An average-sized tunnel boring machine (TBM) might be roughly 6m in diameter and weigh 400 metric tons. Judged on weight alone, even the very largest of TBM components is well within the capabilities of heavy lifting cranes found throughout the world. The real complexity comes into play when logistics is considered — TBMs are often assembled on cramped jobsites, in urban conditions with overhead lines, and amidst components staged in much of the available space. By contrast, in remote, mountainous regions just getting large cranes to a jobsite can be a challenge. Determining the proper crane type and assembly scheme is not always easy in such situations. Whether the TBM is assembled for the first time on-site or initially put together in an assembly shop, extensive pre-planning and collaboration is needed before these complex machines are ready for work.
The cranes used during TBM assembly most commonly fall into two categories: gantry cranes or mobile cranes, with each type contributing its own pros and cons in terms of mobility, accuracy, size, and cost.
It’s all about space
While the size of the TBM determines the lifting capacity of the cranes needed during assembly, the type of crane eventually chosen will have more to do with the available space on the jobsite. TBMs in remote locations could be assembled in large launch pits that allow for full assembly of a back-up system — the network of support equipment that trails the TBM — hundreds of meters long, while a TBM in the middle of a city might be assembled at the bottom of a deep shaft in a starter tunnel.
"What leads to the decision of what type of crane to be employed is the room available at the assembly area and the weight of the components to be handled," says Juan Luis Magro, a TBM and equipment specialist for contractor Dragados USA. With extensive experience of both European and North American tunnel projects, Magro is now managing the assembly of the world’s largest TBM (17.5m in diameter) at Seattle’s Alaskan Way Viaduct Replacement Tunnel. "Jobsites are as small and crowded as ever, and urban projects especially are becoming more popular," he adds.
Tight sites
In terms of size and mobility within a limited space, there are clear differences in the types of cranes to be used. Railmounted gantry cranes are often used in Europe as jobsites there tend to be significantly smaller than for U.S. jobs. These overhead cranes run on a fixed track the length of the site; a compact option compared to a mobile crane, as they do not feature a boom. While there may not be much of a difference in the space taken up by gantry or mobile cranes when dealing with small diameter TBMs, for larger TBMs mobile cranes certainly require more room.
Magro says: "Mobile cranes are more flexible and accurate, and therefore they might work in more confined spaces such as those of urban projects. However, they demand absolute overhead clearance, and a fairly large work area as well if components are to be staged around it." Hence for smaller jobsites with obstructions such as overhead power lines, large mobile cranes aren’t always a completely viable option.
Much of what makes a certain type of crane the best choice for a particular job is the site layout and the way construction materials are arranged. Gantry cranes require a linear layout, with everything for TBM assembly placed in order so that the crane can bring loads back and forth across the site with ease.
Steve Chorley, a field service director for underground construction machinery manufacturer Robbins, has supervised TBM assemblies on projects around the world. He says: "A crawler or wheeled mobile crane is more versatile, and has better positioning. You must be careful with how you stage components if you use a gantry crane.
"Everything needs to be laid out in order for the gantry crane, so it requires detailed planning and more logistics."
Depending on availability, a combination of crane types may be the best choice for certain projects. Construction firm Strabag’s project manager for the Niagara Tunnel Project, Ernst Gschnitzer, was involved in the assembly of the world’s largest hard rock TBM, a Robbins 14.4m diameter Main Beam. He cautions that for large assemblies multiple crane types are usually needed, with gantry cranes being of greater use when used to construct back-up systems. "You will not get away with not using mobile cranes, because of their heavy lifting capabilities. Gantry cranes work better when assembling long parts of the system like back-up systems. You can lay these out along the track and do this much more efficiently with the gantry crane option."
However, both Chorley and Magro agree that the hydraulic telescoping boom used by mobile cranes provides more accurate placement than a gantry crane can, while crawler cranes provide better mobility with loads around the site. "A crawler crane can pick up components from the back of the jobsite and move them to the front. [Some] wheeled mobile cranes aren’t capable of doing this because they need a stabilizer. They can’t just pick up 300t and move it around the jobsite," says Chorley.
Jobsites with ample space
On jobsites with ample space, multiple crane types can be used depending on availability. This was certainly true at the Abdalajis High Speed Rail Tunnels in Spain, a project Chorley previously worked on. Two Robbins/Mitsubishi Heavy Industries 10m ‘double shield’ TBMs were assembled on site using large crawler cranes before the gantry cranes were brought into play.
Chorley says: "We used two crawler cranes simultaneously when lifting the cutterhead and other big components. Gantry cranes were then used to assemble the back-up, and for segment transport later on."
But in remote locations the logistics of crane transport can be a limiting factor, even if the available space is large. "In remote locations, with difficult access to the site, it’s too costly and time consuming to get a crawler crane up there," says Chorley. "Also, the contractor usually needs to have a system that can be also be utilized after machine assembly, which is the gantry crane. The gantry crane can be used for segments and other materials throughout the project."
Crane economics
While it depends on the situation, large mobile cranes tend to be more costly because they are usually rented and require the hire of specially trained crews. "Large mobile cranes are very costly — in addition to the rental there are fees associated with mobilization and demobilization. To mobilize a wheeled crane, a counterweight must be set up on the back of the crane to prevent the crane from falling forward during the lift. It takes two or three days to set up the crane before you can even use it. You [may] also need to hire a crew for counterweights and a crew to set up the lift, in addition to the crane operator," said Chorley.
