Press Coverage

august 2019 csengineermag.com 43 Diverging Diamond Interchange A DDI is an innovative design solution that addresses congestion by al- lowing vehicles to travel more quickly through an intersection. Though a relative rarity in the United States, it is gaining popularity because of its efficiency and safety. The first DDI was constructed a decade ago in Springfield, Missouri. Since then, 88 have been built. DDIs are particularly effective at locations that have a high volume of left-turn traffic. In a DDI, traffic is temporarily shifted to the left side of the road to increase traffic flow by allowing through-traffic and intersect- ing traffic turning left to proceed through the intersection simultane- ously. Consequently, this configuration can accommodate 40 percent more left-turning vehicles than a standard diamond interchange with the same number of lanes. Additionally, a DDI enhances safety by reducing potential crash points at intersections. Compared to a conventional diamond interchange, the DDI reduces vehicle-to-vehicle conflict points by nearly 50 percent, according to the Federal Highway Administration. Other DDI benefits include additional green time at traffic signals to allow more vehicles to pass through the intersection, increased safety for pedestrians and bicyclists due to additional sidewalks, and low-cost construction. "Our traffic analysis showed that in the morning peak hours, the predominant movement is east-bound Slaughter Lane to north-bound MoPac," said Short. "In the afternoon rush hour, the predominant movement is south-bound MoPac to east-bound Slaughter Lane. We considered a variety of intersection types for Slaughter Lane, includ- ing a standard diamond intersection, single-point urban intersection, grade-separated roundabout, and the DDI. Ultimately, we chose the DDI because it can handle substantial left-turn volumes." Environmental Protections The MoPAC Intersections project is located over the Barton Springs segment of the Edwards Aquifer Recharge Zone. The Barton Springs segment of the Edwards Aquifer supplies water to between 50,000 and 60,000 people, provides habitat for two endangered salamander spe- cies, discharges at the Barton Springs complex, and is one of the most studied karst aquifer areas in Texas (the word karst describes an area of irregular limestone in which erosion has produced fissures, sinkholes, underground streams and caverns). To protect the Edwards Aquifer, TxDOT and the project team imple- mented rigorous environmental measures that met, or even exceeded, the Texas Commission on Environmental Quality (TCEQ) compliance requirements. The measures included four categories of environmental controls: environmental compliance management, karst void mitiga- tion, temporary construction controls, and permanent post-construction controls. • Environmental Compliance Management – TxDOT hired an independent environmental compliance manager to monitor construction and ensure that Best Management Practices (BMPs) for environmental protection are implemented and function as designed. This is in addition to construction management and environmental compliance personnel provided by the contractor. This level of environmental compliance oversight is atypical for TxDOT projects and exceeds TCEQ requirements. • Karst Void Mitigation – Construction plans and specifications provide con- struction details and notes directing the contractor on how to proceed if a karst feature is encountered. In addition to specific construction details, the contractor is directed to notify the TxDOT project manager, environmental compliance manager, TCEQ and the City of Austin. • Temporary Construction Controls – The project incorporates erosion and sedimentation controls such as silt fences for perimeter control, rock berms for concentrated flow and construction exits to control sediment onto roadways. • Permanent Water Quality Controls – The project includes a suite of storm water quality measures that, in combination, will exceed the 80 percent Total Suspended Solids (TSS) removal requirement of the TCEQ Edwards Aquifer Protection Program rules. These measures include vegetated filter strips, permeable friction course, and retrofit of the existing vertical sand filters. In addition, the project includes seven batch detention ponds functioning primarily as hazardous material traps, but also providing extended detention to remove the Total Suspended Solids. Accelerated Construction Due to the high traffic volumes in the two intersections, TxDOT wanted to minimize the impact to the traveling public during construction. To this end, the project team employed a number of strategies to expedite construction. One such strategy was implementing Accelerated Bridge Construction (ABC) techniques such as precast bent/abutment caps and post-installed column aesthetic features. "On conventional bridges, you excavate to the top of the foundation, which in many cases are the drilled shafts," said Mario Rosino, Web- ber's project manager. "Then you build the columns up, build the cap in place, set beams and then pour the deck. On Slaughter Lane, we built the bridges essentially at-grade. We built the drilled shafts and the columns through the existing pavement under nightly lane closures. Then we pushed traffic into a new detour section and excavated down to sub-grade, exposing the drilled shafts and then set the pre-cast caps in place." By using pre-cast elements, Webber was able to find savings in con- crete cure time, thereby reducing the total schedule. After completing the bridge, the contractor dressed-up the columns with Class K con- crete so they had a smooth finish with the appropriate form liner. Also, by using a new detour section, Webber was able to build the bridges in Slaughter Lane in one phase instead of two, which accelerated the construction process. To expedite the construction, TxDOT has also employed a disincentive/incentive strategy. Photo: LAN

• Cover