The light at the end of the tunnel for TARP – the massive water-control program launched by the Metropolitan Water Reclamation District of Greater Chicago (District) more than 40 years ago – is within sight. Navigating the final stretch, however, requires both vision and care.

As construction of the Des Plaines Inflow Tunnel (DPIT) nears completion, the District’s plan to collect and divert CSOs and floodwaters via 109 miles (175km) of tunnels in metropolitan Chicago to quarries that have been converted into temporary storage reservoirs prior to treatment is close to completion. Because the TARP tunnels and reservoirs are already operational, making new connections, like connecting the Des Plaines TARP Tunnel to McCook Reservoir, presents unique challenges – especially the complexities associated with two live connections and tunnelling near existing operational infrastructure.

Partnering with the U.S. Army Corps of Engineers (USACE), the District originally connected the Des Plaines TARP Tunnel to the McCook Reservoir using tunnels that were recently constructed to drain the McCook Reservoir back to the Mainstream Pumping Station (MSPS) for treatment. But when it became clear that this infrastructure would limit flows into the reservoir and cause an unacceptable bottleneck in the tunnel system, the District engaged Black & Veatch to develop and help implement an alternate solution that would provide a higher level of protection for 19 municipalities in western Cook County from flooding, basement back-ups, and CSOs in the 32 square mile Des Plaines TARP Tunnel service area.

As the designer and now providing construction phase engineering support, Black & Veatch has worked closely with the District and the general contractor, Walsh Construction Co., to implement the larger, more-functional DPIT project that will protect the Des Plaines service area from a 10-year storm event with the ability to convey 11,000 cfs of combined wastewater and stormwater to the recently commissioned McCook Reservoir.

Making the connection

Without the DPIT, two flow scenarios existed to move water from the Des Plaines TARP Tunnel to McCook Reservoir: pumped flow through the MSPS or gravity flow bypassing the MSPS through the Distribution Chamber and McCook Reservoir Outlet Tunnel. Both scenarios would have significantly limited allowable differential pressure heads and flow rates to prevent potential damages to the existing infrastructure. The DPIT project eliminates these concerns by providing a direct connection between the existing Des Plaines TARP Tunnel and the McCook Reservoir, enabling operation at critical times during large storm events.

The DPIT project requires connecting the Des Plaines TARP Tunnel to the McCook Reservoir when both systems are operational and cannot be taken offline. Currently the Des Plaines TARP Tunnel is filled during wet weather events, and combined storm and wastewater is stored in the tunnel and associated shafts until there is capacity at the Stickney Water Reclamation Plant (WRP) to treat the water. The stored water in the Des Plaines TARP Tunnel is drained via a dewatering tunnel that is connected to the MSPS that pumps water to the Stickney WRP, which has a design capacity of 1.44bn gallons per day.

The McCook Reservoir became operational at the end of 2017 when the McCook Main Tunnel and Gates System construction contract was completed through the USACE. The Main Tunnel System connects the Mainstream TARP Tunnel System with a 30,000 cfs flow capacity to the McCook Reservoir.

Construction of the DPIT necessitated live connections to both the existing southern terminus of the Des Plaines TARP Tunnel at approximately 280ft (85m) below ground and McCook Reservoir about 45ft (13.7m) above the reservoir floor. Walsh Construction is responsible for construction of all DPIT tunnel, shaft, gate, and portal facilities.

DPIT details

Construction of the DPIT requires coordination of many project components:

  • A tunnel (drill and blast from two headings) between the existing southern terminus of the Des Plaines TARP Tunnel and the McCook Reservoir, consisting of approximately 5,400ft (1,646m) of 20ft (6m) finished diameter hard rock tunnel.
  • A temporary construction shaft and tunnel adit to facilitate the live tunnel tie-in, consisting of a 280ft deep shaft and approximately 420ft (128m) of 20ft excavated diameter rock tunnel.
  • A gate shaft for the installation and operation of a primary and backup gate, consisting of an approximate 270ft deep shaft with a diameter of 45ft.
  • A gate structure and gate control building to allow for the control of flow between the Des Plaines TARP Tunnel and the McCook Reservoir. The gate structure consists of a high head primary and backup gate, both approximately 16ft wide by 20ft tall. A gradual transition from a circular tunnel to a rectangular tunnel was required at each end of the gate structure. The downstream transition is steel lined to prevent cavitation damage from high-flow velocities and differential pressure that could occur under partial gate opened conditions.
  • Live connections between an active McCook Reservoir and the DPIT, and between the existing southern terminus of the Des Plaines TARP Tunnel and the DPIT, including the excavation of two rock plugs and demolition of an existing concrete plug.
  • Backfilling of the tunnel adit and construction shaft that are not utilized following construction of the tunnel connection.
  • A tunnel portal at the McCook Reservoir highwall, approximately 45ft above the floor of the reservoir. 
  • An energy dissipation structure to mitigate damage to the floor of the McCook Reservoir caused by the approximate 45ft drop in water to the reservoir floor at the DPIT outlet. The energy dissipation apron slab featured high strength concrete, large baffle blocks and a backsplash wall to prevent erosion and undermining of the portal highwall.
  • Highwall stabilization measures in the McCook Reservoir protecting the portal of the DPIT.

