large commuter workforce, and millions of visitors each year can push demand for essential services beyond capacity. But crowds aren’t always the cause of the overloads: Mother Nature and aging infrastructure can combine to create a huge challenge to the city’s sewer system, which dates to the early 1800s.

    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. Combined sewer overflows (CSOs) result in the release of untreated effluent into area rivers.

    To help mitigate the problem, the District of Colombia Water Authority (DC Water) is building a series of tunnels that will capture and store runoff, then feed the water to treatment plants in a controlled flow. The tunnels ultimately connect to the Blue Plains Advanced Wastewater Treatment Plant where a new pumping station will lift the water from the tunnels and deliver it to the treatment facilities.

    The tunnel into the Blue Plains plant is 5 miles long, 23 feet in diameter, and roughly 170 feet below the surface. It connects to a pair of circular, 132-foot-diameter concrete shafts that will contain piping, pumps, and equipment that make up the tunnel dewatering pump station (TDPS). The joint venture of PC Construction (PC) and CDM Smith has a design-build contract on the pumping station and related facilities.

    Torres described the project as placing a 12-story industrial facility with seven levels of steel, mechanical, and electrical components into a deep, round hole. 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

    The view from the prism pole during layout. The display guides the user to the correct location. Brackets on the shaft walls hold prisms for control points. The total station is visible above the workers to the left of the prism pole.
    The view from the prism pole during layout. The display guides the user to the correct location. Brackets on the shaft walls hold prisms for control points. The total station is visible above the workers to the left of the prism pole.

    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 with the 3D model of the concrete structure that they would build into the shaft. Torres suggested that 3D scanning would be a fast, cost-effective approach.

    Using a Trimble TX5 scanner, PC crews scanned the first shaft, conducting 16 scans connected by control points attached to the shaft walls. After registering the scans to produce a single point cloud in Trimble RealWorks software, Torres could bring in the design model.

    Using the scanning data, the design team merged field measurements with the 3D design model of the pump station. They 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 said. “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.”

    The lower levels of the pumping station model. The pipe opening on the lower level is 13 feet in diameter.
    The lower levels of the pumping station model. The pipe opening on the lower level is 13 feet in diameter.

    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 manages dozens of different file formats to bring the designs together in large model viewer systems such as Navisworks or Trimble Business Center (for earthworks and surveying) 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, placing updated files onto the Trimble Connect server. Then the data goes to handheld field controllers for layout.

    The choice of field software depends on the person and task involved. PC crews use Trimble Access software for all the surveying work and control. Layout and inspection for site grading is done using Trimble SCS900 Site Controller Software and engineers and tradesmen use Trimble Field Link software and total stations for layout inside the shaft. Engineers and tradesmen 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 double-checked against existing conditions and other design elements before going to the field or fabricator. The extensive checking means that errors and costly rework are avoided, while 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, then it’s not going to be laid out in the field,” Torres said.

    Adopting new technologies

    The controller display during layout of anchor bolts. A laser dot marks the location of the bolt. Layout can be checked with independent measurements to the marks.
    The controller display during layout of anchor bolts. A laser dot marks the location of the bolt. Layout can be checked with independent measurements to the marks.

    Torres said that the ability to move information efficiently among the different software and tasks attracted PC to the concepts of modeling, BIM, and integrated field operations. But the company’s effort to integrate BIM with field work encountered some challenges, said Ron Ellison, PC chief field engineer.

    “In all honesty, it was tough at times,” Ellison said. “I think the big thing for us was, ‘Okay, we have this 3D model. How do we get this to the field?’ We had a lot of qualified people who were willing to jump in and learn to use BIM — we were all teaching each other. I think we’re still learning today.”

    New generations of workers are readily adopting the new approaches; Ellison said that even seasoned workers recognize the value of the 3D models in the field. It enables them to visualize the components of the project they are working on and see the relationship to other, yet-to-be-constructed aspects.

    The PC crews combine measurement and modeling throughout the project. Prior to concrete pours, they can verify that forms and rebar are positioned correctly. Post-pour, as-built measurements enable the model to be updated as necessary. Installers and construction workers use the models to install MEP and other components.

    “They don’t need to look at 2D plans and figure out elevations that are in 2D, which doesn’t always make sense to us as humans,” Torres said, adding that misinterpretation of 2D plans is a common source of error. “But with the 3D stuff it’s just amazing,” he said.

    Ellison, who focuses more on machine control, develops site information and constructible models from design documents and sends them to construction surveyors and machines in the field for layout, excavation, and grading. The approach produces significant cost savings.

    “We’re seeing 30 percent savings, maybe more, from what I call the old school 2D way to the modern way,” Ellison said. “There’s more work in the office, but savings in the field more than make up for it. Having a model means it’s set up, checked, and ready to go.”

    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 modeled all the concrete inside the structure, all the anchor bolts, and steel pipe supports as well.”

    The ability to measure and plan in detail delivered big benefits. One example is the installation of a large header pipe that will carry and deliver water from the tunnel to the pumping equipment. Fabricated in sections, the steel header is 13 feet in diameter and runs the width of the shaft. PC rented a special crane to handle the pipe and its supports.

    “We only had the crane for a week and 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 because 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 installation to move smoothly and according to plan.

    Ellison said that’s not an unusual result. “It makes it so much nicer to be able to see something in 3D in the office before it gets to the field,” he said. “That’s our biggest benefit — to catch and eliminate mistakes in the office before they get to the field.”

    Harraka pointed to recent graduates emerging from trade schools and colleges as an enabling force for BIM and virtual construction technologies. “It’s our job to make sure that the people who want to choose that career path can get the education and start working in it. As we go forward, there’s going to be more tech-savvy people coming out of university. This can do nothing but grow.”

    John Stenmark, LS, is a writer and consultant working in the geospatial and associated industries. He has more than 25 years of experience in applying advanced technology to surveying and related disciplines.