8.5 New Pipe Installation Inspection & Acceptance Testing

8.5 New Pipe Installation Inspection & Acceptance Testing

Exam Focus: Inspect pipe installation (e.g., grade, joints, bedding, backfill, pipe defects, pressure test). Acceptance testing is where the utility either accepts or rejects the infrastructure it will own for the next hundred years, and the numbers are highly testable.

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Grade and Alignment

Sewer is laid upstream from the downstream end, with the bell facing upstream so flow does not enter the joint gap.

Grade is established from a benchmark and held with a pipe laser shooting down the invert to a target in the pipe, or with laser-guided grade rods, or with a traditional string line on batter boards. Whatever the method, the check is the same: the measured invert at each structure must match the design invert on the profile.

The defect the inspector hunts is the sag (belly) — a low spot where the pipe deviates below grade. A sag holds standing water, accumulates solids and grease, and becomes a permanent blockage-generating maintenance liability. Sags come from soft trench bottoms, inadequate foundation, and settlement of poorly compacted bedding. They are cheap to prevent and expensive to fix.

Trench, Foundation, and Bedding

Trench width at the top of the pipe is controlled: too narrow and the embedment cannot be compacted, too wide and the earth load on the pipe increases.

Foundation. If the trench bottom is unstable — soft clay, running sand, or saturated soil — it must be over-excavated and replaced with crushed stone, sometimes over a geotextile. Laying pipe on a soft bottom guarantees settlement and a sag.

Bedding classes describe the quality of support beneath and around the pipe. In the classic rigid-pipe system, Class A is a concrete cradle providing the greatest load factor, Class B is a shaped granular bedding brought well up the sides, Class C is a shaped ordinary bedding, and Class D is a flat-bottom trench with no shaping — the least support and generally not permitted for new sewer.

The haunch zone is the critical detail. The haunch is the region beneath the springline, between the bottom of the pipe and roughly the midpoint of its side. This is where flexible pipe transfers vertical load outward into the soil, and it is the hardest area to compact because it sits under the curve of the pipe. Embedment must be worked into the haunch with a shovel slice or a compaction bar. A pipe with voids in the haunch will deflect excessively no matter how well the material above it is compacted — the single most common cause of a failed mandrel test.

Embedment material is typically well-graded crushed stone or granular material per ASTM D2321 for thermoplastic pipe. Initial backfill covers the pipe to a specified depth above the crown (commonly 6 to 12 inches) and is placed and compacted by hand or light equipment; heavy compaction equipment directly over the pipe before adequate cover will crack or deflect it.

Final backfill is placed in controlled lifts — typically 6 to 12 inches — each compacted to a specified density. Compaction is verified by density testing. Under pavement, inadequate compaction produces the settlement trench every driver recognizes.

Dewatering. Groundwater must be kept below the pipe zone during installation, and dewatering must be maintained until backfill provides enough weight to resist flotation. Pipe laid in water floats, joints wash out, and bedding migrates.

Joints and Pipe Defects

The inspector examines each length before it goes in the trench:

• Cracks, chips, and spalls, particularly at the bell and spigot.
• Gasket condition and placement — a rolled, twisted, or dirty gasket is the leading cause of a joint that fails the air test.
• Home marks. The spigot is inserted to the reference mark; under-insertion leaves an open joint, and over-insertion stresses the bell and can crack it.
• Deflection at joints. Pipe must not be pulled around a curve beyond the manufacturer's allowable angular deflection.
• Cleanliness. Mud on the spigot or in the bell is the second leading cause of a leaking joint.

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Acceptance Tests

Low-Pressure Air Test

The standard leak test for gravity sewer, performed per ASTM C828 (clay), C924 (concrete), or F1417 (thermoplastic).

1. Plug both ends of the isolated reach and brace the plugs.
2. Pressurize to 3.5 psig. If groundwater is above the pipe crown, add 0.433 psi for each foot of groundwater above the crown — otherwise the external head masks a leak.
3. Allow the air temperature to stabilize (commonly two minutes or more).
4. Time the pressure decay from 3.5 psig down to 2.5 psig — a 1.0 psi drop.
5. Compare the elapsed time to the specification's table of minimum times, which increase with pipe diameter because a larger pipe holds more air.

If the segment fails, locate the leak — often by isolating shorter reaches or by joint testing — repair, and retest. Air testing is a safety hazard in its own right: an unrestrained plug ejected by 3.5 psi across a large pipe area carries lethal force. Brace plugs, keep personnel out of the line of the plugs, and never enter a manhole while the reach is pressurized.

Deflection (Mandrel) Test

For flexible pipe such as PVC only.

• A rigid go/no-go mandrel sized to 95% of the pipe's base inside diameter is pulled through by hand.
• It is run no sooner than 30 days after final backfill, so that most load-induced deflection has occurred.
• Passage confirms deflection does not exceed the 5% allowable limit.
• A failure almost always means poor haunch compaction. The remedy is to excavate, re-bed, and retest — not to force the mandrel through.

Never pull a mandrel with a winch or power equipment: a mandrel that jams and is then forced can be driven through a failing pipe and destroy it, and the point of the test is to detect resistance, not to overcome it.

Manhole Vacuum Test (ASTM C1244)

• Plug all pipe openings and brace them, then seal the test head at the top of the cone or chimney.
• Draw a vacuum of 10 inches of mercury.
• Shut the valve and time the decay to 9 inches of mercury — a 1 inch Hg drop.
• Compare to the minimum time table, which increases with manhole diameter and depth.

Post-Construction CCTV

A final televised inspection, usually recorded and PACP-coded, is run before the warranty period expires. It documents sags, debris, protruding gaskets, offset joints, unreinstated laterals, and construction debris. Because sags are visible as standing water on the video, post-construction CCTV is the practical check on grade that a passing air test cannot provide — an air test proves the pipe is watertight, not that it is on grade.