To Grout or Not to Grout? In Rock Tunnels encountering High Water Pressure, Grouting can offer Great Benefits over Slurry
When you’re faced with a hard rock tunnel where there are expected significant sections under high water pressure, which tunneling method do you choose?
While Slurry Shield tunneling has a long history of addressing this problem, this method has not always been problem free. I would argue Slurry tunneling in rock is not, in most cases, the lowest risk or the most cost-effective method.
At recent projects around the world, we have seen that two non-slurry methods can be highly effective: use of a shielded, Non-Continuous Pressurized (NCP)-TBM in rock with a comprehensive grouting program, or sequential advance in EPB mode. Both types of tunneling operations have proven themselves safe, and have saved a significant amount of money for both contractors and owners.
Grout vs. Slurry
There are certain inherent traits to a Slurry tunneling operation that appear to give a lower level of risk: the entire operation is sealed; the slurry itself is conveyed to the surface through a system of pipes. But is this low risk truly the case?
Hyperbaric interventions are high-risk operations, particularly as water pressures go up. In water pressures over 6.5 bar, divers are often not permitted to enter the cutterhead, so grout must be used or there must be an alternate plan to bring down the high pressure. Higher pressure hyperbaric interventions up to approximately 12 bars have been successfully performed, but at what risk? Pressures in some tunnels have far exceeded 12 bars and would make hyperbaric interventions even more costly, risky and time consuming or impossible.
In ground with fines, slurry separation can be costly and difficult. Slurry tunneling is also not immune to problems such as blowouts/loss of face pressure when a fault zone or low cover zone is encountered, as is well-known in our industry from projects such as Hallandsås in Sweden and the SMART Tunnel in Malaysia.
Probing and Grouting
In an NCP-TBM operation, crew members may be more exposed to the tunneling environment but risks are not increased. With a good geotechnical baseline report and ground investigation tools, contractors can determine the zones requiring grouting ahead of the machine. It is now common to drill probe holes accurately of plus 100 meters with Down-the-hole (DTH) drills.
While grouting does take time and cost money, this cost has to be balanced against the cost and time to do hyperbaric intervention during slurry tunneling. Even 100% grouting in a rock tunnel could require less time than high-pressure hyperbaric interventions. The practice of pre-grouting has been done for years in drill & blast rock tunnels in Scandinavia and worldwide.
This video shows the basic process of probe drilling and grouting in a shielded rock TBM.
Grouting can also be done from a Slurry TBM of course, and is normally done to set up safe zones. However, it is worth noting that based on having a pressurized face filled with slurry, drilling through the head is very difficult. Sealed pipes/ports need to be installed in advance, eating up space and compromising the working conditions during hyperbaric interventions.
There has been recent development to enact cutter changes by accessing the cutters through the cutterhead under atmospheric pressure. However, this system requires a large diameter machine as well as a deep cutterhead structure. The deep structure severely affects muck flow and substantially increases the need for more frequent inspection and cutterhead repairs. These atmospherically accessed cutterheads do not address the problems of cutterhead repair, changing center cutters, or replacing scrapers, all of which are high wear items in rock tunneling at large diameters.
Are there times when a Slurry TBM has an advantage over an NCP-TBM in rock? Yes. Rock properties can drive the decision: Some rock formations are very difficult or nearly impossible to grout, and therefore the success of pre-excavation grouting will not be a given. If significant water inflows are predicted and the rock will not readily take common grouting material, or chemical grouting is not an approved option, a slurry TBM is the logical TBM selection.
Lining requirements are another potential reason not to go with Slurry: The operation of a slurry TBM goes hand-in-hand with the use of an (often expensive) segmental lining. Pre-excavation grouting using an NCP-TBM offers tremendous cost savings when done in a non-lined tunnel or when the liner can be installed independently after excavation.
In cases where a final liner has to be installed with tunnel boring, and often in cases where excessive water inflows are predicted, a slurry TBM may make more sense. Under excessive water inflows a grouting operation may still experience leakage after the initial tunnel construction, making installation of a final liner afterwards potentially costly and time consuming.
In Depth: Slurry Tunneling vs. NCP-TBM Shielded Tunneling in Rock
Cutterhead inspections in rock must be viewed with a different mindset than in soft ground tunneling. When tunneling in abrasive rock with any type of machine, inspections should be performed regularly; once per shift can be a requirement. This is in contrast to tunneling in soft ground, where cutterhead inspections are often planned and based on a set number of meters, for example every 100 m.
The Contractor Experience
Contractors who are used to tunneling in soft ground may not realize that when using a Slurry TBM in rock, inspections must be frequent due to increased cutter consumption. We have seen this borne out on recent projects such as the Hiroshima Expressway Line 5 in Japan. On that project, a 13.7 m diameter Robbins Slurry TBM is boring in granitic rock. The contractor opted for a Slurry machine because that was their historic experience, and they were expecting up to 13 bar water pressure. This high pressure water zone was only in a small section of the overall tunnel length, about 5 percent.
The contractor in Hiroshima had grouted off from the surface a planned safe zone in which to inspect the cutterhead without requiring a hyperbaric intervention, but this strategy did not go according to plan. The abrasive rock damaged the cutters and cutterhead before they could reach the safe zone, resulting in unplanned delays.
By far the biggest benefit of using a shielded NCP-TBM in rock, rather than Slurry, is the ease of cutter and cutterhead inspections. In areas with no pressure and with frequent or continuous grouting, the cutterhead can be inspected regularly and without the requirement of expensive, time consuming, and often risky pressurized interventions or complicated procedures to remove slurry from the cutterhead. Frequent inspections mean that cutter and cutterhead damage can be caught early before they cause significant downtime.
