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agents that are injected through the shield. At the end of the shield, about 30 to 50 feet behind the cutting wheel, precast concrete segmental rings are assembled and extruded behind the machine. The ring passes through brush seals, and annular grout is injected between the concrete rings and the ground. The wire brush seals prevent ground, grout, and water from entering the shield in the gap between the segments and shield. The annular grout, typically a two-part grout, gels quickly after injection and prevents movement of the segments and surrounding ground. The precast concrete segmental rings are typically 4 to 6 feet wide, and 5 to 7 segments make up a complete ring. During assembly, the ring segments are bolted together for temporary support and are connected to the previous complete ring with locking dowels. Each ring is tapered such that, depending on the rotation of a set of rings, any vertical or horizontal curve greater than about 800-foot radius can be navigated. Ethylene propylene diene monomer (EPDM) gaskets are used between the segments to prevent water ingress at the joints between segments and between rings. The combination of high-quality, high-strength, precast concrete segments and durable gaskets allows the tunnel to achieve a 100-year design life and a leakage rate less than 5 gpm per mile of completed tunnel. Within the tunnel, the work environment is generally clean and dry. Water and ground ingress into the tunnel is limited and spoils removed through the auger at the face are directly conveyed into either muck cars on rails or a continuous conveyor belt that runs back to the mining shaft. Materials and precast concrete segments are delivered by locomotive along rails that are assembled behind the machine as it advances. The initial 500 to 1,000 feet of tunnel is considered part of the learning curve, and advance rates are typically reduced due to equipment assembly, troubleshooting, and downtime to install additional support equipment. After the learning curve phase, the TBM can consistently advance 50 to 100 feet a day, with scheduled downtime during the graveyard shift and on weekends to perform maintenance. Toward the end of her run, Lady Bird had a personal best 150-feet-per- day production rate. At the surface, the ground movements have been imperceptible, less than 1/10th of 1 inch on average, as measured by geotechnical instrumentation placed along the alignment. Tunnel Safety Safety is an important consideration in any construction project. Safety in a tunnel environment has many different elements from above- ground construction: a long supply line, congested work area, multiple activities at the face, and special equipment. The job safety analysis must include an evaluation of tunnel access and egress, flood control, ventilation and air quality, illumination, and fire/explosion prevention. Site-specific incident response plans are also prepared in conjunction with local emergency responders, which include periodic tunnel rescue training. Conclusion DC Water's TBMs have been instrumental in meeting project goals and the strict milestones associated with DC Water's consent decree. Without this technology, the project would not be possible. These high-production machines have been quietly constructing a new 21st century sewer system in the District with little to no disturbance of the ground surface. Once completed, the tunnel system will reduce CSO volume to the Anacostia River in an average year of rainfall by 98 percent. n 22 wateronline.com n Water Innovations CONVEYANCESYSTEMS Carlton Ray is the director for DC Water's Combined Sewer Overflow Long Term Control Plan called the DC Clean Rivers Project, respon- sible for implementing DC Water's 25-year, $2.7 billion federally mandated consent decree to control combined sewer overflows (CSOs). Previously, he worked at the Department of Public Works in Indianapolis, IN, as chief engineer, deputy director, and CSO program lead. Ray has more than 25 years of engineering and leadership experience in both public and private civil engineering sectors, and holds a Bachelor of Science degree in Civil Engineering from Auburn University. About The Author Figure 4. The segment handler of Lady Bird easily manipulates the 11-foot- long, 6-foot-wide, 6-ton segment prior to ring assembly. Figure 3. Tunnel boring machine cutting wheel (left) and shield components