Method Statement: Subgrade Preparation and Compaction for Highway Construction – Method Statement

Overview

This method statement defines the procedures, quality requirements, HSE controls, and inspection/testing regime for the preparation and compaction of highway subgrade prior to sub-base placement. Activities include site clearing, formation trimming, proof rolling to identify soft spots, moisture conditioning, leveling, multi-pass rolling with vibratory compactors, in-situ density testing (Sand Cone/Nuclear gauge), level control checks, and inspection hold points leading to release for sub-base.

Extents

• Applies to all highway formation/subgrade areas within the Right-of-Way, including mainline lanes, shoulders, medians, on/off ramps, and tie-ins.

Objectives

• Achieve specified subgrade density and moisture conditions.
• Deliver design levels, crossfalls, and evenness tolerances.
• Identify and remediate weak/soft zones before sub-base.
• Establish documented acceptance through ITP hold/witness points.

Key Acceptance Targets [Verify per project specifications]

• Minimum in-situ dry density: Traffic lanes ≥98% of MDD (ASTM D1557/Modified Proctor); Shoulders/medians ≥95% of MDD.
• Moisture content: within ±2% of OMC from the governing Proctor curve.
• Level tolerance: ±20 mm from design; crossfall within ±0.5%.
• Surface evenness: ≤20 mm under a 3 m straightedge.
• Proof rolling: no pumping, no visible cracking, no rutting/deflection >25 mm.

Exclusions

• Chemical stabilization beyond local soft spot treatment (if full-area stabilization is required, a separate method statement applies).

Approvals and Inputs

• Approved ITP and this Method Statement available on site.
• Latest Issued For Construction (IFC) drawings and cross-sections.
• Laboratory Proctor curves (ASTM D1557 unless otherwise specified) for each soil type/lot.
• Approved borrow/source data and material submittals for replacement granular, geotextile, stabilizers (if any).

Site Readiness

• Utility clearance: Completed by competent utility locator; permits and as-built records reviewed. Mark and protect all services.
• Traffic Management Plan: Implemented for work zones, signage, delineation, and plant crossings [Verify per project HSE plan and local regulations].
• Survey control: Primary control points and TBMs established and verified; working benchmarks protected.
• Weather/ground conditions: Work planned to avoid over-wet conditions; stop rules and contingency defined for rainfall events.

Plant/People Readiness

• Plant pre-start inspections completed; rollers set to required ballast/tyre pressure [Verify].
• Calibration: Nuclear gauge within validity; sand-cone apparatus verified; survey instruments calibrated (ISO 17123 practices).
• Competency: Operators licensed; testers certified; crew briefed via task-specific toolbox talk.

Lot Definition

• Define logical lots by chainage, width, and layer (e.g., 200–500 m lengths per lane) prior to testing and record on ITP.

Hold Points Communicated

• H1: Pre-start readiness and survey verification
• H2: Formation after trimming, before proof rolling
• H3: After proof rolling and remediation, prior to compaction
• H4: Post-compaction density/level results, prior to sub-base release

Key Hazards and Controls

• Hazard: Underground utility strike during trimming/undercut
• Consequence: Electrocution, gas release, service outage
• Engineering/Procedural Control: Utility clearance with latest as-builts; locate with GPR/cable locator; pothole by hand/vacuum; establish exclusion zones and permit-to-dig.
• Required PPE: Helmet, gloves, safety boots, eye protection; dielectric gloves if electrical work [Verify].
• Collective Measure: Marked corridors, barriers, and spotter during excavation.

• Inspection/Permit: Permit-to-dig; Supervisor sign-off; utility owner coordination.

• Hazard: Plant–pedestrian interface (rollers, graders, trucks)

• Consequence: Struck-by, crushing
• Engineering/Procedural Control: Segregated pedestrian routes; banksman/spotter; reversing alarms and beacons; no-go zones around rollers; UHF radio comms.
• PPE: High-visibility vest, hard hat, boots, gloves.
• Collective Measure: Temporary barriers, cones, flaggers per TMP.

