Robbins Crossover TBMs are the adaptable solution to the world’s most challenging tunnel projects. This video explains how Robbins Crossover XREs, a hybrid of Hard Rock and EPB TBMs, work and why they are the solution for mixed ground tunnels worldwide.
On February 28, 2019 a ceremony was held to mark the breakthrough of a machine proven in both soft ground and hard rock. The 8.93 m (29.3 ft) diameter Robbins EPB and continuous conveyor system completed what is arguably one of the most difficult legs of the complex Emisor Oriente tunnel, an epic 62 km (39 mi) long conduit that will revamp wastewater treatment for more than 21.2 million people in Mexico City.
The 6 km (3.7 mi) long Lot 5, one of six lots at Emisor Oriente, required the Robbins machine to be launched from the deepest civil works shaft in Mexico, at 150 m (492 ft), and excavated mixed face conditions as well as abrasive basalt rock. “I am proud of concluding the excavation successfully given that this is the section with the greatest depth on the project,” said José Adolfo Méndez Colorado, Superintendente de Maquinaria for contractor Ingenieros Civiles Asociados (ICA). “Robbins Field Service had an important contribution in satisfactorily concluding this section. The machine operation was of primary importance: Field Service personnel know how to operate the equipment accurately for favorable results. The speed with which they detect a problem is a favorable point to reduce downtime on the excavation.”
The complexity of the ground conditions, continued Méndez, required the correct concentration and selection of polymers, a large challenge in itself. Field Service assisted with other issues and with equipment operation: “There were some issues with the screw conveyor conveying mixed ground and also in the articulation system, which were overcome with Field Service experience. They also knew the correct operation of the continuous conveyor system, which kept us to expected performance.”
“We are very proud of this machine as it has worked in two very different worlds,” said Roberto Gonzalez, General Manager for Robbins Mexico. In 2011 the machine originally slated for Lot 5 was fast-tracked to bore a 3.9 km (2.4 mi) long section of Lot 1b—a critical part of the line that needed to become operational right away to prevent chronic seasonal flooding. “The EPB proved itself while using conveyors in the sticky clays of Mexico City with very high percentages of water content, up to 400%.” The machine achieved rates of up to 592.5 m (1,944 ft) in one month—considered a record among the six EPBs (three of them Robbins) used on the project. After completing the bore in just 15 months, the machine was sent to the Lot 5 site, where modifications were made for a section of mixed ground and rock.
Modifications included a high-pressure man lock capable of withstanding 7 bars, chromium carbide wear plates added to the screw conveyor, and grizzly bars added to the cutterhead along with heavy duty cutting tools.
The machine was assembled in a 28 m (92 ft) long assembly chamber at the bottom of the launch shaft and commissioned in August 2014, requiring all back-up gantries to remain at the surface. Two months later in October 2014, after advancing 150 meters (492 ft), the machine and its back-up gantries were completely assembled in the tunnel. One month later, the continuous conveyor system was installed and running. “The assembly of the EPB at Lot 5 has the record of the deepest assembly of a TBM in Mexico. It required great coordination of ICA engineers, high capacity cranes for the assembly and human resources in order to keep to the ten-week schedule for assembly,” said Gonzalez.
With tunneling at Lot 5 now officially concluded there is one more breakthrough remaining before the Emisor Oriente line is complete: An 8.93 m (29.3 ft) Robbins EPB operating at Lot 4 is scheduled to break through this spring. A third Robbins EPB operating at Lot 3 completed its tunnel in 2018.
The Túnel Emisor Oriente (TEO) is the answer to more than 100 years of sinking in Mexico City as the result of drained lake beds and general lowering of the water table. The city’s buildings, main streets and other structures have sunk more than 12 m (39 ft) in that time, and much of the area is vulnerable to flooding. At the same time the area’s gravity-fed wastewater lines have lost their slope and are severely under capacity.
Once tunneling is complete, the resulting TEO will expand capacity, adding 150 m3/sec (5,297 ft3/sec) to the Valley of Mexico system, as well as alleviating the risk of flooding in critical areas. It will also convey wastewater to the country’s largest wastewater treatment plant. The massive tunnel includes 24 shafts, ranging from 23 meters to 150 meters (75 to 492 ft) in depth, plus an exit portal at the treatment plant in the Municipality of Atotonilco, in the state of Hidalgo.
Excavation of Turkey’s longest water tunnel came to an end on December 18, 2018. To get there, a 5.56 m (18.2 ft) diameter Robbins Crossover (XRE) TBM and the contractor JV of Kolin/Limak had to overcome dozens of major fault zones and water pressures up to 26 bar. The completed national priority water line is set to go into operation in March 2019.
