Microtunnel, HDD or Direct Pipe: which one to choose according to geotechnics, diameter (DN) and curvature (Rmin)?

Q: Microtunnel, HDD or Direct Pipe: which one to choose according to geotechnics, diameter (DN) and curvature (Rmin)?

Microtunnel: recommended in urban environment when slope tolerance is critical and DN is medium-large; offers precise guidance (laser/gyroscope) and face pressure control. HDD: ideal for long and curved crossings with minimal civil works of wells, conditioned by Rmin (relation with DN and material), geotechnical and sludge management. Direct Pipe: competitive in short/medium crossings under rivers or sensitive areas, with high watertightness and continuous thrust; suitable in soft/mixed soils and high water table. Final choice depends on DN, length (L), Rmin, geotechnics, pipe material and site constraints.

Short answer:

Urban environment with a precise slope and medium-large DN → Microtunnel.
Long and curved crossings with minimal well civil works → HDD.
Short/medium crossings under rivers or sensitive areas with high watertightness → Direct Pipe.

Rapid decision by parameters

Critical slope / gravity (sewers, outfalls): Microtunneling.
Demanding curvature (low Rmin) and eluded layout of services: HDD.
High water table or stream crossing with minimal footprint: Direct Pipe or microtunnel with hydro-shield (according to DN y L).
Very large DN (collectors/casing): Microtunnel or Direct Pipe.
L > 1 km with compatible DN: usually HDD.

Indicative ranges (validate with project engineering)

Parameter	Microtunneling (MT)	HDD	Direct Pipe (DP)
Geotechnics	Cohesives, mixed and gravels with conditioner; hydroshearing in high water	Cohesive/stable sands; sensitive to unsupported collapse; rock of specific design	Soft/mixed/alluvial under streambeds; good performance in saturated
Typical DN	~ DN 400-3500+	~ DN 100-1600 (depending on material)	~ DN 800-1600 (larger viable)
Length (L)	Short/medium lengths (hundreds of m) that can be chained together	Hundreds to thousands of m	100-1000 m approx.
Curvature (Rmin)	Straight or smooth curves per joint/shield	Continuous curve; Rmin of the order of hundreds of DN	Straight with smooth corrections
Slope accuracy	Very high (laser/gyroscope)	Medium-high (limited in fine slope)	High on axis; controlled slope

Values are approximate: they depend on DN, L, Rmin, geotechnics, piping and regulations. Require verification of feasibility.

Key risks and how they are controlled

Microtunneling: subsidence/soil loss → control of front pressure, and records, fills and push/torque with QA/QC inspections.
HDD: “frac-out”/backflow → trajectory with adequate covers, slurry rheology and pressure control.
Direct Pipe: local settlements → continuous thrust, The system is equipped with a wide range of controls, precise navigation and controlled bed/support.

For a method-independent overview no-dig (definitions and scope), consult the International Society for Trenchless Technology