EPBs Excavate Capital City’s Emergency Wastewater Line
By Desiree Willis
Ecatepec, an area just outside Mexico City, has had more than its fair share of challenges. Shacks line the Gran Canal for miles, the country’s largest open sewer commissioned in 1910 by President Porfirio Diaz. The gravity canal has lost its slope over time due to sinking lake clays and now backs up every rainy season, causing wastewater to inundate surrounding neighborhoods. These untreated floods, or ‘black waters,’ bring road closures and potential health risks to the local population.
“Our goal is to move the contagions of the black waters away from the general population to what will be the country’s largest treatment plant,” said Ruben Perez, liaison for owner CONAGUA, Mexico’s National Water Commission.
The country’s bid to prevent the flooding has been squarely placed on what is a relatively new method of excavation for Mexico — TBM tunneling. Several TBMs were used on projects decades ago, but prior to 2009 a TBM had not been used in Mexico for 20 years.
The 62-km (38-mile) long Emisor Oriente Wastewater tunnel is located at depths of up to 150 m (500 ft) below the surface, a fact that made TBM excavation the only viable option. “These machines are specially designed for the unique geology of Mexico. They have been engineered to handle mixed face conditions from clay to basalt rock. Their flexible design is a great advantage to the project,” said José Luís Luege Tamargo, director of CONAGUA. Six TBMs, including three Robbins EPBs, are tackling tough ground conditions, high water pressures and tight schedules on what can only be called an epic tunneling endeavor.
Emisor Oriente Wastewater Tunnel
The pipeline, Mexico’s largest infrastructure project, is being built both to prevent flooding and add much needed capacity to the area’s growing population. Ground settlement in Mexico City has caused the existing gravity feed wastewater system, built in 1975, to lose its slope. Much of the main pipeline, Emisor Central, is severely corroded and at high risk of failure — an event that would cause up to 5 m (16 ft) of wastewater to flow into Mexico City’s surface streets.
Despite its obvious need, the project is not without some controversy, particularly for the valley’s farmers. Mexico’s untreated wastewater flows through a system of open canals that feed much of its farmland. The lush bounty of crops is the direct result of fertilization by the black waters, which may be tempered by the commissioning of the new Emisor Oriente line and wastewater treatment plant.
Mexico’s National Water Commission (CONAGUA) recommended immediate construction of a new line to help supplement the struggling system with an additional 150 m3/sec capacity. The line will carry wastewater from Mexico City to several water treatment plants currently under construction in the state of Hidalgo, where it will then be recycled for safe use in agriculture.
Scope of Supply
The construction of the line was divided into six lots — Lots 1, 2 and 5 under Mexican contractor Ingenieros Civiles Asociados (ICA) S.A. de C.V., and Lots 3 and 4 under Carso Infraestructura y Construcción S.A. de C.V. Lot 6 is under construction by Lombardo Construcciones and Constructora Estrella. All six lots were designated for excavation with 8.93 m diameter EPB TBMs, with Robbins supplying three EPB machines and accompanying continuous conveyor systems, and Herrenknecht supplying three EPBs.
The tunnel, designed with a finished diameter of 7 m, requires primary lining with 400-mm thick, steel reinforced concrete segments in a 7+1 pattern. The amount of steel reinforcement in each ring has been designed to vary according to the water pressure in each section of tunnel.
Mixed Ground Machines
The three Robbins machines were built for complex ground conditions including lake clays, volcanic rock, and boulders up to 600 mm in diameter. The ground conditions have been identified as some of the most complex in the world — abrasive basalt up to 80 MPa UCS is mixed with sections of watery clay that have been compared to a soup, while water pressure is estimated in the range of 4 to 6 bar, some of the highest pressures EPBs have worked under.
The custom designed EPBs were engineered with mixed ground, back-loading cutterheads to tack variable conditions. High pressure, tungsten carbide knife bits can be interchanged with 17-in. diameter carbide disc cutters depending on the ground conditions. During tunneling a number of small shafts, spaced every 3 km (2 miles) between the larger launch shafts, can be used to perform cutter inspection and changes. Specialized wear detection bits lose pressure at specified wear points to notify crews of a needed cutting tool change. The knife edge bits are arranged at several different heights to allow for effective excavation at various levels of wear.
The design also allows for bearing and seal removal from either the front or back of the cutterhead. Twenty-five injection ports spaced around the periphery of the machine are used for injection of various additives depending on ground conditions, and for probe drilling. Additives such as bentonite are currently being used to condition the muck for removal by belt conveyor.
Two-Stage Screw Conveyor
High-pressure conditions in concert with large boulders necessitated a two-stage screw conveyor design for the Emisor Oriente EPBs. An initial 900 mm (3 ft) diameter ribbon-type screw is capable of transporting boulders up to 600 mm (2 ft) in diameter up the center shaft for removal through a boulder collecting gate. Each of the three machines may encounter pressures of up to 6 bar, necessitating a two-screw setup with a ribbon screw and shaft-type screw in order to smoothly regulate pressure and maintain water-tightness.
Continuous Conveyors for Limited Space
Muck from all three machines is deposited from the screw to a fabric belt conveyor mounted on the trailing gear, which transfers to a Robbins side-mounted continuous conveyor. The continuous conveyor carries the muck to a vertical belt conveyor located at the launch shaft. Once at the surface, a radial stacker deposits muck in a kidney-shaped pile for temporary storage.
