Difficult Ground Solutions (DGS) is a suite of options that can be added to a shielded hard rock machine or Crossover machine to better enable advance when conditions are unknown or difficult conditions are anticipated. While each of these items is not new, the concept of installing them on a machine from the start, and of using them in concert to provide better visualization and performance in the most challenging ground, is. The uses and advantages of DGS are various and wide-reaching.
Number 1: DGS can act like risk insurance.
Yes, risk insurance. While one can argue that risk insurance is not needed—and of course if a tunnel were in entirely homogeneous, medium strength, self-supporting rock, under a medium amount of cover, I would tend to agree—there is a problem with this argument. That scenario exists only very rarely. At Robbins, we are seeing more tunnels being planned in mountainous, remote regions where an accurate GBR cannot be obtained. More tunnels planned through multiple fault zones and mixed face conditions. More tunnels planned in suspected karst conditions and in rock where large water inflows may occur. It is a trend we’ve observed arcing upward, and with these high-risk projects it seems prudent that an equipment manufacturer should strongly recommend risk insurance. We’ve done that the only way we know how: by calling on our extensive experience to develop, test, and provide various solutions that can be added to a TBM at the beginning of the project. These features mitigate the risk of getting stuck in unforeseen (or foreseen) challenges that might come up in a tunnel drive.
This risk insurance works like any other type of insurance—it must be purchased before the actual event happens. Waiting until a machine encounters a given challenge—where it may become damaged, stuck, or irretrievable—is not an adequate plan for difficult conditions. Sure, some of these options can be added in the tunnel, but such an operation is far from ideal. Risk insurance is best purchased during the TBM design phase, long before the machine is built and launched.
Number 2: DGS can get your TBM through a fault zone or squeezing ground where it might otherwise become stuck.
If fault zones or squeezing ground are known or suspected, or if there is even a possibility of encountering them, this can greatly affect your TBM operation. Shielded hard rock machines protect your crew from the surrounding ground, and they can bore and line a tunnel efficiently, but in fault zones and converging material they need some of the features of an EPB machine to keep advancing. Avoiding a stuck machine is paramount to project success. Luckily, DGS uses several ways to avoid the problem of a machine becoming stuck. The first of these is multi-speed cutterhead drives. These drives effectively give the machine multiple modes of operation—high speed, low torque for hard rock, and low speed, high torque for difficult ground. Designing a machine with high-torque, continuous boring capabilities allows that machine’s cutterhead to restart with break-out torque in difficult ground. The net effect is that the machine can keep boring in the event of a face collapse and can effectively bore through fault zones and running ground where the potential for cutterhead jamming exists. Going one step further, multi-speed gearboxes give the machine the ideal EPB-type torque if larger sections of soft ground are anticipated.
Secondly, TBMs can get through squeezing ground and faults using Continuous Shield Advance. This design utilizes a stepped shield configuration—where each successive shield is slightly smaller in diameter—to avoid becoming stuck in converging ground. External shield lubrication is an added insurance against becoming stuck, using a series of radial ports that can pump Bentonite into the annular space to act as a lubricant in squeezing material.
As a last effort, if a machine has already become stuck the TBM can utilize augmented, or “super” thrust. Additional thrust jacks can be added to supply an extra boost in a short stroke, generating enough force to break loose a trapped shield.
Number 3: DGS can keep your crew safe and save your TBM in the event of a massive inrush of water.
Water is an ever-present part of tunneling underground, but unexpected large inflows can damage a machine and grind a TBM operation to a halt.
In the event of a large inrush of water, a guillotine gate on the muck chute can effectively seal off the muck chamber of a Single Shield TBM to keep the crew safe as well as keep the machine from becoming flooded out. This system is termed “passive” water protection because the TBM is stopped in place (not actively operating). During that time the crew can then work to grout off water inflows and dewater the chamber to control the flow before they begin boring again. The grouting crew also have the added assistance of back pressure to assist in grouting.
Number 4: DGS can improve your visualization of the ground around the TBM.
Probe drilling is an essential part of visualization, and, combined with grouting, it can also fall under the heading of water control. We’ve learned that, in difficult ground, more is better. Multiple probe drills, and more drill ports in a 360-degree radius, are always going to give an advantage. We’ve taken this lesson to heart, and have installed multiple probe drills on many of our new shielded machines, with ports to provide probing patterns in a 360-degree radius. High-pressure grout injection can be done through these same ports to stabilize ground up to 40 m ahead of the face (or more if using specialized drills). The type of grout injected can also be specialized—for example chemical or polymer grout can be used to seal off groundwater. Lastly, a rotary forepole drill can be installed behind the cutterhead support to allow for ground consolidation around the shield periphery. The forepole drill is of particular use in fractured rock and fault zones. These drills are the mainframe of a visualization plan that can ultimately create an in-tunnel GBR as the machine advances: of particular use when no accurate GBR can be created due to topography, high cover, etc.
But what if you suspect the ground around your TBM is converging? What if you want to get a look at the actual rock face in the safest possible manner? There are DGS options for these types of visualization as well. For squeezing ground detection, a hydraulic cylinder can be mounted on top of the shield and connected to the TBM’s PLC. It measures the shield gap in the tunnel crown, so that if squeezing or collapsing ground is detected the crew can take countermeasures. These measures include using bentonite lubrication, crown or face rock conditioning, or planning ahead to use another system in the area before the machine can become stuck. For getting a better look at conditions ahead of the machine, a cutterhead inspection camera can be used to remotely inspect the boring cavity without intervention, and to check water levels ahead of the TBM. While these cameras have been used to monitor mixing chambers and perform cutterhead inspections in soft ground TBMs, their use in hard rock machines has been much more limited. In the new ground investigation system, the probe and injection holes in the cutterhead and front shield are specifically designed to accept these cameras.
Number 5: DGS can save you time and money.
The old adage “the best-laid plans can go astray” applies particularly well to tunneling. There are many unknowns, even with an adequate GBR in hand. Making an initial investment on DGS features when your TBM is still in the shop is far less costly than installing them in the tunnel after a major stoppage. These features can mean the difference between a successful operation and a stuck TBM requiring a bypass tunnel or worse. TBMs with DGS features also produce better advance rates in adverse conditions such as fault zones (see our recent projects Kargi and TEP II as good examples of this). Better advance rates mean your project is more likely to stay on schedule and on budget. And who wouldn’t want that?
