The Brightwater Conveyance System project is a major improvement to Seattle’s regional sewerage treatment system. McMillen Jacobs Associates’ involvement spanned the life of this ambitious project. In addition to preliminary engineering and final design of shafts and tunnels, we assisted the client, King County, in evaluating alternative contract-delivery methods, resulting in an extremely successful low-bid total of $443 million. In joint venture with MWH, McMillen Jacobs Associates provided final design services for all of the conveyance contracts, as well as engineering assistance during the construction phase.
The project includes approximately 13 miles (21 km) of soft-ground tunnels, 40 to 440 feet (12 to 134 m) underground. Tunnel construction employs 14-to-19.5-foot (4.3 to 5.9-m) diameter pressure-face tunnel boring machines, launched and retrieved via deep shafts in glacial tills well below the water table. There are four tunnel drives:
- BT1: 13,873 feet (4,228 m) length, finished ID 16.7 ft (5.1 m)
- BT2: 11,605 feet (3,537 m) length, 14.3 ft (4.4 m) ID
- BT3: 20,000 feet (6,096 m) length, 14.3 ft (4.4 m) ID
- BT4: 21,045 feet (6,414 m) length, 13 ft (4 m) ID
McMillen Jacobs Associates encountered many design challenges, including the high groundwater and compressible soils of BT1’s 83-foot-deep (25.3 m) Influent Pump Station and 74-foot-deep (22.6 m) Influent Structure shafts, which precluded dewatering. Instead of conventional internal-bracing methods, we designed a twin circular-cell shaft using slurry diaphragm walls and a structural invert slab constructed with tremie concrete methods.
The Brightwater system connects 335 miles (539 km) of existing sewage pipes to a new 36-million-gallons/day (136 million liters/day) treatment plant.