To go along with the above point, abrasive wear in any type of TBM is obviously always higher in rock than in soft ground, particularly when the rock has a high quartz or other abrasive mineral content. However, in Slurry machines, which crush the rock and send the rock chips through a system of pipes, abrasive wear is of even greater concern. Even with using durable slurry piping, transfer points and pipe elbows will require higher rates of replacement, causing more delays associated with muck removal than a typical NCP-TBM operation using a conveyor belt.
Dealing with Water Inrushes
If sudden water inrushes at high water pressure are a known risk, NCP-TBMs can effectively be designed to statically hold the pressure using sealable muck chutes in the bulkhead. This type of design can be used as a pressure-relieving gate in semi-EPB mode, opening by pressure and allowing muck to be metered out onto the belt. Or in extreme cases, the sealed gates can be activated and probe/grout drills can be used to forward drill and grout for ground consolidation and to seal off the water. Extra seals around the main bearing can be filled with pressurized grease and other vulnerable points can be sealed off in the same manner.
A Crossover TBM can also be designed to keep boring under pressure by implementing a center-mounted screw conveyor. A long screw conveyor can be used to draw down high water pressures and abrasion resistant hard facing can be added to the screw conveyor flights for abrasive wear. Under such conditions, a machine could operate continuously with, say, 3 bar pressure and sequentially in high pressure of 15-20 bar. An example of this is our ongoing project Mumbai Metro, where two Robbins Crossover TBMs are excavating in mixed ground. In these machines, the center screw conveyor is able to seal itself off/hold pressure so the TBM can continuously bore or operate using the screw conveyor in a sequential fashion. Boring is done when there are not enough fines to form a plug. Take a look at the below figures for a summary of sequential boring operation.
Dealing with Gasses and Contaminated Ground
In Slurry tunneling, dealing with gasses in the tunnel is relatively easy because the gasses are contained in the slurry pipes. Gasses can also effectively be contained and safely dispersed on non-pressurized TBMs using scrubbers and high volumes of air. On a recent Robbins TBM in Australia a machine was capable of operating in open mode with gasses using a bulkhead fitted with suction ports to draw any gas from the top of the cutterhead chamber and directly into a sealed ventilation system.
Contaminants such as asbestos may be better contained in slurry pipes, but many other types of contaminants may not be easily separated from the slurry and therefore easier to deal with using NCP- TBMs. In Slurry operation the quality of Bentonite itself can vary widely, with some lower cost material containing heavy metals, which has the potential to be detrimental to the environment. The slurry solution itself also tends to bind well with heavy metals, contaminating the slurry and making separation difficult.
The conclusions to draw from this discussion are straightforward. Is Slurry tunneling still a valid option in rock with potential of high water pressure? The simple answer is yes. Is it the most cost-effective option? Is it safer than any other option? In many circumstances the answer is no.
Slurry TBMs need a level of expertise in operation that NCP-TBMs simply don’t require. The operation of most NCP-TBMs is both simple and straightforward, which in turn saves on personnel costs.
My hope is that consultants and owners realize that Slurry TBMs are not the only option when high water pressure is expected. Slurry TBMs are not in most cases the lowest cost, and other methods can be just as safe while being simpler to operate. While grouting takes time, so does slurry tunneling with its typically lower advance rates and possible need for expensive, high risk hyperbaric interventions. When Slurry machines operate in rock, the need for frequent cutterhead inspections ultimately makes their use questionable. In most cases NCP-TBMs are the better option.
For Further Reading: Recent Industry Examples
At any given time Robbins TBMs are operating in dozens of countries around the world at all project stages. Thus far, 2020 has been no exception to the rule: From an icy visit to the world’s northernmost TBM to breakthroughs across the U.S. to a vast hydropower project on the verge of completion in China, we’ve got the latest updates from Robbins tunnels around the globe.
TBM Tunneling Above the Arctic Circle
Robbins engineers paid a visit to what is likely the northernmost TBM ever to operate in the world, at the Salvasskardelva HEPP near Bardufoss, Norway, 68.7 degrees north latitude. Robbins personnel and the contractor, Norsk Grønnkraft , have been braving frigid winter temperatures, ice and snow to excavate the 2.8 km long tunnel with a Main Beam machine at an upward gradient of 5.8 percent. As of the first quarter of 2020 they are nearly two thirds complete. Once breakthrough occurs the machine will be moved to bore a second tunnel 1.3 km long.
A Trio of U.S. Tunneling Breakthroughs
Meanwhile in the U.S. multiple machines ranging from 2.2 to 6.5 m in diameter broke through. First up is the Deer Creek Interceptor using a 6.5 m Main Beam TBM and continuous conveyor. The machine holed through on January 29, completing a 6.3 km long tunnel below St. Louis, MO for contractor SAK. Watch this great video from the owner, MSD Project Clear, below.
Also in January, the Turkey Creek Interceptor finished up: a project using a 3.0 m diameter Robbins Double Shield TBM to bore three short drives below Kansas City, MO. Contractor Radmacher Brothers bored a total of 220 m with the machine. Check out the video of the final breakthrough, and image of its first breakthrough several months earlier.
Lastly, in San Antonio, TX a 2.2 m diameter Robbins Double Shield TBM achieved a breakthrough at the SAWS Central Water Integration Pipeline, Segment 5-1. The tunnel, for owner San Antonio Water System, was excavated by contractor Atkinson.
