THE SETTING
Seattle is a city surrounded by water: Puget Sound to the west, Lake Washington to the east, with Lake Union just a couple of miles north of downtown. The waterfronts are the working harbors, public recreation areas, and terminals for the ferries that cross Puget Sound numerous times daily. With water being such an important element of life in Seattle, it is no surprise that preventing water pollution has been a key issue for more than 40 years. In 1958, long before passage of the Clean Water Act, Seattle area citizens began work to build a regional wastewater system that continues to be expanded today to meet the needs of a growing city and region.
Tunnels have been part of the system since the earliest days. In the 1890s, the brick-arch Lake Union tunnel was constructed between the Lake Union area and Elliott Bay. In 1911, the Fort Lawton tunnel was built to support disposal of wastewater to Puget Sound. Both these tunnels are still in service today.
Today the regional system of large pipes and wastewater treatment plants is owned and operated by the King County Department of Natural Resources. The Wastewater Treatment Division runs two large treatment plants and about 275 miles of collection system pipes. The system serves about 1.3 million people, including most of the urban areas within King County and part of southwestern Snohomish County.
Within the City of Seattle, much of the system uses "combined" sewers, which collect both stormwater and domestic wastewater. Although combined systems were state of the art when they were built, they are no longer considered acceptable, because their capacity is routinely exceeded during storms. Overflow points are built into the sewer lines to release combined flows into surface waters, including Lake Union, Lake Washington, and Elliott Bay (part of Puget Sound). The Wastewater Treatment Division has taken steps to reduce these overflows by about 1/3 of the levels experienced in the early 1980s, but more controls are needed to meet state regulations limiting the overflows to one untreated overflow per year at each outfall. The King County system has 37 CSO outfalls, and the City of Seattle has 113.
In the early 1990s, the City and County began looking for ways to control CSOs to Lake Union and Elliott Bay. The largest CSO in the County's system is the Denny CSO, located in Myrtle Edwards Park on Elliott Bay, just north of downtown Seattle. Because of the volume and frequency of overflows at this CSO, the options for controlling it involve storage or treatment in large quantities. Similarly, the City's overflows to Lake Union were challenging to address, requiring storage and conveyance to the collection system along the shores of Elliott Bay. By joining their efforts into one project, the City and County could jointly address the CSOs in these areas with one project using a tunnel to provide the necessary storage and conveyance.
Feasibility studies showed that a tunnel between the south Lake Union area and the Elliott Bay shoreline could provide storage for the combined flows during many storms and allow transfer of the stored flows to the West Point Treatment Plant after the storm subsides. During larger storms, the tunnel would fill completely and a new pump station would begin automatic operation, screening and disinfecting the flows and then discharging them through a new pipeline outfall to Elliott Bay. Reflecting the benefits to both the City and County, the agencies agreed on a method for splitting the costs of the project. They were awarded an EPA infrastructure grant for $35 million, helping to ease the burden of the total project cost of $165 million ($145 million for King County's project and $20 million for City of Seattle improvements).
The project will control Lake Union and Denny Way CSOs in two ways. First, the project will store CSO flows during moderate storms and transfer them to the West Point Treatment Plant after the storm subsides. Second, it will provide on-site treatment at the Elliott West site with discharge of treated flows through a new outfall during heavy rain conditions. Required facilities for the project include:
- A 14 ft 8 inch I.D. by 6,200 ft. long tunnel under Mercer Street between Dexter Avenue and Elliott Avenue West (for CSO storage and conveyance)
- CSO control facilities at the Elliott West site, with floatables removal, disinfection and dechlorination
- Piping and regulators to convey CSO flows from the existing County sewer system to the new facilities
- An outfall into Elliott Bay at Myrtle Edwards Park (to discharge treated flows from the Elliott West facilities
- An extension of the existing outfall at the Denny Regulator at Myrtle Edwards Park (to discharge untreated CSO flows, on average once per year)
Black and Veatch Corporation provided engineering services for the Mercer Street Tunnel, the outfalls and the Elliott West CSO Control Facility. RoseWater Engineering, Inc., was responsible for design of three tunnels that cross under the Burlington Northern railroad tracks, west of the Mercer Street Tunnel's West Portal, and two large diameter pipes that will connect the tunnel to new and existing infrastructure that form part of the overall project. The South Lake Union Pipelines were designed by Cosmopolitan Engineering Group.
