The McCook Main Tunnel connects Chicago’s Tunnel and Reservoir Plan (TARP) Mainstream Tunnel to the McCook Reservoir. The tunnel system consists of a 10 m (33 ft) finished diameter and 490-m (1,600-ft) long hard-rock tunnel constructed from a 27.5-m (90-ft) diameter and 92 m (300 ft) deep main gate shaft. The gate shaft houses six high-head 4.4 x 9 m (14.5 x 29.5 ft) wheel gates installed in the bifurcated and steel-lined section of the tunnel. The tunnel also includes portal and energy dissipation structures as it daylights into the reservoir.
Construction of the McCook Main Tunnel in live flow conditions was a challenging task for the McCook Main Tunnel Project participants, the project owner U.S. Army Corps of Engineers (USACE), local sponsor Metropolitan Water Reclamation District (MWRD) of Greater Chicago, designer Black & Veatch and contractor Kiewit Infrastructure Co. (Kiewit). Once completed in two stages in 2017 and 2029, the McCook Reservoir will hold 38 billion L (10 billion gal) of combined sewer overflows (CSO) and flood waters from the city of Chicago and 36 surrounding communities in Cook County, IL.
The construction of the tunnel system was World’s largest tunnel gates and reservoir connection go online as part of Chicago’s Tunnel and Reservoir Plan divided into two contracts. The first contract for the gate and construction access shaft was completed in August 2011. The second contract included tunnel excavation and concrete and steel lining that was completed in September 2014; installation of gates and hydraulic cylinders that was completed in June 2017; and the last major construction activity was the removal of a temporary concrete plug (bulkhead) and lining that tunnel section to bring the overall tunnel and gates system online before Dec. 31, 2017.
This article focuses on the installation, testing and commissioning of the high head wheel gates. Live tunnel connection details to TARP Mainstream Tunnel were addressed in a previous paper submitted at the World Tunneling Conference in 2016.
Chicago TARP System
The MWRD has been addressing CSOs and flooding in Chicago since the late 1960s and formally adopted the Tunnel and Reservoir Plan in 1972 to protect the region’s most precious drinking water supply, Lake Michigan. Phase I of TARP, which included construction of 175 km (109 miles) of deep storage and conveyance tunnels with diameters up to 10 m (33 ft), was completed in 2006. In addition to the protection of Lake Michigan from CSO discharges, Phase I resulted in substantial improvements in surface water quality as well as the quality of life for lake and riverfront communities in Chicago. Water quality improvements and flooding mitigation will be further enhanced as Phase II reservoirs are placed in service, including the three, large reservoir systems, McCook, Thornton and Majewski, as shown in Fig. 1.
The McCook Reservoir is the largest reservoir in the TARP system. Once completed, this $1.031 billion reservoir facility will receive 38 billion L (10 billion gal) of CSO and floodwater via the McCook Main Tunnel which connects the TARP Mainstream Tunnel to the McCook Reservoir and from the Distribution and Des Plaines Inflow tunnels which will bring flow from the Des Plaines Tunnel of TARP.
McCook Main Tunnel layout
McCook Main Tunnel daylights into the McCook Reservoir at the northeast edge and extends east toward the existing Mainstream Tunnel (Fig. 2). The tunnel was excavated using sequential drill-and-blast and lined with concrete and steel in sections for long-term stability and to minimize infiltration and exfiltration.
The tunnel was excavated in its entirety in bedrock, consisting of massive, relatively homogenous Silurian and late Ordovician dolomites. These rocks form a relatively uniform 100+ m (330+ ft) thick sequence across the site and incorporate the Racine Formation, Sugar Run Formation, Joliet Formation, Kankakee Formation, Elwood Formation and Wilhelmi Formation of Silurian age. The tunnel is located in the Kankakee and Elwood formations.
The McCook Main Tunnel system has the following key components:
- Main Tunnel Section: Approximately 490 m (1,600 ft) long, 10-m (33-ft) inside diameter, hard-rock tunnel, bifurcated into two tunnels for 88 m (290 ft) through the gate shaft section.
- Main Gate/Access Shaft: 27-m (88-ft) diameter, 90-m (295-ft) deep circular shaft located near the midpoint of the Main Tunnel and houses the bifurcated tunnel section. This shaft was used for construction of the tunnel and houses the high head wheel gates for controlling flow between TARP Mainstream Tunnel and McCook Reservoir.
- Construction Shaft (contractor option): A 7.6-m (25 ft) diameter and 87-m (285-ft) deep construction shaft was located at 91 m (300 ft) downstream or west of the Mainstream Tunnel connection. Kiewit elected to build this shaft to facilitate the live connection work. As the tunnel and gate shaft excavation and lining were completed, a temporary concrete bulkhead was installed to isolate the live connection section from the rest of gate and reservoir works to the east.
