Q: How would you define small-diameter tunneling?
A: The term “small diameter” tunnel boring machine (TBM) can be difficult to define as it encompasses a range of sizes, types of applications, and a variety of geotechnical conditions where modes of operation are important. In regard to size from a global perspective, small TBMs can range from 5 ft (1.5 m) to 20 ft (6 m) in diameter. The lower limit of 5 ft (1.5 m) is the smallest size for a machine to be used with an operator inside the tunnel or manned-entry. The upper limit is slightly smaller than a tunnel that would be used for transportation, such as trains or car traffic.
For our discussion focus here in North America, we will define “small” TBMs as machines from 6 ft (1.8 m) to about 12 ft (3.6 m) in diameter that can mine from 2,000 to 20,000 ft in open or closed mode through geology from soft ground to hard rock. Projects with these parameters are typically used for sewer and water transfer utility tunnels, or small hydroelectric power tunnels.
Q: Can small-diameter TBMs be used for the same types of applications as their larger diameter counterparts?
A: The applications for small-diameter TBMs range from water and sewer projects in an urban setting, to small hydroelectric power projects through mountains. In all cases, these machines can be classified by the three rules of tunneling: “geology, geology, and geology”!
It is expensive to buy a TBM for one geotechnical condition when a completely different condition is encountered (or when the project owner “should know” the geology and spend the money to define it before a TBM is specified or purchased). Understanding the geology is the key in determining the TBM’s method of operation. The type of TBM and its operation is also governed by the tunnel design.
A tunnel can be a common two-pass tunnel where a primary liner is built before the final liner is cast in-place, or carrier pipe is installed with the annulus between primary liner and pipe OD backfilled with low-density cellular grout. Or, it can be one single pass or segmentally lined tunnel where precast concrete segments are transported to the tunnel heading and erected to form the final liner. The geology is critical in determining the mode of operation, whether the TBM is open or closed, and so much more. While open mode TBMs such as Main Beam machines are used for tunnels mined at atmospheric face pressure, closed mode machines such as small diameter EPBs are used for mining when the tunnel face is pressurized.
Q: Are there any design aspects that are unique to small-diameter TBMs?
A: Small-diameter TBMs are similar to large TBMs in functionality and operation. They can be continually steered to line and grade and configured to install steel ring beams and boards or precast concrete segments. They have basically the same cutterhead tooling with carbide cutters and rippers for soft ground, and smaller disc cutters for rock. Like large TBMs, their cutterheads can be designed to replace cutterhead tooling and discs from the rear or the front, if the geology is rock and is competent. They can be completely shielded for continuous support of the ground, or open for applications in hard rock where the ground is exposed. Based on the skill level of the contractor, small TBM operation can be simple with hydraulic motors driving the cutterhead and manual hydraulic valve banks for machine control, or more like a large TBM with electric cutterhead motors and push-button type operational control panels. Muck removal is generally by rail-bound rolling stock with diesel engine locomotives. However, to reduce ventilation requirements in the smaller space, battery powered locomotives are common when overall costs for ventilation ducting and locomotive maintenance are considered.
The capabilities of small TBMs have always been similar to large TBMs in soft ground. In hard rock above 20,000 psi compressive strength however, there has been real industry advance. I would say that it has only been within the last 10 to 15 years that rock TBMs below 10 ft in diameter have become a reliable solution for utility tunnel design engineers. The demonstrated capabilities of machines in this range are impressive and include hard rock above 40,000 psi compressive strength. A particular feat was achieved recently with a Robbins 88-in. diameter Double Shield TBM used to bore through 54,000 psi diabase rock in Fairfax, Virginia.
Q: What design changes allowed small-diameter, hard-rock TBMs to catch up to their larger diameter counterparts?
A: About 15 years ago, Boretec (the company that would later purchase Robbins) started to develop small-bore, hard-rock TBMs in the 2.2- to 2.4-m diameter range, mostly Double Shield models. The key to this development was to put the largest diameter cutter ring on the smallest effective bore diameter, and back it up with enough power (2 x 350 hp = 700 hp hydraulic or about 500 hp net), to get enough torque through to the cutters. These larger diameter cutters performed much better than smaller cutters that had been used, which were as small as 12 in. in diameter on earlier models. The larger cutters were placed on the cutterhead with a narrow enough kerf spacing to allow for effective chipping of very hard rock.
