The city of Columbus, Ohio, has a problem. For years, untreated wastewater has overflowed from its taxed sewer system during periods of heavy, prolonged rainfall. To combat this, the city’s Department of Public Utilities is undertaking a mammoth project to create a cleaner, healthier city. Part of that effort is the OSIS Augmentation Relief Sewer (OARS) tunnel, which will intercept overflowing water and carry it to the city’s wastewater treatment plants.
Of course, digging out a 4.5-mile tunnel under city streets, as well as under a river, requires not only heavy equipment, but also power. Royal Electric Construction Corp. is providing all power to light the way, pump water in and out of the tunnel, and maintain wireless connectivity. In addition, Royal is powering a giant by the name of Marsha, a 546-foot-long tunnel boring machine (TBM), which is chewing through rock and clay under the city.
The project is ambitious and has been a long time in development. The city launched a Wet Weather Management Plan with the Ohio Environmental Protection Agency in 2005. The plan was to build three deep-tunnel sewers along Alum Creek, the Scioto River, and in the downtown area. The OARS tunnel, due to finish in 2015, will enable the city’s treatment system to increase its capacity and add high-rate treatment functions.
Royal Electric arrived on-site in November 2010, with an $8 million contract, as sub for general contractor Kenny Construction (acquired by Granite Construction in December 2012), according to Rodger Dalton, Royal’s project superintendent. Royal has already done similar projects in the past and has a crew specialized and trained to manage the TBM-related tasks.
“There is a great deal of high-voltage and data work being done, and they are handling all aspects of the job,” said Bob Rautenberg, Kenny Construction project manager.
Operating the TBM
Marsha, the TBM, is working her way deeper into the tunnel—which drops down to 170 feet beneath the surface—at a rate of up to 40 feet per day, five days per week, 24 hours per day.
The $16 million, two-story machine consists of its 95-ton, 23-foot diameter grinding disk and an excavation chamber where broken rock passes before entering an auger-screw conveyor. That conveyor excavates the material into a slurry system, where a water, rock and clay mix is pumped to the surface. The TBM also has thrust cylinders to hold six 5-foot-long concrete pieces in place, while installing them into what becomes a segment ring in the newly cut hole; those cylinders also advance and steer the machine by pushing off from the concrete segments.
The TBM was assembled, as much as possible, on the surface in segments of 30–40 feet. The machine is hydraulically driven but requires electric power, so the mechanical and electrical contractors rely on each other to keep it operating correctly.
To turn the blade, the machine requires six 350-kilowatt (kW) motors. Additionally, its 546-foot length houses multiple motors for the many oiling and grease systems and the fresh-air ventilation system.
To power the TBM, Royal Electric installed approximately 18,000 feet of machine cable, ranging in size from 1/0 to 500 MCM (thousand circular mils) and more than 30,000 feet of low-voltage control cable for the wireless networks.
The entire project is supplied with a medium-voltage switchyard that was constructed on-site to allow for two 14,400V circuits for the TBM and water-pumping system. Both circuits will travel the entire length of the expanding tunnel, one providing 26,000 kW to the TBM and the other providing 36,000 kW to the pumps that are moving fresh and wastewater through the tunnel. To accomplish this, more than 46,000 feet of 15-kilovolt, tough, oil-resistant, SO cable is being installed as the TBM travels toward the other end of the tunnel.
The TBM’s lower level houses three transformers to convert the incoming 14,400 circuit to 480 volts (V). The upper level contains five large motor control cabinets and an emergency generator for lighting in the event of a power outage.
Staffing the TBM
Royal Electric has a team of five men stationed on-site at all times. Three men per shift are always onboard the TBM, providing 24-hour maintenance to keep it running smoothly. They monitor the TBM’s operations, making any corrections and adjustments and constantly testing functionality, ready to make any changes or halt operations if necessary; two of the workers are assigned to the top level of the machine, looking at and evaluating pressure and temperature readings of the cooling system and the motor control centers.
Two remaining team members provide the ongoing load calculation above-ground to prevent the possible overloading of the many temporary services, as equipment is added and removed. They also maintain electrical equipment on the surface and in the tunnel.
After each shift, a staff member goes over safety issues for his or her replacement.
Temporary power and lights
Royal Electric is installing one 400-watt (W) metal-halide fixture every 100 feet in the tunnel to supply the temporary lighting. When the tunnel is complete, these 250 fixtures will be removed.
