ITP generator — drainage

The scope with the most testable criteria in construction — and the one where a vague ITP is least excusable.

Drainage is unusual and it works in your favour: almost every acceptance criterion on it is a number somebody can measure on the day. An air test either loses more than 75 mm water gauge over five minutes or it does not. A mandrel either passes or it jams. There is very little room here for “visual inspection”, which means an ITP that falls back on it is telling a reviewer something about the author.

It is also materials-heavy at the front, and that front end is where drainage ITPs are usually thinnest. The corpus opens with pipes by diameter and material against their product standards and certification scheme, bedding, backfill, concrete grades, geotextile with each declared property against its EN ISO test method, gullies and gratings with class and waterway area, and rapid-setting bedding with its strength at three hours. All of that is inspectable before anything goes in the ground.

It is free and there is no sign-up. Type the scope, read the whole plan on screen. Where the tool has not been given your specification, project-specific criteria come back marked TBC — EVIDENCE REQUIRED with the document that would close them named. That is the correct answer, not a limitation politely worded — an ITP that quietly fills a tolerance with a typical value is worse than one that leaves the box open, because somebody signs the first one.

What controls drainage

These are the documents an acceptance criterion on this scope has to agree with. A criterion that restates the activity and attaches a reference is doing half the job.

• MCHW Specification for Highway Works, Series 500 — Drainage and service ducts — bedding types, surround, testing. OGL. Again: the Clause gives the rule, the contract Appendix gives the number.
• BS EN 1610 — Construction and testing of drains and sewers — the air and water test regimes.
• Sewers for Adoption / the relevant undertaker’s design and construction guidance — Anything to be offered for adoption. Where the run is being adopted, the undertaker’s requirements sit on top of the contract specification and commonly tighten it. Notice before breaking into an existing sewer is theirs to give.
• BS EN 124 — Gully tops and manhole tops — load classes.
• BS EN 752 — Drain and sewer systems outside buildings — system requirements.

The inspection sequence, and where the gates fall

Hold points belong where the evidence stops being recoverable, and the test is always the same question: what becomes invisible next? On this scope that puts 7 of the 16 steps below at a hold point — the rest are witness points, surveillance, review or recorded inspection depending on your contract and on who is watching.

1. [H] Material approvals — pipes by diameter and material against product standard and certification scheme
2. Bedding, backfill and concrete grades approved; geotextile properties each against their EN ISO test method
3. Gullies, gratings, covers and frames — class, skid resistance, non-rock feature, waterway area, bolt specification
4. Manhole steps, galvanised steelwork, penstocks and flow-control devices approved
5. [H] Notice given to the responsible authority before breaking into an existing sewer or drain — Theirs to grant. It is not a courtesy call and it is not retrospective.
6. Excavation to line and level; temporary works where required
7. [H] Bedding to the correct type and depth — The pipe covers it.
8. Pipe installation — flexible watertight joints, level tolerance, maximum differential
9. Connections by junction pipe; saddles prohibited below the stated diameter; every connection recorded and the previous day’s record handed to the project manager
10. [H] Backfill in even layers with the specified minimum cover — Last chance to see the pipe.
11. Jetting and flushing
12. [H] Air test
13. Water test — the fallback where the air test fails
14. [H] Mandrel through all drainage below the stated diameter
15. [H] CCTV survey of all carrier, foul and filter drains, reviewed for defects, with an NCR raised for any defect found
16. Chambers, headwalls, filter drains and back-of-wall drainage — each repeating the sequence with its own criteria

One rule that catches people and is not negotiable: hold points cascade upward. Where the client, the designer or a third party has identified an intervention as a hold point, it is a hold point in your column too. You cannot accept theirs and keep yours as a witness.

Acceptance criteria that survive a review

The acceptance-criteria column header in one real template is the whole test: what answer should we get if it is right? If the entry does not let a third party, months later, decide pass or fail from the record alone, it is not an acceptance criterion. These are from issued ITPs on this scope — not composed as illustrations:

• Air test — 75 mm water gauge loss over 5 minutes. Water test — not more than one litre per hour loss per linear metre over 30 minutes.
• Spherical mandrel of a diameter 10% less than the pipeline diameter passed through all drainage less than 350 mm diameter.
• Saddle connections not permitted on drainage up to 375 mm diameter; all cuts neat and at 90° to the drain run; joints watertight.
• Granular bedding deposited equally on each side of the pipe and compacted in layers of 100 mm; layers not exceeding 150 mm thickness during laying.
• Gully gratings minimum Class D400, non-rock, minimum depth 150 mm, hinged with the hinge on the side of approaching traffic, minimum clear opening 0.189 m², minimum waterway area 0.12 m².
• Concrete bed and surround (Type Z) to the first 1.5 m of gully outlet, continuing until cover exceeds 0.90 m under a road or 0.6 m under verges; minimum 150 mm surround; compressible fibre board at every pipe joint.
• Rapid-setting bedding for chamber and gully tops: non-shrink, minimum 15-minute workable life, ≥30 N/mm² at 3 hours compressive, ≥5 N/mm² tensile.

Against which, the ones that get marked every time: as per design · as specified in the material approval form · as per manufacturer’s instruction · document accepted · within accepted tolerances · acceptable to use · visual inspection · see notes. They fail for one reason: they defer the criterion to a document the inspector does not have on the deck, and they give the person signing nothing to be wrong about.

What comes back TBC on this scope, and where it comes from

The generator does not have your specification, so it will not invent these. It marks them and names the document that would close them:

• The bedding type per run — and the cover depths at which it changes
• Level tolerance and the maximum permitted differential at a joint
• Which runs are being offered for adoption, and whose test regime therefore applies
• Cover class and skid-resistance value for each location
• CCTV coding standard and who reviews the survey

Other scopes

• ITP generator — in-situ reinforced concrete
• ITP generator — earthworks and fill
• ITP generator — formwork and falsework
• ITP generator — precast erection
• ITP generator — waterproofing

Why the generated draft is a draft, and stays one

Starting permanent works without an accepted ITP is a critical non-conformance in its own right, and proceeding through a hold point gets an NCR raised against you. Before this output is issued it needs the project references and drawing revisions, the criteria above completed from your specification, the responsible parties set against your own contract, the check-sheet numbering aligned — inspection item 4.12 on the ITP has to be item 4.12 on the check-sheet — and a named author, reviewer and approver.

So it says DRAFT — FOR COMPETENT REVIEW on its own title, and should keep saying it until a competent person has been through it against the specification.