To facilitate construction, the work was divided into five bid packages. These bid packages are summarized below:
|
Name of Contract |
Scope of Work |
Estimated or Final Contract Value
(in millions) |
|
Mercer Street Tunnel |
Construction of tunnel, and East and West Portals, three tunnels underneath railroad tracks |
$29.5 (bid) |
|
Elliott West CSO Control Facility |
CSO treatment and pumping facility; addition at Denny Regulator, all connections and project start-up |
$22.12 (est.) |
|
Marine Outfalls |
New outfalls for treated and untreated CSO discharges to Elliott Bay |
$8.0 (bid) |
|
Elliott West Pipelines
|
Diversion pipelines and regulators in Myrtle Edwards/Elliott Bay Park area |
$12.2 (bid) |
|
South Lake Union Pipelines |
Diversion pipelines and regulators in South Lake Union area |
$8.8 (est.) |
THE MERCER STREET TUNNEL
Frank Coluccio Construction Company commenced working on the $29.5 million Mercer Street Tunnel contract in June 2000. Having nearly 90,000 working hours without a single lost time accident, this contractor has demonstrated that safety comes first.
The Mercer Street Tunnel contract includes four tunnels all driven from a common shaft. The shaft is 150-feet long by 70-feet wide, and is 50-feet deep. It is also the principal excavation for the future Elliot West CSO facility.
The smallest tunnel is a 72" diameter auger bored tunnel that is about 150-feet long. It was driven under six sets of railroad tracks, with only six feet of cover, on the westside of the shaft. The second and third tunnels are 96" ID jacked pipe tunnels. One tunnel is to be used for CSO flows entering the Mercer Street Tunnel and the other will transfer treated effluent to the new outfall. Both jacked-pipe tunnels are situated about 50 feet under the railroad tracks and are roughly 350 feet long. Each section of jacked pipe has a 9" wall, is twelvefeet long, and weighs 18 tons. Both of the 96" tunnels were driven with a ten foot diameter Lovat EPB TBM equipped with a conveyor belt, muck ring, and Pressure Relief Gates (PRGs). Lovat also supplied the pipe-jacking frame. Excavation of the three smaller tunnels was completed early in 2001.
The Mercer Street Tunnel (MST) is the largest of the four tunnels. A 16'-8" diameter Lovat EPBM was lowered into the shaft in May of this year and in less than two months it has bored nearly 600 feet and is operating in full production mode. The finished tunnel will measure 14'8" in diameter, be 6,212 feet long, and be about 60-150 feet below Mercer Street. It will provide 7.2 million gallons of CSO storage.
Excavation of the of the Denny Way CSO Tunnels has proven to be a unique challenge. The project site is located on a major arterial roadway near downtown Seattle, which hampers the ability to haul muck during peak traffic hours. The underground crew works a single 10-hour shift followed by an 8-hour maintenance shift. On an average production day the Mercer Tunnel produces nearly 400 cubic yards of muck. Operating a maintenance shift at night has allowed for minimum traffic disruptions because hauling can occur at night.
MERCER TUNNEL CONTRACT FACTS:
Project Name: Denny Way / Lake Union Combined Sewer Overflow Mercer Street Tunnel, Contract #C93001C
Location: King County, Seattle Washington, USA
Setting: Urban
Owner: King County DNR Wastewater Treatment Division
Engineer: Black and Veatch Corporation; RoseWater Engineering, Inc.; Cosmopolitan Engineering Group
Construction Manager: Montgomery Watson Harza
Purpose: Combined Sewer Overflow
Contractor: Frank Coluccio Construction Company, Seattle
Tunnel Characteristics: Total Length 6,212 feet (1,89-km), 14-ft 8-in inside diameter and 16-ft 8-in outside diameter.
Overburden:
Max. @ Station 26+00: 172 feet (invert level)
Min. @ West Tunnel Portal: 66 feet (invert level)
Tunnel Lining: Segment Type Prefab Reinforced Concrete
Configuration: 5 Piece Tapered Segments
Internal Diameter: 14'-8" (4.47m)
Outside Diameter: 16'-2" (4.94m)
Wall Thickness: 0'-9" (0.23m)
Segment Length nominal 4'-0" (1.22m)
Geology: The tunnel will be excavated in glacial deposits consisting of silty clay, fine - coarse sand with occasion al cobbles and boulders. The ground is classified as potentially gassy. The tunnel alignment will be entirely below the water table.
The Earth Pressure Balanced Machine (EPBM)
Manufactured by: Lovat Inc., Toronto Canada
Model: RMP200SE Series 15900
Year Build: 1996
Operating Mode: EPB, Semi-Closed, and Open
Weight: 305 tons
Total Length (w/trailing gear): 259-Feet
Cutterhead Features:
- Mixed Face
- Flood Control Doors
- Face Injection Ports
- Air Lock (for access to pressurized EPB cutterhead chamber)
- Variable Speed Drive System (0 to 4.8 rpm)
- 1,050 Cutterhead Horsepower
- 3,200 Tons of Thrust (maximum)
- 2.63 x 106 ft-lb Maximum Torque
Special Features:
- Segment Erector
- Class I Division II Electrical Systems
- Ground Conditioning System capable of Injecting:
- TACS Guidance System* (from Germany)
- Programmable Logic System* (PLC)
*Capable of recording 32 data channels of the EPBM's operating parameters and gas levels at the tunnel heading (up to 10 samples per second). With computer displays onboard the machine and at both the project and construction managers offices.
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