Ground gases including methane, carbon dioxide, and carbon monoxide are produced by decomposing organic material in landfill sites, peat deposits, and made ground. These gases migrate through the soil and can accumulate in excavations, basements, and confined spaces, creating explosion and asphyxiation risks. Monitoring using portable gas detectors and installed borehole monitoring wells is essential on sites where ground gas is known or suspected. The results determine the design of gas protection measures for the completed building.
Key Hazards
Methane accumulation in excavations creating an explosion risk from ignition sources
Carbon dioxide displacing oxygen in below-ground areas causing asphyxiation
Carbon monoxide in ground gas causing poisoning in poorly ventilated spaces
Inaccurate monitoring results from poorly calibrated or incorrectly used equipment
Control Measures
Carry a calibrated multi-gas detector monitoring CH4, CO2, CO, O2, and H2S during all excavation work.
Bump test the gas detector at the start of every shift to confirm sensor response.
Monitor the atmosphere in all excavations and below-ground areas before entry and continuously during work.
Install borehole monitoring wells at the locations and depths specified by the ground investigation report.
Record all gas monitoring readings on the project register with date, time, location, and conditions.
Calibrate gas detectors at the intervals specified by the manufacturer and maintain calibration records.
Ventilate excavations where elevated gas levels are detected before permitting worker entry.
Brief all workers on the ground gas hazards identified in the site investigation before starting work.
Evacuate the excavation immediately if any gas reading exceeds the alarm threshold level.
Emergency / Rescue
If gas readings exceed the lower explosive limit alarm, evacuate the excavation immediately and move upwind. Remove all ignition sources from the area. Call your supervisor and do not re-enter until conditions are confirmed safe.
Remember
Methane is explosive at concentrations between 5% and 15% in air and accumulates in excavations.
Carbon dioxide is heavier than air and displaces oxygen in low-lying areas causing asphyxiation.
Bump test your gas detector every shift — a faulty sensor gives false confidence in unsafe conditions.
Ground gas conditions can change with weather, barometric pressure, and excavation depth.
All gas monitoring readings must be recorded for the project register and design validation.
If any alarm triggers during monitoring, evacuate immediately and do not investigate the cause yourself.
Applicable Legislation: CDM Regulations 2015 · COSHH Regulations 2002 · CIRIA C665 (Assessing Risks from Ground Gases) · Health and Safety at Work etc. Act 1974