In an industry that looks to innovations that guarantee faster, safer and more efficient tunneling, the continuous conveyor system is an existing option that fits the bill. In fact, historically speaking, at least 75% of all world records ever set by TBMs were done with a continuous conveyor system in tow rather than muck cars. The Indianapolis Deep Rock Tunnel Connector (DRTC) was no exception: Multiple world records were achieved in 2014 in the 6 to 7 m diameter range. These included “Most Feet Mined in One Day” (409), “Most Feet Mined in One Week” (1,690), and “Most Feet Mined in One Month” (5,755).
What many other factors were at play, including a good TBM and knowledgeable crew, the good conveyor system availability and speed of muck removal in such a long tunnel certainly played a part. The TBM completed all tunneling, including two extension tunnels, on March 5, 2015.
The custom conveyor system, manufactured by The Robbins Company, enabled continuous tunneling in a difficult layout that included two 90-degree curves and two S-curves. Spanning more than 7 miles, the system included nine booster drives plus a main drive. A vertical belt moved muck up the 250-ft deep shaft to a radial stacker for temporary storage. The system, one of the most complex in North America and the first to operate in 90-degree curves, made swift tunneling possible.
The DRTC is the first phase of a nearly 28-mile long network of deep rock tunnels being built 250 ft beneath the City of Indianapolis, Indiana by Citizens Energy Group. The $179 million DRTC is a 7.5-mile, 18-ft ID tunnel constructed by S-K JV, a joint venture between J.F. Shea Construction and Kiewit Infrastructure Co. The tunnel is the first segment of a conveyance and storage system to provide overflow relief during wet weather events. The project is estimated to be completed in 2017 and will initially deliver 53 million gal of the planned 264 million gal of CSO storage for treatment, as well as provide future connections to other tunnels in the overall system.
S-K JV opted to use a hard rock Main Beam TBM for the tunnel, which was mined through a combination on dolomite and limestone formations. The 20-ft diameter Robbins TBM, owned by S-K JV, was refurbished and redesigned for the DRTC. Originally built in 1980, the TBM had previously been used on at least five other hard rock tunnels including New York City’s Second Avenue Subway. The additions for the DRTC included new 19-in. disc cutters, variable frequency drive (VFD) motors, a back-loading cutterhead, and a rescue chamber. Due to the tunnel length and complexity, S-K JV additionally selected a Robbins continuous conveyor system for muck removal behind the TBM.
Conveyor System Design Parameters
The length and design requirements of what would become one of the most complex continuous conveyor systems in North America was governed by several parameters. These included the requirements by S-K JV for three runs, which included two extension tunnels in addition to the original DRTC. They were as follows:
Pleasant Run Extension: Total Length – 36,184 ft ; Length of Curve Sections – 6,857 ft; 19% Curve
Initial run as bid: Total Length – 39,360 ft; Length of Curve Sections – 7,523 ft; 19% Curve
Final Run with Eagle Creek Extension: Total Length – 38,638 ft; Length of Curve Sections – 7,077 ft; 18% Curve
The Pleasant Run alignment and the original alignment posed the greatest challenges as in each of these alignments the conveyor had to negotiate two 90-degree, 997-ft radius curves along with multiple S-curves. These curves were planned due to easement rights in the project area.
The robust conveyor system selected for the project had been used on several previous projects, including the Parramatta Rail Link in Sydney, Australia, and the South Cobb Tunnel in Atlanta, Georgia. As such, the conveyor system came with some equipment specifications that were predetermined:
Belt Speed – Nominal 3.65 MPS
Belt Width – 914 mm
Belt Strength – 600 PIW
Capacity – 600 TPH
It was then determined that an S-shaped vertical conveyor would be used to transfer the material from the tunnel to the surface. Specifications were as follows:
Belt Speed – Nominal 1.52 MPS
Belt Width –1.82 m
Belt Type – Black Standard Steel Cable Cross Rigid
Setup began at the jobsite in 2013. It was initially decided that the main drive, belt storage unit and splice stand would be located on the surface. This setup required that the vertical conveyor along with the horizontal conveyor be integrated into the shaft along with utilities and other equipment, all while still maintaining a window for the crane to lower supplies and materials to the bottom of the shaft. Even with a relatively large shaft diameter, the shaft area quickly became crowded, requiring everything to be closely integrated.
