On May 23 2019 SIX 8.93m-diameter EPB TBMs completed the excavation of Mexico City’s Túnel Emisor Oriente (TEO), ending ten years and 62.1km of tunneling. The TEO is a highly ambitious plan to stem severe flooding while boosting wastewater capacity, and is the country’s largest infrastructure project. The TBMs excavated some of the most complex geology on earth, ranging from abrasive volcanic rock to watery clays.

National water authority Conagua developed a critically designated plan to minimise the potential for catastrophic flooding if a wastewater line should fail. The mainstay of its scheme is the country’s largest infrastructure project, known as Túnel Emisor Oriente (TEO).

TEO is arguably one of the most challenging TBM tunnels in the world today. The six TBMs excavated the tunnel in some of the most complex geology on earth. This monumental work of engineering will create a complementary and alternative conduit to Emisor Central (the city’s existing and ageing main sewage conveyor) and give security to 20 million people.

Connected to the first major wastewater treatment plant in Mexico City, the new 62.1km-long tunnel will expand drainage capacity to reduce flooding risk, reduce the overexploitation of aquifers (which exacerbates the sinking of the Mexico City metropolitan area), and enable wastewater treatment to promote its reuse in agriculture.

The tunnel’s wastewater capacity will be 150m3/sec. Currently, the drainage system of the valley of Mexico has a displacement capacity of 195m3/sec, but with the commissioning of the TEO, it will increase to 345m3/sec. The TEO includes 24 shafts, ranging from 23m-150m in depth. The exit portal is located at the water treatment plant (the plant is the second largest of its kind in the world) which will enable the reuse of clean water for agricultural irrigation, as opposed to the sewage water used now.

Project Challenges

Mexico city’s geology comprises a drained lake bed with clays interspersed with volcanic rock and boulders from long dormant, buried volcanoes in the area. Water pressures on the alignment can be as high as 4-6 bar. After ten years of work, the six EPB TBMs encountered some of the highest pressures EPBs have ever operated under, in conditions ranging from very soft clays to highly abrasive materials, mixed ground, hard rock and boulders under high water pressures. This required frequent hyperbaric interventions in some of the lots and multiple modifications to the existing machines.

Contractual Setup

Conagua awarded the design, construction, and management of the project’s six-lot delivery to Comissa, a consortium of Mexico’s leading heavy civil contractors ICA, Carso, Lombardo, Estrella and Cotrisa (subsequently acquired by ICA). Each construction lot was around 10km long. Dirac and Lytsa were external supervisors as companies with the greatest tunnel supervision experience in Mexico. Conagua also contracted external advisors including experts such as Rick P Lovat, Dr Gabriel Fernandez, Dr Pier Francesco Bertola, and Dr Daniel Reséndiz Nuñez. Other onsite advisors included Pöyry.

Geological Conditions

Originally, the geologic profile was based on 64 borehole tests conducted along the tunnel length, as well as six cross-tunnel locations that were considered (Figure 2).

Original TBM Design Considerations

TBMs and conveyor systems were provided on the basis of the 2008 geological information. The three Robbins machines (for lots 3, 4, and 5) were designed for abrasive basalt sections up to 80MPa UCS, mixed with sections of watery clay that were compared to ‘soup’, with water pressure estimated in the range of 4 – 6 bar (Figure 3).

Adaptable cutterheads

The custom-designed EPBs were engineered with mixed ground, back-loading cutterheads to tackle variable conditions. High pressure, tungsten carbide knife bits could be interchanged with 17in-diameter carbide disc cutters depending on the ground conditions. During tunnelling, a number of small shafts, spaced every 3km between the larger launch shafts, were used for cutter inspections and changes, and to replace tail seals. Specialized wear-detection bits lost pressure at specified wear points to alert crews to a needed cutting-tool change. Knife edge bits were arranged at several different heights to allow for effective excavation at various levels of wear.

Twenty-five injection ports spaced around the periphery of the machine could be used to inject various additives depending on ground conditions, and for probe drilling, with an additional six ports for the foam system. Additives such as bentonite were used to condition the muck for removal by belt conveyor.

Two-stage screw conveyor

High-pressure conditions with large boulders necessitated a two-stage screw conveyor design for the EPBs. An initial 900mm-diameter ribbon-type screw would transport boulders up to 600mm in diameter along the center shaft for removal through a boulder collecting gate. Due to the anticipated high water pressures, a two-screw setup comprising a ribbon screw and shaft-type screw was deemed necessary in order to smoothly regulate pressure and maintain an effective water-tightness.

