Utility Trenching
QA inspection checklist

Excavation

1. 1.

   Trench width and depth to specification

   Record: Width x Depth (mm)
   What to look forTrench width per pipe size + working room (typically pipe OD + 300 mm each side). Depth to give correct cover to top of pipe per utility company (gas 600 mm verge / 750 mm carriageway; electric 600 mm; water 750 mm). Edges saw-cut vertical in surfaced areas.
   Acceptable whenPer NJUG (National Joint Utilities Group) volumes / utility-specific standards: minimum cover gas 600 mm verge / 750 mm carriageway (IGEM/TD/3); electric 450 mm garden / 600 mm verge / 750 mm road (ENA G39); water 750 mm verge / 900 mm road (Water UK Specification).
   StandardNJUG Guidelines on the Positioning and Colour Coding of Underground Utilities Apparatus. IGEM/TD/3 (gas), ENA G39 (electric), Water UK Specification.
   Common defectsCover too shallow — service vulnerable to future worksWidth too narrow — joints cannot be madeWidth too wide — excessive backfill, settlementDepth to invert measured, not to top of pipe — confusionEdges feathered, not vertical — joint fails in reinstatement
2. 2.

   Existing services located and protected
   What to look forPre-excavation utility records consulted (Linesearch, individual utility records). CAT and Genny scan complete before dig. Trial holes at suspect crossings. Exposed services hand-dug. Services protected with timber/concrete tile/proprietary cover before backfill.
   Acceptable whenPer HSE HSG47: utility records reviewed; CAT-scan with locator; trial holes at crossings; "permit to dig" signed by competent person; mechanical excavation halted within 500 mm of any known service.
   StandardHSE HSG47:2014 — Avoiding danger from underground services. CDM Regulations 2015. NRSWA 1991.
   Common defectsNo CAT scan before dig — service strikeService struck and not reportedPlans not consultedTrial holes only after a strikeService exposed and left unprotected
3. 3.

   Trench support adequate for depth and ground conditions
   What to look forTrench support / shoring / battering for depths > 1.2 m. Trench box used for narrow trenches in unstable ground. Support extends to top of trench. Spoil heap > 1 m back from edge. Ladder access every 25 m or less.
   Acceptable whenPer HSE HSG185 / CDM 2015: > 1.2 m supported / battered / boxed. Battering: 1:1 cohesive, 1:1.5 granular as guide. Trench box per manufacturer rating. Spoil ≥ 1 m back. Ladder access ≥ 1 m above top.
   StandardHSE HSG185:1998. CDM Regulations 2015.
   Common defects> 1.5 m unsupported — collapseSpoil at trench edge — surcharge collapseNo ladderTrench box not deep enough — middle bulgesBattering incorrect angle for soil
4. 4.

   Dewatering in operation where needed
   What to look forPump in trench bottom keeping water below pipe level during installation. Discharge to gully or sewer with consent. Settlement / silt control on discharge. Sumps cleared before backfill.
   Acceptable whenPer BS 6031 / Environment Agency: dewatering by sump pump or wellpoint; silt fencing on discharge; consent for groundwater abstraction if > 20 m³/day; water-quality acceptable for receiving environment.
   StandardBS 6031:2009 — Earthworks. Water Resources Act 1991. EA guidance.
   Common defectsPipe laid in water — fines wash from beddingDischarge floods adjacent propertyNo silt control — fines into watercourseSumps not cleared — voids around fittingsPumping under-spec for inflow

Bedding & Surround

1. 5.

   Pipe bedding material correct (Class S/N/F)
   What to look forBedding material per design pipe class — Class S (full granular surround), Class N (granular bed and side), Class F (granular bed only) per BS EN 1610. Grain size 4-10 mm pea gravel typical. Free of fines, oversize, and contaminants.
   Acceptable whenPer BS EN 1610 / Sewers for Adoption: Class S typical for plastic pipes; bedding material 4-14 mm clean granular. Min 100 mm below pipe; surround minimum 150 mm above crown. CE/UKCA per BS EN 13242.
   StandardBS EN 1610:2015. Sewers for Adoption 7th Ed. BS EN 13242 (aggregates).
   Common defectsAs-dug spoil used as bedding — wrong classOversize stones in bedding — point load on pipeBedding contains fines — pumping under saturationWrong class for pipe material (Class B clay pipe vs Class S plastic)No record of source / class
2. 6.

   Bedding depth correct

   Record: Depth (mm)
   What to look forBedding placed and screeded to correct depth before pipe installed. Depth verified with gauge / rule. Pipe laid to falls — not packed up on stones. Bedding flat under fittings (sockets / collars accommodated).
   Acceptable whenPer BS EN 1610 / Sewers for Adoption: bedding ≥ 100 mm below pipe (more for larger diameter); flat surface to receive pipe. Special bedding around fittings to allow socket clearance.
   StandardBS EN 1610:2015. Sewers for Adoption 7th Ed.
   Common defectsBedding too thin — pipe rests on hard spotsBedding lumpy — pipe out of linePipe propped on stones to achieve fall — point loadSockets not accommodated — pipe rocksNo depth check before pipe laid
3. 7.

   Side fill placed and compacted evenly
   What to look forSide fill placed simultaneously each side to avoid pushing pipe out of line. Hand-rammed against pipe haunches. Layers ≤ 100 mm. Compacted by punner / wacker plate (light compaction near pipe to avoid damage).
   Acceptable whenPer BS EN 1610 / Class S surround: side fill in layers ≤ 100 mm, compacted symmetrically. Light compaction within 200 mm of pipe; full compaction above. No mechanical compaction directly on pipe.
   StandardBS EN 1610:2015. Sewers for Adoption 7th Ed.
   Common defectsSide fill on one side only — pipe pushed out of lineHeavy compaction over pipe — pipe deformed / crackedSide fill not compacted — voids around pipe, settlementSide fill mixed with as-dug materialLayer thickness not controlled
4. 8.

