Chicago and 51 other, older municipalities in Cook County share a combined sewer system. Rainfall drains into the system and combines with wastewater output, resulting in an overflow of contaminated water. At times the system designed to treat 2 billion gallons of wastewater per day is inundated with more than 5 billion gallons of rainwater runoff.
Because there are few open areas in Chicago able to absorb the runoff, when the treatment plants are at capacity, the mixture of raw sewage and storm water drains directly into the rivers and canals. The result of their combined sewer overflows results in severe waterway pollution and community flooding.
In 1972, a team of engineers from the Metropolitan Water Reclamation District, the city of Chicago, Cook County and several state agencies devised a plan called the Tunnel and Reservoir Plan, or TARP, to bring the area’s waterways up to federal and state water quality standards in a cost-effective manner.
The $3.7-billion project proposes to solve three problems: waterway pollution, prevention of raw sewage backflows to Lake Michigan and the chronic flooding of 500,000 homes, all due to storm runoff water mixed with raw sewage in the antiquated sewer systems of Chicago. The plan requires large underground tunnels to be dug to intercept storm water overflow and convey it to storage reservoirs. When the storm has subsided, the overflow could be conveyed to a treatment plant for cleaning before going to a waterway.
Although the plan and funding have been approved, the U.S. Environmental Protection Agency and the Illinois legislature must grant allocations for the project each year. Because reservoirs are flood control projects, the U.S. Army Corps of Engineers is building them and providing 75 percent of the funding for the revolutionary project.
“Although the expense is huge, this solution is more economical than replacing 13,500 miles of the combined piping in the sewer systems, and the benefits far exceed the costs,” said Hugh McMillan, general superintendent for the MWRDGC.
Fifteen years since the project began, Chicago has experienced a dramatic improvement in the quality of the Chicago River, the Calumet River and Sanitary and Ship Canal. Game fish have returned and waterfront real estate is again valuable.
Piece by Piece
Although final plans for TARP were adopted in 1972, continued study in 1973-74 divided the project into two phases – one for pollution control and one for flood control – under four separate systems. The Upper Des Plaines System was completed as a pilot in 1980 just north of O’Hare Airport. Its 6.6-mi. (10.6 km) tunnel ranges from 9 to 22 ft. (2.7 to 6.7 m) in diameter. The Calumet was dedicated in 1986 and the Des Plaines system began in 1985. Construction of the Mainstream system began in 1976 and finished ahead of schedule and under budget in 1985.
One of the largest built underground, the pumping station has six pumps 350 ft. (107 m) below ground in twin chambers that are 270 ft. (82.3 m) long, 64 ft. (19.5 m) wide and 90 ft. (27 m) high. About 200,000 cu. yd. (152,912 cu m) were excavated and more than 100,000 cu. yd. (76,456 cu m) of concrete placed. Access shafts were excavated 280 ft. (85 m) deep, then the cavern was drilled and blasted in downward drifts. A corrugated metal ceiling channels seeping groundwater into a sump pump and also serves as an acoustical barrier.
The suction and discharge tunnels are lined with 1- to 1.5-in. (2.5 to 4 cm) steel for 100 ft. (30.5 m) on either side of the pumphouse. Each chamber can be operated separately to provide four modes: pump from the tunnels to the treatment plant; pump to an adjacent reservoir; pump simultaneously to both; or pump from the reservoir.
Phased In
Phase I is a $2.5 billion pollution control project, and includes 109 mi. (175 km) of tunnels ranging in diameter from 9 to 33 ft. (2.7 to 10 m), and three de-watering pumping stations. Phase II, known as the Chicago Underflow Plan, is designed for flood control and involves the construction of three reservoirs with a total capacity of 16 billion gallons to provide an outlet for floodwaters. It will also remove the remaining pollution not captured by Phase I.
TARP Phase I consists of a system of drop shafts, tunnels and pumping stations that will capture combined sewer overflows from a service area of 375 sq. mi. (971 sq km), according to Jack Farnan, MWRDGC chief engineer.
The four major tunnels (Mainstream; O’Hare/ Upper Des Plaines on the north; Des Plaines on the west; and Calumet on the south) follow the course of major rivers so the drop shafts can collect the overflows before they pour into waterways capturing about 85 percent of the pollution caused by CSOs. Tunnel depths range from 150 ft. to 360 ft. (46 to 110 m) below ground. Tunnel sizes range from 9 ft. to 33 ft. (3 to 10 m) in diameter.
Phase II will add 18 mi. (29 km) to the Mainstream. The quarry at the pumping station site will become a reservoir, and another reservoir will be mined near the North Shore Channel. Together, the reservoirs and tunnels will capture entire storms, eliminating flooding without the need to open the gates that separate the inland waterways from Lake Michigan.
When completed, TARP will consist of 109 mi. (175 km) of deep tunnels, 243 vertical drop shafts, 460 hear-surface collecting structures, three pumping stations and 15.6 billion gallons of storage in three reservoirs. Currently more than 100 mi. (161 km) of tunnels are complete or under construction.
Construction is staged so that completed tunnel segments are immediately placed into service.
Work in Progress
At each drop shaft, excess wet weather flow from existing and future city sewers will enter a connecting structure with a sluice gate, which controls inflow to the tunnel system. Intercepted combined sewer overflow is directed into the vertical drop shaft, where it is conveyed to the tunnel below. Each drop shaft is equipped with a louver device that allows air to be vented during rainstorms while preventing air in the tunnel from escaping out the shaft during dry weather.
The connecting structure and upper portions of each drop shaft are located in overburden (ground) and are excavated by conventional ground excavation techniques. The vertical portion of each drop shaft in rock is circular and is excavated by the use of mechanical vertical boring equipment.
