Recent TBM progress on the Second Manapouri Tailrace Tunnel in New Zealand (2MTT) has shown slight improvement. When the 10m diameter TBM was stopped for the Christmas break, the tunnel was 40% excavated (3.8km), with a progress rate for 1999 of 2.2km. The best advances include one day of 30m, best week of 105.4m and a best month (October 1998) of 431m.
The Fletcher-Dillingham-Ilbau JV had to contend with high water inflows while boring through a previously identified wet zone At one point, a total inflow to the heading of 536 litres/s was recorded which included flows from 125mm drain holes advanced into the water bearing zone to reduce pressure and flow. At present, total water inflows of approximately 900 litres/s remain, above the inflows experienced in the first tunnel for the same length of excavation. The TBM advance has sometimes been affected by materials washing off conveyors and from the cutterhead area and water temperatures of approximately 7°C, which affected crews and progress.
Recent penetration rates for the Atlas Copco-Robbins TBM have varied from 600-2400mm/h as the TBM continues with its 9.6km long drive to West Arm through hard rock. A second major cutterhead repair was accomplished during a 21 day shutdown at the end of 1999. The work included repairing cutterhead cracks around cutter boxes, strengthening the cutter housings, installing 40 000 carbide wear buttons and putting additional wear plates on the cutterhead.
