Author: Brad Grothen
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.
It has been argued that the majority of good ground for tunneling in the world already has a tunnel going through it. While this might be an overstatement, it is true that more and more of today’s tunneling projects are being proposed in difficult, mixed ground conditions. Hybrid TBMs, specifically EPB/Slurry and EPB/Hard Rock machines, are increasingly becoming the best solution for these challenging conditions.
Hybrid machines have the potential to lower risk and make difficult excavations possible, as long as accurate geologic information is available. For example, a hybrid EPB/Open-Type machine can be optimized towards either end of the scale—depending on whether the majority of the drive is in soft soils or majority in hard rock—to produce the fastest possible advance rate over the entire project. If the tunnel is 80% soft ground and 20% hard rock, the overall machine design will be optimized towards EPB.
But what happens when the divisions are not so clear cut, or when the geology is 50% rock and 50% soft ground? A real example is the upcoming Sleemanabad Carrier Canal project, which will utilize a 10.0 m (32.8 ft) diameter Hybrid EPB/Hard Rock TBM. The 12 km (7.5 mi) long water transfer tunnel in Madhya Pradesh, India consists of shorts sections of hard rock and soft ground. Geology includes clay, gravel, marble, and hard rock up to 180 MPa (26,000 psi) UCS.
The solution at Sleemanabad is to provide three modes of tunneling: pressurized EPB tunneling, non-pressurized EPB tunneling, and open hard rock tunneling. The first half of the tunnel will be excavated using a specially designed screw conveyor, which can operate well in both the pressurized and non-pressurized environments. The oversized screw can be rotated faster to handle short sections of hard rock while minimizing any loss in efficiency.
When a longer section of rock is encountered, the screw conveyor is removed and switched out for the TBM belt conveyor. Additional welded steel is added to the cutterhead to close up the open soft ground design, and the machine operates as a Single Shield TBM.
In other conditions, such as sections of soft ground and rock mixed with sections of high water content, a Hybrid Slurry/EPB Type machine is desirable. The machines can similarly be optimized towards slurry or EPB depending on the amount of groundwater expected. An interesting example of this type of hybridization took place at the South Central Hillsborough Intertie Tunnel in Tampa Bay, Florida, USA. The machine excavated limestone with significant groundwater using both a screw conveyor and modified slurry removal system. See the case study here.
Hybrid machines are becoming more common, and are likely to gain in popularity over the near future. With greater experience will also come greater optimization of conversion times between modes—one of the biggest challenges during tunneling. While these machines do have some negatives, as all machine types do, the solution of Hybrid TBMs in mixed ground vastly outweighs any difficulties over the life of the project.
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