A Massive Project Nears Completion in China
Two of three long-running Double Shield TBMs have completed their epic drives at China’s Great Hydro Network in Shanxi Province in recent months. The Great Hydro Network sprawls thousands of kilometers and is a feat of engineering. The Robbins machines at Tunnel 2 and Tunnel 4 bored from 15 to 23+ km in length. The machines overcame fault zones, water inflows and karst cavities to forge fast advance rates up to 865 m in one month.
Debbie Swival, Robbins Field Service Support, is no stranger to tunnel sites. Over her last 12 years at The Robbins Company she has worked on TBMs around the world, from the San Francisco Central Subway to the Moglicë Headrace Tunnel in a remote area of Albania to Turkey’s Bahce-Nurdag High Speed Railway and more. But her longest stay was in Newburgh, NY, USA for the Delaware Aqueduct Repair tunnel.
Swival remained on the jobsite from November 2017 until the TBM’s breakthrough in August 2019—a duration of 21 months. Her role in assisting the crew and troubleshooting issues was integral to the project’s success.
Boring Below the Hudson
The 6.8 m (22.3 ft) diameter Robbins Single Shield TBM for JV contractor Kiewit-Shea Constructors (KSC) bored a total of 3,794 m (12,448 ft) over 582 days with instantaneous penetration rates of 6 m (20 ft) per hour. The unique machine was designed to statically hold up to 20 bar pressure as it bored below the Hudson River to repair a section of the Delaware Aqueduct, the world’s longest continuous tunnel (137 km/85 mi long).
The New York City Department of Environmental Protection (NYCDEP), project owner, discovered that a section of the aqueduct below the Hudson River was leaking up to 75 million liters (20 million gallons) of water per day. On average the aqueduct—built in the 1930s and 1940s—supplies about 50 percent of the water consumed by 8.6 million residents of New York City and an additional 1 million residents in four counties north of the City. A swift repair of the tunnel section was essential.
The tunnel depth—ranging from nearly 270 m (900 ft) deep where the TBM was launched in Newburgh, New York to over 180 m (600 ft) deep at the exit shaft—the water volume, and pressure were all challenges. Probe drilling was mandatory ahead of the TBM and required the use of down-the-hole water hammers for accurate boring under pressure.
What was it like to be at the jobsite for 21 months?
Swival: Being on site for that long was a fantastic experience. I typically stay on a jobsite for 6 to 8 weeks so I do not see the machine in full production. Staying for the boring of the length of tunnel allowed me to see the way that the machine is actually used, as opposed to the theoretical understanding of how different operations should be done. This has given me new insight for programming the machines to better meet the customer’s needs.
Being there for that long also gave the opportunity to create a strong working relationship with the customer. I was part of the team, with everyone working together to get the job done safely and successfully.
What were your main roles at the jobsite?
Swival: Initially I worked with the crew on the machine’s electrical systems and PLC (Programmable Logic Controller) programming changes. Other members of Robbins Field Service conducted training on hydraulics and machine operation. We had three operators at the jobsite who needed training, so I also spent time afterwards reinforcing that training they received. Much of it was on-the-job style training.
As the job continued my role shifted to supporting the machine for any issues that came up. I assisted with troubleshooting hydraulic and mechanical issues as well as continuing to work with the electricians for any problems that arose there. I worked with the customer to implement changes that they requested to the PLC program and to the HMI (Human Machine Interface) screens. I also assisted the customer with interfacing with Robbins engineering to obtain information that they needed.
What was a typical day at the jobsite like for you?
Swival: My typical day started with a 7:30 am meeting with the engineers and supervisors to go over the plan for the day and address any technical issues that had occurred since the previous day. At 8:00 am, a quick safety meeting with the crew, then head to the cage where we all got to be up close and personal during the 6-1/2 minute ride down the shaft. Once at the bottom we got on the mantrip to ride in to the machine. Near the end of the drive it took around 20 minutes to get to the TBM.
Once in the machine, it was time to tie in with the night shift for information on what had happened during the night and where they were in the mining process. At that point it was time to start troubleshooting any problems that had occurred. If all was well I went through and checked the machine for any signs of something getting ready to fail such as wear marks on hoses or cables, low tank levels, loose mounting on sensors and other things that could be fixed before they broke and caused an issue. I spoke with the mechanics and operators to see if there was anything they needed assistance with and also got feedback from them on what they liked as well as what could have been done differently to make their job more efficient.
Conditions in the tunnel were as you would expect – loud and dirty, although there were only a few times when dust was an issue and respirators needed to be worn. The crew often said it was hot in the tunnel, but I love the heat so I didn’t really notice it. We ate lunch on the machine. Since we did not leave the tunnel until the next shift came in, something that is normally overlooked is the need for the crew to relieve themselves. On this machine there was a really nice toilet that was kept cleaned, which made life there so much better! Around 4:30 pm “Mantrip coming in!” was announced over the mine phones as the next shift started their way in to relieve us. We got back to the surface a little after 5 pm and it was so nice to be back in the fresh air!
How important are knowledgeable field service personnel for the success of a project?
Swival: It’s absolutely imperative to have knowledgeable personnel. At the Delaware Aqueduct site they had me there because I knew the equipment well and could solve problems quickly. For example, it’s beneficial to have a PLC person on site because it’s a specialized piece of equipment. Although after commissioning there are not many problems with the PLC, when there is an issue it requires the code to be changed. Some PLC hardware failures or changes can also require that the program is modified. When an electrical or hydraulic issue occurs on the machine, I can use the program as a troubleshooting tool, find out what physical device isn’t working then describe the problem to the crew and educate them on how to fix it. This shortens downtime from 12 hours for a crew without PLC personnel on site, to around two hours.
What are some other examples of challenges that you were able to overcome at the site?
