The industry’s use of hybrid TBMs is a trend that follows with the increasing frequency of difficult tunnel conditions in rock and soft ground. Where multiple machine types might have once been used for different sections of geology, a hybrid EPB/rock machine can excavate the entire tunnel. New designs are making this versatile take on tunnelling more efficient, even at larger diameters of 10m or more.

While all types of hybrid TBMs, including EPB/hard rock, EPB/slurry, and open-type/shielded hard rock designs are gaining in popularity, the EPB/hard rock machine offers some intriguing possibilities. A cross between a hard rock single shield TBM and an EPB, these TBMs bore tunnels with sections of both rock and soft ground, and utilise interchangeable cutting tools and muck removal systems to get the job done. "This type of machine allows one to use the most efficient cutting technology in a given geology," explained Doug Harding, Robbins vice president for sales. "A purely EPB machine would suffer from high EPB screw and cutterhead wear in long sections of rock, and a rock machine operating in EPB ground would have many problems in controlling the flowing ground."

Hybrid TBM efficiency: The great debate
The design of a hybrid machine must be customised for the particular geology in order to be the most effective. "Hybrid machines have the potential to lower risk and make difficult excavations possible, as long as accurate geologic information is available," says Brad Grothen, Robbins engineering manager. "For example, a hybrid EPB/hard rock machine can be optimised 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 per cent soft ground and 20 per cent hard rock, the overall machine design will be optimised towards EPB.

When ground types are not divided into solid sections, or when the geology is 50 per cent rock and 50 per cent soft ground, the design of EPB/hard rock TBMs can become complex and potentially influence the overall machine efficiency. "In reality, a hybrid will most often be a compromise of what is best for each option," says Martin Knights, past president of the International Tunnelling Association (ITA) and global lead consultant at Halcrow Group.

Designs for such machines do require a careful balance: in some projects with short sections of rock changing to soft ground, the design might favor a robust EPBM that can bore in rock. In other tunnels with more clear-cut, long sections of soft ground and hard rock, a hybrid EPB/hard rock machine would be designed towards ease of conversion between two distinct modes.

Much of the efficiency, then, in hybrid TBMs lies in the accuracy of the ground prediction. The selected machine must reflect the often complex geology. "Current hybrid designs are effective as they are, but the big issue is in the cutterhead layout: does one use a hard rock cutterhead, or mixed face cutterhead. In addition the technology of the cutters themselves is moving at an exponential rate," says Steve Skelhorn, project sponsor for McNally Construction.

As for speed, Skelhorn believes that choosing the right machine type and operating in the correct mode are key.

"Hybrid machines are the way to go. Segmentally lined rock tunnels eliminate a lot of issues in loose rock. And for projects with both ground types, it makes perfect sense. I think advance rates are comparable to standard EPBMs, as the EPB mode will always be the factor restricting advance rates."

While some contractors are quick to adopt hybrid machines, there is an increased perception of risk by many in the industry, as the technology is newer. "There is some resistance against using EPBMs in non-EPB conditions," continued Skelhorn, "There is a mindset that an EPB tunnel will be slower and will need to use concrete segments, but this is not the case with hybrid TBMs. For example, there is a tunnel coming up that consists of 3.5km of rock with 200m of soft ground at one end. From a practical point of view it doesn’t make sense to use an EPBM for such a tunnel, but there is no reason why you couldn’t use a hybrid EPBM in open mode."

The next generation
New hybrid designs are variations on a theme: the machines are all capable of operating in EPB mode, both pressurised and non-pressurised, as well as in shielded hard rock mode. The key features of these machines include mixed ground cutterheads, robust screw conveyors, a belt conveyor for hard rock, and cutterhead drives that are capable of handling both hard rock and soft ground conditions. Mode conversion between hard rock and EPB modes typically takes up to two weeks in the tunnel, as modifications are done to the muck discharge facilities, cutterhead, and any other critical structures.

In practice, however, many contractors using hybrid EPB/rock machines avoid mode changes because of the downtime associated with it. This type of TBM is a substantial investment, and if not used properly in the ground type each mode was designed for, advance rates will be less than desired. The requirement for smooth and efficient mode changes is thus essential for these hybrid designs, leading to the next generation of mixed ground machines: those that compromise no features in hard rock or soft ground, and allow for ease of mode changes in the tunnel. New setups have been designed with this in mind, for use on large diameter TBMs in the 12 to 15m diameter range, and for tunnels in nearly equal lengths of hard rock and mixed face ground.

Large diameter, dual mode setup
Robbins’ latest hybrid design is for a large diameter tunnel in Ankara, Turkey: It is capable of operating in 100 per cent EPB mode or 100 per cent hard rock single shield mode. The TBM has been designed for sections of hard rock and mixed ground, in highly variable conditions including sandstone, mudstone, claystone, quartzite, schist, soil, and clay. In EPB mode, the screw conveyor operates as in any typical EPB machine. The screw features a replaceable inner liner and replaceable carbide wear bits for abrasion protection. A mixed ground cutterhead is fitted with knife bits that can be switched out with disc cutters in harder conditions. Much of the cutterhead is covered in Trimay wear plate for additional abrasion protection.