Muck from all three machines was 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 carried the muck to a vertical belt conveyor located at the launch shaft. Once at the surface, a radial stacker deposited muck in a pile for temporary storage. Due to the narrow shafts and small launch sites, the conveyor systems were optimized for space efficiency and safety.

EPB modifications

The three Robbins EPBs had to endure modifications to accommodate the mixed ground conditions on Lots 3, 4 and 5. Sections of hard abrasive rock coupled with high water pressures were discovered during shaft construction, and afterwards more borehole studies were undertaken that identified the challenging ground. Modifications included:

  • A seven-bar man-lock with an additional decompression chamber to allow two teams to work at the same time. Also, a material lock to be able to handle cutting tools more easily.
  • A redesigned bulkhead to allow the new configuration of the man and material locks up to seven bars and high pressure in the tunnel.
  • Chromium carbide plates to reinforce the screw conveyor and removable wear plates added to each turn of the screw conveyor in order to withstand abrasive hard rock. The screw conveyor was also able to open up as a ‘coffin’ to be able to check for wear and plates replacement.
  • An air compression system in order to control the water inflows in the chamber during excavation.
  • Grizzly bars in the cutterhead to enable the closing of the opening and rock sizes before entering the cutting chamber, when facing blocky fractured basalt rock.
  • A new design of the rotary union joint that improved the time to change the center disc cutters.
  • A new design of scrapers capable of resisting the load impact in mixed ground conditions in the presence of hard rock.

Project Highlights

Shaft construction

Several methods were implemented to be able to build the starting and receiving shafts. For soft ground, ‘milan’ or slurry walls up to 50m deep were used; after that, secondary lining with a vertical slip form provided rigidness to the structure. Conventional cut-and-cover with steel ribs, mesh and anchors was also used to excavate after the first 50m. According to the design, more or less one rib was installed after every metre.

One of the most important shafts constructed was shaft 20 – the deepest civil works shaft in Mexico, demanding extra attention to Lot 5. The shaft’s construction was quite unique, as the contractor used a hydro-roadheader that could excavate panels or sections of slurry or diaphragm walls up to 100m deep. After constructing the complete circumference of the shaft, the rest of the excavation was achieved by both traditional shaft sinking and cut and cover. Once the bottom of the shaft was reached, a starter tunnel of 28m was pre-excavated, in order to allow assembly of the machine.

TBMs assembled in very deep shafts

At Lot 5, the machine was assembled in the launch shaft and commissioned on August 2014 with the bridge and all the rest of the back-up gantries at the surface. The first back-up structure was then lowered with the hydraulics and the main electrical components. In October, after advancing 150m, the machine and its back-up gantries were completely assembled in the tunnel. One month later, the continuous conveyor system was installed and running.

After only 250m of excavation, new geology was encountered – a sticky greenish clay with very little water, making it difficult to properly extract through the conveyor system. Much of the muck and material ended up in the bottom of the shaft, dropped from the vertical conveyor. The contractor made several stops for cleaning due to the material getting stuck on the muck discharge chutes. The sticky clay material clogged the TBM cutterhead, necessitating the higher usage of additives to reduce wear and improve the performance. After going through the sticky clay material from Shaft 20 to Shaft 19, the ground conditions changed radically, around 100m before Shaft 19.

The TBM faced high water pressure with mixed ground: mostly hard clay, silty sand and isolated gravel. Once the TBM finished the drive through Shaft 19, the excavated material changed from a mixture of clay with silty sand to a complete face of hard rock (basalt) with a high-water flow (200 L/s).

Going through mixed ground conditions at lot 5

Erratic rock fragments and andesite deposits created wear problems for the cutting discs, which required a strict program of cutterhead inspections to inspect, change and analyze the unexpected wear issues.

Furthermore, the watery lake clays combined with abrasive basalt and large boulders created very challenging tunneling conditions. Normally, interventions are mostly for carrying out inspections, but in this case the wear issues and presence of cutting tools in the muck required many interventions over a period of more than 20 days for tool changes when high water flow was at its peak. For the next 1,000m, ground conditions improved but the pumice fragments of all sizes, sand with gravel, vulcanite, lava deposits, alluvial fans with boulders, sand matrix and high water flows resulted in an excavation with much uncertainty. However, despite these challenges, the machine achieved breakthrough on February 28, 2019.

