Joliet CSO Tunnel & Wet-Weather Treatment Facility

Dec. 28, 2021

Cost: $54,760,000
Location: Joliet, Illinois
Year: 2021-12-21
Size: 81,250 gpm
Owner: Joliet, Illinois
Designers: Strand Associates Inc. GZA GeoEnvironmental Inc.
Contractor: Joseph J. Henderson & Son Inc. Triad Engineering and Contracting Co. Midwest Mole Austin Tyler Construction Inc. Williams Brothers Construction Inc.

The Joliet, Illinois’, East Side sewer system is primarily a combined sewer system, spanning both sides of the Des Plaines River. Since 1985, Joliet reduced its number of permitted combined sewer overflow (CSO) outfalls from 32 to eight through successful basin separation efforts. 

Its National Pollutant Discharge Elimination System (NPDES) permit requires reducing the overflow frequency from the remaining eight CSOs per a compliance schedule. Strand Associates Inc. completed a Long-Term Control Plan for Joliet to address these requirements, which was then approved by the U.S. EPA and Illinois EPA. 

This tunneling project is constructed below the Des Plaines River, which is used as a shipping canal and has normal pool elevation controlled by the U.S. Army Corps of Engineers (USACE). Each side of the river has concrete river walls constructed in 1927 that confine the river and the downstream lock and dam. Existing grade on the west side is slightly above normal river level, while existing grade on the east side is 20 feet below normal river level. As such, the tunnel and shaft excavation required close attention lest a breach in the rock create a free discharge of the river out of the east shaft.

This project includes significant conveyance, storage, and wet-weather treatment infrastructure to serve the east side sewer system. The conveyance improvements included the Phase II West Side Interceptor and Des Plaines River Tunnel projects. These projects aim to collect and convey both average-day and wet-weather flow from the west side of the Des Plaines River to the Joliet’s Eastside Wastewater Treatment Plant (WWTP) and the Phase III Wet-Weather Treatment Plant on the east side of the river. 

This project addressed the replacement needs of the single existing siphon sewer crossing the river that was constructed in the 1920s. The new siphon control structure was designed to maintain existing flow; allow for phased modifications of upstream flow contributions; allow for eventual rerouting of combined sewer flow away from the crossing; and open doors to reuse the existing crossing as a future storm water conveyance system.

The river wall is under the jurisdiction of the USACE, so communications and permitting with the Chicago and Rock Island Districts, as well as the Central Illinois office were required.

“On the east side of the river, the river wall is well above your head so the river is actually above the working service,” said Troy Stinson, senior associate at Strand Associates. “Any problem with water would have been a problem out there.”

Phase II included construction of a wet-weather screening structure, inlet and outlet siphon control structures, a circular drop shaft, an 865-foot long, 111.5-inch diameter tunnel, a rectangular drop shaft with future integral wet-weather pumping station, and a Westside Interceptor. Phase III included construction of a wet-weather treatment facility. Flow on the west side is screened via a deflection screen on the west side and flow on the east side is screened with multi-rake bar screens. 

Screened flows from both sides are conveyed to the 117 million gallons per day (mgd) capacity Wet-Weather Pump Station where it is pumped to the influent control structure and the first 4 million gallons of combined sewage is diverted to the first flush tanks for storage and treatment at the Eastside WWTP after the event. When the first flush tanks are full, flow is diverted to two, 180-foot diameter clarifiers and a chlorine contact tank, which have a treatment capacity of 90 mgd.

There were numerous wet-weather challenges and environmental issues during the project’s construction. A hydraulic, horizontal deflection screen was installed on the west side of the river to keep the tunnel clean of debris, avoiding the need to collect screenings at a remote location. 

Tunnel cross sections and specifications were developed based on a drill and blast technique with probing and potential grouting prior to advancement. This method seemed most appropriate because of the relatively short length of the tunnel (865 feet), the required length of the starter tunnel, and the cost of a tunnel boring machine (TBM). The design incorporated two dry weather siphons located in the bottom of the tunnel and used the space above for conveying wet-weather flows all within a single tunnel diameter. 

A full-length, guided, horizontal direction drill was used instead of probing ahead and grouting to provide information on any vertical fractures or expected water inflow during tunneling. In addition, the tunneling subcontractor constructed a TBM launch tube to avoid a long starter tunnel. The design also incorporated the deep east tunnel shaft into an integral wet-weather pumping station, saving millions of dollars in construction costs.

“We had to get into that sewer to divert the wastewater. And so that was a pretty challenging part of the project, you’re digging below the river wall and you have to support the excavation,” Stinson said. “The project had very low change orders. Tunnels are typically notorious for cost increases, you just never know what you’re going to hit. For a tunneling project and wet weather treatment plant, it went pretty smooth.”

To mitigate concerns, the contractor was required to prepare contingency plans and complete probe drilling ahead of tunneling to identify potential issues. The contract documents also placed tight restrictions on blasting by limiting peak particle velocities for key structures, such as the river wall, I-80, and adjacent residential homes.

The West Side Interceptor is more than 800 feet of 72-inch diameter, centrifugally cast, fiberglass reinforced polymer mortar (CCFRPM) gravity pipe constructed to intercept flow from the head of the current city river crossing to the new Des Plaines River Tunnel. The interceptor was routed south within the US Route 6 right of way, and under the I-80 bridge with depths exceeded 20 feet (with up to 16 feet of rock). Design of the project required significant coordination with the Illinois Department of Transportation (IDOT), since the interceptor alignment was in the right of way and very close to the bridge piers for I-80. 

A detailed assessment of the soils and rock as well as development of comprehensive specifications for rock removal to meet requirements set by IDOT Bureau of Structures were completed. A complex vibration testing protocol using specified maximum peak particle velocity was also developed and implemented for both the interceptor and tunnel. 

The interceptor project’s complex flow control structures are designed and phased to enable Joliet to maintain current flow conditions for  existing interceptors that eventually would either be removed, diverted, or abandoned as part of the final piping and tunnel system. 

One structure was a new siphon control structure at the head of the existing river crossing. The head of the existing crossing was integral to the river wall holding back the Des Plaines River. The structural design needed to maintain the integrity of the river wall, account for instability during excavation and construction of the structure, and squeeze dimensions to minimize encroachment on the adjacent private property. These goals were ultimately accomplished after extensive and collaborative efforts.

Project Year: 2021-12-21Contractor: Joseph J. Henderson & Son Inc. Triad Engineering and Contracting Co. Midwest Mole Austin Tyler Construction Inc. Williams Brothers Construction Inc.Designers: Strand Associates Inc. GZA GeoEnvironmental Inc.Owner: Joliet, IllinoisLocation: Joliet, IllinoisCost: $54,760,000Size: 81,250 gpm