- Gates: A total of six wheel gates operating under 100 m (330 ft) of water pressure head were installed in the Main Gate/Access shaft. Each gate measures 4.4 x 9 m (14.5 x 29.5 ft) with associated hydraulic cylinders, power units, and gate controls. Each bifurcated section of the Main Tunnel contains one main gate and two guard gates — one upstream and one downstream of the main gate. The gates, hydraulic cylinders and controls were manufactured under a separate contract and were provided to the contractor as government furnished items. The gates were designed by Black & Veatch and fabricated by Oregon Iron Works (now Vigor Works LLC).
- MainTunnel/MainstreamTunnel connection: This is the connection section of the Main Tunnel to the existing, live 10-m (33-ft) diameter Mainstream Tunnel that remained in service throughout construction. Removal of the temporary concrete bulkhead and lining of that tunnel section completed the connection.
- Main Tunnel/McCook Reservoir connection: The Main Tunnel connection to the McCook Reservoir included portal excavation and stabilization work at the quarry highwall face and an energy dissipation structure. The portal was excavated from the reservoir side and supported with rock bolts, wire mesh and shotcrete.
- Control building: A surface facility to house gate operating controls, hydraulic power units and provide limited storage.
The McCook Main Tunnel system design, construction, commissioning and operation were coordinated with the overall McCook Reservoir water control plan as well as the reservoir excavation, quarry highwalls stabilization, groundwater protection system construction, Distribution and Des Plaines Inflow Tunnel connections, and other reservoir features. Hydraulic structures were designed to withstand erosion or cavitation during reservoir filling and emptying cycles and to handle flows up to 850 m3/s (30,000 cu ft/sec) and velocities approaching to 12 m/s (40 ft/sec).
Working with live tunnel flow conditions
The connection to the Mainstream Tunnel was challenging due to the limited amount of time available to access the connection area as the Mainstream Tunnel had to remain live or in service at all times. The connection is located near the downstream terminus of the Mainstream Tunnel which drains over the 65 km (40.5 miles) of tunnel network virtually encompassing the highly developed city of Chicago. MWRD operates the Mainstream Tunnel system and the Mainstream Pump Station to collect and pump out the CSOs and subsequently treats the flows through the Stickney Water Reclamation Plant. When it rains, the tunnel fills up rapidly, and there is also a constant base flow in the tunnel of about 113 to 150 million L/d (30 to 40 million gpd). Kiewit designed a base flow bypass system consisting of upstream and downstream steel check dams (partial bulkheads) with a 914-mm (36-in.) nominal diameter HDPE bypass pipe across the connection section of the Mainstream Tunnel (Fig. 4). The connection was lined with reinforced concrete starting with the invert, then walls and finally crown. The connection works in-progress were exposed to inundation multiple times where all personnel and equipment were evacuated from the area upon notice from MWRD operators or the weather service dispatcher. All installations were successfully completed despite multiple and complete flooding of work areas, and the tunnel and subsequent reinforcement and lining work (Fig. 5) were successfully completed by the end of 2016 as detailed in the referenced 2016 World Tunnel Congress paper.
Several lessons learned were valuable to the team as the Mainstream Tunnel was opened up for the first time after 35 years of service. The tunnel liner was in near perfect circular shape without any sign of damage, major cracking or water seepage. Despite being downstream, there was no sediment or grit accumulation observed, primarily due to high-velocity tunnel flows created with large dewatering pumps. It is noted however, there was significant grit accumulation in the Main Tunnel once the construction work was suspended over the spring and summer of 2015. The tunnel-liner concrete exhibited very high unconfined compressive strength, on the order of 83 MPA (12,000 psi) compared to initial placement specification of 28 MPA (4,000 psi), also known as the MWRD’s RA mix. Visual inspection, field and laboratory testing of the concrete liner and verification of its existing condition allowed the designer to shorten the limits of excavation for connection by approximately 12 m (40 ft) at the downstream end.
High-head wheel gates
Six high-head, vertical-lift wheel gates were installed in the Main Tunnel to control the flow of CSOs and floodwater between the reservoir and Mainstream Tunnel. The gates are housed in the Main Gate/Access Shaft (MGAS) in the bifurcated section of the Main Tunnel (Fig. 6). Each bifurcation has one main gate and two guard gates — one upstream and one downstream of the main gate. All gates have an opening of 4.4 m x 9 m (14.5 ft x 29.5 ft). The main gates are the primary feature to control water flow and are designed to seal in both flow directions. The guard gates are designed to seal against hydrostatic head on one side only. The upstream guard gate holds back water on the Mainstream Tunnel side and the downstream guard gate holds back water on the reservoir side. The guard gates provide redundancy to the main gates and can also be used to isolate a main gate when needed for maintenance.
