Note: the undergrounding world is as complex as the geologies that lay beneath the earth’s surface — far too complex to cover in one blog post. With that out of the way, today we’ll cover some examples from the industry in an attempt to map the landscape and position trenchless vs tunneling, but this is by no means comprehensive.
Trenchless technology and tunneling technology are often used interchangeably, but they have very different applications and require different construction technologies. While both trenchless and tunneling technologies involve the creation of underground tunnels or passageways, there are some key differences between the two—in many ways, modern trenchless technology has evolved from tunneling technology. Trenchless technology refers to a variety of methods and techniques that are used to install underground pipes and other infrastructure without the need for top-down surface excavation. These methods typically involve the use of specialized equipment to create underground tunnels in a controlled and precise manner. Trenchless equipment is typically smaller and more maneuverable than tunneling machines, and is designed to create narrower and shorter tunnels.
Trenchless tunnels are generally considered to be small-diameter, not accommodating humans inside, whereas full-size tunnels are typically for large-scale transit such as highways and railways and can bore diameters as high as 50 feet or more, although there is crossover in the 48” to 72” diameter range. Phoenix Boring’s trenchless tunnels, for example, are currently 72 inches (6 feet) or less in diameter. Tunneling technology, on the other hand, refers to the construction of underground passageways or tunnels for transportation or other purposes. Both trenchless and its sister industry, tunneling, attempt to address the most effective way to get through the ground. For trenchless there are a number of distinctly different methods such as pneumatic hammers, auger boring machines and horizontal directional drills—which we will explore in later blogs.
Tunneling typically involves the use of large, heavy-duty machines to excavate the tunnels, and is a more complex and costly process than trenchless technology. In tunneling there are several types of TBMs (tunnel boring machines) designed for various geologies, as well as the use of drill and blast or other mechanical excavation (some examples of this are excavators, roadheaders, pineapple heads, etc).Tunneling machines, in part because of their large size and massive propulsion systems, are designed to dig through almost any geologic conditions, including hard rock, soil, and clay. They are also capable of excavating large volumes of material, which is necessary to create tunnels that are several feet or more in diameter.
Tunneling machines are equipped with powerful engines and hydraulic systems, and require a significant amount of energy and resources to operate. Marsha the TBM (a $16 million, two-story behemoth of a TBM, weighing in at 95 tons and capable of boring a 23-foot diameter at a rate of 40 fet per day) requires an entire medium-voltage switchyard that was constructed on-site to allow for two 14,400V circuits for the TBM and water-pumping system. Both circuits will travel the entire length of the expanding tunnel, one providing 26,000 kW to the TBM and the other providing 36,000 kW to the pumps that are moving fresh and wastewater through the tunnel. To accomplish this, more than 46,000 feet of 15-kilovolt, tough, oil-resistant, SO cable was required. To keep the machine cool and excavate the crushed rock requires 24,000 gallons of water per minute — an olympic-sized swimming pool’s worth of water every 30 minutes.
Put simply, TBMs are far more costly to manufacture and operate, meaning project costs are substantially higher. Trenchless equipment, on the other hand, is typically smaller and more efficient at creating smaller diameter trenchless tunnels. This allows trenchless teams to more quickly mobilize their equipment, resulting in substantially less time to complete a trenchless construction project. Large tunnel boring machines generally need to be purpose-built and are often sent back between jobs to be reconfigured for the job and geology, which can add substantial design and manufacturing lead time, and occasionally additional risk, to projects. Some examples of the largest tunnel boring machines, along with their specifications and associated projects, include:
Within the trenchless industry, there are even more categories of machines, as smaller diameters have historically required specialized machinery that is designed to tackle specific geologies, product applications and bore profiles. In our next blog post, we’ll cover how geology, application and diameter generally affect the type of machine a trenchless contractor will use. Some common types of trenchless boring machines:
These machines are smaller and less powerful, but much more easily deployed and far less expensive than the larger TBM’s discussed above. One key difference is that a TBM often requires a large team of over 30 people, whereas a trenchless team can be as small as 4 or 5 people on a job site. To avoid complicating this blog post too much, we’ve so far ignored mentioning microtunneling (put coarsely, microtunneling can be considered a cross between trenchless tunneling and TBM machines) but it’s important to note its existence — it’s easy to quickly see why this is a very complex and specialized industry.
Overall, the main difference between trenchless and tunneling technologies is the size of their bores and the purpose of their tunnels. Trenchless technology is used for small-bore tunnels that house utilities, and the primary advantage of trenchless boring is that it does not disturb the surface and can be deployed with relative ease. In contrast, tunneling technology such as TBM’s is used for creation of larger passageways—primarily for various types of transportation—and tends to be a longer and more involved process, with specially designed machines and large project timescales.
