Lewisville Lake, a 23,280-acre lake located just northwest of Dallas, 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 currently no east-west connecting route exists between the two. Circumventing the lake to get from one arterial to the other takes drivers half an hour or more.
Part of the solution to this problem will be a 2.03-mile-long toll bridge which opened to vehicular traffic in August 2009. 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 Co. a $93 million contract to erect a bridge over the lake. The company began work on a 1,000-ft-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 is 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-ft-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 are 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 were designed to utilize prestressed concrete beams, which have a typical length of 120 ft. Bexar Concrete, San Antonio, precast 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 time frame on Jensen’s contract was only 30 months, meaning productivity was king. Ryan Cheeseman, P.E., the project engineer for Jensen Construction, fully recognized that time was money on this project. “It’s the most work in the least amount of time that we’ve done,” Cheeseman noted.
As a result of the tight schedule, Jensen Construction used equipment and practices that increase construction efficiency as much as possible while maintaining adherence to design tolerances. Two examples were the use of special light-bent and arch-bent footing forms that also worked as temporary cofferdams, and Global Navigation Satellite System (GNSS) receivers for surveying most of the bridge substructure and superstructure.
The use of these items had gone a long way toward keeping the project on schedule as of just before Memorial Day 2008, when the building team reached the halfway point. The bridge had remained on schedule to that point despite challenges such as an unusually wet May in 2007 in which an 8.34-in. rainfall total was recorded at Dallas/Fort Worth International Airport. The area saw even more rainfall in June 2007: more than 11 in.
Bent on footings
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 also are temporary cofferdams. While conventional cofferdams are constructed by driving sheet piling into the bed of a body of water, building a seal around the base of the sheet piling and pumping out the enclosure, the footings on this project have replaced the sheet piling with a concrete footing cast above the surface.
“Conventional cofferdams are very, very tedious and time consuming and they cost a lot of money,” Cheeseman noted. “With this type of footing, we were able to complete that whole footing in about a week and a half, which kept things moving really quickly. It’s just like a temporary cofferdam using the formwork of the footing as the cofferdam.”
Drilled-shaft casings—which are 60, 72, 84 or 96 in. in diameter—were driven into the lake bed by ATS Drilling, Fort Worth, Texas. A 1-ft-thick footing bottom slab was cast on a barge and the footing forms were set on the bottom slab. The forms and slab were then set on top of the drilled shafts and supported by steel hangers welded to the drilled-shaft casing. Workers pumped out water, installed the rebar and placed concrete for the footing. Divers stripped the footing, and skirt panels were hung on the sides of the footing. Finally, a footing cap was placed to get the footing to grade.
The arch bents are hollow and have a thickness of 2 ft 6 in. 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 formed 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 supported 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 being used to check core temperatures vs. external temperatures and safeguard against the potential for structural cracking.
Teaming Russia with U.S.
Jensen Construction used Topcon HiPer Lite+ GNSS receivers to survey the bridge substructure all the way up to the beam seats. Cheeseman pointed 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, he noted, and on the superstructure, the total station was used for deck and paving grades. In these areas, he explained, maximum pinpoint accuracy was essential. Still, the GNSS equipment is normally accurate to within roughly five-eighths of an inch of target on a typical day.
In recent years, surveyors have begun to rely on GNSS equipment for more and more topographical surveying work once control is defined on a work site. These systems use a “rover”—a rugged GNSS receiver/antenna that the surveyor moves from one location to another—and a base station, the latter of which is located at a known stationary point on the site. Satellites send positioning data to the base station and to the rover. The stationary base and mobile rover work 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 as well as the U.S. Global Positioning System 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 does make the use of GNSS surveying equipment a beneficial option where feasible, Cheeseman said. A professional surveying firm was first brought in to define control, and as the first footings and bents were being constructed, Cheeseman and Jensen’s surveying team had several “crow’s nests” constructed along the shoreline. These used 24-in. pipe pile-driven into the lake bed 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 just kind of abandoned them because they weren’t doing us any good,” said 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 into the company’s bridge work. Before work began on the Lewisville bridge 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 said.
The total station maintained its place where ultra-pinpoint accuracy was necessary on this project, but the location of the GNSS receiver was 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 added. Noting that the receiver got 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 ft—he pointed out that signal loss was rare.
The learning curve on the GNSS equipment was short, the surveyors said. Terry Gammill, sales manager at Griner & Schmitz, trained Jensen Construction’s surveying crew on the equipment for a few days following delivery.
“We had a few issues a couple of times and Terry has dealt with another one of our surveyor engineers and was extremely helpful,” said Buller, who has been a surveyor for about 10 years and joined Jensen Construction for her second stint at the start of the Lewisville Lake Bridge project. “He could talk us through how to fix it over the phone and that helped a whole bunch in the beginning.”
The technology was admittedly a bit intimidating at first in that the crew double-checked the accuracy of the readings with the total station. As the total station verified the accuracy of the GNSS equipment, the confidence grew. Buller noted that she checked two control points every morning to ensure accurate references.
“We would go out on the lake and then when we came back, we checked the point as we got on land every time just to make sure it didn’t get switched around,” she added.
The leap in productivity from using the GNSS equipment was noticeably significant, Buller said. “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, she said. “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.”
25,000 cars a day
The structure is expected to handle 25,000 cars a day and drastically reduce commute times for many. Drivers with electronic-collection-capable toll tags pay $1, and others pay $1.25. Undoubtedly, many drivers will gladly pay tolls in exchange for less “windshield time.” For example, the NTTA estimated that the bridge will reduce the 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 is joining drivers on a fast track.