The 31.6 km (19.6 mi) long Gerede Water Transmission Tunnel is an urgently needed project due to severe and chronic droughts in the capital city Ankara. Its final leg, a 9.0 km (5.6 mi) section of extremely difficult ground including sandstone agglomerate, limestone and tuff, was just one section in the middle of a tunnel widely considered to be the most challenging ever driven by TBMs in Turkey. “I’ve had the chance to study and visit the majority of mechanized tunnelling projects in Turkey since the 1980s. The Gerede project is one of the most challenging projects among them,” said Dr. Nuh Bilgin, Professor of Mine and Tunnel Mechanization at Istanbul Technical University and Chairman of the Turkish Tunnelling Society.
The Robbins XRE TBM was called in to complete the tunnel, which was at a standstill after using three Double Shield TBMs from another manufacturer. Those machines encountered incredibly difficult geology including massive inrushes of mud and water. The Kolin/Limak JV had to develop a new strategy given the unexpected ground conditions. They contacted The Robbins Company, who suggested a Crossover (Dual-Mode Type) TBM for the remaining section of tunnel. “The Crossover TBM provided great ease and versatility during the entire project with frequently changing ground conditions. The TBM was equipped with features such as increased thrust, two-speed gearbox, and modular screw conveyor. It was capable of giving the necessary responses in different geologies, which was our most important asset in achieving our goal,” said Barış Duman, Project Manager for the Kolin – Limak JV.
“The challenging part for us was to design and manufacture a TBM that could complete the difficult section of the Gerede Tunnel where two other competitor TBMs had failed,” said Yunus Alpagut, Robbins representative in Turkey. The specialized machine was designed to statically hold water pressure up to 20 bar, a failsafe that none of the standard Double Shield TBMs had been equipped with. A convertible cutterhead was also provided that was designed for ease of conversion between hard rock and EPB modes, and with cutter housings that could be fitted with either disc cutters or tungsten carbide tooling. To cope with difficult ground, the Gerede machine was also equipped with the Torque-Shift System, multi-speed gearing allowing the machine to function as either an EPB or a hard rock TBM. This function is done by adding another gear reduction–heavy duty pinions and bull gears accommodate high torque at low speed, allowing the machine to bore through fault zones and soft ground without becoming stuck.
The Crossover machine was assembled in spring 2016 after crews excavated a bypass tunnel to one side of one of the stuck Double Shield TBMs. An underground assembly chamber allowed the machine to be built in the tunnel using Onsite First Time Assembly (OFTA). “The logistics of getting components through the existing tunnel were the most challenging thing. The assembly chamber was 7 km (4 mi) from the portal. The water inflow of 600 l/s (159 gal/s) made it difficult to get the materials to the machine,” said Glen Maynard, Robbins Field Service Site Manager.
Despite the challenges, the machine began boring in summer 2016 and within the first 50 m (160 ft) of boring had successfully passed through the section that buried the original Double Shield TBM. The machine was required to be used in EPB mode as it encountered water pressures up to 26 bars, alluvium, flowing materials, clay and a total of 48 fault zones. Water pressure was lowered by draining the ground water through the rear shield probe drill ports, which were equipped with normally-closed ball valves. Probe drilling was done on a routine basis to get through the ground conditions. “Together with the difficult geological conditions the travel time to reach the TBM within the tunnel had effects on TBM performance. Despite this constraint, the tunnel excavation achieved a best day of 29.4 m (96.5 ft), best week of 134.6 m (441.6 ft) and a best month of 484 m (1,588 ft),” said Duman.
“We had many challenging areas with water and high pressures up to 26 bar along with alluvial material in fault zones. Ground pressure on the shield body caused squeezing conditions in clay. In these regions, we were able to quickly pass through by keeping the TBM advance rate, cutterhead rpm and screw conveyor rotation speed at the ideal level. Ultimately, we think our decision to select a Crossover TBM was correct,” continued Duman.
With tunneling complete, the pipeline is on track to open in March 2019. The tunnel will convey water from the Gerede River to Çamlıdere Dam, which provides potable water for the Ankara city water system.
Rapid excavation is considered by many to be the ultimate goal in TBM tunneling—machines that reliably complete projects on time (or early) with faster advance rates, regardless of conditions. However, speeding up a project schedule is not as straightforward as pushing a machine harder, working longer hours, or increasing your crew size. The entire project schedule, from initial investigations to the design process, must be considered.
In this succinct, complimentary 40-minute presentation designed to better fit your workday, Robbins Engineering Director Brad Grothen and Elisa Comis, Associate at McMillen Jacobs, discuss what rapid excavation really looks like in the field including case studies on TBM design and real-world examples of project scheduling.
Rapid Excavation: It’s a term bandied about throughout our industry, but what does it mean? It’s considered by many to be the ultimate goal in TBM tunneling—machines that reliably complete projects on time (or early) with faster rates of excavation, regardless of conditions. However, speeding up a project schedule is not as straightforward as pushing a machine harder, working longer hours, or increasing your crew size. The issue is complex, and we’ve put together 7 key points to help you navigate it.
