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 truly the case?
At recent projects around the world, two other methods have proven themselves in hard rock under water pressure: Shielded, Non-Continuous Pressurized (NCP)-TBM tunneling in rock with a comprehensive grouting program, or sequential advance in EPB mode. Both types of tunneling operations have proven themselves safe and cost-effective at projects in the U.S. and around the world.
NCP tunneling, defined as Non-Continuous Pressurized tunneling, may utilize a Single Shield Hard Rock or Crossover (Hybrid Rock/EPB) type machine. These TBMs are capable of sealing themselves off to water pressures above 20 bar using a pressure bulkhead when needed. Whether the machine is designed to statically hold water pressure using a sealable muck chute, or to bore under pressure using a screw conveyor, is up to the requirements of the project.
Key Comparisons: Cutterhead Inspections
Cutterhead inspections in rock must be viewed with a different mindset than in soft ground tunneling. When tunneling in 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 Slurry Shield machines are more commonly used, as this is the type of geology they were originally designed to excavate. In soft ground conditions, cutterhead inspections are often planned and based on a set number of meters, for example every 100 m. 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.
Often, these inspections in Slurry TBMs require hyperbaric interventions—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.
We have seen this borne out on recent projects, including the use of a large, 13.7 m diameter Robbins Slurry TBM that bored through granitic rock in Japan. 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 NCP tunneling with shielded TBMs in rock, rather than Slurry TBMs, 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.
Key Comparisons: Abrasive Wear
Abrasive wear in any type of TBM can be high depending on the abrasiveness of the material—whether rock, sand, or otherwise. 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 than in hard rock machines. The material being excavated by a Slurry TBM is constantly in contact with the cutterhead and cutting tools, increasing the amount of time that abrasive wear can occur. Even with 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 tunneling operation using a conveyor belt.
Ground Conditions: Water Inrushes
If sudden water inrushes at high water pressure are a known risk, non-continuous pressurized 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 the Mumbai Metro, a project that utilized two Robbins Crossover TBMs. 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.
The sequential operation proceeds as follows: The screw conveyor discharge gate is closed, and the cutterhead chamber and screw conveyor are pressurized with water. The muck chute gates remain open so the muck can enter the cutterhead chamber and screw conveyor as the machine mines forward. As the screw conveyor fills up with muck, the water is pushed out of the screw and back into the cutting chamber. Once the screw conveyor is nearly full, the muck chute gate is closed and the water pressure inside the screw conveyor is lowered by emptying it into a holding tank on the back-up. The muck is then removed from the screw conveyor onto the back-up conveyor, the discharge gate closed again, and the screw conveyor refilled with water at pressure. Once again, the muck chute gate is opened so the machine can bore forward. The entire process can be automated to simplify TBM operation in water-bearing ground (see Figure 1 and 2).
Ground Conditions: Rock or Mixed Ground with Low Fines
In rock or mixed ground with low fines where a plug cannot be effectively formed with an EPB screw conveyor, there have previously been limited options. Using slurry TBMs in such conditions has a high risk of blowout at the surface. New concepts have been developed using the Crossover TBM with Positive Pressure Control (PPC). The Crossover with PPC can bore in sequential operation using compressed air and dual screw conveyors when water pressure is high, or in open mode, discharging muck onto a belt conveyor when high pressure control is not needed. The net effect is adaptable for changing ground conditions that require stable face pressure control (see Figures 3-5).
Ground Conditions: 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 the 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 tunneling methods. 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.
Cost Comparisons: NCP-Tunneling vs. Slurry Tunneling
In ground with fines, slurry separation can be costly and difficult. Slurry tunneling is also not immune to problems such as blowouts or the 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.
In any evaluation of cost, the increased power requirements resulting from the slurry separation and transport system need to be considered. In order to make the excavated material pumpable by centrifugal pumps and prevent settling, high levels of flow are required over the length of the tunnel with substantial losses due to friction—this leads to both wear and increased power requirements. Since the pumps in the transport system are carrying the excavated material, high clearance pumps are used which further lowers the efficiency of the systems. Once on the surface the added fluid must then be separated, which requires additional power. Increased power is further required when fine particles like silt and clay are present.
