Bridge to Time Savings

Innovative construction techniques and high-tech surveying equipment kept a project to build a bridge near Dallas, Texas on a fast track. Now complete, it drastically cuts driving time.

By Don Talend

Lewisville Lake, a 23,280-acre lake just northwest of Dallas, Texas, is a favorite of area sailboaters and fishermen, but in recent years, it hasn’t done much for drivers. Two major north-south arterials that stretch north of Dallas, I-35E and the Dallas North Tollway, straddle the lake and, previously, no east-west connecting route existed between the two. Circumventing the lake to get from one arterial to the other took drivers half an hour or more.
Part of the solution to this problem is the 2.03-mile-long Lewisville Lake Toll Bridge, which opened to vehicular traffic in August. Combine a high-profile bridge project, a fast-track schedule, and a big lake, and the contractor needs the most high-tech tools it can find to survey the structure with pinpoint accuracy amid strong wave action.

The North Texas Tollway Authority (NTTA) awarded Des Moines, Iowa-based Jensen Construction Company a $93 million contract to erect a bridge over the lake. The company began work on a 1,000-foot-long flow-easement bridge on the west side of the lake in late 2006 and then started constructing the lake bridge in February 2007. This contract forms the centerpiece of roughly $220 million in congestion-easing road improvements to be made to a surrounding 13.7-mile corridor.

The center of the bridge has a tied arch span that supports the bridge deck with cable hangers. This segment also features a 370-foot-long center span with the bents, i.e., piers, spaced to allow plenty of room for boat traffic to pass under the bridge. Two arch bents supporting the center span, combined with an arched steel truss structure, give the structure a distinctive architectural appearance, as the arch bents themselves resemble sails. Adding to the nautical appearance of the structure will be four more pairs of light bents, which resemble lighthouses and shine light to the north and south of the bridge.

The majority of the spans are designed to use prestressed concrete beams, which have a typical length of 120 feet. Bexar Concrete, San Antonio, precasted the beams, deck panels, and skirt panels and trucked them to the jobsite. At a dock in Lake Dallas on the west side of the lake, the precast elements were unloaded onto barges, shipped, and erected.

The timeframe on Jensen Construction’s contract was only 30 months, meaning productivity was king. Ryan Cheeseman, P.E., project engineer for Jensen, fully recognized that time is money on this project. “It’s the most work in the least amount of time we’ve done,” he notes. As a result of the tight schedule, Jensen Construction used equipment and practices that increased construction efficiency as much as possible while adhering to design tolerances. Two examples are the use of special light bent and arch bent footing forms that also work as temporary cofferdams and GNSS (Global Navigation Satellite System) receivers for surveying most of the bridge substructure and superstructure.

Forms Double as Cofferdams
The most unique design and construction aspects of the Lewisville Lake Toll Bridge are the arch bents, light bents, and the footings supporting these bridge bents. These bent footing forms are also temporary cofferdams. While conventional cofferdams are constructed by driving sheetpiling into the bed of a body of water, building a seal around the base of the sheetpiling and pumping out the enclosure, the footings on this project replaced the sheet piling with a concrete footing cast above the surface. “Conventional cofferdams are very tedious and time-consuming, and they cost a lot of money,” Cheeseman explains. “With this type of footing, we could complete that whole footing in about a week and half, which kept things moving really quickly. It’s just like a temporary cofferdam using the formwork of the footing as the cofferdam.”

Drill shaft casings—60, 72, 84, or 96 inches in diameter—were driven into the lake bed by ATS Drilling, Fort Worth, Texas. A one-foot-thick footing bottom slab was cast on a barge, and the footing forms were set on the bottom slab. The forms and slab are then set on top of the drilled shafts and supported by steel hangers welded to the drilled shaft casing. Workers pump out water, install the rebar, and place concrete for the footing. Divers strip the footing, and skirt panels are hung on the sides of the footing. Finally, a footing cap is placed to get the footing to grade.

The arch bents are hollow and have a thickness of 2.5 feet. Each bent required five concrete placements prior to construction of the bent caps. A vertical section facing the center of the lake was cast. Then a sloped section facing the shoreline was formed and cast. At the top of these two sections, a slab was cast that forms the floor of utility rooms. Another vertical section that forms the utility room walls was cast on top of the slab, and the fifth placement was the roof of the utility rooms. The caps were then constructed on top of the bents and support the beam seats. All columns and caps on the project were mass concrete placements and required temperature-controlled concrete. The concrete supplier, Dallas-based TXI, used liquid nitrogen in the batching concrete to reduce the heat of hydration in the cement paste, one of the most extreme measures available for reducing concrete temperature in massive concrete structures. Temperature-monitoring devices were also used to check core temperatures versus external temperatures and safeguard against structural cracking.