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For underground construction professionals and manufacturers, there is some confusion about the current TBM designations and what they really mean, and for what applications they are to be considered. This is especially noted when it comes to soft and mixed ground type shield machines and if they are to operate in “open” mode, “closed” mode or “semi-open/closed” mode conditions.
The term “open mode” defines defines a mode of TBM operation where no pressure is required to stabilize the boring face; the geology is self-standing and ground settlement will not occur.
The term “closed mode” defines tha mode of TBM operation where face pressure needs to be constantly applied in order to maintain face support. Should the face pressure not be maintained as determined by the geology, under-pressurization will cause ground settlement, over-pressurization can cause surface heaving.
The term “semi-open/closed mode” defines a mode of TBM operation in varying geology where the TBM has the ability to control the face.
The various machine types in question are Mixshield, Hybrid and the Robbins “Crossover”. Are they the same? If not, what is the difference? This blog seeks to clarify and explore those definitions.
Mixshield is a terminology introduced by German manufacturer Herrenknecht for what is essentially a Slurry TBM that uses an air bubble to control and support pressure at the face. The air bubble acts like an accumulator with the belief that this offers better pressure control. This type of machine operates only in “closed mode”. Mixshields are not meant to go from high pressure soft ground to significant rocky conditions with great efficiency, hence the name “Mixshield” is somewhat of a misnomer. They can excavate mixed face conditions, but the machine configuration is not changed so it is a different thing entirely than to excavate significant sections of differing geology. In order to change the configuration to allow more efficient operation in different sections of geology, a Hybrid machine is needed.
“Hybrid”, “Dual Mode”, and “Multi-Mode” are names developed by the industry to try and define machines that feature components of two different machine types. The most common types is a multi-mode between EPB with a screw conveyor (for operation in “closed mode” or “semi-closed mode”) and conversion to Rock Mode with the installation of a belt conveyor (for use in “open mode” operation). Typically, the only changes made to the machine are components such as cutting tools and muck removal method that can be switched out in the tunnel depending on ground conditions. The machines are used for large sections of vastly different geology in one tunnel, such as a section of clay followed by as significant section of rock. However these types of machines are typically designed as EPB machines and then converted, making them not very efficient for use in rock geology.
The “Crossover Series” of TBMs was developed by Robbins in March 2015 using innovative design concepts to effectively excavate between different geologies. The Crossover Series is Robbins’ version of a Hybrid TBM but with many additional features to allow the TBM to effectively cross between different geology types. Operation of the machine can allow the machine to quickly change from “open mode” to “semi open/closed mode” and “closed mode.” The main types of Crossover TBM include the XRE (Crossover between Hard Rock and EPB), XSE (Crossover between Slurry and EPB) and XRS (Crossover between Hard Rock and Slurry). The features include interchangeable cutting tools; single direction and bi-directional cutterhead muck pick-up; multi-speed, multi-torque cutterhead drive systems; durable, abrasion resistant components on the cutterhead and screw conveyor; emergency thrust; tapered shields to reduce shield entrapment in squeezing ground; integrated probe/grout drill system for ground consolidation; and heavy steel construction with components designed for +12,000 hours of life, just to name a few.
The Crossover Series is suitable for tunnel projects that require the ability to excavate different types of geology efficiently with one machine, based on ground conditions. The TBM design is flexible enough to switch from one mode to another. Ultimately, Crossover Series machines are cost efficient in the long run, since the TBMs require less repair costs than a non-customized machine, and they can be used on multiple projects.
Based on the ground situation (whether it’s self-supporting ground, self-supporting ground with water pressure, or unstable ground), the Crossover is optimized towards “open” mode (as in hard rock shielded TBMs) or “closed” mode (as in compressed air, slurry, and EPB TBMs).
Reliable geological information is therefore critical to the TBM design. An accurate geological report is needed to decide when and where to convert the machine as well. During excavation, adequate probe drilling is further essential to determine the ground conditions ahead of the Crossover TBM.
Below are the features specific to each type of Crossover machine.
- Mixed ground cutterhead: with disc cutters & soft ground tools or a combination of tools (interchangeable).
- Two-speed gearboxes: provide high torque at low RPM under soft ground conditions and high RPM under hard rock conditions.
- Cutterhead rotation: Single direction is more efficient in hard ground conditions and bi-directional is more efficient in soft ground to prevent roll.
- Equipped with both screw conveyor and slurry system for muck removal.
- Ground conditions: soft ground containing water under pressure (particularly for water pressures > 5 bar).
- The most universal of the Crossover machines, the XSE can bore in most types of ground.
- Highly adaptable to variable ground conditions; suitable for rock tunnels with water pressure > 5 bar.
- Hard rock machine with a rock cutterhead and slurry system in place.
- Capable of mining rock through high water pressure without grouting off water flows.
With the increase in large TBM projects over the last few decades and a global awareness of their environmental impacts, there has been a greater focus on the origin of TBMs and their parts. The focus has been further highlighted by the ITAtech; a technology-focused committee for the International Tunneling Association (ITA-AITES) that produced guidelines on rebuilds of machinery for mechanized tunnel excavation were released in 2015. While the guidelines are relatively new, Robbins has a long history of delivering robust machines, many of which are rebuilt (many are also 100% new). In this post, I’d like to explore just what a “rebuilt” TBM is, and what that means to Robbins as a TBM manufacturer.
History and Terminology
Throughout Robbins’ history (over 65 years), our TBMs and design philosophies have been based on the understanding that TBMs require a substantial initial investment, and designing machines for single tunnels is neither economical nor sustainable. This realization has resulted in robust, sturdy designs of high quality that–even before the pencil is put to paper–are meant to last for multiple projects. This is clearly shown when you look at the number of Robbins TBMs around the world that are excavating their second, third, fourth or even their eighth or ninth tunnel. There are even Robbins TBMs manufactured in the 1960s that are still in operation!
