Method Statement: Installation of Precast Concrete Sewerage Manholes – Method Statement

Overview

This method covers the end-to-end installation of precast concrete sewerage manholes, including:
- Subgrade preparation, blinding, and in-situ reinforced concrete base slab (where a precast base is not used).
- Delivery, inspection, lifting, and stacking of precast manhole rings/cones/slabs.
- Joint preparation and sealing using hydrophilic waterstops (and/or gaskets) per manufacturer instructions.
- Formation of pipe connection openings by core drilling and installation of flexible boots or hydrophilic mortar seals.
- Application of internal and/or external waterproofing coating systems.
- Final level adjustments for frame and cover, benching/haunching as required, and as-built survey.

Exclusions

• Bulk excavation and shoring design/installation (covered by separate method unless specifically noted here for interface controls).
• Permanent diversion/over-pumping operations.
• Road reinstatement beyond frame and cover seating area.

Key Performance Targets [Verify per project specifications]

• Base slab level tolerance: ±5 mm; surface flatness ≤5 mm under 2 m straightedge.
• Manhole stack verticality: deviation ≤10 mm overall and ≤1:400.
• Ring joint gap uniformity: per manufacturer, typically 3–10 mm.
• Subgrade/bedding compaction: ≥95% MDD (ASTM D698) or ≥98% MDD for granular bedding/backfill layers, as specified.
• Waterproofing total DFT (cementitious): 1.5–2.0 mm in two coats; pinhole-free.
• Watertightness: per EN 1610 or ASTM C1244 acceptance criteria. [Verify per project]
• Cover/frame level tolerance: ±5 mm to finished surface; seating mortar bed 10–20 mm unless specified otherwise.

• Approved method statement, ITP, material submittals, lifting plan, and temporary works design.
• Permits: Excavation, Lifting, Confined Space Entry (if any internal works), Hot Works (if cutting steel), Dewatering/Discharge. [Verify per project HSE plan and local regulations]
• Utility clearance (GPR/records) and permit to dig; trial holes as required.
• Verified survey controls and benchmark; design invert/cover levels issued for construction.
• Ground bearing capacity verified for crane outriggers and working platforms; mats installed as required.
• Shoring/benching installed per temporary works design; safe access/egress provided.
• Materials inspected on delivery with certificates; storage protected from contamination/moisture (especially hydrophilic strips).
• Weather check: avoid coating during rain or substrate < manufacturer min temperature/humidity limits.
• Calibrations in-date (survey kit, gas detectors); test labs arranged for concrete and compaction testing.
• Uplift/floatation check for groundwater: factor of safety against buoyancy ≥1.2 (typ.) or per design. [Verify per project]
• Toolbox talk and task briefing completed with crew; emergency and rescue plan briefed.

Task-specific hazards and controls

• Hazard: Excavation/trench collapse and falls into excavation.
• Consequence: Serious injury/fatality, entrapment.
• Engineering/procedural control: Temporary works design per BS 5975; install shoring/boxes/benching before entry; maintain safe batter slopes where allowed; keep spoil ≥1 m from edge; edge protection and barriers; designated access ladders every ≤6 m.
• Required PPE: Helmet with chin strap, hi-vis, safety boots, gloves.
• Collective preventive measure: Physical barriers/guardrails; exclusion zone.

• Inspection/permit/supervision: Daily excavation inspection by competent person; Permit-to-Dig; TW inspections at each stage; records kept. [Verify per project HSE plan and local regulations]

• Hazard: Crane/rig instability and load drop during ring lifting.

• Consequence: Crushing, fatality, structural damage.
• Engineering/procedural control: Appointed Person lift plan; crane positioned on verified platform with mats; confirm load charts and radii; use correct lifting keys; keep personnel clear of load; tag lines; no lifting in high winds above manufacturer limit (e.g., >9–12 m/s) [Verify].
• PPE: Helmet, gloves, safety boots, eye protection.
• Collective measures: Exclusion zone with barriers and spotters; radio comms.

• Inspection/permit/supervision: BS 7121 compliance; daily crane checklist; lifting accessories with in-date certs; Lifting Supervisor present.

• Hazard: Confined space entry (if internal works/benching/coating).

• Consequence: Asphyxiation, toxic exposure, drowning.
• Engineering/procedural control: Confined Space Entry permit; atmospheric testing (O2, H2S, CO, LEL) before and during entry; forced ventilation; top-man and rescue plan with retrieval system; limit entry time; control water ingress.
• PPE: CSE harness, gas detector (entrant or attendant), gloves, eye protection, boots; respiratory protection if required by readings.
• Collective measures: Tripod/winch, barriers, standby rescue team.

• Inspection/permit/supervision: Calibrated gas detector; CSE permit; rescue drill; competent supervisor.

• Hazard: Contact with hydrophilic primers/chemicals and cementitious coatings.

• Consequence: Skin/eye irritation, chemical burns.
• Engineering/procedural control: Review SDS; use mixing stations; avoid skin contact; provide eyewash; follow curing/ventilation requirements.
• PPE: Chemical-resistant gloves, goggles/face shield, coveralls.
• Collective measures: Spill kits, controlled storage.