This is where gantry cranes deliver a cost benefit over mobile cranes. The ability to perform lifts without counterweights means reduced setup times. In addition gantry cranes offer increased utility on site as they can also be used after TBM assembly, for segment and materials handling, as long as the layout is taken into consideration.
That said, it is usually during the planning stage of tunnel construction projects that the biggest cost reductions are provided. Magro says: "The key is to make sure that whoever is going to design the layout of the jobsite understands what will happen in each area. The civil designer must meet with the TBM assembly team. The cheapest way to make sure that TBM assembly goes well is to properly consider the launch pit/shaft design. For example, if we have a shaft, we could have a gantry crane on top of it. There is not a cheaper or more expensive crane, it is whichever type is able to maximize the TBM assembly."
Does the assembly method matter?
The role of cranes in TBM assembly is pivotal no matter the assembly method. TBMs can be either pre-assembled — for testing by the manufacturer — before final assembly and launch, or assembled for the first time at the jobsite. The latter method, known as On-site First Time Assembly (OFTA), was developed by Robbins in 2006 and has since been used to assemble all types of TBMs on projects worldwide. OFTA offers time and cost benefits over traditional pre-assembly by eliminating the extra time needed as well as the shipping costs associated with components being sent to the assembly shop. Up to five months can be shaved off the total project time in this way, with cost savings for the contractor potentially totaling as much as USD 4M for a large diameter machine.
For an OFTA build the TBM is typically assembled in a concrete cradle. During the assembly process components are shipped to the jobsite to build the machine from the inside out, starting with core components, such as the main beam, and working towards outer components including the outer cutterhead pieces and shoes on the grippers that push the TBM along the tunnel as it is excavated. The method can be modified for a launch chamber, shaft, or pit. Once the machine has been fully assembled it can then crawl forward to the tunnel face.
The process is similar for all types of tunnel boring machines, from open-type hard rock machines to shielded, soft ground TBMs, and is essentially identical to those required for shop-assembled TBMs. Due to the longitudinal setup in the cradle, gantry cranes are easily used. This has certainly been true of recent Robbins jobsites, from the Kargi Hydroelectric Project in Turkey to the Sleemanabad Carrier Canal and AMR Projects in India. All have involved large diameter (10m) machines, either shielded hard rock or soft ground, which were successfully assembled in a concrete cradle using gantry cranes.
For very large OFTA assemblies, overhead cranes alone will not be enough. Commenting on assembly of the TBM tasked with excavating a 6.5 mile-long tunnel under Niagara Falls for Ontario Power Generation, Gschnitzer says, "Our largest was a mobile crane with a 380 metric ton capacity. We also had a gantry crane and lots of other mobile cranes for lighter lifting. If we didn’t have the 380t mobile crane we wouldn’t have been able to assemble the largest TBM components, so this was the most critical piece of machinery. All of the other cranes just sped up the assembly."
Whether a machine is assembled onsite or in an assembly shop, the types of cranes used do not vary significantly. The only variation is in the weight loads and types of components being lifted. "The OFTA process doesn’t really decide whether we use gantry or mobile," says Chorley. "What decides that is the location of the jobsite and availability of the crane."
Perhaps the biggest factor in the success of any TBM assembly, particularly when the OFTA method is used, is coordination. Components can be assembled in smaller pieces for lighter lifts or vice versa, and these need to be planned between the machine supplier and the contractor. The machine assembly sequence must be finely coordinated in order to ensure that all components are placed in the most efficient way.
A successful OFTA project requires that the contractor and machine supplier both understand all of the other’s requirements and expectations at an early stage in the process.
Heavy Lifting Trends
Although the requirement for cranes in the tunneling industry may not change, the capabilities demanded of each type of crane is certainly evolving. As TBMs are built larger and larger, lift planning for various component assemblies becomes more labor intensive. Magro says: "We are thinking of assembling TBMs with fewer components so they are heavier. Heavy lift technology is well developed and you can find huge cranes anywhere in the world. But, they need a lot of room, so it won’t be worth it on every project."
A move towards larger components is certainly evidenced by Magro’s latest project, which involves a giant TBM manufactured by Hitachi Zosen for the Alaskan Way Viaduct Replacement Tunnel.
"The project will be very challenging," says Magro. "It’s one of a kind because of the dimensions of the machine. The weights and dimensions of the components make the TBM assembly similar to those that normally happen for bridges, or nuclear plants, in a large space with a large mobile crane. We don’t have the benefit of extra room or a long schedule here."
Instead, the massive machine will be assembled in downtown Seattle on a seven-acre jobsite crammed full with components, supplies and other necessary project infrastructure. An extra heavy lift gantry crane with a 1,200 metric ton capacity will be used for assembly as there is not enough clearance to rig and maneuver a mobile crane with a similar lifting capacity. According to Magro, assembly of the TBM using fewer, larger components will reduce the number of shipments needed and shorten the assembly timeframe.
TBMs are also occasionally assembled in more unusual environments, such as underground chambers. "In the US, we recently used a heavy lift hoist rolling over beams anchored to the ceiling of our startup caverns to assemble our two ‘hard rock’ TBMs in New York City, which is not a gantry system but could fall into a similar category," said Magro.
The East Side Access tunnel project in New York City involved the transport of 6.7m diameter TBM’s inner core components — including the main beam and main bearing — through a small starter tunnel and into a chamber where hydraulic side and roof supports were staged for final assembly.