• Inspection/Permit: Daily plant pre-starts; traffic management inspection.

• Hazard: Roller overturn or ground collapse on weak edges/embankments

• Consequence: Serious injury/fatality, equipment damage
• Engineering/Procedural Control: Maintain safe offset from edges (≥1.0 m or greater per geotech advice); assess bearing by proof rolling; avoid high vibration near unsupported edges; ramp transitions.
• PPE: Seat belts, high-vis, boots.
• Collective Measure: Edge delineation and berms; spotter when near edges.

• Inspection/Permit: Supervisor authorization; equipment inspection.

• Hazard: Excessive ground vibration near structures/services

• Consequence: Damage to utilities/structures; complaints
• Engineering/Procedural Control: Reduce amplitude/switch to static; increase stand-off distances; vibration monitoring if required.
• PPE: Standard PPE
• Collective Measure: Exclusion zones; monitoring devices.

• Inspection/Permit: Vibration monitoring logs; Engineer instruction.

• Hazard: Dust generation during scarification/compaction

• Consequence: Respiratory irritation, visibility reduction, environmental nuisance
• Engineering/Procedural Control: Pre-wet surfaces; water carts; limit drop heights; halt in high winds.
• PPE: Dust masks (P2/P3) when visible dust persists.
• Collective Measure: Speed limits; wheel wash.

• Inspection/Permit: Environmental inspection records.

• Hazard: Noise >85 dB(A) from vibratory plant

• Consequence: Hearing loss
• Engineering/Procedural Control: Maintain mufflers; schedule exposure; rotate crews; use low-noise settings when possible.
• PPE: Hearing protection (earplugs/earmuffs).
• Collective Measure: Acoustic barriers where practicable.

• Inspection/Permit: Noise monitoring if required.

• Hazard: Working adjacent to live traffic

• Consequence: Vehicle collision
• Engineering/Procedural Control: TMP with lane closures, tapers, crash barriers; flaggers; work during off-peak.
• PPE: Class-compliant high-vis, cut-resistant gloves, boots.
• Collective Measure: Approved road traffic control devices per MUTCD/local.

• Inspection/Permit: Traffic permit/approval; daily checks.

• Hazard: Manual handling of test equipment (sand-cone, trays)

• Consequence: Musculoskeletal injury
• Engineering/Procedural Control: Use trolleys; team lifts; stage equipment close to test points.
• PPE: Gloves, boots.
• Collective Measure: Ergonomic training/toolbox talk.

• Inspection/Permit: HSE inspections; training records.

• Hazard: Hot/cold stress and dehydration

• Consequence: Heat exhaustion/hypothermia
• Engineering/Procedural Control: Work-rest cycles; shade/heated shelters; hydration plan; weather monitoring.
• PPE: Weather-appropriate PPE; sunscreen.
• Collective Measure: Cool/heat stations; first aid.

• Inspection/Permit: HSE monitoring; stop-work for extreme weather [Verify per project HSE plan and local regulations].

• Hazard: Contaminated or soft ground collapse during undercut

• Consequence: Entrapment, slips
• Engineering/Procedural Control: Avoid over-excavation; batter/shore sides where needed; restrict access.
• PPE: Boots with good tread, gloves, helmets.
• Collective Measure: Barricade with signage.
• Inspection/Permit: Supervisor sign-off; excavation permit if depth/conditions require.

General Requirements

• Mandatory pre-start briefing covering sequence, plant routes, hold points, and emergency procedures.
• Only competent, authorized operators to run plant; seat belts mandatory.
• All controls to be verified against Project HSE Plan and local regulations.

Key Environmental Risks and Controls

• Dust Emissions
• Control: Pre-wet formation; continuous water carts; reduce speeds to ≤20 km/h in work zone; suspend works in high winds.

• Monitoring: Visual inspections; complaint log review.

• Sediment Runoff and Erosion

• Control: Perimeter silt fences, straw wattles, temporary berms; avoid directing water to unprotected drains; staged works to limit exposed areas.

• Monitoring: Post-rain inspections; maintain erosion/sediment controls.

