6.4 Cased vs Uncased, Bore Specs & Voids

6.4 Cased vs Uncased, Bore Specs & Voids

Quick Answer: A cased crossing carries the carrier pipe inside a larger outer casing (with insulators, end seals, and vent pipes for gas lines) at roads and railroads where the permit or code requires it; an uncased (direct) crossing installs the pipe directly in the bore with a cathodic-protection-compatible design. The API 1169 inspector verifies casing installation, insulators, end seals, vents, and the casing vent test, and checks for bore voids and grouting and the condition of the pipe inside the casing.

Cased vs Uncased: When Each Is Used

Why Casings Are (Sometimes) Required

Casings were historically required at road and railroad crossings to let the carrier pipe be removed or replaced without disturbing the surface, and to take external loads off the carrier. Some railroad and state permits still require casing; others now allow uncased installations with concrete coating, deeper cover, or HDD. The crossing agreement/permit dictates which is used, and the inspector verifies the as-built crossing matches what the permit authorizes.

Why Uncased Is Now Common

Modern coatings and cathodic-protection systems, plus HDD, have made uncased crossings common — there is no casing-to-carrier short to defeat CP, and no annulus to fill with water. However, uncased crossings require a CP-compatible design and the permit must allow it. The inspector verifies CP test leads are installed and that continuity is confirmed.

Casing Components the Inspector Verifies

Casing Insulators/Spacers

• Insulators (also called casing spacers) center the carrier pipe inside the casing and electrically isolate the carrier from the casing.
• If an insulator fails, the carrier shorts to the casing and CP current is shielded — the carrier can corrode undetected.
• The inspector verifies: insulator type and spacing per spec, insulators installed and undamaged, and the carrier is centered.

End Seals

• End seals close the annulus between carrier and casing to keep water, soil, and debris out.
• A failed end seal lets water into the annulus, which can cause external corrosion of the carrier and freeze damage in cold climates.
• The inspector verifies end seals are installed per spec and sealed at both ends.

Vent Pipes (Gas Lines)

For gas pipelines in cased crossings, vent pipes are required:

• Vents extend from the annulus to above grade so any gas leak into the casing is detected and vented safely.
• Typically one or two vents per casing, per code and permit.
• The casing vent test — the inspector verifies the vent is open (not plugged) and that a test (commonly a low-pressure air test or vacuum) confirms the annulus-to-vent path is clear.

Pipe Condition in Casing

Before the carrier is pushed into the casing:

• Coating inspected — the carrier coating must be holiday-free; any damage is repaired before insertion.
• Pull-through / test holes — the inspector may witness a pull-through test to confirm the carrier does not bind or damage coating.
• After insertion, the carrier is centered and isolated — insulators not displaced, no short to the casing.

Bore Voids and Grouting

At cased crossings, the annular space between the casing and the surrounding soil (or between carrier and casing, where specified) may need to be grouted:

• Grouting fills voids to prevent settlement of the surface above, to lock the casing in place, or to seal the annulus.
• Bore voids — if the bore is over-sized or soil collapses, voids outside the casing can cause surface settlement; the inspector verifies voids are grouted per spec.
• Grout type — cement-bentonite or flowable fill per the crossing spec.
• The inspector verifies grout volume and placement, and that vent and drain paths are not blocked by grout.

Inspector Verification Checklist for Cased Crossings

1. Casing material and size per spec — diameter, wall thickness, grade, and length.
2. Casing installation — bore or open-cut per permit; casing centered in the bore.
3. Insulators/spacers — type and spacing per spec, installed and undamaged; carrier centered and isolated.
4. End seals — installed and sealed at both ends.
5. Vent pipes (gas) — installed, extended above grade, and vent-tested open.
6. Carrier pipe coating — holiday-free before and after insertion.
7. Grouting and voids — annulus or bore voids grouted per spec; vents not blocked.
8. CP isolation — carrier-to-casing isolation verified (no short).

Uncased Crossing Inspector Checks

1. Coating — concrete-weight or abrasion-resistant coating per spec; holiday-free.
2. Cover — depth of cover under the feature per permit.
3. CP continuity — no casing short; test leads installed per the CP design and continuity confirmed.
4. Bore tolerance — within length, cover, and deviation tolerances.
5. Permit — uncased installation allowed by the crossing permit.

Key Takeaways Recap

• Cased crossings use an outer casing with insulators, end seals, and (for gas) vent pipes; uncased crossings install the pipe directly with a CP-compatible design.
• The crossing permit dictates cased vs uncased; railroads and some road authorities still require casing.
• Insulators electrically isolate the carrier from the casing; a failed insulator shorts CP and risks corrosion.
• Vent pipes on gas casings must be vent-tested open; end seals keep water out of the annulus.
• Bore voids and annular spaces may require grouting; the inspector verifies grout placement and that vents are not blocked.