Swival: I helped to optimize the boring parameters, such as reducing thrust pressure, adjusting ring build procedures, and fine-tuning articulation cylinder pressure to avoid downtime and keep the machine moving. There was a section of the tunnel where the rock transitioned from shale to granite, and the difference in rock strength meant we had to set new limits for the thrust pressure to extend the life of the disc cutters.
There was an issue with the segment unloaders where the forward unloader was in the lowered position and the rear unloader was raised. The setup didn’t provide any clearance for when the train moved out. Typical sensors were not able to be mounted in the location. I worked with the mechanical team to find a solution for sensing the positions of the unloaders and prevent this from happening in the future. Lights were added to give a visual indication to the operator of the fully raised and lowered positions, and I changed the logic to incorporate the modifications. Those are just a few examples of what I helped with.
What is your favorite memory from the project?
Swival: It was amazing to be able to climb through the hole in the cutterhead and stand in front of the machine when it broke through. The support from everybody throughout the project was incredible. Everyone worked together, and the level of community on the job was exceptional. It was simply a great place to work.
Video courtesy of NYCDEP (www.nyc.gov/dep).
For more on the unique tunneling operation at the Delaware Aqueduct Repair, check out these resources:
3 Ways to Bore More Efficiently in Extremely Hard Rock: Maximize your TBM Advance through Minimized Downtime
When the rock seems unbreakable, stresses are multiplied: The cutters must be stronger, the TBM more durable, and the operation optimized to keep equipment running smoothly. Once rock hardness rises beyond 180 to 200 MPa UCS, the limits of cutting tools are put to the test.
Given the clear risks of excavating massive, hard rock, how can tunnellers set themselves up for the best possible chance of success? The combination of knowledgeable personnel, properly designed equipment, and rigorous TBM operation and maintenance are making excavation of hard rock—even extremely hard rock strengths of 300 MPa UCS or more—possible.
1: Consider your Cutting Tools
Cutters are a significant factor for efficient excavation when rock is extremely hard. To that end, Robbins has developed Extra Heavy Duty (XHD) rings for projects where Heavy Duty (HD) rings are close to their design limit in terms of the thrust force required to break the rock. The XHD rings resist chipping, mushrooming, and other damage that can occur in very hard rock conditions. Enhanced heat treatment gives the discs increased hardness and strength without the normally-associated reduction in fracture toughness.
The rings have a proven track record: they’ve been put to the test at several jobsites, including Norway’s Røssåga headrace tunnel bored in rock from 200 to 280 MPa UCS. Initially, HD cutters mounted on the Main Beam TBM’s cutterhead experienced low cutter life in the range of 100 to 150 cubic meters bored per cutter. XHD rings were gradually introduced onto the cutterhead to determine what, if any improvement in cutter life could be obtained. It is likely that the performance in the very hard sections was improved by a minimum of 25%. The benefits of the XHD are also likely to explain the superior cutter life for the remainder of the project, even in the relatively softer ground.
2: Optimize Penetration Rate
Harder rock requires equipment that can stand up to high stresses. Penetration rate and thrust are exponential functions. The first rule is: Push as hard as you can. The more thrust the better. A machine with a robust steel structure is needed to take the higher loads without damage.
But penetration rate is trickier: The overall goal in hard rock should be to operate TBMs as efficiently as possible to maximize production. This means increasing penetration per revolution as much as possible. Consumption of cutterhead wear parts is related to the number of revolutions of the cutterhead, so it follows that increased penetration per revolution will result in fewer total revolutions of the cutterhead, reduced consumption of wear parts and fewer cutter changes for the duration of the tunnel.
The TBM Operator should therefore be looking for the best advance at the lowest RPM, because lower RPM reduces wear on the outer cutters and periphery of the cutterhead. Robbins has conducted site tests at multiple sites over several years showing that a lower RPM achieves the same and often better penetration rates than a higher RPM in hard rock. This can be clearly seen in the data: For example, on an 8 m diameter cutterhead, the circumference is 25.12 m. At 10 RPM during TBM operation, this would mean 251.2 m of travel in one minute. If the speed is reduced to 8 RPM, the distance would be 200.9 m of rotation—a full 50 m less in one minute. That is 3000 m less per hour, reducing wear substantially.
Malaysia’s Pahang Selangor Water Tunnel was a good example of this phenomenon. What is now the longest tunnel in Southeast Asia, at 44.6 km, required excavation using three 5.2 m diameter Main Beam TBMs mounted with 19-inch disc cutters. The machines operated in abrasive granitic rock exceeding 200 MPa UCS, up to 1,200 m below the Titiwangsa Mountain Range. RPM trials were conducted on the three TBMs, showing that an increase in penetration rate per revolution of 15 to 20 percent could be achieved by decreasing the RPM from between 11.5 and 12 to just 9.5. While the overall advance rate was lower because of fewer revolutions, downtime was decreased and cutter changes were reduced by as much as 19%. The overall time savings more than made up for the decreased advance rate.
3: Your Crew is the Key
Knowledgeable operators are key in hard rock: there needs to be a balance between cutterhead speed and thrust force. An experienced TBM Operator will be able to identify when ground conditions change and react accordingly. In addition, knowledgeable operators know how to react if varying rock strengths are present in the excavation face. The most effective way to prevent impact loading in such conditions is to reduce cutterhead speed and penetration rate per revolution.
Maintenance is another key point and is especially important for extremely hard rock conditions. At the start of a project it is recommended to set a maintenance period for each day, say four hours out of each 24-hour period, which enables the crews to become familiar with the maintenance regime. This equates to approximately 24 hours of maintenance in a 6-day working week. Cutter change time can take up substantially more than four hours per day in hard rock tunnels, so once the crews are familiar with the maintenance tasks they are best carried out concurrently with cutter change operations.
