THE MOST difficult terrain for TBM tunnelling is mixed ground. It’s a challenging prospect, which can involve excavation in boulder fields, sections of rock, sticky clay, often under high water pressure or changing water pressure. The right cutting tools and cutterhead design is therefore vital in maintaining a rapid advance rate.
The choice of cutting tool is dependent on the anticipated geology. However, disc cutters and knife bits are the primary tools of choice for EPB machines with other add-on tools supporting the functioning of these tools.
The optimal primary protection for EPB cutterheads in soft and mixed ground is the replaceable knife bit. "These come in standard duty and heavy duty, but standard duty is only recommended for geology that is very easy to excavate," says Brad Grothen, engineering manager for Robbins. "Although these tools are replaced relatively quickly and easily once they are accessed from behind the cutterhead, they require interventions to inspect and replace; thus, it is better to choose the highest quality knife bits in order to minimise required changes."
Grothen adds that common knife bits include eight layers of tungsten carbide inserts, a proprietary alloy steel shank, and a complete coating of 64 Rockwell C (Rc) hard facing. These replaceable knife bits are welded to a base plate assembly that is wedge-locked into the cutterhead.
Mixed-face EPB cutterheads are designed to accept either disc cutters or knife bits. "In pure EPB applications, it is recommended to add shorter secondary knife bits in the same path with the primary knife bits so wear doesn’t rapidly advance to the cutterhead structure if the primary knife bits wear out. The secondary knife bit is essentially a shorter version of the primary knife bit. "Cutterhead spokes are designed to alternate between primary and secondary knife bits. It has been found that a radial spacing of these primary cutting tools at a set distance apart is efficient in the breaking up of soft ground. When hard rock or boulders are encountered and these tools are replaced by disc cutters, this same spacing allows the discs to break up the rock and allow the cracked rock in-between cutters to fall out."
Disc cutters were first invented for breaking rock and have the longest life of any cutting tool in rock. Over the years, disc cutters have shown to be the best choice of cutting tool for breaking rock into smaller pieces for easy removal. There are many styles of discs available and many options for the configurations of those discs on the cutterhead.
When large boulders are expected the cutterhead is typically fitted with disc cutters. However, Robbins states that when the tunnel also passes through more traditional EPB materials, it is important to maintain the cutterhead face opening ratio.
Disc cutters take up a lot of cutterhead space compared to EPB picks and bits. The design of the cutters and cutterhead take on great importance for mixed ground tunnels with a probability of large boulders. Restricting the size of rock pieces that may pass through the cutterhead is important to reducing the risk of blockage of the screw conveyor. Also, in such situations, minimising wear is imperative.
High performance appears to be linked to a mixed ground EPB machines being fitted with a cutterhead designed and fitted for mixed ground, for example fitted with disc cutters as well as soft ground tools.
Machines that have to be stopped one or more times in the tunnel to have the cutting head redressed, from soft ground tools to full disc cutters, under pressure often lose time for the retrofit making it impossible to achieve a rapid tunnel excavation.
"In Hybrid TBMs, the ability to manage soft ground tools and disc cutters is of primary importance. Many different cutters and soft ground tools can be used – disc cutters with carbide inserts, soft ground tooling with increasing amounts of carbide as ground becomes more challenging."
Furthermore, increasingly tough, mixed ground conditions are ushering in a new era of cutting tools. "Certainly in mixed ground hyperbaric interventions and tool changes can be a challenge, so optimising tool life and cost is important," says Grothen. More robust tool design is desirable. More hard facing, increased carbide in the tools is desired. Mounting systems must be in place, which are able to interchange between the different tool types. We want a design that is flexible in the tools that it can accommodate.
"The tools themselves can be used in a variety of mounting systems, but these need to work together to mount into a single housing."
The hybrid TBM
The use of hybrid TBMs is a trend that follows with the increasing frequency of difficult tunnel conditions in mixed ground conditions.
Where multiple machine types might have once been used for different sections of geology, today hybrid machines are excavating the entire tunnel.
"Hybrid TBMs are becoming more widely accepted, and are thus becoming more common," notes Grothen. "As more challenging jobs are considered, hybrid TBMs help mitigate some of that risk.
"They are being seen as a viable alternative compared to traditional solutions that might involve using multiple TBM types or combining methods like drill and blast and TBM. There are lots of ways to approach a problem of course, but Hybrid TBMs are being considered a good alternative now more than ever."
While there are various types of hybrid TBMs, 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 complete the project tunnel.
In terms of muck removal, Grothen says that interchangeable muck removal is what defines a hybrid TBM. Looking at a EPB/hard rock machine designed to excavate in open or closed mode, in hard rock or soft ground, Grothen explains: "This type of machine has a screw conveyor to operate in closed mode. Some machines in this category will also be fitted with a belt conveyor for better open mode performance in hard rock. In larger diameter TBMs, both conveyor types can be installed concurrently, and in smaller sizes the conveyors usually need to be exchanged.
This conversion process of course requires some amount of downtime but it can be justified if there are long stretches of rock that require the machine to be in that mode. Another feature that can greatly improve performance is use of variable speed cutter head drives with multi-speed gearboxes that can provide high torque at slow speeds for soft ground , as well as high RPM for rock.