When the main components are located on the surface as was done on this project, there are some key advantages. The setup allows conveyor components to be preinstalled or installed in conjunction with TBM assembly, saving time and money. Once operational, the only time the conveyor system must be shut down is when belting is loaded into the belt storage unit. With the splice stand located on the surface, a roll of belt could be positioned and readied to be installed at any time. Initially the belt is installed in the belt loop using a mechanical fastener. By using mechanical fasteners, a 1,500-ft roll of belt can be added into the system in about 45 minutes. The mechanical fasteners can then be removed and replaced with a vulcanized splice. The vulcanized splice is considered a permanent joint and is good for the life of the belting.
The Conveyor in Operation
Excavated rock was removed with the horizontal and vertical conveyor system between 2013 and 2015. With over 35,000 ft tunnel of conveyer installed in the tunnel, another 250 ft going up the shaft, and the ability to store or let out 2,000 ft of conveyor from a surface mounted storage unit, the system was vast and complex. At the shaft bottom, the loaded belt discharged onto a vertical bucket belt to be hauled up the shaft and deposited onto a stacking conveyor.
Power to the loaded horizontal belt was supplied through a series of boosters with 149-kW drive assemblies. The unloaded belt was powered by return boosters, again with 149-kW drives. The surface storage unit provided easy access to the belt for inspection, and allowed belt to be loaded into the system without having to lower rolls of conveyor down into the shaft or starter tunnel as the TBM advanced. Splices were done by trained crews at the surface in a sheltered enclosure, and mechanical splices were kept to an absolute minimum, with mechanical splices replaced by vulcanized splices.
A 37-kW electric motor that was excited by a small variable frequency drive was used to supply tension in the system at the storage unit, and again, with no moving parts other than the cable due to the use of the VFDs. Each drive assembly underground was powered by an individual VFD as well. Each drive was monitored at the surface by PLC systems tied into a fiber optic cable, with Ethernet access available from the surface. For the system start-up under loaded conditions, various parameters such as timing between boosters, ramp up speeds, belt speeds, and motor loads could be monitored and then altered from an office setting topside, allowing for changes to the system as the belt length increased. By the time the belt had reached its full extension, a total of nine booster drives were installed throughout the tunnel length.
As the conveyor system extends, conveyor designers had to ensure the belt structure was not over-tensioned and would not roll over in curves. As such, the horizontal belt-carrying structure required specialized rollers in curves to keep belt tensions within a manageable range. These rollers, known as self-adjusting curve idlers and patented by Robbins, sense changing belt loads in curves and adjust the belt tension accordingly.
Conclusions and Lessons Learned
The challenging alignment of the Indianapolis DRTC proved a great opportunity to learn valuable lessons for upcoming projects. The first of these lessons was the attention to detail necessary when assembling and installing the equipment during startup and the importance of proper layout, as installation of each piece directly affects the next. Of particular importance was the installation of the horizontal conveyor sections as the TBM advanced. Additional attention and roller adjustment was necessary throughout the many complex curves.
Secondly, as progress advanced, mechanical splices were used when adding additional horizontal belt. These splices were removed and vulcanized as quickly as possible. The mechanical splices created weak points within the belt and were easily able to snag the horizontal belt structure and break. It was best to eliminate these problems before they arose using vulcanized splices.
Finally, and most importantly, it was imperative to have trained and qualified personnel to maintain the belt and belt structure. The belt mechanics were dedicated solely to daily maintenance and adjustment of the belt. Without a properly running conveyor system, the TBM cannot advance.