For Robbins, the term “rebuilt” describes any manner of creating a custom TBM from already existing components. It is the term we use most and continue to use. The ITAtech guidelines introduce different terminologies depending on the extent to which a TBM is rebuilt. They are, very shortly, described here:
- Remanufacturing – Remanufacturing is a process with the aim to start a new life cycle of the product using its current or modified configuration.
- Refurbishment– Refurbishment can be considered a full maintenance, where defect parts are replaced to extend the life of the product in its original configuration or with small modifications.
The guidelines describe the requirements of each process in order to designate a TBM “refurbished” or a “remanufactured”, but in reality the majority of “rebuilt” TBMs may be somewhere in between these qualifications.
The Robbins Philosophy
Over the years Robbins has built a quality assurance system that ensures when we deliver a rebuilt machine, either to the original configuration or a modified one, we still adhere to a design life of 10,000 hours. This standard also includes checks to make sure that all the components are in a functional condition of ‘as new’ or ‘new’. Due to our long experience in rebuilding TBMs, we can offer in principal the same warranty on a rebuilt machine as a new machine.
The Robbins philosophy on this is that, in order to offer the same design life and same warranties on a rebuilt machine, the initial design of the TBM will need to consider that the TBM will be used on several projects. This means that the major structures will need to be strong enough to survive even the toughest conditions and that worn parts can easily be replaced. If the machine is not properly designed for multiple projects, there will be a need to do major work to get the TBM in a working condition either in its original or modified configuration. Robbins strongly believes that considering the total life cycle of a machine, even in its first design stages, is the most economical and sustainable option.
One can argue that project owners typically only have one project and that the condition of the TBM and the suitability of its rebuild is therefore not essential. This is something that is also reflected in many of today’s tunneling projects, where the commercial consideration is often given far more attention than the technical one. We would argue, however, that an initially sturdy and robust design of the TBM will give the project more uptime, higher production rates and better flexibility if unexpected conditions are encountered, making it a good and effective kind of insurance for the project. This effect has been clearly identified in the field, where Robbins has more than 90% of world production records in hard rock.
In terms of the international guidelines, they are certainly necessary and welcomed. However, the strictness of the guidelines makes them hard to adopt worldwide and perhaps not realistic for the majority of TBM rebuilds, which are customized based on project needs. The ITAtech guideline is also missing something else: the definition of what makes a TBM “new”. While opinions from different suppliers vary, Robbins is perfectly clear on this topic: If you are buying a new TBM, then it is a 100% new TBM with only 100% new components. We strongly believe the whole industry should commit to this definition of what makes a TBM “new”, and this definition should be added to the guidelines.
Robbins has throughout the years built up a vast experience in providing the right machine for the right project, whether that means a new or rebuilt machine. As a part of this experience, we are convinced that the life cycle of TBMs should be considered at the earliest stages of the design process. Designing machines with ease of rebuilding in mind ensures that we do not have to start from scratch every time a machine needs work. It also results in time, cost, and energy savings when the time does come to rebuild a machine for a new project. For the industry, this type of perspective is the only economical and sustainable option going forward.
By Sindre Log, Civil Engineer and General Manager for Robbins Norway
TBMs have become well accepted in civil construction tunneling and excavate a high percentage of civil construction projects each year. But each year in the mining industry, far more kilometers of tunnels are excavated for mining purposes than for civil purposes. The amount of tunnels needed for mining operations is staggering.
Some examples from metal mines:
- A large, deep gold mine has over 800 km of tunnels. That is a single mine!
- One small underground metal mine excavates over 13 km of tunnel each year
- In the Sudbury, Ontario mining district, there are over 5000 km of mine tunnels
If tunnels for coal mining were included, the statistics would be even more dramatic. But all these tunnels hold a little-known secret.
The Lack of Mechanized Tunneling
How many of the above thousands of kilometers of mine tunnels do you think have been excavated by TBM? The answer is “Nada”. Compare that to the thousands of kilometers of civil construction tunnels built over the last decades, which required hundreds of TBMs. Why such a great disparity when the objective in both industries is to excavate underground openings as rapidly, economically, and as safely as possible? What are the differences in these industries?
In TBM civil construction, the TBM crew members that know how to make things happen underground, that know how to drive the machines, how to lift heavy components and repair the equipment, how to get the trains in and out to remove the muck and to bring in supplies, are known by a special name. They are known as the “miners”. Yet the only thing they are mining is the muck, no coal, no minerals. But “miners” is a term of reverence for someone who knows how to excavate a tunnel rapidly and efficiently. When a civil tunnel is progressing well, they say “now we’re mining”. (Well, sometimes they also say this sarcastically sometimes when things are going poorly underground.) Why aren’t such talented miners, who know how to make a TBM perform, using their TBMs to excavate thousands of kilometers of mine tunnels each year? What are the differences between civil tunneling and mine tunneling?
TBMs for Civil Tunnels
TBMs have become well accepted for civil tunneling. TBMs designs have become widely adapted for different ground conditions or specialized applications: Hard rock, soft ground, mixed face, pressurized face. Various types of ground support can be installed, according to current geological conditions. More current “hybrid” or “Crossover” TBM designs can handle widely different geological conditions, with the TBM adaptable to cope as conditions change.
Civil tunnels are generally long tunnels, where the efficiency of TBM excavation offsets the longer mobilization and demobilization times. And civil tunnels are generally designed with equipment mobilization/demobilization in mind. Suitable sized shafts are located to allow relatively simple introduction or retrieval of the TBM equipment. The most efficient type of TBM is the full face, rotary type TBM, which produces a circular tunnel profile. The circular profile is nearly universally accepted for civil construction. It is the optimum profile for fluid flow for fresh water, waste water, or hydro tunnels. It is also widely accepted for vehicular civil tunnels that need a flat roadbed. A flat roadway is constructed within the circular tunnel profile and the remainder of the profile within the circle is used for ventilation, services, escapeways, etc.
Some efforts have been made to develop non-circular profile TBMs for the civil sector. These machines include the Mini-Fullfacer, the Mobile Miner, and horseshoe shaped shields with excavator boom or roadheader. Such machines may produce a non-circular profile that is better for that specific job. But usually, there is a penalty in production rate compared to full face, circular TBMs.