Cutterhead inspections should be carried out on a regular basis to enable worn, damaged or blocked cutters to be replaced as soon as possible. Boring with even one blocked cutter can result in a cascading type wipe-out, which will progress rapidly in a chain reaction effect through multiple cutters if not immediately recognized. In hard rock this can also result in damage to the cutterhead over the course of just a couple of boring strokes. The same applies to the inspection of the bucket lips—these should be performed regularly and the bucket lips kept in good condition.
Overall, ensuring success in some of the most difficult rock in the world requires pre-planning with proper machine design, good cooperation by all parties involved, a knowledgeable crew, and dedicated maintenance. More than that, it also takes some experimentation on the part of the contractor or those overseeing the TBM operation. Contractors can look at different cutter types and test the parameters of TBM operation, do regular penetration tests, vary the RPM, and vary the thrust to see what gets the best cutter life and the best TBM performance. Taking the time to do such testing can make all the difference between a successful project and an unsuccessful one.
Lastly, new and innovative cutter designs and housing designs exist for extremely hard rock. Consider XHD cutter rings, cutter mountings with hardened replaceable seats, and other designs that may be in development or available for testing.
Dozens of Robbins TBMs are normally in operation at any given time in countries around the world, but this month’s breakthrough extravaganza is exceptional. No less than six breakthroughs occurred during April 2019, and more are on the way. We take a look at these epic completions in pictures below.
Galerie des Janots, France
On April 3, 2019, a Robbins 3.5 m diameter Main Beam TBM broke through into open space, completing its 2.8 km long water tunnel. It was not the first time the machine had encountered open space: twice during tunneling, the machine hit uncharted caverns, the largest of which measured a staggering 8,000 cubic meters in size.
On April 8, 2019, the first of two 6.5 m diameter Robbins Double Shield TBMs completed its bore for Austria’s Gemeinschaftskraftwerk Inn (GKI) project. A second Double Shield TBM will break through later this year. The 22 km long headrace tunnel near the alpine town of Pfunds was bored under high cover (maximum of 1,200 m) in schist rock.
DigIndy Tunnel System, USA
On April 10, 2019, a Robbins TBM completed the White River and Lower Pogues Run Tunnels, part of the DigIndy project in Indianapolis, Indiana, USA. The 36-year-old, refurbished 6.2 m diameter Robbins Main Beam machine was launched in 2013 in Indianapolis, and has done exceedingly well. Multiple world records in the 6 to 7 m diameter range were broken on the job, including “Most Feet Mined in One Day” (124.9 m), “Most Feet Mined in One Week” (515.1 m), and “Most Feet Mined in One Month” (1,754 m). Over the course of the project the TBM will bore more than 40 km of tunnels.
Bheri Babai Diversion Multipurpose Project, Nepal
On April 16, 2019, the Prime Minister of Nepal and other government officials, contractor COVEC and Robbins gathered to celebrate the breakthrough of the first ever TBM in Nepal. The machine holed through months ahead of schedule after excavating in excess of 1,000 m per month.
Mumbai Metro Line 3, India
On April 18, 2019, the first of two 6.65 m Robbins Crossover XRE TBMs made its first intermediate breakthrough at the Mumbai Metro Line 3. The TBM completed its 1.2 km long tunnel drive from Cuffe Parade to Vidhan Bhawan station, and will now be readied for its second section on the 2.8 km lot.
Los Condores HEPP, Chile
On April 25, 2019, a 4.56 m Robbins Double Shield TBM completed tunneling a 12 km intake tunnel for the Los Condores HEPP in Region del Maule, Chile. Contractor Ferrovial Agroman overcame challenging mixed face conditions and high water inflows to break through into an underground chamber. A second machine—a 4.56 m Crossover XRE TBM—will be launched to bore another section of tunnel later this year.
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.
In Nepal, the greeting ‘Namaste’, while pressing both palms together in front of the chest, signifies both ‘hello’ and ‘goodbye’. It implies a circular concept of time that I rather like—I certainly think I will be coming back to this country of high mountains, valley forests, and yes, tunnels.
The Adventure Begins
When I first learned that I would be visiting Nepal to see a swift-moving tunnel project making an impact in a local community, I was pretty jazzed. I also took it upon myself to overpack. The Bheri Babai Diversion Multipurpose Project (or BBDMP for short) is a 12 km long tunnel that travels below protected forest considered part of the Bardia National Wildlife Reserve. This is an area home to tigers, rhinos, Asian elephants, leopards, and more. And, most worryingly for me, mosquitoes.
Mosquitoes love me. I’m not entirely sure why they love me so much, but let’s just say that if there’s a mosquito within a 2 km radius it will find me. Thus, I decided to be prudent and go all out when packing. I purchased a whole new set of safari clothes, the most potent bug spray I could find in copious amounts, and much, much more. I stuffed everything into two suitcases and set off on my journey.
A full 24 hours of traveling later (the journey from Seattle, USA to Kathmandu is no joke, folks), and I arrived in the Kathmandu airport…with no checked luggage. Through a gross miscalculation (what a way to learn a lesson!) I also had not packed hardly anything in my carry-on bag. I had no clothing with me, and more importantly, no bug spray. What was I to do?
Rolling with It
After discovering that my bags were several days away, I decided to forego any hope of reuniting with them during my trip. Instead, I rolled with it. After a one-hour local flight from Kathmandu to Nepalgunj, we arrived at our destination. Nepalgunj is a frenetic, dusty town located around 8 km from the Indian border and one hour from the BBDMP site. The culture is heavily influenced by India, and we had many meals of delicious spicy curry and fried bread. The streets were lined with small shops and marketplace stalls, but to my dismay, no department stores.