Q: Are there logistical differences when tunneling at small diameters?
A: Absolutely — in my opinion small-diameter tunneling is all about logistics. Ensuring adequate space for trailing gear, ventilation, muck removal, and men and materials transport is key to an efficient operation. This is really where the money should be spent. The TBM will do what it will do, but logistics can make the real difference in advance rates. We have a very long, about 4 km tunnel, in Montreal, and at this length there are always some real challenges. It’s a 3.0-m diameter TBM, with not much room in the tunnel. Simply getting the crew to the front and having enough room for them can be a challenge. We have a crew member driving the TBM, three personnel installing ground support, three installing rail behind the TBM and one running the muck train. The tunnel is too small for a California switch so there is only one muck train and it takes 25 minutes to run a loaded train out of the tunnel and back into it.
Q: What are some of the solutions to muck handling logistics at small diameters?
A: We can design customized muck boxes to maximize muck removal capacity. At some diameters a crown-mounted conveyor is also possible, though for very small diameter tunnels there may not be enough room for a conveyor.
Q: Do you think that small-diameter tunneling is trending upward in North America?
A: Yes, we have seen demand increase dramatically just within the last two to three years. Big cities across the United States and Canada are upgrading their utilities for increasing demand and storage capacity, usually for water and wastewater systems. With those upgrades comes a need for small-diameter tunnels. We have seen a particular need in Canada, in Montreal and Toronto specifically. These cities have rapidly increasing populations and the municipal authorities have extensive plans in place to meet those increasing demands. We have two tunnels in the Toronto area, the Mid-Halton Effluent Outfall and the Upper Centennial Trunk Sewer. At Mid-Halton, the contractor, Strabag, is using a 3.5-m (11.5-ft) diameter Main Beam TBM that has advanced about 650 m on a 6.3-km (4.0-mile) long tunnel, and at Upper Centennial we’ve refurbished a 2.4-m (8-ft) diameter Double Shield TBM for McNally Corp., which is installing a new 5.2-km (3.2-mile) sewer. As a side note, on Upper Centennial we went 11 m (35 ft) in one 10-hour shift using just a single muck box — a very good result!
In Montreal, ForAction is using a 3.0-m (9.8-ft) diameter Double Shield TBM upgrading the Rosemont Reservoir. The tunnel will help modernize the large reservoir structure, which was built in 1962 but never connected. That TBM is about 3.1 km (1.9 miles) into its 4-km (2.5-mile) drive and expected to finish up this year. Similarly, we have a 2.2-m Main Beam TBM that we are refurbishing for the EBC/McNally JV and that will be launched on the Rue Jarry Water Transmission tunnel, another water supply project.
In the United States, there is a similar demand. We have done some work for the Middlebelt Transport and Storage Tunnel in Michigan, where Robbins supplied a new mixed ground cutterhead on a 132-in. diameter Lovat Single Shield TBM. The TBM also has the capability to be used with a screw conveyor if soft ground is encountered. We recently launched that machine and it’s about 18 m (60 ft) in. We stopped to install more effective loading plates to muck out the cutting chamber as we were in soft ground. At our suggestion, the contractor, Ric-Man, is also using simple sawdust to help reduce ground stickiness and it’s going quite well. As for other projects, we recently completed the Corbalis to Fox Mill Water Main in Fairfax, Virginia, that I mentioned earlier, a water transfer tunnel in very hard rock conditions.
Overall, as cities continue to allocate dollars toward upgrading and renovating their utility systems, so will go the small-diameter tunnels. As long as the funding is there, I see small-diameter tunneling increasing. It is a function of our systems having been built between 50 and 100 years ago in much of the United States and Canada. These systems can only function properly for so long without needing a major overhaul. This is all on top of new installations that address ever-expanding residential, commercial and industrial projects as a result of increasing population. So, get ready for more tunneling on a small scale!