Thornton Transitional Reservoir, Thornton, Ill.

The Thornton Transitional Reservoir will provide 3.1 billion gallons in flood relief and is reportedly the largest stormwater reservoir in the southern Chicago area.

The reservoir provides flood control along Thorn Creek and the Little Calumet River, where flooding has caused more than $6 million in damage, and will help improve the quality of the watercourses in the Chicago area.

Since the reservoir is within 2 mi. of the Indiana border, the beneficial effects will extend to the Hoosier State's western part as well.

Structures were designed to divert Thorn Creek stormwater to the reservoir to prevent the flow from overwhelming the stream and the Little Calumet River.

When water rises above a predetermined level in Thorn Creek, it flows to the reservoir through an intake structure, a 24-ft.-dia. drop shaft and 8,000-ft.-long tunnel. After a storm, the reservoir is dewatered via gravity through an 8-ft.-dia. dewatering tunnel connected to an existing tunnel and the Calumet Water Reclamation Plant for treatment and discharge into the Cal-Sag Channel.

Calculation Techniques Used

New engineering and numerical computation techniques were used in the project.
The main issue included the stability of the rock walls between the West Lobe and Main Lobe and between the North Lobe and Main Lobe. The only public land available for the tunnel was directly beneath the Tri-State Tollway, and about 4,000 ft. of the tollway is between the sheer vertical walls of the two 250-ft.-deep lobes.

The key was to ensure the stability of the highway wall and the quarry operator's haul tunnels above and below the proposed diversion tunnel. The walls were geologically mapped and deep rock borings were performed, resulting in two- and three-dimensional maps. Stability was analyzed using a three-dimensional numerical model.

This method would also be useful for highways in mountainous terrain, subject to slop stability problems. The numerical modeling, including block theory, can be applied to solve problems in a variety of situations, such as blocky rock mass for a permanent highway cut, quarry wall stability and high wall cuts in hard rock.

Another element in the operation of the facility was the optimum hydraulic shape of the diversion structure. A high-flow velocity field will develop inside the diversion structure.

Typically, a solution is sought by using small-scale physical modeling in a laboratory, a process that can be expensive and time consuming. Time and money were saved by using a two-dimensional computation model that allowed the team to determine the optimum configuration of the diversion structure.

Creating Shafts, Tunnels

Drop and valve shafts were hollowed before the tunnels were created.

About 25 ft. of soil was removed with excavators. Wood lagging was mounted along the holes' sides to reinforce the lip and prevent mud from falling in.

With the entrance steadied, about 225 ft. of rock was loosened through drilling and blasting with dynamite. Muck was removed with a bucket attached to a crane.

A 12-ft.-diameter shaft was built about 65 ft. away from the center line of the 300-ft.-deep drop shaft to relieve air pressure. The torrential onrush of water can cause pressure to build rapidly unless there is a vent.

A tunnel-boring machine created the passageways. The TBM breaks rock with disk cutters tipped with hard metal.

Scoops captured the rock, which dropped onto a conveyor leading out of the tunnel.
The conveyor was anchored to a tunnel rib.

Because of the lesser cost, muck cars removed rock from the dewatering tunnel. The system was similar to a train, as a locomotive on a rail conveyed boxes filled with rock.
Joints in the form of vertical and horizontal cracks resulted in extra precautions to prevent chunks from causing injury. Strapping was used to protect workers, and a hood shielded the machine.

Methods and Materials

The contractor used unique methods and materials to construct the drop shaft and de-aeration shaft.

Rather than using conventional form work supported from the floor slab, form work was designed and constructed that provides a clear passage at the bottom of the drop shaft to the diversion tunnel.

The 16-ft.-radius, semi-circular roof of the de-aeration chamber was formed with a moveable steel arch form, which rolled on steel beams supported by steel columns attached to the sidewalls. The arch form was constructed of steel with support ribs and access doors for vibrating concrete.

After each of the eight arch pours, the form was lowered by jacks, and the weight was transferred to Hillman rollers. A winch was used to reposition the form for the next pour.

The drop shaft was poured with a 20-ft.-high, 24-ft.-dia. steel form. This was hoisted by four winches mounted at the surface.

A special concrete mix was designed and tested to reduce the heat of hydration and control shrinkage. The mix was low in cement and high in fly ash and allowed the contractor to pour 10-ft.-thick lifts every three days.

Structural elements were designed to provide the greatest possible use in the future of the final Thornton Composite Reservoir, which will provide 8 billion gallons of stormwater storage.

The jury said, "This is an amazing project, unbelievable and innovative in all aspects.
The engineering modeling they did on this job was unquestionably a breakthrough, and the model can be applied to highways in mountainous terrains and other extreme conditions. They were really thinking beyond the scope of this project to develop necessary solutions."

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