After a quick Google search my coworkers and our guests with us for the site visit headed to the nearest approximation, known as Rani Mart (Rani means ‘queen’ in Hindi). To my surprise, I purchased a whole new wardrobe and everything I needed for 1/10th of the price I would have paid for it in the U.S. (though the sizing on the tags was quite a bit larger!) I was feeling very pleased with myself.
A Little Perspective
The next day we were ready to visit the jobsite. I couldn’t help but notice the rolling blackouts that plagued the city and the vast stretches of farmland requiring huge water resources. The people in the surrounding area make do with limited resources in ingenious ways—I was intrigued to see, for example, that every outlet requires you to flip an on-switch before the current becomes available. Perhaps we Americans could save significant energy if we made a setup like this a national standard. Local hotels, including one we stayed at, use solar panels for their power and air conditioning, and harvest rain water in order to reduce their usage.
All of this made me realize how much the BBDMP will impact the surrounding areas. This was confirmed in a meeting we had with Nepal’s Department of Irrigation (DOI), the project’s contractor China Overseas Engineering Group Co. Ltd. (COVEC), and consultant Geodata. The tunnel is sourcing water from the Bheri River to the Babai River, traveling through mountainous Himalayan geology known as the Siwalik Range. The water, as the project’s name suggests, is for multiple purposes. The estimated annual benefit in Nepalese Rupees is $2.9 billion for irrigation, and $4.3 billion for hydropower, making a total of $7.2 billion in benefits once the project becomes active.
The completed tunnel will irrigate 51,000 Ha of land and provide 48 MW annual generating capacity. That’s not to mention the environmental benefits: The Babai River currently swells each monsoon season and then runs extremely low in drier seasons. It is connected directly to the groundwater table, which is being aggressively depleted. With a regulated flow during all seasons, the groundwater table will see less depletion year-round.
The impact, in other words, is huge. Learning all of this made my obsession with my missing luggage seem inconsequential in comparison. I had my bug spray and some clothes. That was all I needed.
The First Nepalese TBM
With such a landmark project for the region, its proponents were willing to look to the latest technology during the planning phase. Our local representatives, MOSH Tunnelling, had been working since the 1990’s to bring a TBM to Nepal, a country known for its Drill & Blast tunneling. While TBMs had been considered multiple times, each time conventional tunneling had been chosen.
When the BBDMP was fast-tracked as one of the country’s “National Pride Projects” feasibility studies showed that Drill & Blast excavation could take as long as 12 years. The DOI needed a faster option, and they found it in TBMs. They began working with MOSH Tunnelling and Robbins to bring what would be the first Nepalese TBM ever into the country—a 5.06 m diameter Robbins Double Shield. The process for the DOI to acquire funding for the project and select a contractor through international competitive bidding took seven years, spanning from 2007 to 2015, when project commencement officially began.
Fast forward to our site visit in November 2018 and the project is far exceeding expectations. Tunneling has topped out at 1,202 m in one month with an average of around 740 m per month. The knowledgeable COVEC team have traversed a major fault zone, the Bheri Thrust, with no problems, and overcome a stuck TBM shield with a bypass tunnel constructed in just five days. Overall, the TBM is far ahead of schedule and the results are of national importance.
The local community, national media, and government are all watching how the TBM excavation plays out at BBDMP. Given the strong performance, they are now considering TBMs for a host of future multipurpose water projects. It’s the kind of result that opens up a whole new marketplace.
Into the Forest
After a great site visit, we chose to relax for one day at a spectacular local lodge that offered jeep safaris into the Bardia Wildlife Reserve. Currently home to nearly 90 tigers (a number that is rapidly increasing through conservation efforts) we were hopeful of perhaps seeing one of the striped cats in the forest. Our guide was knowledgeable, our jeep sturdy and our driver adept as we traversed bumpy roads and forded a few streams on our safari drive. We saw mischievous macaques, tree-dwelling langurs, a host of brightly colored birds, and various deer. We saw several enticing tiger prints…but the tiger itself remained elusive. Obviously, this means that I must go back!
A Lesson Learned
After traversing a tunnel, traipsing through the forest, and some swift sight-seeing in Kathmandu on our last day, I felt I had come full circle. I was ready to head home, but with an eye towards future opportunities to return to Nepal, whether for business or vacation. I will be back. But next time, you can rest assured, I will only bring what I can fit in my carry-on.
Traveling in the tunneling industry is basically a regular commute for some. A lot of us don’t think twice about the destination, we just do it with a mission in mind. My mission as Robbins Marketing Manager was this: a small conference in Ho Chi Minh City. Okay, easy! As the date approached, about a month before, I kept hearing about how well our Main Beam TBM was doing in Vietnam, at the Thuong Kon Tum Hydroelectric project, despite a gauntlet of challenges. I talked to some project engineers and field service personnel who told me it was in a remote location in Kon Tum Province. And then I heard more about traveling to that site: a 6-hour drive from Da Nang, through nauseating windy, narrow roads, climbing elevations with no speed limits to speak of. But that’s cool, I thought, glad I don’t have to go on that ride.
“Wait, why not just go to the jobsite for a video and photo shoot? We can have it ready for the breakthrough in October,” says Desiree Willis, Robbins PR manager a couple weeks later. What!? A few days after that, it was all planned. Ron, our videographer, Ken, our photographer, and Keri Lin, our marketing manager from Robbins China, set off with me on a last-minute adventure. The 17.4 km long Thuong Kon Tum HEPP tunnel will be the country’s longest once complete. A 4.5 m diameter Robbins Main Beam TBM and continuous conveyor system were supplied to bore a section of the tunnel.
