An average-sized TBM might be around 6m in diameter and weigh around 400t. 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 onsite, or preassembled in the shop, extensive preplanning and collaboration are needed to bring these complex machines to fruition.
The cranes used during TBM assembly generally fall into two main types: gantry cranes and mobile cranes. Mobile cranes can be further divided into wheeled-type mobile cranes and crawler cranes. Each type has various 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 and layout of the cranes has more to do with the available space on the jobsite. TBMs in remote locations might be assembled in large launch pits that allow for full assembly of a back-up system 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, TBM & Equipment Specialist for contractor Dragados USA. With extensive experience on both European and North American tunnel projects, Magro is now managing the upcoming 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 continues.
Limited space jobsites
In terms of size and mobility within a limited space, there are clear differences in the types of cranes to be used. Gantry cranes are often used in Europe, because the jobsites tend to be much smaller than US jobsites. These overhead cranes run on a _ xed track the length of the jobsite, making them more compact as they do not need a boom. While there may not be much of a difference in size between gantry and mobile cranes at small machine diameters, for larger TBMs mobile cranes require more room. "Mobile cranes are more _ exible and accurate and, therefore, they might work in more con_ ned 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," says Magro. In smaller jobsites with obstructions such as overhead power lines, large mobile cranes may not be a possibility.
Much of what makes a crane the best for a particular job is the site space and how the materials are laid out. Gantry cranes require a linear layout, with everything for a TBM assembly laid out in order so the crane can bring it forward. "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," says Steve Chorley, Robbins field service director, who has supervised TBM assemblies on projects around the world.
Both Chorley and Magro agree that the hydraulic telescopic boom used by mobile cranes is more accurate than a gantry crane, and that crawler cranes are the most versatile of the mobile types. "A crawler crane can pick up components from the back of the jobsite and move them to the front. Wheeled mobile cranes aren’t capable of doing this because they need a stabiliser. They can’t just pick up 300t and move it around the jobsite," says Chorley.
Depending on availability, combinations of cranes may be the best choice for certain projects. Ernst Gschnitzer, Strabag project manager for the Niagara Tunnel Project, 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: "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."
Jobsites with Ample space
In jobsites with ample space, multiple types of cranes could be used depending on availability. This was certainly true at the Abdalajis high speed rail tunnels in Spain, a project where Chorley worked. Two Robbins/MHI 10m double shield TBMs were assembled using large crawler cranes. "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."
In remote locations, however, the logistics of crane transport may 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. Also, the contractor usually needs to have a system that can be also be utilised after machine assembly, which is the gantry crane. The gantry crane can be used for segments and other materials throughout the project," says Chorley.
Crane economics
While it depends on the situation, large mobile cranes, says Chorley, tend to be more costly because they require rental and special operation. "Large mobile cranes are very costly — in addition to the rental there are fees associated with mobilisation and demobilisation. To mobilise 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 also need to hire a crew for counterweights and a crew to set up the lift, in addition to the crane operator," says Chorley. Gantry cranes have the cost advantage because they can be built onsite without a rental. In addition, gantry cranes can be used after the assembly for segment and materials handling, as long as the layout is taken into consideration. They don’t require a counterweight for lifts, though they are typically not capable of lifting as much weight as a mobile crane.
Overall, though, planning is what may reduce costs the most. "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 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/more expensive crane, it is whichever type is able to maximise the TBM assembly," says Magro.
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 in a shop before final assembly and launch, or initially assembled at the jobsite. The latter method, known as Onsite First Time Assembly (OFTA), was developed by The Robbins Company in 2006, and has since been used to assemble all types of TBMs on projects worldwide. OFTA offers time and cost benefits over a traditional shop assembly, by eliminating the time for pre-assembly in a shop as well as shipping costs for components going to the shop. Time savings can be as much as five months, and cost savings for the contractor can add up to USD 4M for a large diameter machine.
In 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 at core components such as the main beam and working towards outer components such as the outer cutterhead pieces and gripper shoes. 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, other types of cranes will also be needed. "Our largest was a mobile crane with a 380t 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," says Gschnitzer of the Niagara TBM assembly. That Robbins machine was completed in just 17 weeks, less than 12 months from contract signing.
Overall, the types of cranes used don’t vary significantly between a machine assembled onsite and a shop-assembled machine; the variation is only in the various weights and types of components they will be lifting. "The OFTA process doesn’t really decide whether we use gantry or mobile. What decides that is the location of the jobsite and availability of the crane," emphasises Chorley.
Perhaps the biggest factor in success of any TBM assembly, and particularly for OFTA, 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 contractor. The sequence of the machine assembly must be finely coordinated in order to ensure that all the pieces are being dropped in the most efficient way. A successful OFTA project in particular requires that the contractor and machine supplier are clear on the requirements and expectations of both parties from early on.
Heavy Lifting trends
While the cranes themselves are not changing, the demands on them from the tunneling industry are evolving. As TBMs get larger, planning of lifts for various component assemblies becomes more labor intensive. "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," says Magro.
Large components are certainly true of Magro’s latest project, involving the giant Hitachi Zosen TBM for the Alaskan Way Viaduct Replacement Tunnel. "The project will be very challenging; 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 1,200t capacity will be used for the assembly since there is not enough clearance to set and handle a large diameter mobile crane. The assembly of the machine in fewer components, says Magro, will allow for fewer shipments and shorter assembly timeframe.
In addition, TBMs are being 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 NYC, which is not a gantry system but could fall into a similar category," says Magro. The East Side Access project in New York City, which used a 6.7m diameter Robbins Main Beam TBM, involved the transport of inner core components including the main beam and main bearing through a small starter tunnel into the chamber, where the hydraulic side and roof supports and other structures were staged for final assembly.
Overall, the use of cranes will remain instrumental in TBM assembly, and the specific type used will depend on factors from jobsite size to location to component weight. The assembly method used for the TBM, whether onsite or pre-assembled in a shop, is not a determining factor. When asked about a takeaway point regarding cranes and TBM assembly, Magro had this to say: "My overall message is that collaboration by all parties involved is needed to make it all successful"
