Method Statement: Artificial Ground Freezing for Cross Passage Construction Between Running Tunnels – Method Statement

Purpose

Execute artificial ground freezing (AGF) to create a continuous, watertight, and structurally competent frozen arch/wall around the proposed cross passage between two running tunnels, followed by controlled excavation and primary support installation under the frozen condition.

Included Activities

• Survey control, drilling through tunnel linings, and installation of freeze pipes and thermistor strings.
• Supply, installation, and commissioning of brine freezing plant, manifolds, headers, and controls.
• Thermal monitoring, verification of freeze wall thickness and closure, and hold points before excavation.
• Sequential excavation of cross passage under frozen ground, installation of primary lining/support, and temporary works.
• QA/QC documentation, ITP execution, and as-built records.

Exclusions

• Permanent secondary lining and final tunnel systems (unless otherwise specified).
• Liquid nitrogen freezing (not applicable to this method).

Key Constraints

• Works within live tunnels/rail environments; strict interface control with the Operator [Verify per project HSE plan and local regulations].
• Groundwater head management and inflow risk during coring and drilling.
• Limited space and ventilation in tunnels; continuous monitoring of temperature, airflow, and gases.

Performance Objectives

• Achieve continuous frozen wall/arch around cross passage profile with minimum design thickness (typically 1.0–2.0 m) and temperature criterion (e.g., ≤ -5°C to -10°C at verification isotherm) sustained for defined hold time (e.g., ≥72 hours) [Verify per project specifications].
• No measurable water ingress during pilot break-through and excavation (target: zero visible inflow; allowable seepage rate per design [Verify]).
• Excavation-induced tunnel lining movement/convergence within design limits (e.g., ≤ 5 mm at crown/SPR readings) [Verify].

• Approved designs: Freeze design report, pipe layout drawings, structural check of tunnel lining penetrations, and cross passage temporary works.
• Permits to work: Confined space, hot work (for welding), drilling through lining, pressure testing, lifting, and energisation [Verify per project HSE plan and local regulations].
• Operator interfaces: Possessions/isolations, earthing/bonding, communications plan, evacuation routes, and emergency response confirmed.
• Survey control established: Primary/secondary control points, tunnel convergence baseline readings.
• Groundwater and geotechnical data verified: Permeability, groundwater head, fines content, salinity; confirm against freeze design assumptions.
• Utilities/Services mapping in tunnel: Cable routes, signalling, drainage; identify and protect.
• Plant locations and services: Freezer plant footprint, power capacity, heat rejection, ventilation; spill containment/bunding installed.
• Materials submittals approved: Pipes, grout mix designs, brine certificates, valves, thermistors.
• Calibration certificates current: Flow/temperature instruments, pressure gauges, data loggers, survey equipment.
• ITP approved and briefed; hold/witness points understood by all parties.
• Trial mixes/tests (if specified): Thermal grout thermal conductivity verification; thermistor check; plant FAT/SAT completed.
• Emergency contingencies ready: Dewatering pumps on standby, pipe repair clamps, spill kits, backup power, rescue team.
• Pre-condition surveys: Adjacent assets, tunnel lining, and surface structures; set monitoring alarms and thresholds.

Key Hazards and Controls

1) Confined tunnel environment
- Consequence: Asphyxiation, toxic exposure, heat/cold stress.
- Engineering/procedural controls: Mechanical ventilation to achieve ≥6 ACH [Verify]; continuous O2/CO/NOx monitoring with alarms; defined egress routes every ≤50 m; thermal PPE rotation plan.
- Required PPE: Hard hat, hi-vis, cut-resistant gloves, eye protection, safety boots; cold-weather layers during freezing.
- Collective preventive measure: Fixed ventilation and alarm beacons; rescue tripod at access.
- Inspection/permit/supervision: Confined Space PTW; gas test each entry; tunnel supervisor present.

2) Drilling through pressurized, water-bearing ground and tunnel lining
- Consequence: Sudden water/sand inflow, loss of ground, flooding.
- Controls: Stage drilling with casing advance; packers at penetrations; standby dewatering pumps; drilling parameters limits; emergency grout pack.
- PPE: Face shield, waterproof gloves, dielectric boots when near water.
- Collective: Exclusion zone; emergency stop and sump containment.
- Inspection/permit: Drilling permit; daily review of groundwater head; Engineer witness for first holes.

3) Brine system under pressure and low temperature
- Consequence: Hose rupture, frostbite, slips/ice.
- Controls: Hose pressure rating ≥1.25× design; whip-checks; insulation; drip trays; heat tracing where needed.
- PPE: Cryogenic gloves for handling cold lines; eye/face protection.
- Collective: Guarding of rotating equipment; non-slip mats; spill kits.
- Inspection/permit: Pressure test certificate; daily leak walkdowns; isolation/LOTO for maintenance.

4) Welding/hot works on manifolds
- Consequence: Fire/smoke in tunnel; burns.
- Controls: Hot work permit; fire watch; non-combustible blankets; gas detection.
- PPE: Welding helmet, gloves, FR clothing.
- Collective: Portable extinguishers and hose reels; dedicated ventilation extract.
- Inspection/permit: Hot Work PTW; welder qualifications (ASME IX); WPS approved.

5) Lifting and handling in restricted space
- Consequence: Crushing, dropped objects.
- Controls: Rated lifting points; low headroom hoists; tag lines; barricades under suspended loads.
- PPE: Helmet with chin strap, gloves.
- Collective: Exclusion zones; banksman appointed.
- Inspection/permit: Lift plan; equipment inspection certificates.

