SR 154 Bangerter Highway
SR 154 Bangerter Highway
Along the southern boundary of Riverton, UT, on the southern edge of the Salt Lake City metropolitan area, two major commuter routes intersected at grade in the middle of a rapidly-growing bedroom community and its supporting commercial development. The resulting traffic nightmares on both east/west Bangerter Highway (SR 154) and north/south South Redwood Road (SR 68) necessitated replacing this signalized intersection with a grade-separated interchange. Utah DOT (UDOT) required a single point urban interchange, with seismic design requirements of 0.66g for walls within 50 ft. of the abutments and 0.43g for walls elsewhere, and suggested a 250 ft. long bridge taking Bangerter over Redwood. The project, advertised in early 2014 as design-bid, was to be awarded based on price + time scoring and specifically required maintaining all existing traffic lanes and movements.
The winning bidder, Wadsworth Brothers Construction (teamed with Wilson & Company, engineers, and Fehr & Peers, transportation consultants), proposed a 350 ft. long, 3-span continuous bridge due to its relatively shallower depth, resulting in shorter abutments and lower approach fill embankments on this settlement-prone site. To fit the interchange into the neighborhood required retaining walls; Wadsworth Brothers selected Reinforced Earth® mechanically stabilized earth (MSE) retaining walls from The Reinforced Earth Company (RECo). Why Reinforced Earth walls? Because they offer low and even bearing pressure, rapid and simple construction, a variety of architectural options and, of course, an economical price. But the clincher was RECo's high level of service, including the ability to deliver the first wall in just 8 weeks.
The settlement-prone foundation soils significantly influenced geotechnical design of this project. Although the lower-height abutments and approach fills somewhat reduced the expected foundation loading, geotechnical engineer Ryan Cole (of Gerhart Cole Inc., Geotechnical Engineers) wanted to assure long-term embankment stability by forcing additional settlement to occur during the construction process. But a typical way to force settlement, by preloading with or without wick drains, was not possible because there was neither space for the necessary preload embankments nor time to wait for the settlement to occur. So instead of using extra weight to produce more settlement, the designers opted to use lightweight fill to produce less settlement. The lightweight cellular concrete product Cell-Crete, with its ±32 pcf unit weight (only 25-30% the weight of ordinary fill) was selected for the lower portion of the embankments, but these embankments were also the backfill of the Reinforced Earth structures. Therefore, it was left to RECo to design their walls using enough Cell-Crete to achieve the required load reduction, then switch to soil backfill for the upper portions of the walls.
Sufficient weight reduction could be achieved by using cellular concrete from the bottom of the required undercut (which varied in thickness) up to just below the tops of the traffic barriers that would be constructed in front of the walls. By using its Terratrel® (TRL) wire faced wall system (instead of precast panels) behind the barriers, RECo reduced the overall facing cost and gave the contractor a little flexibility in locating the multiple layers of reinforcing strips which would run through the cellular concrete (which was the backfill for the TRL walls). The leveling pad for the precast panel facing would then sit atop the Cell-Crete and only the panel portion of the wall would be visible above the traffic barrier. As expressed by Susan Rafalko, P.E., RECo's Western Division Engineering Manager, "The design for the bottom of the wall was challenging because each wall's foundation elevation had numerous steps, the thickness of cellular concrete necessary for reduced bearing pressure varied with the height of each wall, and we had to be sure the TRL facing was not exposed. We also had to keep in mind that the cold joints in the cellular concrete could not coincide with the reinforcing strip levels. This was not easy given all the foundation steps and the frequently changing elevations of the strip levels along the bottom of the wall."
The process of constructing the cellular concrete portion of the walls had its own challenges. The several layers of reinforcing strips – that would be embedded in the Cell-Crete at different elevations along the length of the walls – had to be connected and held in position in thin air while Cell-Crete was placed and cured. To position these strips, the contractor drove vertical rebar stakes approximately 4 ft. apart parallel to the strips' alignment (Fig. 1). Horizontal crossbars, parallel to the wall face, were tied to the vertical stakes to support the strips in an essentially horizontal position, as if they were lying on the surface of compacted backfill. Lift thickness of the Cell-Crete mixture varied, but every layer of reinforcing strips had to have a minimum of 6 in. of Cell-Crete both above and below, to avoid strips being in or near a cellular concrete cold joint. Normal reinforcing strip lengths of 70% of structure height were used throughout the project, except within 50 ft. of the abutments where the UDOT-specified increased seismic acceleration required a 0.75H strip length to provide increased pullout resistance.
Two walls along the north side of the project were required early in the job as part of the maintenance of traffic plan. Originally conceived by the contractor as two-stage walls (first build a flexible, wire-faced MSE wall, allow it to settle, then add a permanent concrete facing), RECo's ability to use the cellular concrete and build these walls in one continuous process promised to save both time and money. But it was RECo's ability to design and deliver wall materials in only 8 weeks that clinched the deal, allowing Wadsworth to get farther into the project sooner and have more work completed prior to the inevitable slowdowns of winter in Salt Lake City. And drivers benefitted by having traffic continue to flow in the direction to which they were accustomed, without confusing detours and out-of-direction travel.