What size casing pipe is required for a pipeline road crossing?

The casing pipe must be at least 2 inches larger in nominal diameter than the carrier pipe to provide adequate annular space for insulators and allow venting. For example, a 12-inch carrier pipe typically requires a 16-inch or 18-inch casing. The casing must also withstand external earth and traffic loads without exceeding allowable stress limits per API 1102.

When is a cased crossing required for a pipeline?

Per 49 CFR 192.325-327, cased crossings are required at railroad crossings and may be required at road crossings depending on the jurisdiction and pipeline class location. Many state DOTs and railroad companies mandate cased crossings. Water body crossings may also require casing depending on environmental regulations and crossing method (HDD, open cut, etc.).

What is the Marston load theory for buried pipe?

Marston load theory calculates the earth load on buried pipe based on soil unit weight, depth of cover, trench width, and a load coefficient that accounts for soil arching effects in the trench. For ditch conduits, the vertical earth load is W = Cd * gamma * Bd^2, where Cd is the Marston load coefficient, gamma is soil unit weight, and Bd is trench width.

How do you calculate live load on a buried pipeline at a road crossing?

Live load from traffic is calculated using AASHTO H-20 or HS-20 wheel loads distributed through the soil to the pipe depth. The surface load is spread at approximately 1.75:1 through the soil, and an impact factor (typically 1.5 at shallow depths, decreasing with depth) is applied. At depths greater than 8 feet, live load becomes negligible compared to earth load.

Pipeline Crossing & Casing Design Calculator

Carrier Pipe Properties

Carrier Pipe Outside Diameter

Carrier Pipe Wall Thickness

Carrier Pipe Grade (SMYS)

Carrier Pipe MAOP

psig

Casing Pipe Properties

Casing Pipe Outside Diameter

Casing Pipe Wall Thickness

Casing Pipe Grade (SMYS)

Crossing Configuration

Crossing Type

Depth of Cover (to top of casing)

Min 3 ft for roads, 5 ft for railroads (49 CFR 192)

Trench Width (at pipe crown)

Typically casing OD + 12" to 24" each side

Soil & Loading

Soil Type

Soil Unit Weight

pcf

Impact Factor Override

Leave blank for auto-calculation based on depth

Understanding Pipeline Crossings

Why Cased Crossings?
Casings protect the carrier pipe from external loads at road and railroad crossings, allow pipeline inspection and replacement without disturbing the roadway, and provide a secondary containment barrier.

Key Standards:
API 1102 governs steel pipeline crossing design. DOT 49 CFR 192.325–327 establishes minimum cover and casing requirements for gas transmission pipelines at road and railroad crossings.

Design Considerations:
External loads include earth pressure (Marston theory) and live loads from traffic. The casing must withstand these loads while the carrier pipe must pass combined stress checks for internal pressure plus external loading.

📚 Learn the Theory

Understand Marston load theory, API 1102 methodology, cased vs. uncased crossings, and cathodic protection considerations

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Results

Casing Stress Ratio

of allowable

Casing Adequacy -

Earth Load (Marston) -

Live Load (Traffic) -

Total External Load -

Casing Ring Stress -

Carrier Hoop Stress -

Carrier Combined Stress -

Carrier Stress Ratio -

Annular Space -

Vent Pipe Size -

Casing Seal Type -

Min Depth of Cover -

Key Formulas

W_e = C_d × γ × B_d²

W_e = Earth load on pipe (lb/ft)

C_d = Marston load coefficient

γ = Soil unit weight (pcf)

B_d = Trench width (ft)

Standards & References

API RP 1102
Steel Pipelines Crossing Railroads & Highways
49 CFR 192.325–327
DOT Pipeline Safety — Cover & Casings
AASHTO H-20
Highway Bridge Design — Truck Loading
AREMA Cooper E-80
Railroad Loading Standard
AWWA M11 / M23
Steel Pipe Design — External Loads

Engineering Notes

Minimum cover: 3 ft for roads, 5 ft for railroads per 49 CFR 192
Annular space: Minimum 2" radial clearance between carrier and casing
Casing vents: Required at both ends for gas pipeline cased crossings
Casing seals: Wrap-around or link-seal type to prevent water ingress
CP shielding: Cased crossings can shield cathodic protection; consider CP test stations at each end
Insulators: Install casing spacers/insulators every 10–20 ft to center carrier in casing

Quick Reference — Minimum Cover

Class 1 location: 30" (normal), 36" at crossings
Class 2 location: 30" (normal), 36" at crossings
Class 3/4 location: 24" (normal), 36" at crossings
Railroad crossings: 60" minimum to top of casing
Drainage ditches: 36" below bottom of ditch