TBMs for Mine Tunnels
There have been some notable successes to the application of circular profile “civil type” TBMs for mining projects. Benefits have been lower costs, quicker access, and improved safety. Some examples include:
- Magma Copper, San Manuel Tunnels
- Stillwater Mines (four TBMs used)
- Grosvenor Coal Mine, Two Inclined Access Drifts to the coal seam
However, application of TBMs for mine tunnel construction has remained surprisingly limited. Why is this? Tunnels for mining are often not so long, or a developed in shorter phases, with the excavation front moved from place to place within the mine. TBMs and their constituent components are large and heavy. It is not easy to mobilize a TBM deep underground in a mine at a remote face. Better efforts must be made to make TBM transport and mobilization within the mine practical. This includes considerations for steep ramp roads and other restricted cross sections within the mine.
Efficient TBMs are highly productive, but require a lot of power, ventilation, cooling, and support services. These need to be part of the mining plan so that the TBM has the necessary support and can provide the full benefit. Operating personnel with proper skills are also essential. If a TBM is introduced into a mining environment, either the mine personnel need proper TBM training or motivation, or the TBM drive must be isolated as a “stand alone” operation within the mine and be given proper priority of skilled personnel and the necessary services so the full benefit can be realized.
Mines often do not accept the circular profile produced by the most efficient, full face rotary TBMs. Mine tunnels are usually designed with a flat invert to allow for passage of rubber tired vehicles during the production phase of the mine tunnel. Many efforts have been made to provide TBM type equipment that produces a flat invert. Some have been relatively successful. But generally these machines do not provide the same productivity of efficient, full face circular profile TBMs. Rail bound mining vehicles can be used in the circular tunnel to take advantage of this TBM efficiency. Or, precast invert slabs, poured in place concrete, or partial invert filling can be used in a circular tunnel to provide a flat roadway. The cost/benefits must be analyzed and presented to the industry for a change to occur.
Mining plans often have tunnels with steep gradients and sharp radius curves. On steep gradients (up to 12-15 degrees), the most efficient haulage is usually by belt conveyor. However, the belt system is not effective if there are sharp radius curves. And typical TBM curve ability is limited. Special TBM designs can be made that allow for excavation in sharp curves, but there is a compromise in reduced TBM performance.
Mines need Versatile TBMs
It seems the mining industry needs the benefits that TBMs can provide. Open pit mines are becoming depleted, and mining activities are reaching deeper. Longer access tunnels are needed. Safety and speed of development are paramount. TBMs can offer these advantages, but have limitations. Special TBMs can be developed that meet special requirements, but usually there is a penalty in reduced TBM performance. The mine planners and the TBM equipment designers must work together at an early stage in the mine planning to determine the optimum compromise between most desirable mine plan, and most beneficial application of TBM equipment. A good partnering approach is necessary in the planning stage, as well as the operating stage, to allow for the most efficient application of TBM equipment to the needs of the mine.
By Dennis Ofiara, Chief Engineer
The Real Field Experiences that Resulted in a Versatile Design
This blog is the first in a series called “Hidden Underfoot”, exploring the little-known history and behind-the-scenes happenings of the tunneling industry.
Many technological breakthroughs have been the direct result of the necessity to solve a problem; the creation of the first Crossover TBM was no exception. While much has been written about Central Turkey’s Kargi Kizilirmak Hydroelectric Project, we interviewed Field Service personnel who were there working in the conditions every day to get the untold story behind the genesis of the versatile Crossover TBM.
Behind the Scenes
The Kargi HEPP, now complete, generates 470 Gwh of power annually for project owner Statkraft, and supplies an estimated 150,000 homes.
Robbins supplied a 9.84 meter (32 ft) diameter Double Shield TBM and continuous conveyor system to Turkish contractor Gülermak for the project. The machine was to bore an 11.8 km (7.3 mi) headrace tunnel to divert water from the dam to powerhouse. Initial geological reports predicted softer ground for the first 2.5 km (1.6 mi), which would be lined with pre-cast concrete segments. The remainder of the tunnel was to be supported by a combination of shotcrete, rock bolts, and wire mesh in more competent rock.
The project became arduous soon after startup, when the machine encountered blocky rock, sand, and clays that were not initially predicted. “We realized modifications needed to be made as soon as we started experiencing flowing materials and squeezing ground,” said Glen Maynard, Robbins Site Manager, who worked on the project throughout the challenging conditions. Maynard’s prognosis was apt, as 80 meters (262 ft) into the bore the TBM became trapped in a section of collapsed ground. “The machine faced serious blockages,” added Maynard, “there was no one fix, each problem needed its own solution. Thrust, cutterhead, and ground support all needed adjustment.” The machine was freed but continued to struggle, requiring not just one but seven bypass tunnels to free it each time it encountered collapsing ground.
Concerns that the machine would need to be buried were quickly mollified. Robbins President Lok Home told the Kargi team to build a “wish list” of all the materials they would need to modify the machine and get it moving again. Robbins and Gülermak worked together to determine what changes would need to be made, and parts were shipped to the jobsite as quickly as possible. This of course, was not the first time a machine had required modifications while in the tunnel, and Robbins engineers were able to pull from past projects in order to know what needed to be done.
The contractor, with the assistance of the field service team, installed a Robbins custom-built canopy drill and positioner to allow pipe tube support installation through the forward shield. This allowed drilling with a distance of up to 10 m (33 ft) ahead of the cutterhead while 90 mm (3.5 in) diameter pipe tubes provided extra support across the top 120 to 140 degrees at the tunnel crown. Injection of resins and grout protected against collapse at the crown while excavating through soft ground. “There were a lot of risks to the modified machine, but we gave comprehensive instructions on how to operate it to the contractor,” said Maynard. “The contractor team was open to change and had great cooperation. They were a really hard working team.”
