A world-renowned combined sewer overflow (CSO) conveyance and storage system in Chicago, Illinois, is in the final stretch, with construction of two reservoirs and tunnel connections remaining. The deep-tunnel system, which has already helped the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) intercept billions of gallons of CSOs, mitigate flooding and divert polluted water to area water reclamation facilities, will be enhanced with completion of the large reservoirs.

TARP Overview
The MWRDGC adopted the Tunnel and Reservoir Plan (TARP) in 1972 to cost effectively enable the Chicago area to comply with federal and state water quality standards in the 375-square-mile (603.5sqkm) combined sewer area consisting of the city of Chicago and 51 suburban municipalities in Cook County. The main goals of TARP are to protect Lake Michigan—the region’s drinking water supply—from raw sewage pollution, improve water quality of area rivers and streams and reduce street flooding and basement backups.

Construction of the Phase I tunnel systems commenced in 1975. The tunnel systems were placed in service as portions were completed, beginning with the Mainstream Tunnel System in 1985. By 2006, Phase I tunnel work and dewatering pump stations were completed and in operation. The overall system consists of 109.4 miles (176km) of deep, large-diameter rock tunnels primarily driven by tunnel boring machines (TBMs). The tunnels collectively provide 2.3 billion gallons (8.7 billion L) of volume to capture CSOs that previously discharged at hundreds of outfall locations. These underground tunnel systems have proven to be a long lasting sustainable solution by preserving the surface space for use by future generations, and using open pit rock quarries for the reservoirs.

Phase II of TARP consists of three reservoirs and associated connection tunnels that are intended primarily for flood control but will also considerably enhance pollution control benefits provided under Phase I. The Majewski, McCook, and Thornton Composite reservoirs will store CSOs and floodwaters when deep-storage and treatment capacities are exceeded until the water can be treated. The 350-million-gallon (1.3-billion-litre) Majewski Reservoir, formerly called the O’Hare CUP Reservoir, was completed by the U.S. Army Corps of Engineers in 1998 at a cost of USD 45M. As of December 2010, it has yielded USD 207M in flood damage reduction benefits to the three communities it serves.

The McCook and Thornton Composite reservoir projects are progressing as planned. The McCook Reservoir will have a capacity of 10 billion gallons (37.9 billion litres) upon completion and will provide more than USD 90M per year in flood damage reduction benefits to 3,100,000 people in 37 communities. The McCook Reservoir will be brought on-line in two stages. The first stage will have a capacity of 3.5 billion gallons (13.2 billion litres) and will be completed in 2017, allowing for partial benefits to be realized while the remaining volume is being mined and constructed.

The Thornton Composite Reservoir will provide 4.8 billion gallons (18.1 billion litres) of expanded storage for CSOs and 3.1 billion gallons (11.7 billion litres) of expanded storage for stormwater in southern Chicago and Cook County, with completion planned by 2015. This reservoir will provide USD 40M per year in benefits to 556,000 people in 14 communities.

The reservoir is being completed under three major contracts. The first contract protects existing groundwater resources by creating a water retaining engineered barrier in and around the reservoir and protects the remaining quarry from being flooded, by plugging two haul tunnels and an open gap with concrete plugs and an RCC dam; the second contract will connect the reservoir to the existing CSO tunnel and provides isolation and inflow/outflow gates; and the third contract will connect the reservoir to an existing stormwater diversion tunnel, and provide additional work required to make the reservoir operational.

Thornton Composite Reservoir Tollway Dam, Grout Curtain & Quarry Plugs
Under a contract awarded on December 3, 2009 for USD 67.8M, the joint-venture team of F.H. Paschen, S.N. Nielsen & Associates and Cabo Construction is serving as general contractor for construction related to the conversion of the 300ft (91.5m) deep, 90-acre (364sqm) dolomite quarry into a reservoir. Black & Veatch designed the groundwater protection system that encircles the west, north and east sides of the reservoir. The system consists of an inclined double row 550ft (167.6m) deep grout curtain approximately 70 to 120ft (21 to 37m) back from the quarry highwalls; improvements to the reservoir access ramp on the north quarry highwall; and multi-level groundwater monitoring wells located beyond the grout curtain to measure the performance of the grout curtain and to monitor groundwater quality. The grout curtain is being constructed using state-of-the- art technology and equipment and will be one of the deepest, if not the deepest, in the United States.

The remaining work on this contract was designed by MWH and includes the south side grout curtain, two quarry tunnel plugs, and a 116ft (35m) tall RCC dam along the Interstate 294 tollway. The grouting subcontractor is Hayward Baker. A 320ft (98m) long test section of the grout curtain is nearing completion and production grouting on the inner row of grout holes is proceeding on the west side of the quarry. Construction is on schedule, with an expected completion in May 2014.

Thornton Composite Reservoir Connecting Tunnels & Gates
Construction of 1,000ft (305m) of 30ft (9m) diameter drill-and-blast tunnel connecting the existing Calumet System TARP tunnel to the Thornton Composite Reservoir is underway. Pre-excavation grouting has been completed at the gate shaft and excavation of the 63ft (19m) diameter, 340ft (104m) deep shaft is ongoing. This project was designed by MWH and the construction contract was awarded to a joint-venture team of Walsh Construction Company and II in One Contractors on May 6, 2010 for USD 135.5M.