The gates are operated by a hydraulic operating system. Each gate is raised and lowered by a hydraulic cylinder that is mounted above the gate in a vertical position. The cylinders are actuated by two hydraulic power units (HPUs) at ground level in the control building near the MGAS.
Each gate consists of three sections (leaves) that are pinned together to form the gate. There are four-pin connections between the lower and middle leaves and four-pin connections between the middle and upper leaves. The upper leaf is connected to the hydraulic cylinder by means of a single pin. Each leaf has four wheels, two on each side (Fig. 6). These wheels are the bearing portions of the gate when it is subjected to load.
The gates use neoprene seals to control the water. The seals along the sides and top of the gate are center bulb seals. The seals have a fluorocarbon coating on the bulb to reduce friction during gate operation. This coating will eventually wear away but does not compromise the sealing function of the seal. The seals on the main gates have a 0.95-cm (0.375-in.) preset deflection for flow toward the reservoir, and a 0.32-cm (0.125-in.) preset for flow in the reverse direction.
The seals on the guard gates have a 0.95-cm (0.375- in.) preset. A pressure groove behind the seal is provided so hydrostatic pressure will force the bulb against the sealing surface. Holes in the seal bars allow pressure to build up in this groove. This groove also assists in lessening wear on the seals during gate movement when no differential pressure is present. A wedge seal is mounted at the bottom edge of the gate. The joint between the gate sections is sealed with flat natural rubber seals. All seals are split at the joint between gate sections to allow installation and removal of the gate in sections. The seals are detailed to allow a 0.16-cm (0.0625-in.) preset compression between the gate sections in order to minimize leakage at these joints.
Each leaf is a steel structure made up of welded horizontal plate girders that span from wheel to wheel. It is composed mostly of A572 grade 50 steel, with some portions that are 304L stainless steel. The gate is metallized with a zinc-aluminum coating to help protect it from corrosion. The metallized coating is sealed with a vinyl sealer. The gates are open on their loaded side (Mainstream Tunnel side for the main gates). This keeps the gate from being buoyant when submerged. Drain holes in the girder webs and in the gate bottom keep water from ponding on the girders as the water level goes down.
The wheels for each gate are 0.9-m (3-ft) in diameter and made form ASTM A705, UNS 13800, Condition H1025, 380 BHN stainless steel. They rotate about a fixed axle that is of the same material but tempered to a harder condition (430 BHN) so that wear will more likely occur in the wheel. The wheels are also softer than the wheel track plates so that wear will more likely occur in the wheel. The wheels are crowned with a 15-m (50-ft) radius so that they bear continuously on the wheel track plates even when the gate deflects under load. The wheels have a force fit bronze bushing that rolls on the greased interface between the bushing and the fixed axle.
The gate guides position each gate correctly and provide a seating and sealing surface for the gate. They consist of upper and lower guides. The upper guides are only used during gate installation and removal. They help to control gate motion in the upper part of the shaft. The upper guides are bolted to the concrete of the gate well walls. They consist of ASTM A36 angles that have been galvanized to provide a durable coating.
The lower guides are the primary traveling surfaces of the gate. The wheels of the gate run along wheel tracks. There is a front track and a back track. The distance between the tracks is slightly larger than the wheel, to keep the wheel from binding but also to reduce play in the gate itself. The wheel tracks run from the tunnel invert all the way to the cylinder support level. These wheel tracks are ASTM A693, UNS 13800, Condition H950, 430 BHN stainless steel. This is a harder material than the gate wheels so that damage occurs in the wheels rather than in the track. On the side of each guide is a roller track plate on which the guide roller contacts. These track plates are bolted to an ASTM A304L grade stainless steel that forms the guide slot. This also is the sealing surface of the slot. When seated, the gate presses up against the wheel track and compresses the seals against the sealing surface. The guides are anchored to the shaft concrete with embeds and rebar anchors cast into a 34 MPA (5,000-psi) second placement concrete.
The MGAS concrete consists of 34 MPA (5,000) psi reinforced concrete. This concrete anchors the steel liner and forms the sides of each of the six gate wells. It was designed to resist water pressure around the exterior of the shaft. Also, the walls between the gate wells were designed to resist the full height differential water pressure on them (i.e., one gate well full and the adjacent one empty).
The concrete was placed in sections. Between each vertical construction joint is a PVC waterstop system to resist leakage through the joint. The horizontal construction joints were prepared to allow for bonding between the concrete lifts and do not have a waterstop.
