Channel Tunnel, Texas Style

Two tunnels are being constructed in soft ground beneath the Houston Ship Channel and Greens Bayou as part of the Houston area 's water delivery system. When completed the tunnels will convey 320 million gal.day of raw water to one of Houston 's water purification plants. The initial tunnel specifications emphasized the minimization of risks to lower costs either through the initial bid or as a result of claim mitigation. Seismic investigations helped delineate clay zones and showed the need to place the tunnels deeper. Pipeline depth and cost considerations dictated a primary lining of bolted steel segments with a 10 ft, 2 inch diameter to provide the necessary clearance for the 108 inch steel carrier pipe. Deflection studies of linear deformation caused by ground stress showed that overstress was not occurring and the values obtained came in close agreement with the 0.5 inch design criteria. Shafts were constructed using circular concrete caissons sunk from sheetpile and ring wale starter pits. The measured heave resulting from deep excavations ranged between 1.7 inches and 2.2 inches, indicating that the ground and tunnels were behaving as expected. A 133 inch diameter Lovat TBM, which features the ability to convert to an earth pressure balance shield when necessary, was used for the deep tunnel drives. Since the TBM was designed with a stroke allowing simultaneous erection of two 2 ft. wide rings, it was possible to reduce ring erection time considerably by prebolting adjacent segments of two rings at the fabricator shop. Advance rates averaged 30 ft per 12 hour shift, with a maximum of 64 ft in a single shift. Tunneling is expected to continue into 1990, with the deep Greens Bayou tunnel being the last drive to be completed. (Male-PTT)