Method Statement – Granular Sub-Base (GSB) Installation (Type 1 / Type 2 Unbound Mixtures) – Method Statement

Overview

This method statement defines the procedures, resources, and controls for constructing granular sub-base (GSB) layers using Type 1 or Type 2 unbound mixtures beneath pavements, access roads, footways, hardstandings, and similar works. Activities include material receipt and verification, stockpile and segregation control, formation verification, geotextile (if specified), setting-out, moisture conditioning, spreading, compaction, field density and moisture testing, surface level/regularity checks, cross-fall verification, rectification, protection, and handover.

Objectives

• Achieve specified layer thickness, density, moisture content, surface tolerances, and cross-fall within the approved design and project specifications.
• Prevent material segregation and contamination.
• Ensure HSE-compliant execution and environmental protection.

Typical Acceptance Targets [Verify per project specifications]

• Material: Type 1 or Type 2 unbound mixtures conforming to BS EN 13285 and BS EN 13242 grading and quality requirements.
• Layer compacted thickness per drawings (typically 125–200 mm per layer). Loose thickness generally not exceeding 200–225 mm to achieve compaction.
• Moisture content during compaction: within approximately -2% to +1% of Optimum Moisture Content (OMC) determined by BS EN 13286-2 (Heavy/Modified Proctor) or ASTM D1557/AASHTO T180 equivalent.
• Compaction: ≥95% of Maximum Dry Density (MDD) by BS EN 13286-2 (or ASTM D1557) unless otherwise specified.
• Surface regularity: 3 m straightedge deviation ≤10 mm; local depressions to be corrected.
• Level tolerance: typically ±10 mm (sub-base) from design; verify per drawings.
• Cross-fall: typically within ±0.3% of design gradient; verify per drawings.

Approvals and Documentation

• IFC drawings, pavement design, and specifications approved.
• Material submittals (Type 1/Type 2 aggregate) approved with source, grading, and quality certificates.
• Method Statement and ITP approved.
• Calibration certificates for survey instruments and testing equipment in date.

Site Conditions

• Formation/subgrade inspected and accepted: correct level, compaction, drainage, and stability; defects rectified. Proof-rolled as required.
• Utilities located/marked; protection measures in place. Permit to Dig if trimming formation adjacent to services. [Verify per project HSE plan and local regulations]
• Weather forecast checked; avoid compaction during heavy rain or freezing conditions.

HSE

• Task-specific risk assessment and toolbox talk completed for plant–people interface, dust/noise, WBV, radiation (if nuclear gauge), and night work.
• Traffic management and plant exclusion zones established and briefed.
• PTWs obtained where required (e.g., night work, radiation, work near live traffic). [Verify per project HSE plan and local regulations]

Quality

• Lot boundaries defined (e.g., by area/chainage/daywork) for traceability.
• Target tolerances and test frequencies agreed with the Engineer.
• Trial section (if specified) completed to confirm roller type/passes and test parameters. [Verify per project specifications]

Task-Specific Hazards and Controls

• Hazard: Plant–pedestrian interface (dumpers, rollers, graders).
• Likely consequence: Struck-by/crush injuries or fatalities.
• Engineering/procedural control: Segregated work zones with barriers and banksmen; one-way haul routes; REVERSE alarms and flashing beacons; radio contact; spotter for reversing; exclusion zones around operating plant (≥5 m or as per risk assessment); dedicated pedestrian walkways.
• Required PPE: High-visibility clothing, safety boots, hard hat, gloves, eye protection.
• Collective preventive measure: Physical barriers and traffic management plan with signage; lighting for night work.

• Inspection/permit/supervision: Daily plant inspection; traffic management inspection; competent banksmen; TBT records. [Verify per project HSE plan and local regulations]

• Hazard: Vibration exposure (whole-body vibration to operators).

• Likely consequence: Musculoskeletal disorders, fatigue.
• Engineering/procedural control: Select modern rollers with vibration isolation; limit exposure times; rotate operators; maintain seat suspensions; adhere to manufacturer’s speed/amplitude settings.
• PPE: Anti-vibration seat, appropriate clothing; hearing protection as needed.
• Collective preventive measure: Task rotation schedule and monitoring of exposure.

• Inspection/permit/supervision: Supervisor to track exposure durations.

• Hazard: Dust generation during spreading/compaction.

• Likely consequence: Respiratory irritation, reduced visibility.
• Engineering/procedural control: Pre-wet material; water bowser with fine spray; reduce drop heights; halt works in high winds if visibility impaired.
• PPE: Dust masks (minimum FFP2/N95) when dust is visible; safety glasses.
• Collective preventive measure: Bowser on standby; speed limits to reduce dust plumes.

• Inspection/permit/supervision: HSE inspections; environmental monitoring as required.

• Hazard: Noise from plant operations.

• Likely consequence: Hearing damage, nuisance to neighbors.
• Engineering/procedural control: Use compliant equipment; schedule noisy activities within permitted hours; maintain mufflers.
• PPE: Ear protection (SNR as per assessment).
• Collective preventive measure: Noise barriers if required near receptors.

• Inspection/permit/supervision: Noise monitoring logs where specified. [Verify]

• Hazard: Underground/adjacent services and structures (edge compaction).