Five thousand feet of 4-inch PVC with approximately 20,000 feet of cable, ranging from 4/0 to 750 MCM, is needed for general use to provide the temporary power throughout the 15-acre main project site. Royal Electric also installed two 800-ampere services to feed the surface area, one for the office trailer complex and one for the air compressor tent.
The monthly cost to power the entire site for the temporary power averages between $75,000 and $100,000, and that average is expected to double toward the end of the project.
In fact, this volume of electricity consumption causes a major challenge for contractors.
“The biggest hurdle is managing the incoming power,” Rautenberg said. “We are limited on the amount of electrical energy that the city can supply to us, and we need to balance that energy between the TBM and the 13 pumps (six at 500 horsepower, seven at 300 hp) that need to be installed in the tunnel. We are working with Royal to optimize the positioning of all this equipment.”
The TBM requires a freshwater supply for cooling and for removing the crushed stone from the cutting head of the machine. The freshwater begins its journey at a local stone quarry more than a mile from the main project site. It uses a series of six 225-kW pumps to supply the TBM with up to 24,000 gallons per minute, keeping the machine cooled and removing the crushed stone. Removal of the crushed stone is done by a series of ten 400-kW pumps that push it to the surface.
Employing laptops in the tunnel has been essential for construction crew and managers, so several wireless transmitters were installed on the TBM to enable the team to move around freely on the machine while diagnosing problems.
Royal Electric crew members on the TBM’s top level lay out an additional 25,000 feet of fiber optic cable in 2,000-foot intervals on the ceiling of the tunnel. The additional cable will be used in the tunnel to maintain constant communication with the TBM. More than 7,000 feet of 2-inch PVC provides the raceway for the fiber optic cable communications network on the surface.
The fiber optic cable is truly the backbone of the tunneling operation; it allows the TBM operator to control the supply and discharge water pumps in the tunnel and the pump on the surface. It also gives the operator the ability to control the shaker, which is a piece of electrically controlled equipment that is used to filter the water and separate the stone before sending the water back to the quarry. The fiber also allows for constant monitoring of the TBM from the surface, for navigation, production and critical maintenance.
Dalton said multiple issues can arise in this kind of work. The machine comes equipped with several hundred different pressure sensors to prevent problems.
The highly intelligent machine is self-diagnosing and can shut itself down when it senses danger. Such a case requires human interaction to diagnose the nature of the problem and fix it.
“Every day, there could be some challenge capable of shutting the machine down,” Dalton said.
Thus far, however, delays have been minimal. The Royal Electric team takes full advantage of the latest test equipment, such as sensors; workers also make use of an infrared scanner to check the many motor control centers and motors on the TBM, helping them detect possible loose connections, an overloaded motor or a worn bearing.
“It’s just a very demanding job. If things go wrong, they have to be repaired very quickly,” Dalton said.
When the machine is stalled due to a problem, the cost of labor alone can have a negative effect on the whole project, he said. Any time the TBM shuts itself down, it may require some effort to calculate why.
“It certainly can shut itself down, and you can’t ask it why,” Dalton said.
Also, problems can arise that have nothing to do with the TBM. For example, at one point early on in the project, excess water began interfering with work in the tunnel, Dalton said. Kenny Construction resolved the problem by first allowing the shaft to flood, then bringing scuba divers in to place pipe needed to force cement grout into the fractured stone to dam up the water.
When it comes to materials, Dalton said, the team tries to anticipate what spare parts might be needed in the event of a component failure. Because of the high cost of the material, the company has had to be selective in what may be of greatest urgency.
And there are other challenges; the machine was built in Germany, so all the diagrams and schematics are composed in the metric system. As a result, crews have found it necessary to not only to acquire metric tools, but also to think in metric. It poses a complication by pulling them out of their element.
“Our team has had to interpret machine diagrams that are written in French and Italian as well,” Dalton said.
They accomplish this by working with a lead person who is well-versed in those languages.
Dalton said that they have a meeting every morning to discuss the upcoming workday and any problems that may have arisen before. The team checks protective gear and discusses any near misses.
“We’re very much safety-oriented,” Dalton said.
Ultimately, the team needs to be flexible and prepared for any new event as it happens.
“There’s no instruction manual for this kind of work,” Dalton said.