Per ASCE 36, the North American design and construction standard, a method qualifies as microtunneling if it is remote-controlled from the surface, uses a guidance system (typically laser-guided) to hold line and grade within tight tolerances, and provides continuous support to the excavation face as it advances. That combination distinguishes it from simpler pipe-jacking or auger-boring methods. Despite the name, qualification does not depend on tunnel diameter.
The two main variants are slurry microtunneling, which uses a closed-loop slurry system to remove excavated spoil, and auger (pilot-tube) microtunneling, which manages the excavation face through auger rotation and soil conditioning. Because it holds precise line and grade without surface disruption, microtunneling is typically reserved for larger-diameter gravity sewers, deep installations, or crossings where settlement tolerance is tight (highways, rail lines, and sensitive utilities above the alignment). It is more expensive per foot than open-cut installation and usually only pencils out where full excavation is not realistic.
Microtunneling projects carry some of the highest per-foot costs and the tightest technical requirements in the trenchless toolkit. Depth, diameter, soil conditions, and crossing length matter before committing significant estimating time. Nonlinear pulls that detail out of drawings and geotechnical reports.
