What Is Underground Piping?

Quick Answer: Underground piping refers to any piping system installed below grade (buried in soil or encased in concrete). It includes process piping within plant boundaries, utility systems (fire water, cooling water, sewers), and cross-country pipelines. Underground piping requires specialized design for external corrosion protection, soil loading, thermal expansion, and accessibility for inspection and maintenance.

Common Underground Piping Applications

Application	Typical Material	Governing Code
Fire water mains	Ductile iron, HDPE, carbon steel	NFPA 24, FM Global
Cooling water	Carbon steel (coated), FRP, HDPE	ASME B31.3 (within plant)
Oily water sewers	Carbon steel (coated), FRP	Plant standards
Process piping (within plant)	Carbon steel with 3LPE/FBE coating	ASME B31.3
Cross-country oil pipelines	API 5L carbon steel (coated + CP)	ASME B31.4, API 1160
Cross-country gas pipelines	API 5L carbon steel (coated + CP)	ASME B31.8
Sanitary sewers	PVC, HDPE, vitrified clay, concrete	Local codes, ASTM standards
Potable water	Ductile iron, HDPE, PVC	AWWA standards

Design Considerations

Parameter	Guideline
Minimum burial depth	0.9-1.2 m (process), 0.6-1.0 m (fire water), per project spec and local codes
Soil loading	Dead load (soil weight) + live load (traffic, equipment); calculated per AWWA M11 or Spangler’s Iowa formula
Thermal expansion	Restrained by soil friction; thrust blocks or expansion loops at above-ground transitions
Bedding	Compacted sand or gravel (150-300 mm below and around pipe)
Backfill	Select backfill free of rocks, debris; compacted in layers per specification
Trench width	Minimum pipe OD + 300-600 mm on each side
Crossing other services	Minimum vertical separation 300 mm; sleeve or concrete encasement at crossings

External Corrosion Protection

Buried carbon steel piping corrodes rapidly without protection. Two complementary systems are used:

Coating Systems

Coating Type	Thickness	Max Temp	Application
FBE (Fusion Bonded Epoxy)	300-500 microns	80 degC	Standard for most buried piping
3LPE (3-Layer Polyethylene)	1.8-3.0 mm	80 degC	Pipelines, superior mechanical protection
3LPP (3-Layer Polypropylene)	1.8-3.0 mm	140 degC	High-temperature buried service
Coal tar enamel	3-6 mm	60 degC	Legacy coating (declining use)
Polyethylene tape wrap	0.5-1.5 mm	50 degC	Low-cost, field-applied; limited to low-risk service

Cathodic Protection (CP)

Coating alone is not sufficient for carbon steel—holidays (coating defects) always exist. Cathodic protection provides electrochemical corrosion control at exposed metal:

CP Type	Method	Application
Sacrificial anode	Zinc or magnesium anodes connected to the pipe	Short pipe runs, low soil resistivity
Impressed current	External DC power source drives current through anodes to the pipe	Long pipelines, high soil resistivity, large pipe surfaces

Pro Tip

Verify pipe-to-soil potential at test stations (minimum -850 mV Cu/CuSO4 per NACE SP0169) during commissioning and at regular intervals. A potential more positive than -850 mV indicates inadequate CP and active corrosion. This is the single most important measurement for buried steel pipe integrity.

Material Alternatives

For highly corrosive soils or where CP is impractical, non-metallic piping eliminates external corrosion concerns:

Material	Advantages	Limitations
HDPE (plastic pipe)	No corrosion, flexible, fusion-welded joints	Temperature limit ~60 degC; limited pressure
FRP (fiberglass)	Corrosion-resistant, lightweight	Brittle; sensitive to point loads and UV
Ductile iron	High strength, long service life	Requires polyethylene encasement in aggressive soils
PVC	Low cost, chemical resistance	Low pressure; temperature limit ~40 degC

Inspection Challenges

Underground piping is inherently difficult to inspect after installation. Common integrity assessment methods include:

• Cathodic protection monitoring (pipe-to-soil potential surveys)
• Close interval potential surveys (CIPS)
• Direct current voltage gradient (DCVG) for coating defect detection
• Guided wave ultrasonic testing (GWUT) from above-ground access points
• Internal inspection (in-line inspection tools / “smart pigs”) for pipelines
• Excavation and direct examination at suspected anomalies

Common Mistake

Assuming that coated underground piping does not need cathodic protection. No coating is perfect—installation damage, soil stress, and aging create holidays where corrosion concentrates. CP is mandatory for all buried carbon steel piping in oil and gas per NACE SP0169 and ASME B31.4/B31.8.

Underground piping accounts for a significant portion of the total piping material on many EPC projects, particularly for fire water, cooling water, and interconnecting process lines.

Read the full guide to pipe types