   Cover to pipe correct

   Record: Cover (mm)
   What to look forCover from top of pipe to ground level (or carriageway top) verified at multiple points. Per utility minimum (gas, electric, water, telecoms differ). Marker tape at correct depth above. No reduced cover at high points without protection slab.
   Acceptable whenPer NJUG / utility specs: gas 600 mm (verge) / 750 mm (carriageway); MV electric 600 / 750; water 750 / 900; telecoms 250-450; HV electric 900-1200. Reduced cover requires concrete slab protection per ENA G39 / TD/3.
   StandardNJUG Guidelines. IGEM/TD/3 (gas). ENA G39 (electric). Water UK Specification. PIA Code of Practice (telecoms).
   Common defectsCover below minimum — service vulnerableReduced cover not protected with slab — strike risk in future worksCover varies along run — high spots dangerousNo cover recordProtection slab missing where required
5. 9.

   Warning / marker tape installed at correct depth
   What to look forWarning tape (colour-coded per service: yellow gas, blue water, red electric, green telecoms) at design depth above pipe — typically 250-300 mm above. Detectable type for plastic pipes (foil core). Continuous run, no gaps. Marker posts above gas/electric where appropriate.
   Acceptable whenPer NJUG / HSG47: warning tape 250-300 mm above service. Detectable tape mandatory above plastic pipework. Marker posts at boundaries / changes of direction per utility-specific requirement.
   StandardNJUG Guidelines on Underground Utilities Apparatus. HSE HSG47. BS 1710 (colour coding).
   Common defectsWrong colour tape — wrong utility identified in future digTape > 600 mm above service — too high to warnNon-detectable tape over plastic — no CAT responseTape discontinuous — gap in warningNo tape at all

Reinstatement

1. 10.

   Backfill material matches specification
   What to look forBackfill material per SROH / utility spec — typically Class A or Type 1 above bedding, with subgrade replacement above. Source declared and traceable. No clay, organic, or contaminated material.
   Acceptable whenPer SROH 4th Ed Annex / utility-specific: granular Class A backfill above bedding to within sub-base; Type 1 sub-base in surfaced areas; surfacing per road category. Recycled aggregates per SROH Annex.
   StandardSROH 4th Ed Annex. SHW Series 600 / 800. NRSWA 1991.
   Common defectsAs-dug spoil reused (cohesive) — not allowedWrong class — strength does not developContaminated material reusedNo material ticketsOversize material in backfill
2. 11.

   Compacted in correct layer depths

   Record: Layer (mm)
   What to look forBackfill placed in lifts ≤ 200 mm. Compaction passes per Table A1 SROH / Table 6/4 SHW. Each layer compacted before next. Plant size matched to material.
   Acceptable whenPer SROH Table A1: lift thickness per material and plant — typically 100 mm for vibrating plate, 150 mm for trench rammer. Passes per Table A1.
   StandardSROH 4th Ed Table A1. SHW Table 6/4.
   Common defectsLift thickness exceeded — only top compactsNumber of passes not countedPlant too lightCompaction not symmetricalNo compaction record
3. 12.

   Interim reinstatement level with surrounding surface
   What to look forInterim surfacing flush with surrounding ±6 mm. Joints cleanly cut. No depression at edges. Cold-lay or hot-lay material per SROH section A11. No "humps" left as trip hazards.
   Acceptable whenPer SROH Section A11: interim reinstatement flush ±6 mm carriageway, ±10 mm footway. Permitted material per Section A10 — cold lay acceptable for interim only. Convert to permanent within 6 months.
   StandardSROH 4th Ed Sections A10, A11.
   Common defectsInterim sunken — settlement before permanentCold-lay used for permanent — early failureEdges feathered — fail at jointPatch trip hazard — proud of surfacePhoto record of layers missing
4. 13.

   Permanent reinstatement completed within timescale
   What to look forPermanent reinstatement carried out within statutory time (typically 6 months from interim). Notice of permanent reinstatement issued via streetworks IT. Materials and method match SROH for permanent (hot mix surface course, full depth).
   Acceptable whenPer SROH Section A18 / NRSWA: permanent within 6 months of interim (24 months for trenches > 1.5 m where additional excavation required). 2-year guarantee starts from permanent date (3-year for deep).
   StandardSROH 4th Ed Section A18. NRSWA 1991. Streetworks UK Code of Practice.
   Common defectsPermanent overdue — non-compliance, finesNotice of permanent not issued — guarantee disputedCold-lay permanent — early failureSurface course mismatched with surrounding roadPermanent reinstatement guarantee unclear
5. 14.

   NRSWA compliance — permit valid and S74 not triggered
   What to look forPermit / notice valid for the duration of works. No S74 overrun charges incurred. Works completed within permitted extent. As-built submitted within statutory time. Defects period started.
   Acceptable whenPer NRSWA 1991 / Traffic Management Act 2004: permit / notice valid; no overrun beyond permitted duration; variation if extension needed; as-built submitted via EToN within statutory window.
   StandardNRSWA 1991. Traffic Management Act 2004 Section 74. Streetworks UK Code of Practice.
   Common defectsNo permit — fixed penaltyPermit overrun — daily S74 chargesAs-built late — finesVariation needed but not submittedDefects period reset due to repeated re-cuts

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