1. Consider the Entire Project Schedule
First of all, consider that increasing the excavation rate may not be the only way—and indeed may not be the best way—to speed up a project schedule. The generalized graphic below illustrates my point: TBM excavation often makes up around 25% or less of the total time to complete a public works tunnel. In fact this is a conservative value as by many estimations the total project time is often 15 years. Even if we were to increase the excavation rate by several times what TBMs are currently capable of, it wouldn’t significantly speed up project delivery.
Shortening the decision-making process or the design and consulting process is much more feasible than creating a “super-fast TBM” and would have a bigger impact on the project schedule as well.
2. Know the Facts about TBMs
TBMs are fast, and they’ve been fast for decades. In fact, 50% of all known TBM world records were set more than two decades ago. Much of the seeming lack of progress is illusory–it has to do with the fact that modern tunnels are being built in ever more difficult geology, while more stringent health and safety standards put necessary limits on the excavation process, among other things. Today’s TBMs are capable of boring in harder rock, in higher water pressures, in mixed ground conditions and a host of other environments that would have been impossible in the 1970s and 1980s. And they do it while performing well; indeed, at much higher rates than conventional excavation. The below chart is a good illustration of just how far TBMs have come in recent years.
3. Know That Productivity Has Vastly Improved
There have been some recent articles looking at decreasing productivity in the construction industry overall, such as this article in The Economist. While the productivity of the overall construction industry is up for debate, productivity is not decreasing in the tunneling industry. Moreover, productivity is incredibly reliant on each project’s limitations and requirements. When considering productivity, think about logistics, geology, and data.
Based on decades of field data, we’ve found that a typical TBM heading is two to three times faster than a drill & blast heading. This effect is more pronounced the longer the tunnel drive, and more than makes up for the typically longer lead time to acquire and mobilize a TBM.
So it’s safe to say that TBMs are the way to go for more productive tunneling in all but the shortest tunnels. Logistics is the other key: scheduling of crew and materials, particularly in long tunnels, is so important. This is doubly so if using muck cars. For this reason, using continuous conveyors for muck removal is more efficient, as the removal process does not need to stop for personnel and material movements. In fact at least 75% of all TBM world records were set while using a continuous conveyor for muck removal.
Lastly, consider geology when planning the construction schedule. Even a customized machine with streamlined logistics will bore more slowly in fractured volcanic rock with significant fault zones than in competent sandstone. Setting the excavation schedule requires a close look at geology and the excavation rates of recent projects in those conditions.
4. Identify the Bottlenecks
The bottlenecks must be identified and alleviated if productivity is to be increased. Think about the operations that can be done simultaneous with boring that are now done separately:
- Applying a Concrete Lining: Continuous concrete lining can be done concurrent with boring in many cases. This type of lining eliminates the separate operation of lining a tunnel with segments. Waterproofing membrane can be applied with a membrane gantry if needed
- Increasing Automation: Processes such as cutter changes and segment erection can and are being fully automated on research projects in the industry. Full automation could significantly reduce downtime
- Eliminate re-grip time: When setting segments and thrusting off rings, elimination of re-grip time could be key to increasing advance rates. New innovations such as helical segments are promising to do this through a simple change in segment architecture
5. Understand the Limitations
There has been talk in our industry of making TBMs excavate up to ten times faster. While this is all well and good to aim for, in many cases it may not be realistic. For example, when boring in soft ground using EPB TBMs the penetration rate is limited by material flow and additive permeation. Boring at faster rates could cause heave in front of the TBM followed by subsidence at the surface.
So how could we bore faster in softer ground? It would require a change in the mechanism of excavation—no short order. It would require a better way of holding pressure than the screw conveyor can currently achieve. This is just one of many examples where physical limitations are the barrier to speed, not efficiency.
6. Think Outside the Circle
The possibilities for tunnel construction in the future are intriguing. Consider non-circular tunneling machines, of rectangular, square or other shapes. How much efficiency could be gained by creating a tunnel that requires no back-filling or invert segments to create a flat tunnel invert? Robbins has been exploring these types of machines for decades, with machines such as the Mobile Miner, seen here.
7. Promote Industry R&D
Lastly, there are things all of us in the industry can do to advance technology towards faster and safer tunneling. R&D in our industry is necessarily incremental as technology must be tested for safety and efficacy. But the rate of advancements could be sped up with better funding and closer cooperation between owners, consultants, contractors and TBM suppliers.
Watch a 9.26 m Robbins Crossover XRE TBM, dubbed “Rosie” in honor of Rosie the Riveter, tackle tough ground on the Akron Ohio Canal Interceptor Tunnel (OCIT).
- The Crossover in 3D: Robbins XRE TBMs
- Akron’s Crossover TBM: Powering Through Tough Geology
- Overcoming Fault Zones and Water Inflows: Thuong Kon Tum Hydro Project
- Moving Projects Forward: Ohio Canal Interceptor Tunnel
- Overcoming Tough Ground: Akron Ohio Canal Interceptor Tunnel