Maybe because I was bracing myself for the worst as far as the drive goes, I didn’t feel like it was that bad. The scenery was jaw dropping with lush jungle, rice paddies and scenic villages, and so many distractions on the sides of the road that I just completely avoided looking ahead. Problem solved! The coolest distractions were the scooters driving on these mountain roads: what they were carrying, how many people were on them, their driving technique. My favorite site was seeing a whole family of people—a 5-year old kid in the front, the dad driving, the mom in the back and a toddler sandwiched in between the parents—on a run-down 125 cc scooter. Then there’s the animals: a litter of stray dogs, adorable black piglets making road crossings that almost made me swear off bacon, herds of cows, goats, etc.
But how the heck did they get the machine, assembled by Onsite First Time Assembly (OFTA), to the site? Apparently, the previous contractor built portions of the roadway to the jobsite while components were staged at the port site. Several bridges, previously built only for light traffic with scooters and carts, were rebuilt or reinforced in order to carry the heavy TBM components. In a road conditions survey report by logistics firm SDV Vietnam Company Ltd., the original bridges were described as “having no shoulder”, and as having “blind corners and sharp curves”. The OFTA process allows a TBM to be assembled on location, saving time and money in terms of shipping costs and project schedule. The machine had successfully made it to the site, as it had launched in 2012.
Upon arriving at the jobsite, I was immediately impressed with the site and the surroundings, especially after hearing about all of the challenges the project had back when the TBM was launched, under a different contractor. The TBM launched into complex rocky conditions that turned out to be quite different than in the original geological study. The machine sat abandoned 2.6 km into the tunnel with equipment sitting idle for months, waiting for a change in circumstances. In 2015, Robbins signed a contract with the project’s new contractor CC47 to supply full refurbishment and to operate the equipment with a full Field Service team. Since then, Robbins Field Service has generally been kicking butt.
The machine was running as expected, so filming interviews and the machine went smoothly. I learned a lot from the interviews, mainly from the Robbins guys we interviewed—PN Madhan, Robbins Engineering Geologist, and Greg Adams, Robbins Field Service Manager. “It’s a workhorse of a machine and still in great condition, considering all of the difficult ground it’s been through,” Greg mentioned. Massive granitic rock wasn’t the only challenge they faced—the team also endured a handful of major fault zones requiring rock bolting and the use of the McNally Roof Support System, as well as some huge water inflows.
Greg did warn our photographer and videographer that they were going to face heat with extremely high humidity. When John McNally, Robbins Field Service Manager-Asia, took over the project site, he managed refurbishment of much of the equipment inside the tunnel along with the camp’s living quarters. This included adding a ventilation tube, keeping the tunnel at a cool 34°C. That’s the temperature AFTER installing the ventilation. Read more about the challenges here.
The crew at the camp quarters, and the jobsite in general, were very welcoming. Greg and Taylor Hwang, Robbins project manager, arranged for a special dinner just for us on the first night—steak and French fries, an American staple! The crew on site mainly consists of English-speaking expats, Chinese, Thai, Vietnamese, Indian and Philippine guys, so the cuisine varied every day. We hung around that night for a while, listening to stories from some of the field service crew. This is while we were fawning over the pet falcon that one of the field service guys owned. I think the falcon liked living there, because his string wasn’t even tied to the perch at one point. Taylor told us that there were some families and children of the crew living there, who had to find ways to pass the time in this little remote village. He even told us they had pet monkeys at one point. I peeked into a living quarter and saw a drum set made with those Danish butter cookie tins that you get at Christmas. That’s one way to pass the time! Before we were about to leave, some of the Thai people at the site had brought in a bucket with a cover over it. I was curious and went to take a peek, because they started adding salt to the buckets. It was about 30 river snakes (possibly river eels, but they called them snakes) writhing violently because of the sodium touching their skin. Then they started preparing them one-by-one, sticking a nail into their heads and gutting them. It was quite mesmerizing, really.
We only had two full days of shooting video and photos. Before the 6-hour drive back to Danang, all of us got caffeine-drunk on two strong Vietnamese coffees served with condensed milk each. I suspect it’s the perfect combination of the tenaciously strong espresso dripped into a puddle of pure sugar syrup (condensed milk) that singlehandedly causes adult ADHD from the first sip. During the drive down (which was much worse, I didn’t realize how much elevation we climbed going up), we were already reminiscing about the stories and experiences shared at the jobsite. When talking to the field service crew, they were all excited for the coming breakthrough later this year, but it seemed they were just a bit melancholy too. They were at this site for years, in a tiny remote village in the mountains of Vietnam, building friendships and routines. With only a handful of meters left to go on the tunnel, they’ll soon be off to the different parts of the world where they came from. I’m fortunate to have experienced everything I did on this trip and can’t wait to come back to this beautiful country.
At any given time, Robbins TBMs are operating at dozens of jobsites around the world. Our dedicated Field Service personnel take video and pictures of the TBM progress often, so we’ve decided to offer a periodic roundup of what’s going on in a picture-based blog–from assembly of a massive Slurry TBM in Japan, to machine assembly for India’s Mumbai Metro to extremely hard rock encountered in Vietnam. Read on to found out the latest.
MEGA TBM ASSEMBLY IN HIROSHIMA, JAPAN
Onsite First Time Assembly (OFTA) of a 13.67 m (44.8 ft) diameter Robbins Slurry TBM is underway for Hiroshima, Japan’s Expressway Line 5 project. The geology of the 1.4 km (0.9 mi) long tunnel is predicted to include maximum 190 MPa (28,000 psi) UCS granite with high water pressure of up to 13 bar. The Robbins Field Service crew is nearing assembly completion and will begin testing of the equipment in August.
