Method Statement – Vibro-Compaction Works for Granular/Sandy Soils – Method Statement

Purpose

Provide the procedure to execute vibro-compaction (deep vibratory compaction) for densifying loose to medium-dense granular or sandy soils to reduce settlement and liquefaction potential, increase bearing capacity, and improve stiffness.

Extent

• Treatment of designated plan areas to the specified design depth using downhole vibroflot with water jetting.
• Establishment of compaction grid, trial area calibration, production compaction, and rework (if required).
• Post-treatment verification by CPT and/or SPT, and settlement monitoring.

Key Performance Objectives

• Achieve target relative density Dr (typ. ≥ 65–80% [Verify per project specifications]).
• Meet post-treatment CPT qc and/or SPT N60 criteria at specified depths.
• Demonstrate acceptable residual settlements and differential movements per design.

Exclusions

• Vibro-replacement (stone columns) and grouting (not applicable unless instructed).
• Treatment of cohesive or high-fines soils not suitable for vibro-compaction (fines content threshold to be confirmed by the geotechnical design).

Approvals and Documentation

• Approved method statement, risk assessment, lifting plan, and ITP. [Hold]
• Design package specifying treatment depth, target Dr/qc/N60, and rework criteria. [Hold]
• Utility survey, mark-out, and Permit-to-Dig/Excavate. [Hold]
• Jetting water source and discharge permit including turbidity/pH limits. [Verify per project HSE plan and local regulations]
• Access/platform certificate: platform CBR ≥ 5% or bearing capacity ≥ 200 kPa; gradient ≤ 3%; thickness/compaction verified. [Verify]
• Calibration certificates: gauges, flow meters, RCDs, depth encoders, data loggers, CPT/SPT rigs.

Site Readiness

• Grid coordinates issued; primary/secondary control established and verified.
• Exclusion zones and traffic management set; lighting for night shifts.
• Baseline monitoring installed (settlement plates/benchmarks) and initial readings recorded.
• Pre-treatment CPT/SPT locations approved and executed where required.
• Trial area (e.g., 20 m × 20 m) demarcated for parameter calibration. [Witness]

Suitability of Ground for Vibro-Compaction

• Predominantly granular soils with low plasticity; fines content typically <10–15% passing 0.075 mm sieve; PI ≈ 0. [Verify per design].
• Groundwater level known; sufficient saturation anticipated to facilitate rearrangement of particles during vibration.

Principal Hazards and Controls

1) Underground utility strike (penetrating vibroflot and jetting)
- Consequence: Electrocution, explosion, flooding, service outage.
- Engineering/Procedural Controls: Up-to-date utility plans; GPR/CAT scanning; marked exclusion; hand-dug trial pits at each first-row grid point; Permit-to-Dig; max penetration limited until clearance confirmed.
- Required PPE: Dielectric gloves when probing; helmet, eye protection, safety boots.
- Collective Measures: Physical barriers/spotters; lock-out of known live services.
- Inspection/Permits/Supervision: Permit-to-Dig signed by Supervisor daily; HSE Officer verification; records retained. [Verify per project HSE plan and local regulations]

2) Crane/rig instability and platform failure
- Consequence: Overturning, crushing, fatalities.
- Controls: Geotechnical platform design; CBR ≥ 5% or plate bearing ≥ 200 kPa; thickness and compaction checks; track mats where soft; daily plant walkaround; wind limits per OEM (e.g., ≤ 12 m/s for lifting).
- PPE: Helmets, hi-vis, steel-toe boots.
- Collective Measures: Exclusion zone (radius ≥ swing radius + 5 m); banksman control; slew restrictors.
- Inspection/Permits: Platform certificate; lifting plan and pre-lift checklist; crane certification in date.

3) High-pressure water jetting and hose whip
- Consequence: Laceration, injection injury, eye damage.
- Controls: Rated hoses and whip-checks; pressure regulators/relief valves; guards; depressurize before disconnect; colored strobe for pump running; minimum 2 m exclusion around hoses/jets.
- PPE: Face shield + goggles, waterproof gloves, cut-resistant gauntlets, waterproof clothing.
- Collective Measures: Physical barriers; emergency stop within reach; signage.
- Inspection/Permits: Pressure test certificates; daily hose inspection log; permit for high-pressure jetting.

4) Electrical hazards (vibroflot power cables, generators)
- Consequence: Electric shock, arc flash.
- Controls: RCD protection; IP67 connectors; cable routing away from water pathways; lock-out/tag-out during maintenance; residual current tests.
- PPE: Electrical gloves (as needed), dielectric boots for maintenance, standard PPE.
- Collective Measures: Barriers around distribution boards; dry platforms for gensets; spill containment.
- Inspection/Permits: Electrical test sheets; competent electrician; permit-to-work for electrical tasks.

5) Struck-by / caught-between during lifting and rigging
- Consequence: Serious injury or fatality.
- Controls: Certified slings/shackles; tag lines; no hands on load; defined communication; lift plan with load chart.
- PPE: Helmets with chin straps, gloves.
- Collective Measures: Exclusion zone; banksman authority to stop work.
- Inspection/Permits: Lifting permits; pre-lift checks; operator certification.

6) Open craters/soft ground after compaction
- Consequence: Falls, equipment bogging.
- Controls: Immediate backfilling/leveling; barricade until stable; proof-rolling where specified.
- PPE: Standard PPE; fall protection if near edges >2 m deep.
- Collective Measures: Edge protection, signage.
- Inspection/Permits: Supervisor sign-off before traffic.