To further mitigate the effects of squeezing ground or collapses, custom-made gear reducers were ordered and retrofitted to the cutterhead motors. They were installed between the drive motor and the primary two-stage planetary gearboxes. When the machine encountered loose or squeezing ground the reducers were engaged, which resulted in a reduction in cutterhead RPM and a doubling of the available torque. The net effect of the modifications allowed the Double Shield TBM to operate much like an Earth Pressure Balance Machine in fault zones and squeezing ground with high torque and low RPM—these methods effectively kept the machine from getting stuck. In addition, short stroke thrust jacks were installed between the normal auxiliary thrust to double total thrust capabilities.
The results of the modifications led to astonishing results. An advance rate of 600 m (1,986 ft) in one month was achieved in March 2013 and a project best of approximately 723 m (2,372 ft) was achieved in spring 2014, including a daily best of 39.6 m (130 ft) in April 2014. The TBM bored 7.8 km (4.8 mi) of the tunnel in total, making its final breakthrough in July 2014. The remainder was excavated by drill and blast—of which it is notable that the modified TBM achieved advance rates more than twice that of the traditional mining operation. “It was enjoyable to find a way to overcome this challenge,” added Maynard.
Mixed Ground Legacy
A project that had started out as a disheartening one transformed into one that encouraged others. The insights gained by implementing changes to the Kargi machine were quickly applied to other projects facing mixed ground conditions that would otherwise require multiple tunneling machines. From these creative solutions came the design of a new line of Robbins dual-mode machines—TBMs that contain features of two machine types—termed Crossovers. Everything from multi-speed gearboxes to canopy drills, and emergency thrust are regular features of Crossover machines, and they owe their genesis to the hard work involved at Kargi HEPP.
This blog is the first in a series called “Insights in Brief” that aims to boil down complex concepts into bite-sized facts and key points.
While probe drilling and pre-grouting have a long and successful history in drill and blast applications, their adoption for TBM technology has been more tenuous. Continuous probe drilling and pre-grouting was first pioneered in Norwegian hard rock D&B tunnels, where they have since been used with great success to detect ground conditions and consolidate weak rock ahead of the excavation face. If these methods have great potential to allow TBMs to excavate in difficult conditions, then there must be a knowledge gap. Here, we highlight four key things to know in order to get the most out of probe drilling and pre-grouting.
Number 1: There are always Pros and Cons
Much of the reluctance to adopt probing and grouting is based on a belief that the overall impact on the project schedule and TBM advance does not usually make up for the benefits provided. However, the benefits can be dramatic when compared to alternatives such as stuck TBMs, bypass tunnels, and other costly delays to the project schedule. The ability of a grout curtain to cut off or reduce water ingress and stabilize weak zones is unique to the method. This result has also been proven on hundreds of D&B projects over decades. When tunnel projects such as India’s Tapovan-Vishnugad Hydroelectric Project are considered, where a Double Shield TBM was brought to a halt following a massive influx of mud and water, the benefits seem clear.
There are other barriers towards industry acceptance besides time, however, and that is the perception of cost (both of course being related). In particular, in mountainous conditions or when tunneling downhill, probing may be the only practical approach towards risk mitigation, and pre-grouting may be the best possible option to control water inflows. When the cost of a stuck TBM is considered as the alternative, continuous probe drilling is looking pretty good.
Number 2: The Right Program and Machine Design can make all the Difference
Though the industry view of probing and pre-grouting tends toward the conservative, there are multiple ways to reduce impact to time and budgets while maximizing the benefits:
Plan and optimize the downtime for maintenance and cutter changes to minimize the downtime caused by probing and grouting
Proper scheduling may be one of the easiest ways to reduce downtime compared with current industry standards. To efficiently perform probe drilling and potentially pre-grouting in a TBM process, it is essential to plan the interventions and remove them from the critical path of the TBM process. Detailed planning should be done to coordinate the maintenance and cutter changing stops to the probing intervals. As an example a daily maintenance shift could be sufficient time to complete a grouting umbrella, with the correct TBM set up.
Analyze the drilling performance in detail
To get the most out of probe drilling and pre-grouting, detailed measurements of the advance rates of the probe drilling and the grouting pressure should be done. These measurements enable proper prediction of the ground ahead of the TBM. The drilling could be measured manually or automatically with Measurement While Drilling (MWD) systems, which are commonly used in D&B applications. The MWD system is used to analyze the rock in detail (hardness, water content, rock mass properties, etc.) and can be used to generate 3D-models of the rock mass in order to decide on the rock support or for documentation purposes.
Choose the right TBM Design
Ultimately, choosing the right TBM type can significantly cut time and cost. A customized machine, whether shielded or open-type, can be designed for accurate and continuous probe drilling. While it might seem that a shield machine would have limited drilling trajectories, machines like the Single Shield TBM for New York’s Delaware Aqueduct Repair offer 360-degree probe drilling paths using multiple drills, as well as probe drilling under pressure using down-the-hole hammers through ports sealed with ball joints. No matter what is needed, planning during the TBM design phase will most certainly cut downtime and costs later on.
Number 3: It has been Successful on Difficult TBM Projects
Though probe drilling and pre-grouting have not yet been used extensively on TBM projects, successful examples can be found worldwide. At Canada’s Seymour Capilano Water Filtration Tunnels, using two Robbins Main Beam TBMs, 100% probe drilling (with minimum overlap) and pre-excavation grouting were specified as part of the twin down drives of the Seymour Capilano tunnel project. This was due to the down gradient of the tunnel under high cover where there was a moderate risk of encountering significant inflows. Fortunately the actual groundwater inflows were much less than originally anticipated and so very limited pre-excavation grouting was required. Probe drilling was continuously carried out during TBM excavation, and while it had an impact on progress in the early days, once the crews became familiar with the equipment and procedures the work activity became efficient, and was successfully implemented with minimum impact to progress. Success stories like this are common in deep rock tunnels around the world.
Number 4: Knowledge Level is Key to Success
As with many developments in the tunneling industry, probe drilling and pre-grouting are seen from the view of risk sharing. Risk sharing could entail clearly defined specifications and payment provisions that allow for fair compensation to the contractor so they will not be reluctant to accept the approach, for example. Clear design and environmental criteria need to be established, so the execution of probe drilling and pre-excavation grouting involve the opinion of the contractor, the design engineer, and equipment supplier. In some cases, for example, the contractor may choose to accept moderate inflows that are manageable and do not impact excavation progress.
