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
Event Name: Bauma CONEXPO India
Dates: November 3-6, 2020
Location: New Delhi, India
Come visit Robbins at Bauma CONEXPO India in New Delhi to get expert advice and information from our team of experienced staff. Stay tuned for more information on what you can expect to see from Robbins at the show.
Small hydroelectric projects, with installed capacity up to 10 megawatts (MW), are a relatively untapped but potentially game-changing source of renewable energy in North America. In Norway, hydro projects are pioneering the use of small diameter TBMs in hard rock. Compared with drill and blast, TBMs offer increased production rates and reduction in cross section, among other benefits. The uniquely designed machines are engineered to take on steep gradients, up to a 45-degree angle in some cases.
On May 23, 2019, the last of six 8.93 m diameter EPBs completed excavation at Mexico City’s Túnel Emisor Oriente (TEO), a feat marking the completion of ten years and 62.1 km of tunneling. The TEO is a critically-designated plan to stem severe flooding while boosting wastewater capacity, and is the country’s largest infrastructure project. The six EPB TBMs excavated some of the most complex geology on earth, ranging from abrasive volcanic rock to watery clays.
The Metropolitan St. Louis Water District’s Project Clear is a 28-year program targeting water quality and wastewater capacity throughout St. Louis, MO. The extensive program involves multiple tunnels, including Deer Creek, a 6.3 km long tunnel being bored with the largest TBM ever used in the St. Louis area (6.5 m in diameter). Another tunnel, Jefferson Barracks, is using TBM components that have bored over 40 km of tunnel since 1981.
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.
Event Name: Bauma China
Dates: November 24-27, 2020
Location: Shanghai, China
Come visit Robbins at Bauma China in Shanghai to get expert advice and information from our team of experienced staff. Stay tuned for more information on what you can expect to see from Robbins at the show.
Event Name: Cutting Edge Conference
Dates: November 9-11, 2020
Location: Dallas, Texas, USA
Come visit Robbins at the Cutting Edge Conference in Dallas, Texas and hear relevant talks on industry hot topics while getting expert advice and information from our team of experienced staff. Sign up for the field trip to see the largest hard rock TBM ever to operate in the USA at the Mill Creek Project. The 11.6 m (38.1 ft) diameter Robbins Main Beam TBM is excavating an 8 km (5 mi) long tunnel. Stay tuned for more information on what you can expect to see from Robbins at the show.
Event Name: TAC 2020 Toronto: Vision Underground
Dates: September 28-30, 2020
Location: Toronto, ON, Canada
Come visit Robbins at the Tunnelling Association of Canada (TAC) Conference in Toronto, Canada and hear relevant white papers while getting expert advice and information from our team of experienced staff. Stay tuned for more information on what you can expect to see from Robbins at the show.
In January 2020, a Robbins 5.97 m (19.6 ft) diameter Main Beam TBM cleared its final hurdle when it broke through in Guangxi Province, China. The TBM excavated its first of two tunnels, an 11.9 km (7.4 mi) long conduit for Lot 1 of the North Line Water Irrigation Project, Letan Water Reservoir, Drought-Relief. The tunnel was marked by a gauntlet of challenges, from karst cavities to fault zones and water inflows. The workers on the jobsite, contractor Guangdong No. 2 Hydropower Bureau Co., Ltd., and the owner, Construction Management Bureau for the Letan Water Reservoir, had much to celebrate after completion of what is widely regarded as the most complex and longest tunnel on the North Line project.
Boring with the Robbins Main Beam TBM and continuous conveyor system began in summer 2015. “There was no precedent in this province for using a Main Beam TBM to excavate a tunnel longer than 10 km. We didn’t have relevant local experience to use for reference,” explained Yongjiu Jin, Deputy Manager of the Project for contractor Guangdong No. 2 Hydropower Bureau Co., Ltd. The machine did encounter a number of difficult geological obstacles as it bored through limestone rock, but was still able to achieve advance rates up to 40 m (130 ft) per day in good ground.
Much of the geology consisted of lightly weathered limestone in rock class II to III, with some sections in class IV to V rock that required the heaviest amount of ground support, ranging from rock bolts to ring beams and mesh. “Our team encountered a coal seam, gasses in the tunnel, two large water inrushes, three fault zones up to 103 m long, 11 karst cavities, and more. In order to solve the ground problems, there were more than 160 special technical research meetings held,” said Yongjiu.
Throughout tunneling, the contractor expressed thanks for Robbins Field Service staff. “Robbins personnel provided good technical support from equipment installation and commissioning through to tunnel completion. After the equipment was handed over to our company, they still helped us with equipment usage on our project, which makes us very satisfied with the Robbins after-sales service. Robbins really delivered: the after-sales phase was not the end of service, but the beginning of site service,” said Yongjiu.
While the completion of the first tunnel—the longest single-heading construction on record for water tunnels in Guangxi—is a milestone, there is more to do. The Robbins machine will be inspected and relaunched to bore a second tunnel 4.2 km (2.6 mi) in length. The ground conditions are predicted to be equally challenging, but the tunneling operation has some help from ground prediction methodology. Tunnel Reflection Tomography (TRT)—consisting of ground prediction using seismic waves—is being used to detect changing conditions ahead of the TBM. The method can predict the distribution and scale of joints and fissures, allowing the crew to plan ahead.
Located near Laibin City, the North Line project provides much needed drought relief using a network of tunnels totaling 29.4 km (18.3 mi). “This tunnel will realize the dream of drought control that people in Central Guangxi have had for many years. The breakthrough is the most important milestone event in this first phase of the North Line project,” said Yongjiu.
- Tough TBM breaks through after navigating Faults, Karst, and More
- Robbins unveils Largest Hard Rock TBM in the U.S. at Mill Creek
- Two Robbins Crossover TBMs to Bore Second Severomuysky Tunnel
- Compact TBM bores longest Rock Tunnel at 2.46 m Diameter
- Major Milestone for Delaware Aqueduct Repair as Robbins Single Shield Completes Excavation