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TR N EW S 288 SEPTEM BERâO CTO BER 2013 41 The author is Senior Acoustic Engineer in the Raleigh, North Carolina, office of RK&K Engineers. When the North Carolina Department ofTransportation (DOT) widened a 6.2-mile corridor of I-40 west and southwest of Raleigh in 2009 to 2011, the designâbuild team1 recognized several design changes that could improve the acoustical performance, aesthetic appeal, and context sensitivity of the noise wall near Jones Franklin Road. Although the wallâs initial design complied with the stateâs 2004 traffic noise abatement policy, North Carolina DOT was amenable to the proposed changes. The western section of the initial noise wall design was approximately 40 feet beyond the top of the slope of the roadway section cut. The designâbuild team saw an opportunity to reduce noise levels further without increasing the wall area, by shifting the horizontal alignment of the noise wall toward the highway; North Carolina DOT approved the request. The initially recommended design for the noise wall was based on a Federal Highway Administration Traffic Noise Model (TNM) that set the barrier seg- ments to nominal heights; in this case, however, the irregular elevations of the horizontal alignment ground line would have created a jagged, acoustically inefficient top-of-wall profile, as depicted in Figure 1 (above right). The reevaluation revised the heights of the modeled barrier segment to set the tops of the panels to whole-foot elevations, as depicted in Fig- ure 2 (above right). The new design not only met the requirements for acoustical performance but also achieved a more aesthetically appealing profile. The plans for the initial project called for the full clearing of vegetation between the edge of the pave- ment and the right-of-way in the vicinity of the noise wall. This would have fully exposed shaded residen- tial backyards to the sun. The revised design required clearing only the areas necessary for access by con- struction personnel and equipment for the con- struction of the noise wall; as a result, the shade cover was preserved to the greatest degree, as seen in the photograph below. The initial traffic noise analysis did not investigate whether the maintenance gap was technically war- ranted. North Carolina DOT Division 5 Maintenance and the Traffic Noise and Air Quality Group, along with the designâbuild team, conducted a thorough field inspection. Although the findings showed that a maintenance gap was warranted for the I-40âJones Franklin Road noise wall, the field investigation influenced North Carolina DOT to change its prac- ticeâthe agency no longer includes maintenance gaps in all lateral shifts of noise wall horizontal align- ments unless the gaps are functionally necessary. The improvements to the I-40âJones Franklin Road noise wall, depicted in the photograph at left, confirmed the viability of many technical aspects of the 2011 North Carolina DOT traffic noise abate- ment policy and of the Traffic Noise Analysis and Abatement Manual. The 2011 policy revisions have improved confidence in noise impact determinations and abatement performance, and the public is better served with the implementation of noise abatement measures that are more effective and aesthetically appealing. Effective Noise Barriers Case Study from North Carolina J O E R A U S E O 1S. T. Wooten Corporation,Wilson, North Carolina, and RK&K Engineers. Birdâs-eye view of I-40â Jones Franklin Road noise wall. I-40âJones Franklin Road noise wall: elevation view at maintenance gap. FIGURE 1 I-40âJones Franklin Road noise wall with an initial jagged, acoustically inefficient top-of-wall profile. FIGURE 2 Revised Traffic Noise Model profile of I-40âJones Franklin Road noise wall with whole-foot top-of-wall elevations.