A wear detection pipe on the cutterhead monitors any wear occurring to the cutterhead structure itself, while wear detection bits on the cutterhead and periphery tell the operator about tool wear, and if a gage cutter has been lost. The machine design includes a man lock for cutterhead inspection and changes, and mixing bars inside the mixing chamber.

To convert to hard rock mode, the mixing bars and initial portion of the screw conveyor can be optionally retracted. Interchangeable EPB knife bits must be replaced with disc cutters on the cutterhead, and the EPB scrapers on the cutterhead must be replaced with hard rock bucket lips. Muck paddles are installed in the cutterhead to allow the muck to fall into the muck chute. A hydraulic muck ring allows a chute attached to the bulkhead to move forward and down at a diagonal angle, directing rock chips into the chute and through the screw conveyor onto the TBM belt conveyor.

Optimization towards Hard rock
Going the other direction, a separate hybrid design has been developed for large diameter (8m and up) machines in majority hard rock tunnels. In 2013, an 8m diameter Robbins hybrid single shield/EPBM is gearing up for the Grosvenor Decline Tunnel at Australia’s Anglo-American Coal Mine. The machine is being built using Onsite First Time Assembly (OFTA) in order to fit within a tight project schedule. The design allows operation in a decline through possibly gaseous conditions.

Two decline tunnels, at grades of 1:6 and 1:8, will be used for mine access to new coal seams. The hybrid machine will tackle mixed ground conditions ranging from sand and clay to varying grades of hard rock up to 120MPa UCS, as well as coal seams. Methane gas is expected to be present throughout the tunnel, so the machine has been designed as Explosion Proof Compliant to ERZ-1. Because of this, it was decided not to include a man lock, though one could be included. Only about 300m of ground are expected to be in EPB mode, while the rest will be bored in hard rock mode. Thus, the design was optimised towards hard rock excavation.

In EPB mode, the machine utilises a two-stage, centermounted screw, with a replaceable inner liner and carbide bits for abrasion protection. A mixed ground cutterhead is fitted with interchangeable knife bits and Trimay wear plates for abrasion protection. To keep the mixing chamber spark-safe in the presence of methane the chamber is filled with water, foam, and other additives. To deal with the watery muck, one screw conveyor is run faster while the second screw conveyor is run slower, creating a muck plug in screw conveyor number two, which pushes the water out of the screw conveyor.

"The machine essentially uses its EPB technology to deal with any methane gas safely," says Harding. If any methane leakage is detected, an evacuation system called a ‘snuffer box’ will draw methane out of the end of the screw conveyor and directly into the ventilation system.

To convert to hard rock mode, a hydraulically operated muck chute is deployed around the screw. The muck is then picked up by paddles in the muck chamber to load the screw. Interchangeable EPB knife bits must be replaced with disc cutters on the cutterhead, and the EPB scrapers on the cutterhead must be replaced with hard rock bucket lips.

A skew ring offsets the torque of the machine in hard rock, allowing for more efficient single direction cutterhead excavation and muck pickup. Mini grippers on the rear shield allow the machine to bore 400 to 600mm forward, and then be retracted for cutter changes.

A final interesting aspect of the machine is a specially designed ‘Quick Removal System’. As no ground in Australia can be left unsupported and the machine is boring a blind tunnel, it will be able to retract in one piece from its shield, leaving the shield in place. "The core of the machine is a bolted design and separates from the shield, in a process that does not require a cutting torch," said Harding. The machine will then be walked up the decline tunnel on a set of specially designed transport dolleys and sent by rail to the second decline tunnel, where another shield will be waiting for machine assembly prior to launch.

One direction
The future use of all hybrid TBMs is predicted to increase, as more tunnels are slated for areas in mixed ground conditions. Other types of hybrids than those discussed here, such as EPB/slurry and open-type/shielded hard rock TBMs, are also increasing in popularity.

Skelhorn sees some barriers to the acceptance of hybrid rock/slurry and EPB/slurry designs however: "I think they have their place, but slurry is very expensive, and the size of the slurry plant and its requirements is tremendous. I think with advances in machine technology, we are closing the gap between slurry and EPB, and the hybrid design lends itself better to EPB conversion anyway."

Harding believes the initial investment of a hybrid EPB is offset by their versatility: "The machine may initially be expensive, but it is likely the machine will be able to be used and reused on a variety of projects afterward. It will also be able to perform better in difficult ground conditions."

When asked about the likelihood of the long-predicted ‘universal TBM’ able to operate in all modes and convertible from within the tunnel, attitudes are quite mixed. "There will never be a universal TBM that can equal the performance of a uni-purpose TBM," says Knights, indicating that the efficiency of such a machine would be quite low. For Skelhorn, the possibility is there, but it would require some advances in technology to make the design useable: "I think it’s probably 15 years away. There is not a huge difference between the various TBM types, so it is possible. The biggest difference would be in converting such a machine for very hard rock tunnels where robust disc cutters are needed. Replacing cutterheads and cutting tools will be the biggest challenge."

For now, hybrid machine use seems destined to increase. Above all, Grothen says that effective hybrid machine design is about minimising complications: "Any job underground is not a simple one. Hybrid machines must be able to mine efficiently in the prescribed conditions while minimising cost to all involved".