"Cutterheads can also be designed to be converted to/from bidirectional to single direction mucking."
Efficiency
The design of a hybrid TBM must be tooled for the geology in order to be most effective. Although hybrid machines provide the possibility to make difficult tunnelling projects possible, the machine must be geared towards either soft soils or hard rock.
"A Hybrid TBM is a balancing act between two different types," says Grothen. "This is particularly true of Rock/EPB hybrids and Rock/Slurry hybrids as the designs are so distinct from one another. Depending on geology, the design is then optimised to work most efficiently through the whole project. If most of the project will be in rock, for example, we will skew cutterhead design towards rock geology so it can get through short sections of soft ground while being highly efficient in rock.
"It’s all a compromise, skewed for the best overall effect. Some examples of modifications for rock include abrasion- resistant wear plating installed in high-wear areas of the cutterhead, protective structures for cutters in blocky ground, and the use of disc cutters themselves."
Much of the efficiency in Hybrid TBMs, therefore, lies in the accuracy of the ground prediction.
Grothen adds: "However, If the geology is 50 percent rock and 50 percent soft ground, this will then depend on the lengths of each section. The machine will be a true compromise — and if there are very short sections of rock and soft ground switching back and forth, then the machine must be optimised towards swift conversion of muck system and cutting tools."
Mexico’s Big Project
Work on Mexico’s Emisor Oriente sewerage project started in April 2009 after decades in planning and design. The desperately needed infrastructure is designed to replace, underground, an open canal that conveys waste and stormwater from Mexico City to a discharge on the Tula River to the east of the city.
For many years engineers believed the ground would be too difficult to excavate – the soil is made up of watery clays running up to 80m deep with the water table just 2 to 3m below the surface. The ground also contains boulders up to 600mm in diameter.
Custom designed EPBs were designed for the difficult mixed ground conditions they would be facing on the job site. The machines were engineered with mixed ground, back-loading cutterheads, with carbide cutter bits to deal with variable ground conditions, and ribbon-type screw conveyors to remove large boulders.
"The mixed ground heads are performing well, although the geology has changed over time and has become more challenging. The Mixed ground design has been optimized for difficult geology, but the ground is more difficult than we initially thought," says Grothen.
Alcala adds: "We have found during tunnelling that the ground conditions can change dramatically in as little as 50m. For this reason, we have backloading cutterheads in order to change cutting tools safely and effectively. We are currently doing geological surveys at Lots 3 and 4, which have encountered some unexpected very hard rock conditions including abrasive basalt and boulders. We are constantly adjusting based on which points we expect the basalt to be encountered at during the tunnel drives so we can find the best way to operate the TBMs.
"Efficiency at interchanging cutters has improved dramatically throughout the project. Now that we are past the learning curve, crews can change 48 cutters in 2 to 3 days. At some points the machines are in full-face basalt so cutters can be changed at atmospheric pressure. Lot 4 did require some hyperbaric interventions using divers due to ground water."
Furthermore, cutting tools wear – especially in mixed ground conditions – and besides disc cutters and/or knife bits, additional tools are needed to encourage muck flow, protect cutterhead structures, and monitor for excessive wear.
On the TEO project hydraulic wear detectors have been installed at various locaions and levels, while there is wear detection covering the cutterhead structure throughout the entire face in the form of a wear detection pipe. Grothen says: "The installation of wear detectors at varying heights on the face of the cutterhead can give warning of excessive wear or of the need to replace cutting tools before damage occurrs to the cutterhead structure. Wear detectors use hydraulic pressure that is released when a certain amount of wear occurs and the hydraulic line is sheared, sending a signal to the machine operator.
"Another very important wear detector type is the wear tube. This tube is routed along the surface of the cutterhead structure from the center to the outermost structure, and is protected by the cutterhead wear plate on both sides."
Robbins states that the company invests more on R&D for cutters than any other company in the industry.
However, Aaron Shananhan says that improving the overall project through greater wear prediction capabilities could benefit from more research.
"There will always be the potential for incremental improvements in tool performance through the material, processing, and design of tools but I believe that there is greater potential for improving the overall project through greater wear prediction capabilities. Disc cutter wear prediction models are fairly well developed at this point. Soft ground tool wear models take only a few inputs and generate a basic estimate of cutting tool wear. I’m not aware of any predictive model that can accurately predict cutting tool performance in a true mixed face condition."
New developments
Generally, while some designs for high-pressure cutters used in conditions of plus three bar exist in the industry, their capability is limited in certain conditions. For example, while the standard seal is rated for about three bar of pressure differential between the atmosphere and the bearing cavity, which is reached at 30m below the water table, many tunnels are far deeper.
Various methods have been tried over the years to try to compensate the internal pressure in cutters to match the exterior pressure, in order to overcome the pressure differential problems. "Our approach for handling high pressure situations is to equalise the pressure across the cutter seals so there is no great pressure differential acting against the seals."
"We accomplish this through the use of a patented pressurecompensating device that is compatible with all standard cutter designs. Previous applications of pressure compensation devices in EPB applications have generally not fared well because they utilised small surface area devices that were highly prone to plugging.
"The device has a much greater surface area, requiring very little movement to achieve complete pressure compensation across the seals, negating the need for special high pressure seals"