SIMULTANEOUS BUILDS FOR MUMBAI METRO
Mumbai, India’s massive Metro Line 3 project requires multiple TBMs on various contracts. A total of four Robbins TBMs will bore on two separate contracts–two 6.65 m (21.8 ft) Crossover (XRE) TBMs for contract UGC-01, and two 6.65 m (21.8 ft) Slurry TBMs for contract UGC-03. Assembly of the first Crossover TBM and the first Slurry TBM on each contract are occurring simultaneously, and are being overseen by our experienced Field Service crews.
EXTREME HARD ROCK IN VIETNAM
Robbins Field Service crews operating the Main Beam TBM at Vietnam’s Thuong Kon Tum HEPP have encountered granitic rock exceeding 300 MPa UCS and high water inflows. Despite the challenges the tunnel is more than 85% complete.
AN EPIC PROJECT IN MEXICO
With one of three Robbins lots completed at Mexico City’s massive 62 km (39 mi) Emisor Oriente tunnel, Field Service efforts are focused on completing Lots 4 and 5 in difficult mixed ground conditions. The customized EPB TBMs are boring through some of the most challenging conditions in the world, from abrasive basalt to watery clays and boulders.
A JOB WELL DONE IN ALBANIA
TBM assembly and breakthrough is often discussed, but disassembly can be just as complicated. Following the recent breakthrough of the Crossover (XRE) TBM in Albania (the first machine of its kind in Europe) for the Moglicë Headrace Tunnel, Field Service crews are working to disassemble the machine in an underground launch chamber.
The idea of transparency is one that we most often hear touted in politics and policies. But transparency is a concept that applies to our underground industry as well. Widespread knowledge sharing can and should be the policy in our industry, but all too often jobsite politics, confidentiality agreements, and fear of poor public opinion limit what is ultimately divulged. I argue that transparency in tunneling is a help, not a hindrance, and we can make steps today towards clearer communication.
Why It’s Needed
- Learn from Experiences in the Field: Tunneling professionals deserve full access to the successes and the problems that have been encountered in the field. If we were to employ universal knowledge sharing, tunneling operations themselves could become safer and more efficient and overall project costs could be reduced.
- Improve Tunneling Technology: Knowledge of advance rates, performance in specific ground conditions, wear rates for cutters and other surfaces in contact with the cutting face, performance of ground conditioners and foams, and many other types of info are imperative to improving technology. Tunnel boring could be made faster, safer, and more cost effective with such knowledge. It could also improve willingness to try new and emerging technologies in the industry, which benefits all stakeholders. With a good knowledge of TBM performances in specific ground conditions, TBM specifications could also be written to greater accuracy and result in using the most cost effective solutions.
- Improve Risk Sharing: Risk is often apportioned unfairly on today’s tunneling projects. In many cases the TBM supplier is required to shoulder a larger part of that burden than their potential returns. With a better understanding of the risks through knowledge of past projects, as well as knowledge of past risk sharing strategies, this problem can be resolved.
- Ensure Fair Financial Practices: With transparency in terms of contractual pricing and project spending, payments for work performed, etc. and other unsavory practices can be lessened.
- Reduce Litigiousness: Claims in tunneling projects are on the rise worldwide. Recent projects have resulted in separate and duplicate claims against different parties, such as the equipment manufacturer and the owner, for the same issues and without their mutual knowledge. Such claims are unfair, and through transparency and equal access by all parties to a Dispute Review Board, the frequency and extent of lawsuits in our industry could be reduced.
From the Equipment Supplier Perspective
As equipment suppliers, we strive to share the information that is available to us. We’ve opened up about stuck TBMs, challenging drives, and how we’ve overcome those issues. We’ve also tried to share whenever possible why we think fast advance rates and good efficiency were achieved on various projects.
Unfortunately knowledge is sometimes not shared even within parties on the same project, such as the contractor, project owner, owner’s representative, and equipment supplier. Time and again we see that communication between all parties is key to overcoming challenging conditions. The road to transparency starts here: with clear communication between all coordinating parties on a given project.
Not Without Precedent
Transparency is not an untested idea in our industry. A great recent example is that of the U.K.’s Crossrail project. The project follows governmental codes of practice for data transparency and the country’s Freedom of Information Act. Details such as project and equipment costs, spending budgets and records, safety records, and more are available for download on their website, and special requests can be made for additional data. Multiple reports have also been up front about TBM performance, advance rates, and other experiences on the project. The Crossrail project was highly successful and, arguably, transparency played a large part in that. Crossrail is also just one example of transparency at work in the U.K.—governmental programs such as Transport for London’s Transparency Strategy aim to give clear and consistent information to the public about all road and public transit spending.
A Call for Clarity
Transparency is possible in the tunneling industry, but it may require things like international regulations, or at the very least certifications that could be provided by an international organization like the ITA. Such a certificate would be highly beneficial to all stakeholders: It could certify corruption-free practices, and guarantee knowledge sharing. Transparency is not an easy thing to achieve, and there are certainly barriers to the process in various areas of the world. But we can start with transparency requirements within projects, and then move outward. To be clear: knowledge sharing is something that can only benefit our industry. It is our recommendation we start today.
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- On the Move: Robbins TBMs Around the World
- What it’s Like to Live at a Jobsite for a Year (or More): An Interview
- 3 Ways to Bore More Efficiently in Extremely Hard Rock: Maximize your TBM Advance through Minimized Downtime
- A Bevy of Breakthroughs: The Robbins Jobsite Roundup Featuring 6 Epic Projects