6) Shotcreting and excavation under frozen ground
- Consequence: Falling ground/ice, rebound injuries, silica dust.
- Controls: Short rounds; immediate support; mesh/ice-scaling prior to work; dust suppression; rebound shields.
- PPE: FFP3 respirator, eye/face protection, hearing protection.
- Collective: Temporary support designed by engineer; dedicated scaling tools.
- Inspection/permit: Daily ground control inspection; nozzleman certification.

7) Electrical hazards and energisation of plant
- Consequence: Electric shock, arc flash.
- Controls: IP-rated distribution; RCDs; earthing/bonding to Operator’s standard; LOTO procedures.
- PPE: Electrical gloves for testing; arc-rated PPE where required.
- Collective: Barriers around panels; dry platforms.
- Inspection/permit: Electrical PTW; test and tag records current.

8) Noise and vibration
- Consequence: Hearing damage, hand-arm vibration.
- Controls: Low-vibration tools; duty time limits; acoustic enclosures on plant.
- PPE: Hearing protection SNR per risk assessment; anti-vibration gloves.
- Collective: Noise barriers; monitoring at tunnel limits.
- Inspection/permit: Noise/vibration monitoring per plan.

9) Emergency flooding/ingress scenario
- Consequence: Drowning, asset damage.
- Controls: Flood boards; check valves at penetrations; emergency grout packs; remote shutoff valves.
- PPE: Life jackets where required near sumps; headlamps.
- Collective: Muster points; emergency pumps on auto start.
- Inspection/permit: Emergency drill prior to drilling; weekly readiness checks.

[All controls to be verified per project HSE plan and local regulations.]

• Brine containment: All manifolds and plant within bunds sized for ≥110% of largest vessel; drip trays along hoses; spill kits every 25 m; brine compatible absorbents only.
• Leak detection: Daily visual checks, pressure/flow trending alarms for suspected leaks; immediate isolation and recovery SOP.
• Drill cuttings and water: Closed-loop water capture for coring; filter press or settlement tanks; dispose as controlled waste per permit; analyze chloride content before discharge [Permit to Discharge – Verify].
• Noise and vibration: Acoustic enclosures around chiller/compressors; work hours per consent; noise monitoring at set receptors.
• Energy and heat rejection: Optimize setpoints and ΔT to reduce power; route condenser exhaust away from personnel; record kWh/ton for reporting.
• Dust and rebound: Use wet methods and extraction during shotcrete; clean-down at end of shift; prevent dust migration to operational tunnel sections.
• Ground movement: Monitor settlement and tunnel convergence; adjust freezing rate if heave/settlement trends exceed trigger values [Verify thresholds].
• Waste management: Segregate steel offcuts, grout bags, hoses; maintain waste transfer notes.
• Water resources: Prevent saline contamination of drainage/sumps; use dedicated IBCs for waste brine; contractor to provide licensed disposal route.
• Flora/Fauna and community: If surface plant near sensitive receptors, install temporary screens and lighting control (downward-directed, low-spill).

• Document control: All drawings, calculations, and procedures under ISO 9001 with revision control. Redline as-built maintained daily.
• Materials QA: Mill certificates for pipes; brine certificate and lab test; grout mix design and preconstruction trial (thermal conductivity and bleed). Thermistor calibration certificates (<12 months).
• Drilling QA: Deviation surveys at completion of each hole; as-built coordinates; bore logs retained with bit size, casing, drilling fluid, and returns.
• Grouting QA: Record volumes/pressures; cube/beam tests as specified; verification of top-up where settlement/void suspected.
• Piping QA: WPS/PQR approvals; welder qualifications; NDT extent per risk (e.g., 10–100% of critical welds) [Verify]; pressure testing and leak tests documented.
• Instrumentation QA: Channel mapping cross-checked; baseline data ≥24 h; alarm thresholds and escalation tested.
• Freezing QA: Daily freeze report with supply/return temps, flows, ΔT, plant capacity; temperature contour plots; model vs. measured variance documented.
• Closure verification: Formal certificate signed by Freezing Specialist and Engineer at Hold Point HP-3.
• Excavation QA: Round length, support timing, and thickness measured; convergence readings per round; shotcrete QA per EN 14487 (panels/cores rebound, fiber count if used).
• Nonconformances: NCR raised for deviations (e.g., pipe position beyond tolerance, temp shortfall); corrective action plan with re-freeze or additional pipes if required.
• Handover: As-built pipe/thermistor coordinates, pressure test certs, grout results, temperature logs, closure certificate, and primary lining QA pack.

• Freeze Design Report and Thermal Model Outputs (design isotherms, wall thickness).
• Pipe Layout Drawings, Manifold/Header General Arrangement, Penetration Details.
• I&M Plan (thermistor locations, alarm thresholds, data management procedures).
• HSE Documents: Risk Assessment/Method Statement (RAMS), Confined Space Plan, Emergency Response Plan.
• Pressure Testing Procedures and Certificates.
• Grout Mix Designs and Test Certificates (thermal conductivity, strength, bleed).
• Shotcrete QA Procedures, WPS/PQR, and Welder Qualifications.
• Survey Control and As-Built Records.
• Operator Interface Agreements, Permits, and Communications Protocol.
• Daily Freeze Reports and Closure Certificate.
• Environmental Permits (discharge consent, waste transfer notes), Spill Response Plan.