Gas does not accumulate only at the lowest point
At an underground excavation site, it is dangerous to reason that “gas is heavier than air, so measuring only at the bottom is enough.” Low points are important measurement candidates, but they are not the only candidates. Where gas actually remains depends on the location and quantity of generation, temperature, leak pressure, excavation geometry, fissures in the strata, permeability of backfill, groundwater, ventilation airflow, equipment operation, and time. When different gases mix with air, simple density layers may not persist for long, and fan-generated airflow may carry contaminants into an unexpected dead end or the worker’s breathing zone.
The U.S. OSHA standard for underground construction requires factors such as distance from fuel tanks, sewers, gas lines, former landfills, coal seams, and wetlands; soil type and permeability; the history of nearby sites; diesel engines, blasting, and welding; and ventilation volume and flow to be considered together when determining which substances to measure and how frequently. This list is a starting point for building the site’s gas map. Before asking “Which gas will be at which height?”, ask “Where is it generated, along which path does it move, and where does dilution stop?”
First, divide the excavation into several air zones
Even within one excavation, atmospheric conditions are not uniform. An open trench, the inside of a shored excavation, a work shaft, the bottom of a vertical shaft, a cross passage, a tunnel face, a sump, a drainage pit, a stair landing, an equipment room, and a void behind the lining are different air zones. Areas behind muck or material piles, beneath an excavator body, between formwork and a wall, and in corners not reached by ducts can also become small stagnant zones. Even if fresh air is felt near the top, it cannot be assumed that the same air reaches the shaft bottom or the corner of the face.
At a minimum, mark on the drawing the entrances, deepest points, dead ends, vertical and horizontal connections, sumps, existing utilities and manholes, temporary covers, ventilation supply and exhaust openings, and equipment parking positions. Then overlay the work sequence. Air paths change as excavation advances, additional walers are installed, covers are closed, and temporary partitions and materials are placed. A measurement point that was safe yesterday may not represent today’s worst location. The measurement plan must therefore be an airflow map updated with each excavation stage, not a fixed coordinate table.
Check low points, but do not select locations by density alone
Sumps, shaft bottoms, reverse-grade sections of utilities, and the deepest ends of trenches are always worth checking. Leaked liquids collect there, natural convection is weak, and ventilation ducts are often physically difficult to extend into them. However, measuring locations must not be determined simplistically by gas density, as in “only below because it is a heavy gas” or “only at the roof because methane is light.” Leak jets, the buoyancy of hot exhaust, cooling, moisture, mechanical ventilation, and the movement of people and equipment alter mixing and transport. Oxygen deficiency can also occur broadly because of ingress of another gas, oxidation, combustion, or biological activity, rather than as one distinct gas layer.
In practice, measurements include upper, middle, and lower levels, with separate measurements near sources, in the worker’s breathing zone, and along recirculation paths. At deep locations, use a sampling tube or remote probe for advance measurement rather than having a person insert their body first. Allow sufficient time for the suction delay due to tube length, pump flow, and sensor response. The Korea Occupational Safety and Health Agency’s confined-space work safety material advises that, in spaces with little airflow, concentration can vary greatly by location even within the same space; measurements should therefore be distributed with area and depth in mind, and the air inside the sampling tube must be sufficiently displaced by sample air.
Vertical shafts and connections are both chimneys and traps
A vertical shaft is not simply a deep hole. Temperature differences and wind pressure can create upward or downward flow, while cage movement, material hoisting, and the opening and closing of covers can act like pistons that displace air. If the wind direction at the surface entrance changes, an opening regarded as an intake may temporarily become an exhaust path. Conversely, an open top does not mean the bottom receives natural ventilation. Intermediate work platforms in the shaft, spaces behind the lining, the crown and floor of a connecting cross passage, and areas around drainage equipment may all lie outside the main airflow.
Place measuring points at several elevations at the shaft entrance, mid-level, and bottom; at cross-passage connections; at the face; in sumps; at the supply-air terminal; and along the return-air path. Also check whether contaminated air from an adjacent zone rises through the shaft into an access route. The OSHA standard requires continuous monitoring for flammable gases at the face, ribs, and return-air duct when a rapid-excavation machine is used. This arrangement is intended to show changes from the source area to the return, rather than a concentration at one point. When site conditions change, extend the duct closer to the face, remeasure airflow volume and direction, and compare results for each shaft-cover condition.
Utilities and backfill are invisible gas pathways
In urban excavation it is easy to think that only gas-pipe leaks need to be found, but sewers, storm drains, telecommunications and power conduits, utility tunnels, abandoned pipes, and backfilled zones from past excavation also serve as migration paths. If gravel or sand backfill is more permeable than the surrounding native soil, vapors and gases can travel long distances along the utility. Penetrations, manholes, valve chambers, junction boxes, cracks, and drains are where those paths meet the current excavation. Because abandoned pipes or damaged connections may be absent from drawings, survey results must be considered together with what is exposed in the field.
The U.S. EPA’s description of vapor-intrusion pathways shows that vapors from contaminated soil, groundwater, sewers, and drains can migrate through foundation cracks and utility penetrations. Although this material addresses building investigations, the migration principle is also important for excavation risk assessment. When a new excavation cuts through an existing high-permeability backfill layer, a previously closed migration route may open into the work area. Before construction, therefore, check utility-owner information and contamination history. If odors, discolored soil, an oil sheen, waste, or an unexpected void appear during excavation, stop work and revise the conceptual model and target substances before proceeding. The absence of odor is not evidence of safety.
