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mitigating the frequency, magnitude, and duration of sewer flooding and basement backups, and thus address chronic sewer flooding in one of the District's largest drainage areas that have been plagued by flooding since the early 1900s. How Do The Tunnel Boring Machines Work? The TBMs provide an effective method of constructing tunnels in soft ground by actively supporting the ground during excavation, providing continuous ground support, and constructing behind itself a watertight final lining with a 100-year design life in a single pass. The TBMs excavate the dense sands and hard clays encountered along the tunnel alignment with a cutterhead or cutting wheel that resembles a pinwheel. The arms of the wheel are lined with ripping teeth and, depending on the ground conditions, the faces of the arms can include disc cutters. The ripping teeth, or drag bits, are effective at removing clayey and sandy soils whereas the disc cutters engage when harder materials, such as the concrete walls at shaft penetrations, cemented soils, ground improvement zones (jet grout, frozen ground), and rock, are encountered. The combination of drag teeth and disc cutters allows the TBM to handle the variety of soils and materials that are encountered along the alignment. The cutterhead is also designed to support the soil during excavation, distribute and mix ground conditioning agents, and convey the excavated ground to the extraction point at the auger. Conditioning agents (foams, polymers, or bentonite) are added during excavation to turn the soil into a cohesive mass with a consistency similar to toothpaste. This consistency helps maintain and evenly distribute ground support (face pressures) during excavation and allows the auger to efficiently extract the excavated spoil in a controlled manner. The auger is the one point where excavated ground is removed from the face; the rate and direction of auger rotation controls the face pressure, which supports the unexcavated ground to control ground movements around the tunnel. The TBM advances through the ground by pushing off the lining system with the use of hydraulic jacks. As it does so, undisturbed ground passes over the shield, which is a few inches smaller in diameter than the cutting wheel. The ground is supported by conditioning wateronline.com n Water Innovations 21 CONVEYANCESYSTEMS Figure 2. TOP: The 23-foot cutting wheels of Lady Bird, Lucy, and Nannie (left to right) are designed for each contract's specific requirements and ground conditions. BOTTOM: The shield sections handle the excavated ground, provide hydraulic thrust for advancing, steer the machines, and erect the final concrete lining.