Carrier Pipe Pull Through a Flooded Highway Casing
A 12.75-in. OD, 0.250-in. wall carrier pipe is being pulled through a 20-in. casing under a highway. The bore is 70 ft long. The casing contains about 3 ft of water, and the annulus seals are installed at both ends; the two 2-in. casing vents are capped. The carrier has 8-in. casing spacers every 10 ft and weighs 33 lb/ft bare. The contractor plans to pull the empty carrier with a backhoe and a wire rope bridle, then pressure-test the annulus. The spacers are rated for 1,000 lb each. Weather is windy and 35°F. The carrier has no concrete coating. The inspector must decide whether the pull can proceed as planned, what to do about the water and vents, and whether the spacer spacing is adequate. The contractor wants to finish the crossing before the end of the day.
1. Calculate the net buoyancy of the empty carrier in the flooded casing and decide whether the pull can proceed with water present.
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The carrier OD is 12.75 in. = 1.0625 ft. Displaced volume is π/4 × (1.0625)² = 0.887 ft³/ft, so buoyant force is 0.887 × 62.4 = 55.3 lb/ft. The bare carrier weighs 33 lb/ft. Net uplift is 22.3 lb/ft, or about 1,561 lb over the 70-ft bore. The empty carrier will float upward inside the casing, which can bind the spacers, damage the coating, and make the pull force unpredictable. The pull should not proceed with 3 ft of water unless a calculated hold-down system is used. Dewater and clean the casing before pulling. If water cannot be removed, add controlled weight to the carrier or use an engineered hold-down, but do not rely on the spacers to resist buoyancy.
2. Evaluate the spacer spacing and the capped vents.
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At 10-ft spacing over 70 ft there are about seven spacers. Their combined 7,000-lb rating exceeds the 1,561-lb uplift, but spacer strength is not the only issue: spacing must limit carrier sag and keep the carrier centered in the 20-in. casing. Check the manufacturer's allowable span for 12.75-in. pipe in a 20-in. casing; 10 ft may be acceptable only if the carrier is supported and not floating. The capped vents are a separate problem. Pulling a close-fitting carrier can act like a piston and pressurize the casing. Open both vents before the pull and keep them open. After the seals are installed, the vents must be piped above grade and left unobstructed.
3. What is the correct installation sequence?
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Dewater and clean the casing, verify the bore is clear, inspect the spacers, and confirm the vents are open. Pull the carrier with a controlled bridle and monitor the pull force; do not exceed the spacer, pipe, or rope ratings. If water returns or the pull force rises sharply, stop and investigate rather than pulling harder. Install the annulus seals, leave the vents open and piped above grade, and pressure-test the annulus only as specified. Document the water condition, spacer spacing, pull force, and vent status. Finishing before the end of the day is not a reason to skip dewatering or venting.