|Jensen Construction is using these bent footing forms as temporary cofferdams. The footings on this project have replaced the sheet piling of conventional cofferdams with a concrete footing cast above the surface. Photo courtesy of Jensen Construction.|
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 due to open 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, IA-based Jensen Construction Co. 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 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 will have a tied arch span that will support the bridge deck with cable hangers. This segment will also feature 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, will give the bridge 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 will resemble lighthouses and shine light to the north and south of the bridge.
The majority of the spans are designed to utilize prestressed concrete beams, which have a typical length of 120 feet. Bexar Concrete, San Antonio, is precasting the beams, deck panels and skirt panels, and trucking them to the job site. At a dock in Lake Dallas on the west side of the lake, the precast elements are unloaded onto barges, shipped and erected.
The timeframe on Jensen's contract is only 30 months, meaning productivity is king. Ryan Cheeseman, P.E., the project engineer for Jensen Construction, fully recognizes that time is money on this project. “It's the most work in the least amount of time that we've done,” Cheeseman notes.
As a result of the tight schedule, Jensen Construction is using equipment and practices that increase construction efficiency as much as possible while maintaining adherence to design tolerances. Special Light Bent and Arch Bent footing forms double as temporary cofferdams. Global Navigation Satellite System (GNSS) receivers survey most of the bridge substructure as well as the 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 2007, when an 8.34-inch rainfall total was recorded at Dallas/Fort Worth International Airport, followed by more than 11 inches of rainfall in June 2007.
|The Lewisville Lake Bridge was designed by Turner & Braden, Inc. The two "Arch Bents" supporting the center span, combined with an arched steel truss structure, will give the bridge a distinctive architectural appearance as the Arch Betns themselves resemble sails. Inset Photo by Don Talend.|
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. The footings on this project have replaced the sheet piling of conventional cofferdams 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 notes. “With this type of footing, we're able to complete that whole footing in about a week and half, which keeps things moving really quickly. It's just like a temporary cofferdam using the formwork of the footing as the cofferdam.”
Drill shaft casings – which are 60, 72, 84, or 96 inches in diameter – are driven into the lake bed by ATS Drilling, Fort Worth, Texas. A 1-foot-thick footing bottom slab is cast on a barge, and the footing forms are 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 feet 6 inches. Each bent requires five concrete placements prior to construction of the bent caps. A vertical section facing the center of the lake is cast. Then a sloped section facing the shoreline is formed and cast. At the top of these two sections, a slab is cast that forms the floor of utility rooms. Another vertical section that forms the utility room walls is cast on top of the slab, and the fifth placement is the roof of the utility rooms. The caps are then constructed on top of the bents and support the beam seats. All columns and caps on the project are mass concrete placements and require temperature-controlled concrete. The concrete supplier, Dallas-based TXI, is using 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 are also being used to check core temperatures vs. external temperatures and safeguard against the potential for structural cracking.
Jensen Construction is using Topcon HiPer Lite+ GNSS receivers to survey the bridge substructure all the way up to the beam seats. Cheeseman points out that GNSS is being used where possible to address productivity and logistical issues. A Topcon GTS-235W total station is used for profiling each of the girders for setting the decking, he notes, and on the superstructure, the total station is used for deck and paving grades. In these areas, he explains, maximum pinpoint accuracy is essential. Still, the GNSS equipment is normally accurate to within roughly 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. 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 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 says. 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-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 just kind of 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. 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 says.
The total station maintains its place where ultra-pinpoint accuracy is 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 can spend more time surveying from a wider range of locations without devoting as much time to equipment setup. Signal reliability has not been 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 learning curve on the GNSS equipment was short, the surveyors say. Terry Gammill, sales manager at Griner & Schmitz, trained Jensen Construction's surveying crew on the equipment for a few days following delivery.
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 notes that she checks two control points every morning to ensure an accurate references.
The leap in productivity from using the GNSS equipment is 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, she says. 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,” she says. “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.”
When completed, the structure is expected to handle 25,000 cars per day and drastically reduce commute times for many. Drivers with electronic collection-capable Toll Tags will pay $1 and others will pay $1.25. Undoubtedly, many drivers will gladly pay tolls in exchange for less “windshield time.” For example, the NTTA estimates 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 is joining drivers on a fast track.
|Don Talend of Write Results, West Dundee, IL, is a publicity and communications project manager specializing in the construction industry.|