Mixed ground conditions also limited production on the other lots. Abrasive material and high-water flow were constants. The machines were modified and the capability to change from disc cutters to cutting tools, as well as the capability to open or close the cutterhead using grizzly bars, helped the machines to face the changing ground conditions.

Lots 3 and 4: abrasive basalt rock

Lot 3, for contractor CARSO, was tunnelled for 9.2km from Shaft 10 to Shaft 13. The TBM was launched in February 2012. After several kilometres, the machine encountered worsening conditions with partial rock and soil at the face, causing impact loading on the cutters and the cutter mounting system, and severe wear on the cutterhead and cutting tools beyond what was expected. The machine also encountered a large amount of fines in the excavation face, causing clogging and requiring significant quantities of foam to be used.

As such, the contractor and Robbins proposed a new set of modifications, which were carried out at shaft 11. A new screw conveyor was fitted with Trimay wear plating to better handle abrasive rock chips, and a newly-designed cutterhead featured more wear plating and slightly different cutter spacing. The redesigns worked, and the machine ultimately achieved breakthrough in 2018.

One of the biggest tests for the EPBs came at Lot 4. The 10.2km-long lot ran from Shaft 17 to Shaft 13 at depths of up to 85m. The TBM was assembled in launch shaft no. 17 and commissioned in August 2012, with the bridge and all the backup gantries at the surface. Two months later, after advancing 150m, the machine and its back-up were completely assembled in the tunnel, and in November, the continuous conveyor system was installed and running.

After 405m of excavation, the presence of rocks, scrapers, parts of mixing bars and other wear materials in the excavated muck prompted a cutterhead inspection. With high pressure up to 3.5bar, it was determined that a hyperbaric intervention was necessary, and on 2 June 2013 the first hyperbaric intervention through an EPB in a tunnel was performed in Mexico. More interventions followed, forcing the contractor to perform hyperbaric interventions in order to change the cutting tools in a very complicated and harsh environment.

After about 50 hyperbaric interventions, the remainder of the project’s interventions were carried out in open air. Despite the challenges of pumping water of up to 180L/s and cleaning fines from the tunnel each time the operation was performed, atmospheric interventions were still lower in cost and quicker than those done at hyperbaric pressure. By the time of breakthrough on 23 May 2019, the machine had achieved a project record of 30m in one day, not to mention a high of 528m in one month.

Quick installation of secondary lining

The installation of cast-in-place secondary lining was undertaken with very good advance, thanks to the telescopic form that ensured a continuous cast tunnel slip-form for installation of the 350mm-thick lining. At 45m long, the form achieved over 180m/week.

Learning from Mexico

Lessons learned during tunneling were as varied as the geology. The basalt encountered was very abrasive. Reinforcement of high-wear components, such as screw conveyor flights and screw casings was critical when the machines were boring in rock conditions. Contractors also found that while operating in soft ground, excavation rates were higher, but more additives were used, while in mixed ground the machine advance needed to be slowed down to avoid impact damage and excessive wear to the cutters.

Of all the lessons learned, the most consistently mentioned advantage was the use of continuous conveyors rather than muck pumps. The contractors noted that advance rates were achieved thanks to the conveyor design. The tunnel conveyor was composed of elements such as the booster, vertical belt, curve idlers, and advancing tail piece, as well as elements on the surface.

Events leading to an extended project schedule and higher costs

The following are noted:

  1. Prior to the job starting, there was insufficient, precise geotechnical information and project design details available.
  2. Shared risk created many grey areas and conflicts between the contractors and the owner.
  3. Placing shafts according to geological profiles (rather than at equal spacings) would have allowed bespoke TBM design to suit each lot’s geology. This would have mitigated the effects of geological changes.
  4. Spare parts were not readily available at various sites.
  5. The lack of maintenance and resulting downtime could have been avoided if the responsibility rested, for example, with the contractor only.
  6. Unforeseen site events e.g a flood damaged a standing TBM; hyperbaric interventions; vertical conveyor failure; high wear to all EPBs, requiring modifications; and the presence of high-pressure water.

Conclusion

The Emisor Oriente Tunnel is a project that is both logistically complex and geologically daunting. The conditions tested the limits for EPB tunneling and limited advance rates. However, the lessons learned, now the project is completed, will be invaluable in terms of mechanized tunneling projects, contracting, regulations, machine design and mega project management for projects in the future.