• Likely consequence: Service damage, utility strike, structural disturbance.
• Engineering/procedural control: Utility plans and CAT/Genny scans; hand-expose within tolerance; restrict vibratory mode within agreed offsets to services/structures; use small compactor near sensitive assets.
• PPE: Standard PPE; insulated tools when near electrical services.
• Collective preventive measure: Permit to Dig and mark-out.

• Inspection/permit/supervision: PTD approval; supervisor sign-off before compaction adjacent to services. [Verify]

• Hazard: Slips/trips and uneven surfaces.

• Likely consequence: Sprains/fractures.
• Engineering/procedural control: Maintain tidy access; define walkways; illuminate at night; promptly repair potholes/soft spots.
• PPE: Safety boots with ankle support.
• Collective preventive measure: Good housekeeping regime.

• Inspection/permit/supervision: Daily site walk inspections.

• Hazard: Radiation from nuclear density gauge (if used).

• Likely consequence: Radiation exposure, contamination if damaged.
• Engineering/procedural control: Licensed operators only; gauge in lockable transport case; exclusion during readings; follow manufacturer SOP.
• PPE: Dosimeter (if required), gloves when handling.
• Collective preventive measure: Radiation safety plan and spill kit for damaged source.

• Inspection/permit/supervision: Radiation permit/licensing, leak tests, calibration in date. [Verify per local regulations]

• Hazard: Hot weather/cold weather working.

• Likely consequence: Heat stress/hypothermia; poor compaction results.
• Engineering/procedural control: Adjust shift times; provide hydration/warm shelters; postpone compaction during freezing/heavy rainfall.
• PPE: Weather-appropriate PPE.
• Collective preventive measure: Environmental working limits in method.

• Inspection/permit/supervision: Supervisor weather checks and go/no-go decisions.

• Hazard: Manual handling of test equipment and straightedges.

• Likely consequence: Strains, pinches.
• Engineering/procedural control: Use team lifts, mechanical aid where possible; store tools close to point of use.
• PPE: Gloves, safety boots.
• Collective preventive measure: Manual handling training.
• Inspection/permit/supervision: Supervisor to enforce safe lifting techniques.

Controls and Mitigations

• Dust: Maintain moisture conditioning; use water bowser with fine spray; limit tipping heights; reduce vehicle speeds; suspend if dust migrates beyond boundary. Record dust control in daily logs.
• Noise and Vibration: Work within permitted hours; maintain equipment; consider temporary acoustic barriers near receptors; monitor PPV at sensitive structures if required (typical limit ≤5 mm/s PPV at nearest structure) [Verify per project specifications].
• Stormwater and Sediment: Prevent runoff from stockpiles with silt fences/berms; do not allow muddy water to enter drains; place stockpiles away from watercourses; provide temporary sumps and pump through silt bags if dewatering is required.
• Material Management: Separate material types; prevent fuel/oil leaks; maintain spill kits at fuelling points; refuel over drip trays. Any contaminated material to be segregated as waste and disposed per waste management plan.
• Public Interface: Wheel wash/road sweeping to prevent track-out; signage and barriers maintained; respond to complaints promptly.
• Resource Efficiency: Optimize haul distances; avoid over-watering; prevent wastage by correct layer thickness planning.
• Ecology/Archaeology: Stop work and notify if protected species or finds are encountered. [Verify per project HSE plan and local regulations]
• Documentation: Maintain environmental inspection checklists and incident logs.

Quality Controls

• Pre-Works: Verify approvals, material certifications, and equipment calibrations. Establish lot sizes and testing frequencies.
• Process Control: Record roller passes, moisture conditioning actions, and any rework. Maintain uniform layer thickness.
• Testing Regime [Verify per project specifications]:
• Laboratory MDD/OMC: 1 per source change or at least per 1,000 m³.
• Field density/moisture: Minimum 1 per 200–250 m² per layer; include edges and wheel paths; increase after failures.
• Grading checks: 1 per 500–1,000 tonnes or per lot.
• Surface regularity: 100% of area by 3 m straightedge sweeps at 10 m intervals or tighter.
• Level and cross-fall: Survey on 10 m grid (carriageways) or 5 m for footways/critical areas; spot checks at grade changes.
• Thickness: At least 1 pit per 500 m² or per lot; correlate with survey deltas when practical.
• Acceptance Criteria [Verify per project specifications]:
• Material complies with BS EN 13285/13242 and approved submittal.
• Relative compaction ≥95% of MDD (BS EN 13286-2 or ASTM D1557) with moisture within -2% to +1% of OMC.
• Surface: 3 m straightedge ≤10 mm; level ±10 mm; cross-fall ±0.3%.
• Thickness: Average ≥ design; local tolerance -10 mm/+15 mm.
• Nonconformance Management: Raise NCR for repeated failures; define corrective actions; re-test to confirm compliance.
• Records: Delivery tickets, test reports, survey/as-built data, ITP checklists, IR approvals, roller pass logs, TBT and HSE records.

• Approved IFC drawings and pavement design details.
• Approved material submittals and supplier certifications (Type 1/Type 2).
• Trial section report and established roller pass regime (if conducted).
• Traffic management plan and plant–people interface plan.
• HSE risk assessment, method-specific toolbox talk, and PTWs.
• Calibration certificates for survey and testing equipment.
• ITP checklists, field density/moisture forms, surface regularity logs, survey reports.
• As-built survey files (native + PDF) and lot acceptance records.