Due to the narrow shafts and small launch sites, the conveyor systems have been optimized for space efficiency and safety. The belt is surrounded by a guard with recycle hopper to prevent hazardous falling muck while returning the material to the vertical conveyor.
A unique vertical belt cassette allows for splicing of belt with a footprint 170 percent smaller than a typical horizontal belt cassette. The 34 m (111 ft) tall belt cassette is used to splice in a 450 m (1,500 ft) length of belt, which takes roughly 12 hours and allows the machine to advance for roughly 200 to 225 m (650 to 750 ft).
Complex Geology Crops Up
Despite rigorous geologic testing, excavation of the shafts in 2009 and 2010 revealed much more difficult geology than originally anticipated. These changes in geology resulted in some modifications to the designs of the machines, depending on the ground conditions in each lot. Lots 3 and 4 were found to include more abrasive basalt, volcanic rock, and higher water pressure than originally expected, for example. In 2011, modifications to the machine designs were done that included redesigned man locks, pressure bulkheads to accommodate higher water pressures, hard facing for screw conveyors and casings expected to go through rock, and hydraulically operated boulder collecting gates that replaced manual gates for faster removal of numerous boulders.
Although six TBMs were slated to bore at each lot, problems in Shaft 0 at the critically designated Lot 1 site prompted a swift machine change in 2011. “The first piece of this project is the critical 10 km (6 mile) section at Lot 1. The first Herrenknecht TBM started in Shaft 0 in 2009, but was stalled for six months due to a flood. In order to finish in time, meaning this year, we brought in a Robbins TBM originally slated to work on Lot 5, Shaft 20. The TBM is now boring at Lot 1, Shaft 5 in the opposite direction toward the TBM in Shaft 0,” said David Juarez, Site Manager for ICA. The Robbins TBM is boring a 3.5 km (2 mile) tunnel and was expected to reach Shaft 4 at the 1.8 km mark in March 2012. The entire section is slated to be complete in June 2012, after which the TBM will be disassembled and sent to its original Lot 5.
Due to the loss of slope, the Gran Canal at Lot 1 now has a positive vertical alignment below Mexico City and the Ecatepec area — the portion of the canal farther outside the city has not been affected. Increased volumes of water have the potential to overload current pumping stations and send the untreated water back into the city.
“We are currently building a treatment plant and a pumping station at Shaft 5 of Lot 1, to pump the water diverted into Emisor Oriente back into the Gran Canal where the slope has not been affected,” said José Miguel Guevara, General Supply Coordinator for Potable Water and Sanitation at CONAGUA. The pumping station will go into service as soon as tunneling is completed at Lot 1 and the finished section will be sealed off from the rest of construction.
In February 2011, the Robbins EPB boring at Lot 3 was launched to excavate a section of mixed ground giving way to nearly 4 km (2.5 miles) of highly abrasive basalt. The machine encountered this rock after just 78 m (250f ft) of boring and, as of early March 2012, had been stopped in order to execute a planned change out of the cutterhead knife bits for tungsten carbide disc cutters.
Meanwhile, the Robbins machines at Lots 4 and 5 are scheduled to launch in 2012 and 2013, respectively. The machines will be launched from very deep shafts — nearly 100 m (330 ft) for Lot 4 and 150 m (500 ft) for Lot 5.
The Lot 5 launch shaft (Shaft 20) had to be excavated in two stages because of its depth, using slurry walls at 20 m (65 ft) diameter for the first 46 m (150 ft), and conventional excavation at 16 m (50 ft) diameter for the remaining depth. The diameter variation enhances stability, a concern due to the great depth and ground conditions.
As the deepest tunnel, Lot 5 is expected to be the most challenging excavation. Water pressures in this section are expected to top out at 6 bar, requiring careful monitoring and high-pressure machine design. “Lot 5 has very high cover and mixed ground conditions with rocks and boulders in the soft ground. High pressure modifications may need to be done to the TBM after it completes its excavation at Lot 1,” said Roberto Gonzalez, General Manager for Robbins Mexico.
Despite challenges, excavation at Lot 1, Shaft 5 is going well. “At Shaft 5, we are almost at the 1 km mark, and we’ve had no real performance issues. The machine is doing much better than any other machine in Mexico right now,” said Andrew Courtney, Robbins Field Service Manager.
Juarez agreed: “The performance is very high compared with the other machines that are working here. The field service people of Robbins have been very helpful — they monitor advance every single day. There is a person specifically assigned to each machine to monitor advance.”
Though a sludge pump was initially used at startup due to limited space in the shaft, the conveyors were installed and began running in 2012. “We are expecting to bore 25 m (80 ft) per day minimum with the conveyors running — so far the performance has gotten better and better since we have switched over. This is the first time ICA has used conveyors, and the material is being conditioned just enough for transport, in order to keep the material from sticking in the conveyor,” said Juarez. To date advance rates have been up to 15 m (50 ft) per day, with no settlement detected.
While tunneling at the Emisor Oriente project is expected to take several years to complete, the fast-tracked Lot 1 will finish in 2012. The swift completion of the project is of critical importance to CONAGUA’s improvement plans, according to Guevara: “There are over 20 million people who live in the Valley of Mexico, and at this moment they are vulnerable. The treatment plant will treat up to 60 percent of the city’s water, but we need to look for alternatives to supply fresh water to the valley. We are working on pieces to the problem, but the problem is not solved yet.”
Desiree Willis is a technical writer for The Robbins Company.