Testing, Commissioning and Inspection of Drainage Systems
Prove it before you bury it
Inspection is staged: pre-cover inspections by the building consent authority, plus the drainlayer's own checks. Tests available include water (hydrostatic) testing - plug the low end, fill to a nominated head above the highest point of the section, hold for the specified period and check for loss; air testing with a calibrated gauge and a permissible pressure drop; smoke testing to find illegal connections and leaks in existing systems; dye testing to trace connections; CCTV inspection with recorded survey for buried and existing lines; deflection testing with a mandrel for flexible pipe; and, for mains, infiltration/exfiltration or low-pressure air tests under NZS 4404. Always use the acceptance criteria in the current standard or the consent conditions - not a remembered figure.
Defects found must be repaired and the section re-tested; a repaired joint is not "probably fine".
Commissioning covers pump stations, alarms, grease traps, detention tanks, backflow devices and on-site wastewater systems: verify operation, set levels, confirm maintenance arrangements, and hand over as-builts and O&M information.
Documentation
Testing records, as-laid plans with invert levels, producer statements and photographs support the Code Compliance Certificate application and protect you later.
Traps
Testing before solvent joints have cured (a joint that failed in test may simply have been too green). Testing too long a section in one go. Leaving branch openings unplugged. Failing to record the test result at the time. Confusing water supply pressure testing with drainage testing.
Worked example: IL at inspection point A is 12.450 and at B is 11.950 over 24 m. Fall = 0.500/24 = 1 in 48 ≈ 2.08% (20.8 mm/m), steeper than the 1:60 minimum for DN100 - acceptable, but confirm velocity and scour at the outlet are also satisfactory.