Microtunneling 101: Good things come in small packages
October 19, 2021
October 19, 2021
Why is this trenchless technique gaining traction, and does it make sense for your project? A look at some key considerations around microtunnels.
Microtunneling. It’s one of the most rapidly expanding techniques in tunneling, and my team has used it on several successful projects. If you’ve heard about microtunneling but don’t know much about it, let me lay out some key facts for you.
Microtunneling is a trenchless construction technique. It’s used for constructing small-diameter utility tunnels, which can be 20 to 160 inches in diameter. To construct these tunnels, you need special boring equipment—similar to the larger boring machines we’ve used on big tunnel projects. Microtunnels involve the use of microtunnel-boring machines (MTBMs), which are operated remotely from a control room typically at ground level, with no operator in the machine.
Curious about the installation process? Here’s what it looks like, in a nutshell: You put the MTBM into an entry point in the ground—known as the entry eye (shaft)—and then push pipes behind it. The pipes, which will eventually be installed into the tunnel, advance the machine forward as material is removed from the cutter face. As the machine proceeds, its cutting head removes the soil at the face into smaller bits and—after introducing additives, if they’re required—mixes it into a slurry blend. This material is then removed from the machine cutting face with the slurry mixture typically pumped back to the surface.
Microtunneling could work well for installing water mains, sanitary sewers, or many other types of utilities. Is there a busy highway or sensitive wetland near your project site? A microtunnel might be the right choice. Below we dive into the “why?”
Let’s look at some key considerations around this trenchless technique, so you can decide whether it’s the right one for your project:
One of the biggest benefits of microtunnels? The level of accuracy and control. Microtunneling’s accuracy has increased tremendously over the past few decades, which makes the technique useful when working within smaller easements or installing curved alignments with a consistent grade. In the old days, your tunnel would be a straight line, but tunnel-boring machines have the capability to tackle radiuses. MTBMs give you the ability to correct your alignment very quickly and maintain the line and grade. That greater sense of control is handy.
This accuracy means that you know exactly where the machine is located, and you can control it. This means you can get closer to other infrastructure like sewers or buildings. With the traditional jack and bore tunneling method, there’s a greater chance of hitting something near your tunnel project.
During microtunneling, you can monitor the pressure at the cutting face, so there’s a smaller likelihood of settlement problems. This can occur when you remove the material around a sewer or water main and the pipe can’t hold itself up—leading to issues like sink holes in the street.
Here’s a benefit to consider when it comes to microtunneling: the safety that comes from remote work. Remote control naturally leads to greater safety for the controller, because instead of sitting near the head of the machine underground—like they would for a larger tunnel-boring machine—a person can operate the MTBM from above ground. That way, if you face a safety issue—if you run into gases, or something fails in the pipes—there isn’t a controller present underground.
Microtunneling can serve as a good option when dealing with difficult ground conditions, such as softer soils. The MTBM is essentially set up for softer ground, and it works well below the groundwater table. You can also use it to bore through many types of soil, sand, and clay. It’s possible to use it to bore through hard rock, but you may not be able to go a long distance if you’re dealing with high rock compressive strength and hardness. We’ll touch more on distance below.
Microtunnels involve the use of microtunnel boring machines (MTBMs), which are operated remotely from a control room typically at ground level.
Another perk of microtunnels? The lack of surface disruption, especially compared to open-cut construction. If you need to tunnel through environmentally sensitive or contaminated areas, microtunneling—like other trenchless tunneling options—will reduce the amount of contaminated material you need to handle. This can be a big factor in some situations, like if you’re performing a deep excavation and it’s expensive to dispose of the contaminated material.
In previous decades, the cost for microtunneling may have seemed high, especially compared to other methods like jack and bore. But advances in the industry—and the increasing number of companies that have microtunneling capabilities—has led to competitive pricing for microtunneling. And the MTBMs are less expensive than larger tunneling machines. The industry essentially took the benefits of larger tunneling machines and downsized them.
Even though the use of MTBMs has become more economical, you need to figure out what you want for your project. For certain conditions, locations, and alignments, microtunneling makes a lot of sense. The accuracy is worth it. But it depends on what you’re trying to do. Microtunneling could be seen as an expensive option if you don’t really need it—and if another technique, like jack and bore, could work just as well.
Here’s one of the drawbacks of microtunneling: Distance. You’re limited in terms of how far you can go. Other techniques—like horizontal-direction drilling—allow you to change cutter heads. But it’s extremely difficult to reach the face in smaller diameter microtunnel machines, which means that you don’t want to wear out your cutter head. You need to understand the geotechnical and hydrogeology components (e.g., groundwater conditions) of your project before the tunneling begins. Then, you design your cutter heads to help you reach your desired distance comfortably, without wearing the cutter heads out. This can also be accomplished by having shafts strategically located to allow for cutter head replacements.
If you do wear out a cutter head during a microtunnel project, you may have to build a rescue shaft to change it, since you can’t back it out. And that rescue shaft could be in the middle of a busy downtown intersection, or an environmentally sensitive area. It’s not ideal.
Here’s a good example of a microtunnel in British Columbia, Canada. The project, known as South Surrey Interceptor - Johnston Road Section, featured a sewer that utilized the largest diameter microtunnel-boring machine used in Canada and North America to date. This project provides a good example of how microtunneling can be used to efficiently complete large-diameter curved pipeline installations in urban areas while minimizing disturbance and environmental impacts.
Here’s some history: The South Surrey Interceptor was constructed in the 1970s to convey sewage from South Surrey, White Rock, and Langley to the Annacis Island Wastewater Treatment Plant. Metro Vancouver began twinning the interceptor in 1999 to increase capacity for the area’s rapid growth and to improve environmental protection by reducing sewer overflows.
The Johnston Road section was constructed in 2018 and 2019 as one of the final portions of this twinning project. Our geotechnical and environmental engineers, tunneling specialists, environmental scientists, and archaeologists worked with Metro Vancouver’s engineering and capital project teams on the installation of a large-diameter sanitary interceptor sewer.
An environmental assessment of the project identified portions of the project that contained naturally occurring metals above normal disposal limits, requiring additional material handling for disposal. The decision to construct the entire project using microtunneling proved to be prescient in this sense. If the project had been open cut, the amount of material that would have had to be disposed of at an appropriate landfill would have resulted in significantly higher project costs.
I hope you’ve learned more about this intriguing trenchless tunneling technique. For the right project, microtunneling can serve as an excellent option. Like I said, it all depends on what you want to achieve and what ground conditions you encounter. Contact me if you’d like to discuss the use of microtunneling for your next project.