Needing to optimise the 700MW power generating potential of the Manapouri hydroelectric power station on New Zealand’s South Island, construction of a second tailrace tunnel started in 1998 (T&TI April 2000, p31). Excavated using a 10m diameter Robbins TBM, the 10km long tunnel runs parallel to the original tailrace tunnel built with the power station in the 1960s.
One of the most critical operations for the project, awarded to international joint venture of Fletcher (NZ)/Dillingham (US)/Ilbau (Austria) (FDI) in 1997, was connecting the new tailrace to the existing tunnel. The substantial contribution of the Manapouri power station to owner Meridian Energy’s generation capacity made it necessary to hold the length of any outages to a minimum. The original contract conditions called for a maximum of 21 days.
Because of the substantial risk involved, the construction contract allowed for separate and specific liquidated damages of US$108,125/day for any overruns on the outage period and a bonus of US$54,060/day for potential early completion.
The planned connection was within an area of the tailrace outlet channel that would normally be partially under water during shutdown of the power station. This called for temporary dewatering while the connection was completed in a cofferdam across the tailrace channel at the Deep Cove portal.
However, shortly after mobilising on site the contractor’s subsurface borings and pump tests indicated large, highly permeable rockfall deposits underlying the cofferdam location. This brought into question the viability of the cofferdam system.
An alternative method for dewatering the connection area was, therefore, devised. This consisted of a temporary bulkhead wall installed by divers within the existing tunnel near the power station draft tube manifold. The bulkhead comprised five buoyant steel tanks, the largest of which was 1.75m high, 9m wide and 1m deep, and weighed 10.5t.
The bulkhead system had several major advantages. It significantly reduced the pumping capacity required, and the pieces of the bulkhead could be manufactured off the critical path of the outage, greatly reducing the risk for both the owner and the contractor. It provided an easy, repeatable method for dewatering the tailrace tunnel bifurcation area in the future for inspection and maintenance purposes.
Because of the unique features of the bulkhead system, Electricity Corporation of New Zealand had to retain a variety of subconsultants to come up with a functional and easy to construct design. The team included Meritec (formerly Worley Consultants), New Zealand Diving & Salvage and OCEL Engineering.
Divers performed several field trials in Lake Manapouri in a specially manufactured steel mockup of the tailrace tunnel, to avoid having to go through learning curve stages during the actual outage time. The trials also allowed final modifications to the bulkhead sections based on any difficulties encountered.
Once the power station was shut down for the outage, the bulkhead sections were transported to the water’s edge on a special trailer, launched and floated into place with divers manoeuvring and stacking the pieces close to the existing tunnel. Valved tanks inside allowed the injection of water or air to ballast and trim each piece and assist installation.
The perimeter and the areas between each section were fitted with rubber seals to control leakage through the bulkhead. A manway in the top bulkhead section allowed the divers to pass through to place perimeter seals on the downstream side. Work went without a hitch with installation completed by eight divers in 13 hours.
Divers then used float bags to transport and attach four Flygtt BIBO 4150 pumps on to a by-pass manifold in the bulkhead base section to dewater the connection area. With a combined capacity of 880 litres/s, the pumps took about three and a quarter hours to pump 10 million litres of water to the downstream side of the bulkhead wall.
From within the dewatered manifold the two tunnels were connected by conventional drill and blast methods. Leakage around the installed bulkhead was limited to about 30 litres/s, easily handled by one of the dewatering pumps. At the end of the operation the manifold was rewatered and divers then dismantled and removed the bulkhead sections.
A permanent stop-log gate was constructed as part of the new tailrace tunnel contract. This allowed the finished connection to be kept separate from the new TBM tunnel and allowed the TBM to hole-through at the power station end of its drive in the dry.
The total outage time was 11 days, cutting 10 days off the original schedule. This resulted in a substantial bonus for FDI and helped Meridian Energy to bring the facility back online earlier than anticipated.
Related Files
Figure 1: Perspective
Figure 2: Longtitudinal section
Figure 3: Plan of the junction area
