Air test loses pressure before the driveway pour
60 m of new 100 mm uPVC foul drain has been laid at 1:60 and backfilled except for a 3 m section under a driveway crossing. An air test on the whole run holds initially but loses pressure slowly; the joints are solvent-cemented. The drain connects to an existing gully trap and has two inspection openings. Groundwater is 300 mm above the pipe invert at the downstream end. The builder wants to pour the driveway and "test it later".
1. Interpret the air test result. What is the correct response?
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A slow, continuing pressure loss is a failed test, not a pass with a minor leak. The test pressure and duration must be those specified in the consent and AS/NZS 3500.2; the loss exceeds the permitted allowance and means the run is not proven watertight.
The correct response is to investigate and rectify before covering the driveway section. "Test it later" is not acceptable because the driveway would conceal the pipe and make access, repair and inspection difficult, and because the test is part of the evidence for certification.
Do not simply repeat the test and accept a better-looking result. First check the test setup - plugs, caps, gauge, stabilisation time and temperature effects - then isolate and retest in sections.
2. What are the likely causes, and how would you locate the defect?
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Likely causes include:
- an incomplete or poorly made solvent-cement joint;
- a damaged pipe or fitting, especially at the driveway crossing;
- a leaking test plug, inspection opening cap or gully trap connection;
- groundwater entering an open or poorly sealed section, which can mask or mimic a leak;
- a defect at the connection to the existing gully trap.
Investigation sequence:
- Verify the test equipment and plug seals; confirm the gauge is reading correctly.
- Isolate the run into sections using the inspection openings and temporary plugs, and test each section.
- Where groundwater is above the invert, distinguish water ingress from air loss - a water test may be more informative on the affected length.
- Expose suspect joints at the driveway crossing and inspect for gaps, misalignment or damage.
- Repair and retest the affected section to the specified criteria before backfill.
3. What inspection and documentation requirements apply before the driveway is poured?
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Before the driveway is poured you need:
- the pipe laid to the approved line and grade, with inverts checked;
- bedding and surround inspected and approved in the open section;
- a satisfactory test on the complete run, or on properly isolated sections, recorded with method and result;
- territorial authority inspection/sign-off of the drainage before covering;
- photographs and as-built levels of the crossing section, including any concrete protection or encasement required by the consent;
- the record of work/producer statement and test certificate held on file.
If the crossing requires special protection (for example concrete encasement), confirm the detail with the designer and record it before pouring. The driveway pour should be programmed after the inspection and test, not before.