What’s the difference between microtunneling vs. guided boring?

Q:

What’s the difference between microtunneling vs. guided boring?

A:

Microtunneling and guided boring are often erroneously used interchangeably in the construction industry. While both these trenchless boring methods may seem identical, there are a number of key differences between the two that need to be considered when deciding which technology is better suited for a particular project.

Microtunneling, as its name implies, is a trenchless technique used to bore relatively small tunnels. The diameter of the bores are usually too small to have an operator in the machine itself; instead, a remotely controlled microtunneling machine is used. During the microtunneling process, a laser-guided cutting head is launched in an entrance pit and subsequently advanced by a jacking rig. Sections of product pipe are lowered into the entrance pit and jacked in sequence behind the cutting tool until the product pipe reaches the exit pit.

In contrast, the guided boring method (GBM) involves first drilling a pilot hole using a hollow pilot tube attached to a steering head and guidance system. When the pilot hole is drilled from the entrance to the exit pit, reaming heads or conventional auger boring rigs are used to enlarge the hole (in one or multiple passes) to the desired diameter. A pipe adapter is then installed at the back of the reamer or auger, and the final pipe sections are jacked from the entrance to the exit pit.

Microtunneling can achieve drive lengths of up to 1000ft, with longer distances possible with upgraded equipment. While GBM can reach drive lengths of up to 700ft, it's recommended that distances are limited to less than 400ft to maintain precision and improve the chances of drive completion.

Microtunneling is also ideal for almost all ground profiles, including challenging and varied conditions, as well as sites with groundwater. GBM, on the other hand, is better suited for cohesive soils above the groundwater table. GBM is also incompatible with granular soils (gravel, cobbles) and harder materials such as boulders and rocks.

GBM, however, can be performed with much smaller equipment than its microtunneling counterpart. The drill rack (which consists of the thrust and pull back carriage assembly) used in guidance boring is considerably more compact than the jacking rig used in microtunneling. For shorter tunneling distances (300ft or less), GBM entrance and exit pits can be as small as 8ft in diameter. These smaller pits are ideal for urban environments where construction disturbances must be kept at a minimum.

Additionally, compared to microtunneling, the equipment used in GBM is relatively more straightforward to set up, has a faster drill time, and requires less skilled site labor. These factors make GBM significantly more cost-effective than microtunneling.

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Written by Krystal Nanan
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Krystal is a civil engineer and project manager with an MSc in Construction Engineering and Management. Her experience includes the project management of major infrastructure projects, construction supervision, and the design of various infrastructure elements including roadway, pavement, traffic safety elements and drainage.

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