Thornton Composite Reservoir Final Reservoir Preparation
All of the remaining work required to make the Thornton Composite Reservoir operational was designed by Black & Veatch and included on the Final Reservoir Preparation contract. Awarded on December 2, 2010 for USD 50.8M, to the joint-venture team of Walsh Construction Company and II in One Contractors, work will begin in early 2011 on the construction of a 1,080ft (329m) long, 20ft (6m) diameter drill-and-blast tunnel to connect the Thornton Composite Reservoir to the existing 22ft (6.7m) diameter unlined Thorn Creek Stormwater Diversion Tunnel. This connection will include a 340ft (103.6m) deep shaft to access and vent the 50ft (15m) drop of up to 6,200 cubic feet (175.6cu.m) per second of flow. A 168ft (51m) long, 32ft (8m) wide deaeration chamber with a roof that slopes from 25 to 64.5ft (7.6 to 19.7m) in height is located at the drop to facilitate air removal.

Initial support of the chamber will consist of rock dowels and bolts on a 5ft (1.5m) pattern in the arch and in the walls above 20.5ft (6m) of the chamber floor. The tunnel portal incorporates highwall stability measures to the top of the reservoir and an energy dissipation apron to protect the reservoir floor from flows in excess of 20ft (6m) per second. A rock plug will remain between the connection and existing tunnels until the reservoir is ready to accept water. At that time, the rock plug will be removed to connect the reservoir to the diversion tunnel and a reinforced concrete plug will be installed under live conditions in the existing Thorn Creek Stormwater Diversion Tunnel to redirect the flow to the reservoir.

The alignment of the connection tunnel was designed to allow for the decommissioned diversion tunnel underlying Interstate 294 in the tollway dam to be converted into a drainage adit to reduce seepage pressures on the downstream face of the rock dam. Drainage holes will be drilled upward at a 30-degree inclination from the crown of the drainage adit to facilitate interception of seepage water. A smaller concrete tunnel plug will be installed in the west side of the drainage adit, and during reservoir operation water collected in the adit will be drained by gravity most of the time using the dewatering tunnel presently being used to dewater the Thornton Transitional Reservoir (TTR). When gravity flow cannot occur, a pump in the existing dewatering valve chamber that services the dewatering tunnel will be activated. A 10ft (3m) diameter shaft and tunnel will be constructed to access the existing dewatering tunnel and drainage adit as necessary for maintenance.

To prevent the reservoir from overfilling, reservoir level and tunnel inflow instrumentation will be installed in both of the Thornton tunneling contracts. As the reservoir is nearing its highest allowable stage, a signal will be sent to close the gates in the connecting tunnel gate shaft and the slide gates present on the existing diversion tunnel structure present on Thorn Creek. This contract will also include decommissioning the TTR that has been used for the past eight years to store flood waters from Thorn Creek. It will include removal of all rock support from the abandoned tunnel portal and will allow the quarry owner to resume full production mining in this lobe.

McCook Reservoir Projects
The McCook Reservoir is functionally similar to the Thornton Composite Reservoir, but serves a much larger area. The U.S. Army Corps of Engineers is responsible for the design and construction of the McCook Reservoir, with MWRDGC acting as the local sponsor and future owner. The next major contract to be awarded under this project is the Main Tunnels contract, being designed by Black & Veatch. The McCook project includes 1,800ft (549m) of 33ft (10m) diameter drill-and-blast tunnel. The circular tunnel is bifurcated into two rectangular shaped tunnels to transition flow from the connection tunnel through the gate shaft and back to the circular tunnel for discharge into the reservoir. Tunnel bifurcation and gate shaft features include steel lining in the bifurcation and two chambers that house six 120-ton, 14.5ft wide by 28.5ft tall (4.4m x 8.7m), high head wheel gates that will control CSO flow into the reservoir from the Mainstream System TARP tunnel. Construction of the bifurcation will occur in a 300ft (91.5m) long by 60ft wide by 32ft tall (18m x 10m) cavern. This tunnel work will include a live connection into the existing Mainstream TARP tunnel utilizing a separate construction shaft for access. Construction of the live connection at McCook will follow completion of the downstream tunnel, gates, and in-reservoir energy dissipation structure. The McCook Reservoir Connecting Tunnels and Gates contract is scheduled to bid in the first half of 2011.

Conclusion
Success of TARP is evidenced by dramatic improvements in the water quality of the Chicago and Calumet rivers as well as other waterways. Game fish have returned, marinas and riverside restaurants abound, river recreation and tourism are booming, and waterfront real estate values have skyrocketed as Chicago area residents see the river system as a major asset rather than an embarrassment. TARP has earned recognition and acclaim at all levels as a way to effectively and sustainably mitigate storm- elated overflows. Continued progress on the mega-reservoirs moves MWRDGC a giant step closer to its goal of completing TARP and further improving the water quality and flood protection in the Chicagoland area.


The McCook Reservoir, now under construction, will provide flood control and water quality benefits upon completion in 2024. Tunnels for the Thornton Composite Reservoir. Phase I TARP tunnels, completed by 2006 TARP projects along Lake Michigan. The Thornton Reservoir