• Inspection/permit/supervision: SDS available; COSHH assessments; HSE inspections.

• Hazard: Silica dust, noise, and splash from core drilling.

• Consequence: Respiratory injury, hearing loss, eye injury.
• Engineering/procedural control: Wet coring with water suppression; local slurry control; noise barriers where practicable; rotate operators.
• PPE: EN 166 eye protection, hearing protection (SNR per site assessment), cut-resistant gloves, FFP3 respirator if dry work unavoidable.
• Collective measures: Extraction/vacuum; designated coring zone.

• Inspection/permit/supervision: PAT test for electrical tools; check water supply/containment; HSE monitoring.

• Hazard: Groundwater ingress and floatation uplift.

• Consequence: Instability, movement, flooding.
• Engineering/procedural control: Dewatering plan with standby pumps; check anti-floatation factor of safety ≥1.2 [Verify]; sequential installation to minimize open time; install check valves where needed.
• PPE: Standard PPE; life jackets if working near deep water.
• Collective measures: Controlled discharge with settlement tank.

• Inspection/permit/supervision: Dewatering permit; environmental monitoring.

• Hazard: Manual handling/pinch points when aligning rings/frames.

• Consequence: Crush injuries, musculoskeletal disorders.
• Engineering/procedural control: Use mechanical means; keep hands clear; use pry bars with caution; team communication.
• PPE: Gloves, boots.
• Collective measures: Training, lift aids.

• Inspection/permit/supervision: Supervisor oversight and task briefing.

• Hazard: Traffic/plant interface.

• Consequence: Vehicle-person collision.
• Engineering/procedural control: Traffic management plan; segregated walkways; banksman for plant movements; reversing alarms.
• PPE: Hi-vis class 3, boots, helmet.
• Collective measures: Physical barriers, signage, lighting.
• Inspection/permit/supervision: Daily TMP checks, night-work lighting inspections.

• Groundwater and dewatering discharge: Route to silt bags or settlement tanks; maintain turbidity within consent limits; no discharge to surface water without permit. [Verify per local regulations]
• Concrete washout: Use lined washout area; no wash water to ground; collect and dispose via licensed contractor.
• Core drilling slurry: Contain at source; vacuum collect; dispose as non-hazardous waste unless contamination indicated; prevent entry to drainage.
• Noise and vibration: Plan noisy works during permitted hours; use acoustic barriers; maintain equipment; monitor if near receptors.
• Dust control: Wet methods; wheel washing for trucks; cover stockpiles.
• Waste segregation: Separate concrete, steel, packaging; maintain waste transfer notes.
• Material storage: Store hydrophilic strips dry and covered; protect coatings from frost/heat; secondary containment for chemicals (110% volume).
• Spill prevention: Spill kits at mixing and fueling areas; refuel on drip trays; immediate containment and reporting.
• Ecology/archaeology: Stop work and notify if sensitive finds encountered; follow project protocol.
• Carbon/efficiency: Optimize crane picks; batch deliveries; consider low-carbon concrete where approved. [Verify per project]

Quality controls

• Hold and witness points as per ITP, including base reinforcement, joint waterstop pre-closure, watertightness testing, and final survey.
• Material verification: Certificates and approvals for precast units, gaskets, hydrophilic strips, boots, coating system, concrete, and rebar.
• Testing frequencies:
• Compaction: One test per 50 m³ or per lift per 20 m length around structure [Verify].
• Concrete: Slump each truck, temperature each truck, compressive strength 1 set/50 m³ with minimum daily set [Verify].
• Waterproofing: Record WFT each coat per face; adhesion pull-off if specified (≥0.7 MPa, EN 1542) [Verify].
• Watertightness: As specified (EN 1610 or ASTM C1244). Results recorded with location, date, method, acceptance.
• Tolerances:
• Base level ±5 mm; flatness ≤5 mm/2 m.
• Manhole plumb ≤1:400 and ≤10 mm overall.
• Pipe invert level ±5 mm; frame level ±5 mm.
• Documentation: ITRs, checklists, survey reports, photographs, delivery dockets, calibration certificates, NCR/CAR where deviations occur.
• Protection: Maintain joint cleanliness; prevent premature wetting of hydrophilic strips; protect coatings during backfill with protection boards if required.
• Training/competency: Certified riggers and crane operator; approved waterproofing applicators; confined space trained entrants/attendants where applicable.

• IFC drawings and details for manholes (plans, sections, reinforcement, pipe inverts).
• Temporary works/shoring design and approvals.
• Lift plan with crane charts, rigging drawings, exclusion zones.
• Material approvals: precast units, hydrophilic waterstops, gaskets, pipe boots, waterproofing system, concrete mix, rebar mill certs.
• SDS for primers, coatings, sealants, grout.
• Calibration certificates (survey instruments, gas detectors, torque wrenches).
• Manufacturer installation guides for hydrophilic strips, boots, and coating system.
• ITP forms, checklists, and test report templates.
• Risk assessments and method-specific permits (Excavation, Lifting, Confined Space, Dewatering).