Training in the operations of probe drilling and pre-grouting will also necessarily lead to greater acceptance. While there is some training available at colleges that offer mining and tunneling degree programs, much of the training for such operations is necessarily hands-on and experience based. With an experienced workforce, the negatives of probe drilling and pre-grouting are greatly reduced. Such hands-on training is currently being provided by Robbins on several projects using a combination of classroom instruction and jobsite operation for crews who are not familiar with the methods.
Ultimately, the adoption of probe drilling and pre-grouting on TBMs is something that must be recognized as an overall benefit to the industry in difficult ground conditions. The technology is well developed in D&B tunneling, and it has been field-tested for decades. It is, in our opinion, the best method of accurately detecting and treating poor ground conditions in front of the TBM.
It has been some time since I have written on the Robbins blog page, but I am inspired to do so by the announcement that Elon Musk is entering our business—the tunnel boring business. It is great to see people with a vision of an improved world enter our industry. I agree with Musk that the advance rate of tunnels can be significantly improved if development money comes into the industry. Development money in tunneling, however, is at best minimal and is more often essentially nonexistent. Nearly all tunnels are heavily specified to avoid risk taking by owners (therefore discouraging new development). Nearly all tunnels go to the low bidder and low bidders try to buy the TBMs at the lowest price; a further discouragement of development. The industry has therefore been slow to improve advance rates, but with Musk bringing the issue into the spotlight, perhaps things will change.
Risk Aversion and How to Reverse it
There are some exceptions to this practice of risk aversion for new technology, and one is the Delaware Aqueduct Repair. This tunnel corrects heavy water leakage occurring from the 1940’s-built aqueduct tunnel for New York City. We are just completing Factory Acceptance in our plant in Solon, Ohio of this unique Single Shield TBM. The tunnel is at significant depth (approximately 300 m / 900 ft) with the distinct possibility of encountering very high water pressure (up to 30 bars). The contractor JV of Kiewit/Shea have shown their willingness to move forward with several new developments for this project. The concept of grouting off high water pressure as the primary means to allow advance in such conditions, rather than use an EPB or Slurry TBM, is in my view a significant step forward for our industry. Granted there have been halfway attempts with a combination of grouting and pressurized tunneling at recent projects like the Arrowhead Tunnels and Lake Mead Intake No. 3, but these have come at high cost and sometimes long delays. The Delaware Aqueduct TBM, by contrast, is designed to hold up to 30 bars of pressure while grouting occurs. Boring and cutter changes are done in atmospheric pressure.
Chemical grouting and grouting technology in general have advanced multifold in recent years, and it is commendable to see it used extensively on several aspects of the Delaware Aqueduct Project. It’s a great example of what can be done when a contractor is willing to use new technology to address potential risks—it appears it can actually reduce risk in the long run. It is a great honor to be working with the capable Kiewit/Shea JV team to be a part of advancing technology.
Areas Ripe for Change
The Delaware Aqueduct Repair project is a flagship project for what I hope will become more common in the industry: instead of low bidding with the cheapest possible machine, offering a reasonable bid with a specialized TBM that has a higher initial investment, but ultimately a lower cost overall. The project’s use of technology is wide-reaching, particularly atmospheric cutter changes and chemical grouting, which have the potential to reduce downtime and increase safety. I do not see the future of rock tunneling under high water pressure being left to divers to change cutters and repair the cutterhead. We all know it is not cost effective to send divers to work in confined spaces over 10 bars. It should be noted that the long-duration Hallandsås Tunnel, for example, finished the majority of its TBM advance by relying on effectively this technique of grouting and advance after failing with a Slurry System. There are lots of tunnels to be built with above 10 bars pressure that will use this technology. The industry needs to automate cutter and bit changes as much as possible, and increase the integration of chemical grouting in tunneling.
Certainly there are many areas for advancement in our industry, and major public figures like Musk drawing attention to it is ultimately a good thing. After all, getting the general public to think about solving traffic by going underground is no easy feat. Even more so, getting the tunneling industry to think about its own risk-averse practices has a big potential benefit. Hopefully all of this attention will result in more tunnels, more business, and better infrastructure. Musk’s willingness to take a risk aimed at making the underground construction industry potentially faster and more stable is a good bet to take.
Tunneling is a worldwide business with often-changing markets. At the moment, everybody is watching the markets in Asia and in particular various huge projects in China and India, as well as the Middle East. With these busy hot spots, it’s easy to overlook projects elsewhere.
In this blog, I want to refocus and take a look at Europe, where there are some very interesting and challenging mega-projects underway for the Trans-European Network for Transport (TEN-T) of the European Union. The TEN-T connects all economies in the European Member Countries from West to East and North to South (Figure 1). This master plan for connectivity in Europe includes nine core network corridors with several projects crossing the borders of multiple European countries. The cross-border project implementation is complicated because of the different technical, financial and statutory requirements.
TEN-T core network projects
The core projects of the TEN-T master plan all serve as missing links that would connect the European Member Countries, like the Koralm and Semmering Railway Tunnels (both in Austria), Lyon-Turin Railway Project (connecting France and Italy), Fehmarn Belt Crossing (connecting Denmark and Germany), Brenner Base Tunnel (connecting Austria and Italy), Ceneri Base Tunnel (connecting Switzerland and Italy) and also some completed projects like the Oresund Bridge, Tunnel Road and Rail Fixed Link (connecting Sweden and Denmark), Milano-Roma-Napoli Railway (Italy), Gotthard and Lötschberg Base Tunnels (Switzerland), Betuwelijn Railroad (connecting the Netherlands and Germany) and the Channel Tunnel (connecting United Kingdom and France).
Financial support from the European Union in an amount estimated at 500 billion euro is required to complete this Trans-European Network and to widen some of the bottlenecks within it. The Brenner Base Tunnel is one of the most interesting cross-border projects and a core project of the TEN-T master plan (Figure 2).