• Water Use and Discharge

• Control: Meter water use where required; prevent ponding; manage discharge to designated points; no discharge of turbid water without treatment [Verify].

• Monitoring: Discharge turbidity/clarity checks if stipulated.

• Noise and Vibration

• Control: Maintain equipment; avoid night operations unless permitted; limit vibration settings near receptors; install temporary acoustic screens where practical.

• Monitoring: Noise/vibration monitoring if required by permit.

• Waste and Spoil Management

• Control: Segregate topsoil for reuse; dispose unsuitable material to approved tip with haul permits; prevent spill from trucks (cover loads).

• Monitoring: Waste transfer notes; tip receipts.

• Hydrocarbon and Chemical Spills

• Control: Refuel in designated bunded area; spill kits on plant; immediate containment and cleanup.

• Monitoring: Spill log; weekly inspection of refueling points.

• Protection of Flora/Fauna and Watercourses

• Control: Respect exclusion zones; no stockpiles within [25–50] m of waterways unless protected [Verify]; rapid stabilization of exposed areas.

• Monitoring: Environmental walkdowns; photo records.

• Archaeology/Heritage Chance Finds

• Control: Stop work, protect area, notify Engineer/Authorities.
• Monitoring: Induction includes procedure.

Quality Objectives

• Achieve specified density, moisture, level, crossfall, and evenness criteria prior to sub-base.

Lotting and Testing Frequencies [Verify per project specifications]

• Lot size: Define by uniform soil type and work front (e.g., 200–500 m length per lane). Minimum 3 density tests per lot.
• Density testing: ≥1 per 500 m² per layer; increase frequency for variable conditions or after rework.
• Referee method: Sand Cone (ASTM D1556). Nuclear gauge (ASTM D6938) allowed with correlation (at least 1 sand cone per 5 nuclear readings initially; adjust per approved correlation plan).
• Moisture testing: 1 per density test minimum (ASTM D2216 oven-dry or D4944 speedy).
• Level checks: Grid or stringline at 25 m intervals per lane and at edges; additional checks at transitions.
• Optional stiffness checks: DCP 1 per 2,500 m² if specified.

Acceptance Criteria [Typical—Verify]

• Density: Traffic lanes ≥98% MDD (ASTM D1557); Shoulders/medians ≥95% MDD.
• Moisture: OMC ±2% at time of compaction/testing.
• Levels: ±20 mm; crossfall within ±0.5%.
• Evenness: ≤20 mm under a 3 m straightedge in any direction.
• Proof rolling: No pumping; no visible cracking; rutting/deflection ≤25 mm.
• Lift thickness: ≤200 mm loose per compaction layer (unless otherwise approved).

Calibration and Records

• Nuclear gauge: Calibration and leak test current; correlation documented; daily standard counts recorded.
• Sand cone: Cones and sand calibrated; density of sand verified.
• Survey: Instrument calibration certs current; backsight checks recorded.
• Records: ITP checklists, test reports, survey sheets, proof-rolling logs, NCR/CAR (if any), Engineer approvals.

Nonconformance and Corrective Action

• If any acceptance criterion fails: Identify lot boundaries, stop subsequent layer placement, investigate cause (moisture, lift thickness, equipment settings), recondition (scarify, adjust moisture), re-compact, and re-test. Document via NCR and close-out with evidence.

Protection of Finished Subgrade

• Restrict traffic; maintain drainage; re-proof/re-test after heavy rainfall or if surface dries/cracks before sub-base placement.

• Approved ITP for Subgrade Preparation and Compaction
• Latest IFC drawings and setting-out data
• Proctor (ASTM D1557) lab reports for each soil type/borrow source
• Calibration certificates: Nuclear gauge (incl. leak test), sand cone apparatus, survey instruments
• Traffic Management Plan and permits
• HIRA/Task Risk Assessment for subgrade works
• Manufacturer data sheets: rollers, geotextiles (if used), stabilizers (if used)
• Proof-rolling log template; density test forms; survey check sheets
• NCR/CAR templates and sample close-outs
• Environmental management plan excerpts relevant to earthworks