Microtunneling: The next big thing
October 15, 2021
October 15, 2021
The benefits of microtunneling are hard to beat. Not only for the environment, design, and construction teams, but for the rest of the community.
By Anil Dean and Nick Goodwin
Water infrastructure is largely invisible. Because it’s out of sight, it’s out of mind for most people. Construction of this essential infrastructure often is not. However, microtunneling is changing the game.
The practice came to North America from Japan and Europe in the 1980s, taking decades to catch on. It is not as disruptive in nature—replacing continuous pipe trenches with widely spaced shafts—and boasts a lower carbon footprint and enhanced environmental controls compared to open-cut pipeline installation methods.
Now, its popularity is growing in North America. Why? It is considered a fast alternative to traditional pipe installation systems to replace aging water, sewer, gas, electricity, and telecommunications infrastructure.
Microtunneling is a trenchless technology that describes itself with its name—it’s a technique typically used to bore smaller tunnels, mostly used for pipelines of 0.6 - 2 meters (m) in diameter.
However, it’s beginning to be a bit of a misnomer. Microtunnels are getting longer, bigger, deeper, and curvier. In 2019, the largest diameter microtunneling project in North America was designed by Stantec and completed at 3.68m in diameter.
The path to a successful microtunnel installation starts well before the first pipe is ever pushed into the ground. Microtunneling is like pipe jacking, but it is remote controlled and has a sealed and pressurized face with a slurry system used to retrieve excavated material (referred to as spoil). This technique can be used for pipelines that need a high degree of installation accuracy, in terms of line and grade, over a long distance.
Microtunnel boring machines (MTBM) are very similar to conventional tunnel boring machines (TBM), but on a smaller scale. These smaller diameter tunnels are generally controlled remotely with laser-guided steering by an operator at the surface rather than sitting in the machine itself. MTBMs use high-end guided systems with live monitoring for real-time correction capability. Through a computer console and precise control equipment, the operator receives continuous data about the location and orientation of the MTBM as well as data about the other parts of equipment.
A big advantage to microtunneling is that it is very accurate on line and grade compared to other trenchless methods. Hitting the mark of plus/minus one inch is achievable. While we always strive for this type of accuracy, this is especially important for gravity pipeline/sewer projects.
In most microtunnel projects, a MTBM is pushed by hydraulic jacks from a launch shaft and pipe sections are pushed behind the machine using the jacking frame. This process is repeated until the boring machine reaches the reception shaft. As the machine advances, more pipe sections are pushed from the launch shaft. Interjacking stations (IJS) can be inserted into the pipe string to provide additional jacking capacity, if needed.
Working underground, even with the best pre-project reports and studies, always brings with it a sense of the unknown.
A surging trend in the microtunnel market is curved drives. The first curved drive in Canada was constructed in 2013. In combination with longer drive lengths, curved drives can lead to the elimination of intermediate shafts that can lower the cost of construction and reduce disruption on the surface. Longer drive lengths, compound curves, and now, tighter radii and larger diameters increase the range of what can be accomplished with microtunneling techniques.
This method is best used in difficult ground conditions—like softer soils—but can be used to bore through all types of soil, sand, clay, and even hard rock. It works especially well below the groundwater table due to its sealed and pressurized face. This prevents groundwater and soil from flowing into the machine. This is one reason we used microtunneling on The Port – PL6 Stormwater Storage Tank Project (PL6) in Cambridge, MA as we were in soft Boston Blue Clay below the groundwater table.
The City of Cambridge Department of Public Works overall objective for the PL6 project was to mitigate historical flooding problems and sanitary sewage backups into residential and commercial basement properties in the Port neighborhood. Excess flows would be conveyed via the tunnel to an adjacent catchment with available capacity.
PL6 presented a distinctive design challenge. It needed a single trenchless crossing threaded between two buildings—one of which was under construction at the time—and under the Massachusetts Bay Transit Authorities (MBTA) Red Line subway. Primary design concerns were completing the trenchless drive, limiting settlement to the nearby structures, and the long-term performance of the system given the challenging geotechnical conditions.
The pressurized face MTBM limits settlement. Why is this important? A benefit of microtunneling is continuous pressure provided at the face of the excavation balancing groundwater and earth pressures. This was a key factor for PL6 as for 60 feet, we passed just 1.8 meters below the MBTA Redline Subway Tunnel in downtown Cambridge. This is a structure and service you don’t want to displace!
As tunneling progresses, the face of the excavation is stabilized with slurry or conditioned soil, and cuttings are removed in a closed system consisting of a pipeline or screw auger. Pilot tubes or pre-installed rails are supplemental methods often employed to both support the TBM or casing and to prevent sinking during the drive. Sometimes in troublesome underground conditions, directional controls are not as effective at maintaining elevation and grade and these added approaches support accuracy.
In the instance of PL6, the 194-feet long tunnel was designed to be a 63-inch steel casing with an invert elevation nearly 50-feet below the ground surface and about 21 feet below the bottom of the existing Red Line commuter rail tunnel.
Most underground infrastructure capital improvement projects are driven by capacity issues. These situations often bring with them dense urban environments and critical existing infrastructure making for a limited working area. Working underground, even with the best pre-project reports and studies, always brings with it a sense of the unknown. Microtunneling may be a bit more expensive than other trenchless applications but is also one of the most capable. The accuracy, reliability, and lower maintenance cost of pipelines post-installation can make it a better choice.
Want to know more and see if your project is a good microtunneling fit? Reach out.
About the Authors:
Anil considers each of our underground projects in terms of how to best meet client and community goals, while maintaining compatibility with ground conditions.
All trenchless projects pose design challenges, Nick’s varied experience brings unique solutions to this sometimes volatile underground world.