API 1102 & 49 CFR 192 — Road, Rail & Water Crossings
Min 3 ft for roads, 5 ft for railroads (49 CFR 192)
Typically casing OD + 12" to 24" each side
Leave blank for auto-calculation based on depth
Understand Marston load theory, API 1102 methodology, cased vs. uncased crossings, and cathodic protection considerations
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.
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.).
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.
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.