7) Noise and ground-borne vibration
- Consequence: Hearing loss, nuisance, potential building damage.
- Controls: Maintain plant; acoustic shrouds where feasible; schedule noisy works day-time; monitor LAeq and PPV per BS 5228/BS 7385.
- PPE: Hearing protection (SNR ≥ 25 dB) and vibration-damping gloves for operators.
- Collective Measures: Noise barriers; community notice.
- Inspection/Permits: Noise/vibration monitoring logs; complaint register.

8) Slurry/sediment ejection and slip hazards
- Consequence: Eye injury, slips, environmental discharge.
- Controls: Splash guards; graded working area draining to settlement tanks; housekeeping.
- PPE: Eye/face protection, waterproof boots.
- Collective Measures: Silt fencing; bunded discharge lines.
- Inspection/Permits: Water discharge permit; turbidity spot checks.

9) Night/low-visibility operations
- Consequence: Vehicle/personnel interface incidents.
- Controls: Task lighting ≥ 50 lux general / ≥ 200 lux at workface [Verify]; reflective PPE; traffic plan.
- PPE: Hi-vis Class 3.
- Collective Measures: Dedicated spotters; reduced speeds.
- Inspection/Permits: Night-work permit; lighting checks.

10) Heat stress/dehydration (high water use environment)
- Consequence: Illness, reduced performance.
- Controls: Work/rest regime; hydration points; shade; acclimatization.
- PPE: Sun protection, breathable PPE.
- Collective Measures: Cool-down area.
- Inspection/Permits: HSE monitoring; first aid available.

Water Management and Discharge

• Provide lined settlement tanks/bays sized for ≥ 30 minutes retention at peak flow; fit baffles and silt bags on outlets.
• Monitor turbidity (NTU) and pH per permit; targets e.g., turbidity rise ≤ 50 NTU over background; pH 6–9. [Verify per permit]
• Use flow meters on discharge and makeup water; log daily volumes; prevent uncontrolled runoff.

Groundwater and Adjacent Asset Protection

• Control jetting to avoid excessive uplift or ground loss; observe for heave/settlement at boundaries.
• Where sensitive structures exist, install vibration geophones and settlement points; PPV trigger values typically 5–10 mm/s at foundations [Verify]. Actions: reduce energy/increase spacing or pause.

Noise and Air Quality

• Apply BS 5228 guidance; set project-specific noise limits; use temporary barriers and schedule high-noise works.
• Dust control for access roads: water bowsers, speed limits ≤ 20 km/h; cover stockpiles.

Waste and Spills

• Classify silted water and sediments; dispose via licensed facilities; keep manifests.
• Bund all fuels/chemicals at 110% of largest container; spill kits at pumps/generators; train crews in spill response.

Ecology and Community

• Avoid jetting near watercourses without silt curtains and permits; inspect daily for turbidity plumes.
• Advance notification to neighbors; maintain complaint log and response actions.

General

• Implement ITP with defined Hold/Witness points. All instruments must have valid calibration certificates (≤ 12 months or per manufacturer).
• Maintain daily rig logs capturing: point ID, coordinates, start/finish times, penetration depth, jet pressure/flow, amperage/power draw, dwell times, pull rates, remarks.

Tolerances and Operating Parameters

• Grid point location: ±0.15 m.
• Verticality: ±2° from vertical.
• Depth of treatment: to design depth ±0.5 m.
• Jet pressure/flow (trial to set): nominal 8–15 bar and 150–250 m³/h; maintain within ±10% of approved values.
• Step extraction: 0.5 m increments with 20–60 s dwell; average pull rate 0.2–0.5 m/min.

Verification Testing and Acceptance

• CPT Acceptance (where specified):
• Achieve minimum qc at target depths, e.g., qc ≥ 8–12 MPa in upper treated zone and ≥ 12–18 MPa at depth bands as designed [Verify per design and soil type].
• Demonstrate improvement factor (post/pre) typically ≥ 1.5–2.0 where baseline available. [Verify]
• SPT Acceptance (where specified):
• Corrected N60 values meeting target bands, e.g., N60 ≥ 25 in upper zone and ≥ 30–40 at depth [Verify].
• Relative Density (if used): Dr ≥ 65–80% [Verify]. Compute from ASTM D4253/D4254 index densities and in situ density.
• Test Frequency (minimum): CPT 1 per 800–1,200 m²; SPT 1 per 2,000–3,000 m² or as directed. Edge and interior coverage required.

Rework Criteria

• Any test location failing targets triggers either: (a) two additional nearby tests to define extent; and (b) re-compaction with reduced spacing and/or altered parameters. Re-test until conformity or escalate via NCR.

Records and Deliverables

• As-built grid with coordinates and achieved depths.
• Rig parameter time histories per point.
• CPT/SPT reports (raw + interpreted), monitoring reports, nonconformance and corrective action reports.
• Final summary with compliance matrix against acceptance criteria.

• Sample Daily Vibro-Compaction Log (point-based parameter sheet).
• Trial Area Parameter Optimization Report template.
• CPT/SPT Location Plan and Logs (pre- and post-treatment).
• Settlement Monitoring Procedure and Readings Log.
• Working Platform Certificate Template.
• Permit-to-Dig form.
• Lifting Plan and Rigging Certificates.
• Calibration Certificates (pressure/flow gauges, encoders, RCDs).
• Noise/Vibration Monitoring Plan and Results.
• Nonconformance/Corrective Action Form.
• As-built Drawing Template and Compliance Matrix.