Brenner Tunnel Conversations
The Brenner Base Tunnel (BBT) is a big challenge for all involved parties. In 2015, I had the great opportunity to interview Prof. Konrad Bergmeister, who has been the CEO of the BBT SE since August 2006 (Figure 3). Bergmeister was previously the technical director and head engineer of the company managing the Brenner Highway and was responsible for the planning and construction of new infrastructure and for maintenance of the existing structures. For the past 22 years, he has taught construction engineering at the University of Natural Resources and Applied Life Sciences in Vienna (Austria). He has been the President of the Free University in Bolzano (Italy) since 2010.
Interview with Prof. Bergmeister
Could you give me a brief overview of the Brenner Base Tunnel?
The Brenner Base Tunnel is a ground-breaking engineering project for the 21st century. This underground structure has a total length of 64 km and consists of three tubes: the exploratory tunnel and two main tubes with four lateral access tunnels connecting them together (Figure 4). The tunnel itself will become the longest railway tunnel in the world once complete.
The history of the tunnel is a long one: 160 years ago, an Italian engineer came up with the idea to go beneath the Brenner Pass. After World War Two, the project was reassessed but never proceeded. Finally, between 1987 and 1989 a feasibility study was carried out. The preliminary project was approved in 2002, and between 2005 and 2008 project details were worked out. In 2009 the environmental and technical approvals were obtained in Austria and in Italy. So we started with the first exploration of the whole rock situation in 2006, drilling vertical bore holes that totalled more than 28 km when we sum it all up. And this preliminary information was used in order to study the final layout of the tunnel in order to do the definitive design.
What, for an engineer, is the most challenging aspect of the Brenner Base Tunnel?
The most challenging issues for me are the following:
First of all we developed the so-called design guide in order to have the right basis to execute the design in both countries. This design guide was based on some new design ideas taking into account all the safety issues that have been proposed with the Eurocodes on a European level.
Second we developed internally a so-called chance and risk analysis, which should allow us to monitor the development of the project in terms of possible chances and risks. How we are doing that? Well we are trying to evaluate and double check the actual ongoing construction with our project managers and external experts and we are trying to identify all the possible risks and chances that might occur as tunneling proceeds. This analysis is part of a database that will be actualized on a yearly basis so we have additional information in terms of possible risks, additional costs, cost savings and scheduling.
The third issue is something that deals with some novel forecast technologies. We are using digital mapping and photogrammetric mapping as forecasting methods, particularly for the ongoing drill and blast operations. This is called the Tunnel Control System and is able to predict complex excavation shapes by using statistics regression analysis. It is based on 3D point clouds of the rock surfaces captured after each excavation, and the precision of previous excavated profiles is also analysed. The previous blasting data and the corresponding geological information are used for the prediction of the upcoming upper and lower blast profiles. For the forecast a certain number (minimum five previous blasts) is taken into account for a linear or nonlinear regression analysis. Through this optimization procedure up to 65% reduction in over-break has been achieved.
And the last issue is actually that we have been trying to develop ideas for the reuse of excavation material. The concrete mixture must be sustainable for the next 200 years since this is the lifetime of the tunnel. We have been studying the material of the exploratory tunnel (Figure 6), especially the schists, in the central zone of the project and we were able to modify the preparation methodology and optimize the concrete mixture to re-utilize 100% of the excavation material. Five years ago, all of the geologists and specialists during the approval phase told us that the excavation material must be disposed of. So this is actually a real step forward into optimizing the project excavation.
What is your opinion about the on-site TBM assembly of Robbins compared with the re-assembly of factory-built machines on jobsites?
From an engineering point of view we are only interested that the machine works very well on site. So we are interested to see, when the machine really starts, the performance in a certain excavation length. We are interested in the results and not so much on the specific assembly situation. There are different trade-offs: On the one hand, if you pre-assemble the machine in the production facility, this can guarantee you certain tolerances and probably certain pre-operation errors can be adjusted and finally avoided. But, the major problem is that you have to transport the semi-assembled machine on site and you have to re-assemble the machine on site again and then you have to start excavation. However, with Robbins for example, they can bring all the individual pieces on site and very often they are adjusting the machine itself directly under site conditions. It is easier, I think, to do the adjustment on site directly for specific logistics reasons. We can say that you might have more flexibility if you assemble directly on site compared with doing the assembly at the production facility.
About the Guest Author: Roland Herr has a background in civil engineering and is an international freelancing journalist. He has over 20 years of experience on engineering and construction projects all over the world, and is especially interested in tunneling.
Those working in tunnelling understand that this is an industry more fascinating than any other. What is it about tunnelling that makes it so exciting to those in the field?
Curious to know more, I discussed this question with some very experienced European tunnelling specialists: Frode Nilsen (Norway), Managing Director of LNS, and Dr.-techn. Klaus Rieker (Germany), Managing Director Tunnelling Division of Wayss & Freytag Ingenieurbau AG. Both conversations took place at different places and times, but with strikingly similar results.
Frode and Klaus both have extensive backgrounds in the underground industry. Frode has been working with tunnels since he left university, the Norwegian Institute of Technology, in 1988, and Klaus has been building tunnels for 25 years. Comparatively, I am a “youngster” with 14 years of experience with tunnels and the tunnelling industry, but no less enthusiastic.
Read on for the results of my Q&A sessions with Frode and Klaus:
One characteristic shared by most tunnellers is that they work internationally, on many different projects with varying levels of responsibility. Tell me about your international experience.
Klaus: [At Wayss & Freytag] I was assigned to different projects in Singapore, Malaysia and Taiwan. As a young engineer working in Asia, it was very hard to gain acceptance, particularly because in the Asian culture, older people are typically responsible for project management. It was difficult to convince the contractor that I could handle it, but step-by-step I proved myself with my performance and knowledge. I remember with pleasure my time on a metro project in Singapore; altogether we had about 30 nations involved on the project. It was really amazing!
Frode: Our most impressive project [at LNS] was SILA for the Iron Ore company LKAB. We blasted 12 silos out of the rock and built a 600 m long unloading hall and a 2.8 km tunnel for iron transport from the silos to the harbour. Our most famous project was the Svalbard Global Seed Vault in Spitsbergen for the UN, where we built tunnels and 3 rock caverns in the permafrost with an even temperature of -18° Celsius to store samples of the world’s seeds.