Sequencing and construction strategies

Overlap with the quarry mining and three other construction projects, a tight schedule, proximity to existing infrastructure, and live tunnel connections called for careful planning and scheduling to successfully construct the DPIT. The establishment of interim milestones enabled the team to complete the work inside the McCook Quarry/Reservoir before the reservoir began operation. This early completion date prevented potential flooding of the work area during reservoir-filling storm events and resulting schedule delays. In addition, tight windows for construction were specified for the live connections to McCook Reservoir and the Des Plaines TARP Tunnel to limit disruptions to existing operations and mitigate construction delays and damages. Construction of the tunnels, portal, and gate shaft, including gate installation had to be completed prior to making the live connections and placing the DPIT into operation.

McCook Reservoir components

The highwall stabilization, tunnel portal, and energy dissipation structure inside the McCook Reservoir were completed on an expedited schedule prior to the reservoir becoming operational. The construction contract was awarded to Walsh Construction Co. in June 2016, with mobilization to the site occurring in late summer 2016. The in-reservoir structures were completed by June 30, 2017, simultaneous to the completion of other McCook construction projects and six months prior to McCook Reservoir becoming operational.

The McCook Main Tunnel System, the McCook Reservoir Final Preparation Inlet/Outlet Tunnel, and reservoir excavation were all under construction at the same time, and they all shared the same quarry access ramp with the mining operations of Vulcan. Tunnelling from the portal was completed roughly 48ft into the reservoir highwall before demobilizing from the reservoir. The remainder of the tunnel was excavated and lined from the gate shaft to maintain isolation of the underground work from the reservoir until the live connection was constructed.

Shaft and adit

The DPIT design required installation of a construction shaft and tunnel downstream of the Des Plaines TARP Tunnel terminus to facilitate removal of the rock plug and tie-in to the existing Des Plaines TARP Tunnel. The construction shaft enabled the movement of equipment, materials and employees; provided an emergency egress; and will facilitate rapid access during the live connection to the Des Plaines TARP Tunnel.

The project team expedited installation of the shaft by completing the adit tunnel excavation concurrently with the main tunnel and excavating the shaft by raise bore. This approach limited surface impacts at the shaft, located next to a busy thoroughfare, and helped shorten the project schedule.

Gate shaft and tunnel

To abbreviate the construction schedule, Walsh excavated the gate shaft concurrent with construction activities in the reservoir, using conventional drill-and-blast methodology from ground surface to final depth. The shaft was supported through the overburden with steel liner plates, ribs, and piles that were socketed into bedrock. Pre-excavation grouting of the deeper Elwood Formation was completed in the gate shaft to limit groundwater infiltration from this higher permeability bedrock unit.

Excavation of the DPIT commenced from the gate shaft near the mid-point of the tunnel. Drill-and-blast excavation advanced from the gate shaft simultaneously on north and south headings to maximize productivity. The tunnel was excavated at full face without benching, typically in 12ft lifts. Excavation was completed around the clock, in three eight-hour shifts with blasts occurring in daylight hours.

The tunnel was excavated between March 20, 2017, and January 24, 2018. In-tunnel vibration monitoring was required when blasting adjacent to overlying and underlying operational tunnels.

After completing the tunnel excavation up to the rock plugs that were left in place, the construction contractor placed the concrete liner in 100ft (30.5m) intervals using travelling steel forms and installed the steel liner downstream of the gate shaft through the transitional geometry of the tunnel. The steel liner was manufactured in six sections, assembled at the facility for quality assurance, shipped to the site as separate sections, and reassembled in the tunnel prior to being cast in concrete.

The upstream and downstream DPIT tunnel headings were excavated to within 38ft and 20ft of completion, respectively. Leaving a rock plug at both ends of the tunnel enabled construction to proceed while the McCook Reservoir and the Des Plaines TARP Tunnel are operable. The work sequencing of the project required the rock plugs to remain in place until all other construction is complete and the project is ready to be placed into operation.

Live tie-in tunnel connections

The rock plug at the McCook Reservoir will be the first to be removed because of the reduced risk of flooding from the reservoir compared to the Des Plaines TARP Tunnel. Installation of the concrete tunnel liner will follow removal of the rock plug. The final tunnel construction activity will be associated with the live connection to the existing Des Plaines TARP Tunnel. To make this connection, the contractor will remove the rock and existing concrete plug and then install the tunnel liner. This tie-in work will be staged through the construction shaft and adit, which will be backfilled upon completion. Significant coordination will be necessary to monitor and forecast weather conditions during this final live connection. In addition, water levels within the Des Plaines TARP Tunnel will be monitored closely by the District’s MSPS engineers and operators to ensure that safe working conditions are maintained.

Work sequencing adjustments

At the time of this writing the two high-head gates are being fabricated but are not yet ready for installation. To minimize the delay in project completion, Walsh Construction is in the process of installing two concrete bulkheads in the lined tunnel to isolate the gate shaft so the live connections can be completed. When done, the concrete bulkheads will be removed following gate installation for project commissioning.

Nearly there

The DPIT will connect the existing Des Plaines TARP Tunnel to the McCook Reservoir. Upon project completion in 2020, the additional conveyance of the DPIT and storage capacity provided by the McCook Reservoir will further reduce the risk of flooding in the greater Chicago combined sewer system area and further improve the quality of local waterways and the environment. The District also plans to commission Stage 2 of the reservoir to be able to use the maximum reservoir capacity of nearly 10bn gallons in 2029.

The District and its contractors and consultants continue to dig deep to address these challenges as another critical component of the award-winning TARP program nears completion.