Each of the guard gate wells have large access spots, enough to access the tunnel invert using a four-person crane basket. The main gate wells have access spots as well, but much smaller.
Hydraulic operating system details
The cylinders raise and lower the gates through the use of hydraulic pressure generated by the HPUs. Fluid pressures in the cylinder are monitored by two pressure transducers, one on the bore side of the piston near the top of the cylinder and one on the rod side of the piston near the bottom of the cylinder. The position of the cylinder rod is monitored by a position sensor. Each cylinder is made up of a shell a piston head, a rod, a clevis, supply and return piping, and a manifold with check valves. The bore diameter of the cylinder is approximately 1 m (3 ft), the rod diameter is approximately 0.3 m (1 ft) and the total stroke in the cylinder is approximately 9.5 m (31 ft) (Fig. 7). Each cylinder weighs approximately 65 kips and is capable of producing a jacking load of 2,100 kips. The cylinder rod is coated with a protective anti-corrosion metallic coating. The cylinder shell is coated with epoxy paint. The hydraulic fluid is carried by piping between the HPUs and cylinders.
The cylinders are actuated by two HPUs in the control building near the MGAS (Fig. 7). Each HPU typically controls three gates on each side of the bifurcated tunnel. Each HPU consists primarily of an internal reservoir, two pumps, a manifold, a PLC cabinet with screen and various internal instruments for the function of the HPU. There is a manual crossover between the HPUs to allow for redundancy in hydraulic operation.
Gate testing and operation
Several tests were conducted to check installation and operation of the gates, as follows:
- Gate leaf dry run test: While suspended from a crane, a gate leaf was slowly lowered and raised between the ground surface and tunnel invert to verify the proper alignment of the upper and lower gate guides and sill plate at the invert.
- Functional performance test: The fully installed gate system was tested to verify the operation of the HPUs, hydraulic cylinders, gates, and hydraulic piping under operating conditions.
- Intermediate (flat) seals air pressure test: The flat seals between the gate leafs were subjected to air pressure up to 1.2 MPA (175 psi) to verify performance of those seals prior to filling tunnel with water.
- Wet test: A 79-m (259-ft) column of water obtained from a canal was pumped into the gate shaft and tunnel on one side of a pair of guard gates and then one side of the main gates to test sealing of gates at full hydrostatic head conditions.
The normal condition for all gates is the raised position, allowing flow to move unimpeded between the reservoir and Mainstream Tunnel. The gate speed is approximately 0.3 m/min (1 ft/m), so it takes approximately 30 minutes for the gates to move from the open position to the closed position. When it is desired to isolate the reservoir from the Mainstream Tunnel, both of the main gates will be closed. In the event that the main gates do not close (or only one closes), the respective guard gates will be closed (upstream guard gates if the Mainstream Tunnel water level will be higher, downstream guard gates if the reservoir water level will be higher). The gates are designed to hold back large differentials of water pressure; however, the gates will typically be moved or operated when there is relatively equal water levels on both sides of the gate.
The motion of the gates is controlled by the HPUs in the control building at the ground surface. Generally, the gates are operated in pairs (i.e., both main gates move together, both upstream guard gates move together, both downstream guard gates move together). The gates can be operated locally from the control building or remotely from the Stickney Water Reclamation Plant via SCADA.
Removal of the temporary concrete bulkhead and final stages of commissioning took place from late 2017 through the first quarter of 2018. Once the bulkhead was removed and construction of the remaining reservoir features was completed, including an inflow/outflow connection and aeration facilities, the reservoir became available to take water in December 2017..
The McCook Main Tunnel and Gates are one of a kind underground structures that are used for management of high-volume and high pressure flows in tunnels. Design, fabrication, storage, delivery, installation, and commissioning of tons of concrete and steel structures at depths up to 100 m (300 ft) were truly an engineering and construction feat for the many that worked on this project, and for those who will be benefiting from the water quality improvements and flooding mitigation for years to come.
The authors acknowledge the efforts and contributions of all project participants. In addition to authors listed for this article we specifically acknowledge Mike Padilla and Gordon Kelly (USACE), Carmen Scalise and Kevin Fitzpatrick (MWRD), Matt Trotter, Brent Bridges and Mark Petermann (Kiewit) and Charles Strauss and Clay Haynes (Black & Veatch), for their contributions to this article.
Oksuz, F., and Trotter, M., 2015. McCook Main Tunnel Construction, New York: George A. Fox Tunneling Conference.
Oksuz, F. and Trotter, M. 2016. McCook Main Tunnel Connection, San Francisco, World Tunneling Conference.