Nearly 75% of TBM world record rates of progress have been set using continuous conveyor mucking systems. The combination proved the point again recently in Indianapolis where a Robbins 6.2m (20.2ft) diameter main beam TBM completed an additional 2.8km (9,175ft) long drive within the same progam as the initial 12.5km long tunnel.
The rebuilt machine with its continuous conveyor mucking system started the first 12.5km (7.8 mile) long Deep Rock Tunnel Connector (DRTC) project in November 2012 and completed excavation a year ahead of schedule. Tunnel contractor Shea/Kiewit (S-K) JV were then engaged by project owner, Citizens Energy Group, to complete the extra 2.8km long Eagle Creek Tunnel for the DRTC CSO control system.
The machine was launched on the initial drive from a 76m (250ft) working shaft and bored through limestone and dolomite to achieve world records in its size class of 6m to 7m (20ft to 23ft). These included:
At the end of the extension drive, there was much to celebrate. Tim Shutters, Construction Supervisor for Citizens Energy Group, said there were two main factors for the high performance. “First, the TBM cutterhead performed very well, and secondly, favorable rock conditions provided for optimal mining operations and fast production.”
Key also to achieving record rates was application of an efficient continuous conveyor muck removal system. Built and supplied for the JV by Robbins, this extended into a complex system that comprised 25km (82,000ft) of belt at its longest distance, included horizontal and vertical conveyors, and passed through S-curves and two opposite direction 90-degree curves on the alignment as it continued to haul muck away from the TBM.
Dean Workman, Vice President, Conveyor Systems at Robbins reports that conveyor systems have proven themselves and are now standard systems with longer TBM drives for hard rock projects and increasingly for EPBM operations. "Initially contractors were cautious of these systems because of the always changing consistency of the muck, but if properly treated before it is handled and if transfer points are enclosed to control wetter materials, the systems work well."
Negotiating sharp curves introduces the safety issue of belt coming off the idlers. “Placing the boosters in the correct locations to control the tension on the belt ensures safety and lengthens the life of the system. If the tension is too high it can cause premature failure through the curve,” explained Workman.
Early detection of problems is key for successful use of conveyor systems. “If a major component is damaged, all the best is going to cycle through it in a relatively short time,” said Workman, “and replacing damaged belt takes time. Electrical monitoring and alert systems for detecting faults early have become standard accessories to monitor the temperature of the electric motors and gear boxes, vibrations in the system that can warm of something occurring, and the torque of all the drives in the system. If one begins to pull harder than the others an alarm will go off to highlight the need for attention.”
For the Indianapolis installation, Robbins used its patented curve idlers which self-adjust to the load. For the future Workman sees significant developments in the use of longer and faster belts as well as more sophisticated electrical controls systems and improved components. “There are many developments underway at the moment,” said Workman.
At the Indianapolis project, Stuart Lipofsky, Project Manager, for S-K JV said: “I am proud of our world records, and most of all, of our men and the hard work they have done as a team, working together to accomplish a project of this size. We finished the first 12.5km (41,00ft) almost a year ahead of schedule and while the extension added time, what is remarkable is that we were still able to finish within the original contractual dates. It was unusual also to see a belt system perform as well as this one did,” he added.
Once complete, the deep tunnel project will reduce the amount of raw sewage overflows and clean up tributaries along the White River. After the early completion of the Eagle Creek Tunnel, the project will be moving into its next two tunneling phases. The White River Deep Tunnel will continue 8.5km (5.3 mile) north of the completed DRTC and pump station and the Lower Pogues Run Deep Tunnel will split off 2.7km (1.7 miles) from the White River Deep Tunnel heading east. Two additional tunnels, including Fall Creek and Pleasant Run, are anticipated to be built in 2020, and the total CSO project of 27km (17miles) of tunnels is expected to be fully completed by the end of 2025.
Construction Supervisor Shutters described the environmental benefits the project will provide for the Indianapolis community, explaining: “I have lived in Indy all of my life and the White River has never been a focal point for the city as there is a lot of pollution. I really think that once it has been cleaned up, people will want to visit and property values along that body of water will go up. Being able to finally utilize the river is key for us.”
The completion of the first leg of the much larger tunnel system targets three critical CSOs that flow into the nearby White River, and will go online in 2017. The completed tunnels bring the city one step closer to achieving its consent decree with the US Environmental Protection Agency (EPA) to prevent these combined sewer overflows into the natural rivers and waterways by 2025.