Next Stop: A Virtual Jobsite

February 29, 2012
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Surveyors combine BIM and imaging total stations to streamline underground construction of a university rail system.



The Sound Transit station at the University of Washington (UW) in Seattle is part of Sound Transit’s 3.15-mile light rail University Link (U-Link) extension project. Located adjacent to Husky Stadium, the underground station will provide regional access to the UW campus and UW Medical Center, nearby sports venues, and surrounding neighborhoods via twin-bored tunnels to Capital Hill in downtown Seattle.

Early U-Link construction activities began in February 2009 with the relocation of utilities and new concession stands near Husky Stadium, including new storm drains under the parking lot south of the stadium, new water, power and communications lines in the landscaped area southwest of the stadium, and demolition of two small buildings housing the ticket office, concession and restrooms.

Crews excavated to the bottom of the station (approximately 120 feet underground) and assembled two tunnel-boring machines to begin digging the tunnel. After the tunnel boring machines got underway, the Sound Transit Capital Committee selected Hoffman Construction as the general contractor/construction manager to build the station.

Construction includes completion of the station box excavation, floors, stairs, mezzanines, elevators and the above-ground entrance to the station as well as a pedestrian bridge from the station over Montlake Boulevard Northeast to the University of Washington campus. Technology is playing a crucial role in keeping the project on track.


An aerial rendering of the University of Washington Station.

Prior to excavation and construction of the station, Hoffman Structures Inc. (HSI), a subsidiary of Hoffman Construction and the concrete subcontractor, developed a building information model (BIM) of the structural system and proposed station in Autodesk Revit. HSI developed the model from preliminary as-built data gathered from the site with the idea that the model would be used to streamline field activities. The MEP subcontractor provided a 3D model of the mechanical, electrical and plumbing systems, which was added to the structural model.

Soon after, Hoffman Construction brought in Ming Surveyors, a Hoffman subsidiary, to perform the survey activities. Ming used the HSI-developed as-built-conditions site model combined with the MEP model to verify site details and lay out points off specified gridlines. “The coordination facilitated by the model environment puts a lot more eyes on the layout and other aspects of the project prior to construction,” says Daniel Ellis of Ming Surveyors. “It allows us to construct the station virtually. I like to use the model to lay out complex entities and locate beam and whaler positions off a defined grid.”

Traditional methods would have required the team to calculate thousands or even tens of thousands of points in the field to mark everything from bolts to corner radii. Technology has streamlined this process substantially. “I still have to spend time verifying accuracy of points in the model,” Ellis says. “However, once that information is confirmed, moving into the field is greatly simplified.”

Within the 3D model, Ellis selects the positions and downloads them to a Leica TS15 imaging total station, a 1-second-accuracy robotic total station that incorporates a 1,000 meter range reflectorless EDM and SmartWorx Viva onboard software with a full VGA color touch screen. Throughout construction, a Ming surveyor is on site with the TS15 total station, checking positions and elevations, among other activities.

A cross section of the University of Washington Station. Image courtesy of LMN Architects.

Construction crews work in a top-down approach for the station. Initially, crews pour the roof structure, dig underneath for the first floor, and pour the beams and whalers. The surveyors’ job throughout construction is multi-fold. They must monitor the grid and grade of the station site continuously, working closely with the subcontractors as the crews pour the rat slab on dirt without forms or steel rebar. Once that is complete, crews set beams and whalers and pour another slab on top. Then crews excavate the dirt underneath the rat slab and remove the rat slab to give the ceiling a clean smooth finish.

Per the contract, site monitoring must be completed to less than .0625 inch, while the concrete slabs must be within .5 inch--an easy target for the Ming crew. “We’re getting far better accuracy than .5 inch, primarily because of the work put into the modeling process, which was in a controlled environment,” Ellis says. "We worked out the kinks well in advance of moving into the field. There is still human error, the fact that things move during concrete pours, and potential for changes. However, we eliminated potential clashes and streamlined the construction sequence as much as possible.”

Beyond grid and grade monitoring, Ming performs a number of other activities. Working directly with the trade specialty contractors, the survey team lays out all beams, whalers, embeds and elevations during rat slab pours for finishers, chamfer and similar activities to help speed construction and drive accuracy.

For instance, during rat slab pours, Ellis’ team uses the reflectorless tracking capabilities of the TS15 to measure small pads as the finishers are pouring concrete. The rat slabs must meet strict smoothness requirements since they act as the station’s beams. The onsite tracking allows the finishers to make adjustments on the spot. “The elevation of that rat slab is critical, so we’re shooting as the concrete crews pour,” Ellis says. “With the BIM/total station system, we have been able to deliver far more accurate results faster than using conventional methods.”

Ming also makes use of the TS15’s surface scan feature. For instance, surveyors recently used it to help the excavation contractors calculate soil removal progress. At the contractor’s request, the survey crew would measure points within the excavated area using the TS15, draw a polyline and scan the surface. With the surface model, the excavation contractor could perform quantity takeoffs to determine how much material still needed to be removed and schedule the appropriate trucks for hauling.


A model of one of the basement level pours, created in Revit.

Ming also used the surface scan feature to locate existing slabs and walls prior to a beam pour to ensure proper tolerances between the escalators and other features in the station.

“The TS15 has an easy learning curve, and the R1000 EDM is phenomenal for tight control and easy setup,” Ellis says. “It is tools like this that are changing the survey profession for the better by driving accuracy and speed. Better yet, capabilities such as the surface scanning allow us to deliver more services on the day-to-day activities of a project to help keep it on track and on budget.”

Ellis says he would like to incorporate the imaging functionality for image assisted surveying and documentation that will be available for the TS15 in the future. The dedicated image processing board precisely calculates the exact exposure time and white balance settings for capturing optimal camera images of the total station live view. Image assisted surveying will allow Ming to use the reflectorless option from the field controller side as well as store images of every measurement captured by the instrument. This level of documentation can prove critical for survey crews when they have work around obstructions at a later date and is also an invaluable resource for as-built information.

For now, Ellis and the rest of the University of Washington Sound Transit Station survey team are making the most of the tools and techniques that are available to them. Construction of the UW Station is about 70 percent complete and is expected to be finished in 2014.

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