The boot areas located at the bottom of each drop shaft approximately 274 ft. (83.5 m) below ground are excavated by tightly controlled blasting because of their irregular geometry. These small areas and the construction shaft are the only locations where controlled blasting are allowed.
Mole Hole
The tunnels and shaft are mined almost entirely within Silurian strata, except for the top portions of the shafts, which are mined through overburden. Total thickness of the entire Silurian with the South Chicago area ranges from 350 to 430 ft. (107 to 131 m).
The host rock for the tunnels is the Silurian dolomite, which constitutes the bedrock throughout the area. The tunnels are approximately 310 ft. (94.5 m) below existing grade elevation. The rock projects about 80 to 100 ft. (24.4 to 30.5 m) below ground surface, with glacial drift and lakebed deposits overlaying the bedrock. The dolomite has very good tunneling properties. The unconfined compressive strength varies between 14,000 and 18,000 psi (966 and 1,242 bar).
Because boring causes less rock disturbance, noise and vibration than does blasting, several tunnel boring machines (TBM) were implemented. The same underground tunneling technology used to mine the Chunnel under the English Channel is digging Chicago’s Deep Tunnel. The 33.5-ft. (10.2 m) diameter “mole” shaves off pieces of limestone and soil that then exit the machine on a conveyor belt through the tunnel and up to the surface.
The big mole and eight other borers left “mountains” of dolomite, shale and limestone along the tunnel route. The rock aggregate is recycled into area construction projects. Ted Budd, vice president of Kenny Construction, project manager and tunnel division manager, said that the limestone is IDoT-approved for highway usage.
The TBMs performed beyond their then-proven capabilities, working three times faster than the 2 ft./hr (.61 m/hr) that was predicted. Officials credit the contractors for setting records with the moles, revising procedures and suggesting design changes such as new cutters.
A total of 14 soft ground and rock TBMs have been used on the projects. The combined tonnage of tunnel spoil transported by the conveyor systems equates to 5.5 million tons (4.95 million t). Conventional rail haul was used for the non-mainline tunnels.
The TBM brought out of “moth balls” for work on the Calumet Tunnel is a refurbished Robbins TBM originally purchased in 1978. At 32 ft., 4 in. (10 m) diameter, it has completed 107,420 ft. (32,742 m) of tunnel. For the Des Plaines Tunnel, the TBM was downsized to 24 ft., 3 in. (7.4 m) and changed from a mainbeam configuration to a diagonal thrust propel/steer system. The machine was upsized again to 27 ft., 4 in. (8.3 m) for the Torrence Avenue Tunnel, remaining in the diagonal thrust configuration, with 3,000 hp (2,236 kW) on the cutterhead and a complete conveyor muck-handling system. The equipment has continually set records as capacity and efficiency increase.
Kenny Construction Company of Wheeling, IL, Peter Kiewit Construction Company of Omaha, NE, and J.F. Shea Company Inc. of Walnut, CA, formed a joint venture to build the north leg of the Des Plaines tunnel system. After 13 months of tunneling, crews completed the 8.7-mi. (14 km), $142 million tunnel in 1997, using four soft-ground and two hard-rock TBMs.
The shale and soft ground areas required installation of steel ribs and bracing the mined sections until a concrete liner could be poured. In some soft ground areas, the mole had to push 60-in. to 96-in. (152 cm to 244 cm) precast concrete pipe into place.
Budd indicates a tunneling speed of 200 ft. (61 m) per day, with round-the-clock shifts.
Once a tunnel is bored and grouted, a 12-in. (30.5 cm) concrete liner is poured into telescoping forms that permit the full circle to be poured at once. In the Mainstream and Calumet sections concrete was pumped through lines up to 3,000 ft. (914 m) long. In the Des Plaines system, trains delivered the concrete to a pour deck.
Current Status
Work continues on the 34,150-ft. (10,409 m) Torrence Avenue Tunnel – by far one of the most successful tunnels ever driven utilizing conveyor system muck transport.
Mining crews have completed nearly 1 mi. (1.6 km) a month.
The Torrence Avenue Leg Tunnel is part of the 36.5-mi. (58.7 km) long system. The $140.7 million contract was awarded in April 1998, with scheduled completion due in November 2002. Along the southeast side of Chicago along the Calumet River from 91st Street to 138th Street, approximately 8.1 mi. (13 km) of 25-ft. (7.6 m) and 15-ft. (4.6 m) diameter concrete-lined tunnel will be laid in rock 270 to 310 ft. (82.3 to 94.5 m) deep. There will be eight dropshafts from 7 to 25 ft. (2 to 7.6 m) in diameter, one construction shaft, a 31-ft. (9.4 m) diameter wheelgate shaft and an access shaft. The total storage volume amounts to 143 million gallons of water.
Crews have added a rock screening/gradation and crushing plant at the end of the last conveyor on the surface. Once the TBM mines the rock, the cutterhead dumps it on the conveyor and the spoil starts its journey to the surface. For 52 minutes it travels across a short transfer belt, in buckets on a vertical belt, a muck chute, an overland rotating stacker belt and into a surge pile where it is dumped into the sizing/processing plant.
At the completion of the Torrence Avenue Tunnel, the belt system will have hauled all of the 5.5 million tons (4.9 million t) of cuttings from the mainline tunnel drives.
“The unprecedented system utilization and performance is due to reliable conveyor components and system control. But most important is the dedicated personnel that install, monitor and maintain the system with a genuine pride of accomplishment. Their accomplishments on the TARP contracts and especially on the Torrence Avenue Tunnel will be tough to match or surpass in the near future,” Budd said.
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