Surveying Tools Improve Productivity
Jensen Construction used Topcon HiPer Lite+ GNSS receivers to survey the bridge substructure all the way up to the beam seats. Cheeseman points out that GNSS was used where possible to address productivity and logistical issues. A Topcon GTS-235W total station was used for profiling each of the girders for setting the decking, and on the superstructure, the total station was used for deck and paving grades. In these areas, he explains, pinpoint accuracy was essential. The GNSS equipment was normally accurate to within about five-eighths of an inch of target on a typical day, Cheeseman reports.

In recent years, surveyors have begun to rely on GNSS equipment for more and more topographical surveying work once control is defined on a worksite, with a stationary base and mobile rover working together to provide accurate topographical data. Recently, these systems have become even more reliable and accurate as they have added compatibility with the Russian GLONASS satellite constellation to the GPS satellite constellation. This dual-constellation capability roughly doubles the number of signals available to the GNSS antenna/receivers and provides a high degree of positioning accuracy.

Working on water with strong winds and currents makes the use of GNSS surveying equipment a beneficial option where feasible, Cheeseman says. A professional surveying firm was first brought in to define control, and as the first footings and bents were constructed, Cheeseman and Jensen’s surveying team had several “crow’s nests” constructed along the shoreline. These used 24-inch pipe pile-driven into the lakebed and small iron work platforms welded to the top of the pipe. But the wave action on the lake caused slight movement of the crow’s nests and compromised surveying accuracy.

“We used the crow’s nests just enough to get the control traversed from one side to the other and got coordinates defined, and from that point, we abandoned them because they weren’t doing us any good,” says Cheeseman. “They moved so much with the wave action that we couldn’t set up an instrument and be confident that every day we were going to repeat our locations.”

Cheeseman, along with Jensen Construction surveyors Laine Buller and Marcus Marion, had already spearheaded efforts to start incorporating the use of GNSS surveying equipment in the company’s bridge work. Buller, who has been a surveyor for about 10 years, joined Jensen Construction for her second stint at the start of the Lewisville Lake bridge project. Before work began on the project, Jensen Construction purchased the HiPer Lite+ unit from Griner & Schmitz, a distributor of surveying and construction equipment in Kansas City. “From a productivity and constructability standpoint, we went to the GNSS knowing we could get to within a tenth of a foot or better every day, so we just ran with it,” Cheeseman reveals.

The total station maintained its place where ultra-pinpoint accuracy was necessary on this project, but the location of the GNSS receiver is less dependent on a level, stable surface than the total station, so Jensen Construction’s surveying crew could spend more time surveying from a wider range of locations without devoting as much time to equipment setup. Signal reliability was not much of an issue on this project, Cheeseman adds. Noting that the receiver gets signals from the base station located on high ground all the way to the other side of the lake—a distance of about 10,000 feet—he points out that signal loss is rare.

The technology was admittedly a bit intimidating at first in that the crew double-checked the accuracy of readings with the total station. As the total station verified the accuracy of the GNSS equipment, the confidence grew. Buller notes that she checks two control points every morning to ensure an accurate reference. “We go out on the lake and then when we come back, we check the point as we get on land every time just to make sure it didn’t get switched around,” she adds.

The leap in productivity gained from using the GNSS equipment proved noticeably significant, Buller says. “I think it would have taken two other surveyors” to maintain the level of productivity that Jensen Construction enjoyed without the use of the equipment. “We love this GNSS because you carry it out there, and there’s no setting it up and going back to shoot your backsight. It’s excellent, especially on this water.”

Buller notes that time truly is money on a project with a fast-track schedule such as this. “It cuts your survey cost in half. You’re always looking for places to cut costs. You have your initial cost of buying the GNSS equipment, but your labor is cut in half after that.”

Now complete, the structure can handle 25,000 cars a day and drastically reduces commute times for many. The NTTA estimates that the bridge reduces driving time from Lake Dallas, the location of the overflow bridge on the lake’s west side, to Little Elm on the east side from 45 to 10 minutes. Thanks to innovation and technology, the Lewisville Lake Toll Bridge itself has joined drivers on a fast track.

Don Talend of Write Results in West Dundee, IL is a publicity and communications project manager specializing in the construction industry.

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