Audience: project managers, site engineers, foremen
Goal: build drainage that passes inspection first time—no digging twice, no surprise ponding later.

Why drainage rework happens

• Wrong datum/benchmarks → every level after that is wrong.
• Uncontrolled trench formation → bed thickness drifts, pipes go off grade.
• Rushed joints & dirty sockets → leaks on test, contaminated rings.
• Poor backfill/compaction → settlement, broken falls, cracked paving.
• Missing as-builts → disputes at handover.

The five-stage playbook

1) Pre-construction (paperwork + control)

• Confirm information: latest drawings, invert levels (IL), cover levels (CL), pipe sizes/materials, falls, manhole schedules, outfall details, permits/utility plans.
• Set your datum: establish at least two permanent benchmarks (PBMs) clear of works; check them daily.
• Plan the sequence: downstream outfalls first; interfaces with existing live drainage; temporary over-pumping if needed.
• RAMS & ITPs: list hold points—formation level, bed check, first pipe line/level, manhole base, tests, backfill, as-builts.

2) Excavation & formation

• Excavate to formation, not “roughly near” it—leave the bed thickness to tolerance (per spec/manufacturer).
• Keep trench width controlled; if you exceed the design width, escalate (you may need stronger embedment or protection).
• Remove soft spots/wet pockets; stabilise or over-excavate and reinstate with approved material.
• Keep water out: sumps, pumps, silt control—dirty water must not enter existing drains.

3) Bedding, laying & alignment

• Place and level granular bed (or proprietary system) to required thickness and grade; compact in thin layers.
• Use a pipe laser/level line and check every length—don’t “average” later.
• Clean sockets/spigots; use the correct lubricant; push to witness mark; rotate if required by the manufacturer.
• Keep joint gaps consistent; protect open ends from debris at all times.
• Manholes: level bases, correct channel falls and smooth benching to prevent turbulence/standing water.

4) Backfill & protection

• Sidefill evenly and compact in layers up to springline and above, maintaining line/level—no crowbarring pipes into position.
• Segregate materials: no large stones, frozen material or rubbish in embedment.
• Install markers/tape and protection boards where specified; keep service corridors coordinated.

5) Testing, evidence & handover

• Pressure/air/water tests to the project spec and relevant standards (e.g., pipelines commonly tested in accordance with BS EN 1610).
• For flexible pipes, carry out any required deflection/ovalisation checks after a settlement period.
• CCTV where specified (post-construction or as a condition of adoption).
• Record as-built IL/CL, manhole coordinates, cover types, pipe sizes and materials.
• Compile test sheets, cube/compaction results (where applicable), photos and red-lines into the O&M pack.

Quick gradient guide (rule-of-thumb numbers)

• 1:40 ≈ 25 mm fall per metre
• 1:80 ≈ 12.5 mm per metre
• 1:150 ≈ 6.7 mm per metre

Check: Fall (mm/m) = 1000 ÷ gradient denominator.
Use this to sanity-check your laser setup before laying.

Typical hold points (drop into your ITP)

1. Benchmarks established and verified
2. Trench formation level inspected
3. Bedding thickness/grade confirmed
4. First pipe on line/level (engineer sign-off)
5. Manhole base and channel falls
6. Sectional test (before backfill where required)
7. Embedment and sidefill compaction checks
8. Final test (air/water) + CCTV if specified
9. As-built survey and O&M index

Common defects & how to prevent them

• Low spots (“bellies”) → control formation; don’t re-grade with soft bed; re-set pipe if tolerance exceeded.
• Leaking joints → clean/inspect every socket; use correct lubricant; never force alignment with bars.
• Silted systems during works → cap ends daily; use silt socks on pumps; keep temporary connections under control.
• Settlement at manholes/roads → correct embedment and layer compaction; don’t mix stone sizes; proof-roll where specified.
• Wrong connections (foul to surface water) → colour-code drawings and on-site tags; independent check before backfilling nodes.

When to stop and escalate

Groundwater inflow, unstable sides, unexpected utilities, trench width exceedance, or any variance that pushes you outside the specified embedment/cover requirements—pause, make safe, and seek an instruction.

Site checklist (print to A5)

• Latest drawings & schedules on file
• PBMs marked & verified
• Laser calibrated; battery/spares
• Trench width/formation to spec
• Bed placed & compacted to grade
• Joints clean, witness marks met
• Sidefill compacted in layers
• Tests passed; results filed
• As-builts completed; O&M updated

Photos that pass audit (what to capture)

• Formation with staff/level visible
• Bed thickness with rule and note
• Joint close-up showing witness mark
• Laser target on grade line
• Sidefill at springline—both sides
• Manhole channel & bench fall
• Test setup with gauges/kit ID
• Final as-built points being surveyed

(Use consistent filenames: area_chainage_date_description.jpg)

Final word

Getting drainage right is mostly discipline around levels: control the datum, protect the formation, and prove the falls as you go. Do that, and rework disappears—and your test sheets become a formality, not a gamble.

Need a template ITP, test sheets or a photo checklist? Email [email protected]