In general, Slurry TBMs need a level of expertise in operation that NCP tunneling with a shielded machine simply doesn’t require. The operation of most shielded rock TBMs is both simple and straightforward, which in turn saves on personnel costs. Crew members may be more exposed to the tunneling environment in NCP tunneling operations, 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 percent 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.
Grouting can also be done from a Slurry TBM of course, and is normally done to set up safe zones. However, 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 a 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.
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 in NCP tunneling from a shielded TBM offers tremendous cost savings when done in a non-lined tunnel or when the liner can be installed independently after excavation. A slurry TBM may make more sense in cases where a final liner has to be installed with tunnel boring, and often in cases where excessive water inflows are predicted. 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.
Recent Industry Example: Delaware Aqueduct Repair
One of the best recent examples of correctly applied NCP tunneling can be seen at the Delaware Aqueduct Repair tunnel in New York, USA. On that project the contractor won as the lowest cost bid using NCP tunneling with a hard rock Single Shield TBM and grouting because they understood the risks and the geology of the project. They anticipated significantly less water impacts than the maximum indicated in the bid documents, as well as less grouting efforts after careful analysis of all available geotechnical data. However, Robbins and the contractor included redundant pre-excavation grouting plants on the TBM in the event of possible high water flows. These redundant plants were ultimately seldom used during tunneling.
The 6.8 m diameter Robbins Single Shield TBM featured a unique setup to deal with water pressure. The tunnel was bored from 270 m to 180 m below the Hudson River and the machine featured a bulkhead for sealing in case of high water inflows at the tunnel face. The closeable bulkhead allowed the excavation chamber to be sealed off in the event that groundwater inflows (shunt flows) from the excavated portion of the tunnel caused washout of the annulus grout. If the bulkhead was closed, the groundwater flows could be stopped and secondary grouting of the precast liner could be performed, effectively cutting off the flow path.
When water inflows exceeded contract-allowable values, grouting was required to reduce water inflows to acceptable levels. The TBM could then advance inside the grouted area of the alignment. To accomplish this feat, the TBM was equipped with two types of grouting systems. The pre-excavation grouting system was a mono-component grout system used to grout ahead of the TBM. The two-component (A+B) grout system was used to backfill the annular gap between the segmental lining and the bored tunnel. The machine was equipped with two drills in the shields for drilling through the cutterhead in 16 different positions and a third drill on the erector to drill through the shields in an additional 14 positions. Additionally, water-powered, high pressure down-the-hole (DTH) hammers allowed for drilling 120 m ahead of the machine at pressures up to 20 bar if necessary.
The setup was a novel use of DTH hammers in a North American TBM tunnel (the drills have been used on other projects internationally). The contractor needed to be able to bore two to four probe drills up to 120 m ahead of the machine, then mine 115 m, then drill out 120 m again. The straightness of the DTH drill holes is a huge advantage, as DTH hammers can be maintained within the tunnel alignment even at this distance. More typically, when top hammer drills are used, meaning that the hammer action is on top of the drilling rod, the hammer action only allows the drill string to accurately reach 45 to 60 m ahead of the TBM.
Ultimately, the project was highly successful, with the TBM achieving instantaneous penetration rates of 6 m per hour, and boring safely through zones of fractured rock with high pressure groundwater.
Are there times when Slurry tunneling has advantages over NCP tunneling 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 machine is the logical choice.
The conclusions to draw from this discussion are straightforward. Slurry tunneling is a valid option in rock with potential of high water pressure. However, is Slurry tunneling the most cost-effective option? Is it safer than any other option? In many circumstances the answer is no.
Slurry TBMs are not in most cases the lowest cost when water is expected in a tunnel, 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 tunneling is the better option.
This article was contributed by The Robbins Company.