Bridges & culverts
The challenge of a lifetime
In the shadow of last century's greatest civil projects, designers and engineers overcame unique challenges to forge the new Hoover Dam Bypass out of the river and rock in one of the nation's most unforgiving climates. Here's how they did it.
Engineers and builders relied on nothing but their expertise in designing a 1,060-foot bridge spanning the Colorado River near a national landmark.
The Hoover Dam Bypass Project is a 3.5-mile corridor that begins at milepost 2.2 in Clark County, Nev., and terminates in Mohave County, Ariz., near milepost 1.7. A massive undertaking, the project has proven to be a once-in-a-lifetime endeavor for all involved. The bridge, officially named the Mike O'Callaghan-Pat Tillman Memorial Bridge, has been nearly 15 years in the making.
Although it will be completed on time and on budget, it hasn't been without its challenges.
Once engineers moved past the first hurdle - how to frame the main bridge design and approaches on either side of a rugged canyon - they needed to address the articulation of the bridge itself.
Among the concerns were the framing at the crown of the arch, the high rise of the arch ribs, the use of composite deck construction, and the logistics of form-traveler construction, all of which led to the use of an open spandrel crown as opposed to an integral crown.
Both elastic and long-term deflections for a 1,060-foot-long arch are considerable and had to be accommodated in design of both the spandrel columns and the integral deck system.
This led to the need for bearings on the shorter piers but allowed fully integral deck connections on taller piers. The extreme height of the taller spandrel columns - approximately 290 feet of precast segmental columns - required that either bracing be provided to the columns, or that an integral deck system be engaged to provide system-wide bracing. The latter had clear advantages and was designed to provide lateral bracing from both the shorter piers to the taller piers, and from the rigid abutments using the deck as a diaphragm.
Facing staggering financial and schedule consequences if just one load of concrete mix failed to achieve strength, the Obayashi/PSM JV construction team's quality control efforts exceeded industry standards.
The team learned from the footing construction that traditional ready-mix batching wouldn't meet the quality requirements of the arch concrete. Too many variables, such as the batching efficiency of a truck's mixing drum and the drive time, could go awry. The team decided to set up a portable batch plant incorporating a 5-cubic yard pan mixer on site.
Pan mixers use high-speed paddles to pre-mix the concrete before discharging into the truck. They're usually used in precast yards but were appropriate for this application because quality was more important than volume per hour. The plant's proximity also made it extremely easy to adjust flow rates during a pour.
The team had two options for concrete delivery: a pumping system or cableway.
Delivery by bucket to the point of placement, the same methodology used to build the Hoover Dam, was rejected. Buckets would tie up a critical resource for several hours nearly every day. Nearly every operation on the project during arch construction was dependent upon cableway hook time. Every minute the cableway was tied up with an operation was a minute that another operation was being affected.
The team decided on a pumping system but had to work around the large aggregate size of the mix, the means to place in the restricted pour windows because of high temperatures, and delivering concrete to the pump.
Trailer pumps specially modified by Putzmeister to handle the aggregates were selected. Delivery to the pump was easy on the Nevada side of the gorge: The pump could be set up on the roadside near the arches, and the concrete could easily be delivered by truck.
The steep cliffs on the Arizona side were another story. The trailer pump was set up on the base of the arch in conjunction with a 5-cubic-yard remixer. Concrete was discharged from the delivery truck into 8-cubic-yard concrete buckets supported by the cableway and lowered to the remixer where the buckets were discharged. The buckets could be rehoisted nearly immediately to receive the next load of concrete.
From the trailer pump the concrete was pumped up the arch through a 5-inch heavy wall slickline up to 600 feet horizontally and 250 feet vertically to a 32-meter placing boom mounted atop the arch near the form traveler. A typical arch segment pour took 4 to 5 hours.
Consolidation of the concrete in the forms was a major concern. The geometry of the arch (many segments were poured at 45-degree angles) required the use of top-surface forms for all pours. Pour windows were established in the forms not only for placement, but also to allow use of high-cycle concrete vibrators. In addition, external vibrators were mounted under the bottom soffit form and along the side forms to ensure good consolidation. It worked. The team found few honeycombs when the forms were removed.