For Frode, the timeline and longevity of underground projects is also an amazing feat worthy of note—tunnels can be built in difficult ground conditions over a period of years, but the hard work pays off in that many tunnels have a design life of 100 years or more.
Working in tunnelling also provides a unique perspective on emerging markets. Which countries or markets are currently experiencing rapid development when it comes to tunnel construction?
Frode: In Europe, Norway is one of the most interesting nations, especially for drill & blast, with hard competition and low prices. In Asia, China is on top, and South America is also growing, in particular with the mining industry in Chile. Much of the future of tunnelling lies in mining: studies show that in 2034 around 60% of mining will be done in underground mines. That means tunnels for access, ore haulage, and more.
Klaus: Germany is now no longer really a market for tunnelling, and in Central Europe tunnel construction is declining. Meanwhile, Asia is the growing market. In Singapore for example, 10 to 20 machines are running annually for the metro system extension. China and India are also huge markets right now. I find North America to also be an interesting market with many current and upcoming projects.
Every industry has its own set of challenges. What do you find most challenging about working in the tunnelling industry specifically?
Klaus: A defining characteristic of tunnellers is that we love a challenge. We thrive on new, very difficult situations that demand utilization of all our knowledge and problem-solving skills, in order to find the best technical solution.
Frode: Client demands can be challenging, and are often accompanied by environmental and technical limitations as well as financial constraints.I am happiest when projects are profitable and everybody involved is satisfied.
Frode and Klaus’ comments on their experiences led me to one overarching conclusion: engineers working in tunnelling are some of the world’s brightest and most experienced, with shared passions for overcoming difficult situations and ever-expanding their world views. For adaptable and driven engineers, the tunnelling industry offers a challenging yet rewarding career with job security, as projects and new markets continue to emerge globally.
Seattle is the founding city of The Robbins Company, and a place where I lived for nearly 15 years and commuted on SR99 while working at Robbins early in my career. As such, the new SR99 Viaduct Replacement Tunnel Project is of great interest to me.
The industry is all too familiar with Seattle’s SR99 Tunnel and its TBM, known as “Big Bertha”. More specifically, much has been written with regards to the TBM needing repairs after about 300 m of boring. The TBM is the world’s current largest at 17.5 m in diameter, and is excavating a 2.7 km long drive.
Robbins was a relatively new entry into the EPB/soft ground tunneling business when tenders were called for the latest SR99 project in 2011, and we made a concerted effort to get the order for this particular TBM. We teamed up with Japanese TBM manufacturer Mitsubishi Heavy Industries (MHI) to get the order. Robbins has had an association with MHI for more than 20 years, with jointly-designed machines operating around the world on projects in India, China, the U.S., and more. MHI has built over 1,000 EPB machines and in my opinion, the Japanese TBM manufacturers are further advanced in EPB technology than their European and American counterparts.
Through the process of trying to receive this order, we learned a lot about the geology, as well as the contractors’ and TBM’s specification requirements. The contractor Dragados, one of the JV partners and very well-experienced in soft ground tunneling technology, developed a high-level specification for the TBM suppliers. All of the prospective TBM suppliers were required to quote and if successful, supply to this standard. We eventually stepped out of the tendering process to supply this TBM, as the lower prices and greater assumption of contract risk offered by our competitors made the TBM supply an impractical business option for us.
The current situation at the SR99 project is more positive than media tend to paint it. The project design consultant did a commendable job on laying out the tunnel route and building in a contingency plan. Boring through glacial till, even with modern TBMs, is never an easy task as previous projects like the Brightwater Conveyance Tunnels have taught the city of Seattle. This is doubly so along the Seattle waterfront, which includes manmade fill, utilities, and buried refuse. In such ground, TBMs can encounter rapidly changing geology; pockets of groundwater; abrasive soil; and manmade objects such as unmapped disused pipes; foundation piles; etc.
Aware of the problems that can develop while using an EPB TBM in glacial till under a city with a lot of backfill, the SR99 designer wisely developed a contingency plan. The strategy, in addition to pre-planned safe havens, involved a “shake down” stretch of tunnel, which ran under no buildings. If problems did occur repairs to the TBM could be made by sinking a surface access shaft at this location. Unfortunately the need for that repair event occurred shortly after the machine commenced excavation.
Why there were failures of the cutterhead seals, and potentially the cutterhead main bearing, is yet to be determined. I doubt there will be any signs of failure of the main bearing when the crews get to inspect it. However, all parties involved are wisely taking precautions and installing a new main bearing in addition to the seals.
The Seattle Tunnel Partners and WSDOT have in place a panel of experts to advise them on the highly technical details of the TBM design. I personally know several of these experts and they are well qualified to recommend and supervise the necessary repairs and procedures to get the TBM into a condition where it is able to finish this tunnel.
Having been in the TBM supply business for quite a few years, I unfortunately have to admit having been in a similar (fortunately not as well published) situation as the TBM supplier on more than one occasion. This situation–significant TBM problems at the beginning of boring—can result from many different factors and is not unique to the SR99 project. In fact, Robbins recently had a similar situation (admittedly on a smaller scale in terms of both public and financial impact) on a project in Turkey known as the Kargi HEPP. Despite extensive pre-planning, unexpected ground was encountered, which resulted in several in-tunnel stops and machine modifications in the first few hundred meters of the tunnel. What happens in these situations is you pull in the best minds with the most experience and immediately analyze the problem. The ultimate fix often ends up as a multi-level solution. You must ensure you have the problem under control, plus take additional measures to monitor the vulnerable components and operating procedures. At Kargi, this process resulted in the remainder of the project being finished without significant TBM problems. Without a doubt a similar process is going on at SR99 with Hitachi Zosen engineers, the contractor’s specialists, and the city’s board of experts.
Being one who is keenly interested in this project, I believe that this TBM will soon be back to boring with a new completion date, which will be fulfilled. I am optimistic that this project will one day be seen as a positive in the tunneling industry, where many lessons were learned and advancements were made. Such advancements will be put to use in Seattle and in other cities that greatly benefit from the excavation of more underground infrastructure.
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