For strata and soil gas, consider both sources and mobility
A soil or rock name is not a definitive table of hazardous gases; it is a clue for prioritizing investigation. Consider the potential for methane or hydrogen sulfide in organic-rich peat soils and wetland ground; methane, carbon dioxide, hydrogen sulfide, and volatile substances in reclaimed, filled, or waste-bearing ground; and petroleum vapors and volatile organic compounds at sites with a history of fuel stations or chemical handling. Rock joints, sand layers, drainage layers, and utility backfill can accelerate migration, while clay layers and the groundwater table can block or redirect it laterally. Changes in pressure caused by excavation and dewatering may open a route different from the one found during an earlier investigation.
New Zealand WorkSafe’s excavation safety guidance distinguishes potential atmospheric contaminants in peat ground, filled land, landfills, urban roads, and areas around fuel stations, and recommends gas monitoring and natural, forced, or mechanical ventilation. In the field, overlay borehole logs, soil and groundwater contamination surveys, past land use, landfill boundaries, permeability by stratum, groundwater levels, and the dewatering plan. Record not only gas concentration but also measurement depth, water level, atmospheric pressure, weather, ventilation status, and work status so that causes of variation can be interpreted.
Equipment exhaust is a new hazard generated on site
Even if no gas was originally present in the excavated ground, internal combustion engines continuously produce carbon monoxide, carbon dioxide, nitrogen oxides, and diesel emissions. Even when excavators, loaders, generators, compressors, pumps, concrete cutters, and haul vehicles remain at the surface, polluted air can enter underground if their exhaust points toward an opening or ventilation intake. Vehicle idling locations, noise barriers, tents, covers, and surrounding buildings can also inhibit exhaust dispersion. Hot exhaust initially rises, but after cooling and mixing into ventilation airflow it can return into the work shaft, so bottom-only measurements may miss it.
NIOSH’s guidance on workplace carbon monoxide hazards warns that small gasoline engines, generators, and similar equipment can create hazardous CO concentrations in a semi-enclosed space within a short time. It recommends placing engines away from outdoor air intakes and using periodic measurements and personal alarms. For underground work, electrically powered, battery-powered, or compressed-air equipment should be considered first wherever practicable. When diesel equipment is necessary, incorporate its manufacturer-specified condition, exhaust aftertreatment, the number of units operating simultaneously, load, and operating time into the ventilation design, and monitor actual emissions such as CO and NO2 at worker locations and in return air.
Blasting is temporary but can also be a major source. A NIOSH investigation of CO poisoning after blasting during sewer excavation describes an incident in which a worker who entered a manhole 45 minutes after blasting and coworkers who attempted rescue collapsed. Re-entry must not be approved merely because a prescribed waiting period has elapsed. Ventilate the blast point and muck pile, adjacent manholes and utilities, and the return path, and then confirm conditions through measurement.
Verify the ventilation delivery path, not just the airflow rating
The airflow in a fan capacity table is only a starting point. The key is whether fresh air reaches the work face and contaminated air is discharged to a safe outdoor location along a path that does not pass people or ignition sources. If supply and exhaust are too close, short-circuiting occurs and only clean air circulates. Duct leakage, crushing, sharp bends, insufficient extension, an advancing face, and the opening or closing of doors and covers reduce terminal airflow. If an exhaust opening is close to the exhaust of surface equipment or a building HVAC intake, contaminants discharged outdoors may be drawn back in.
The OSHA underground-construction standard requires enough fresh air to prevent the accumulation of hazardous dusts, fumes, mists, vapors, and gases, and it requires mechanical ventilation unless natural ventilation is demonstrated to provide the necessary quantity and flow of air. After ventilation stops or is reduced, all affected areas must be reinspected rather than allowing re-entry merely because a restoration signal appears. Field verification should include not only one air-velocity reading at the intake but also terminal duct airflow, airflow direction in each zone, smoke testing or another appropriate tracing method at the face and corners, return-air concentrations, and conditions with a fan stopped, a door open, and equipment at maximum load. Also confirm that a ventilation change does not push contaminants toward another crew.
Build the measurement plan around sources, pathways, and receptors
A monitor is not a device that stamps a space as safe; it is a tool for testing a hypothesis. First list the anticipated sources, draw migration paths through strata, utilities, openings, and airflow, and then mark receptors where workers and ignition sources are present. Place a measuring point at every link. Basic targets are oxygen, flammable gases, and anticipated toxic gases. Depending on site history and the work, add CO, H2S, CO2, NO2, VOC, or specific substances. Because some flammable-gas sensors may give unreliable indications when oxygen is deficient, confirm the instrument principle and test sequence.
OSHA’s permit-required confined-space standard for construction requires conditions to be evaluated before entry, atmospheric hazards to be monitored continuously in principle during work, and testing to proceed in the order of oxygen, flammability, and toxicity. Not every open excavation is legally a confined space, but this management logic is useful for work shafts, manholes, and vertical shafts with limited entrances and hazardous atmospheres. Before measurement, confirm bump-test and calibration status, sensors appropriate to the target gases, interfering substances, humidity and temperature ranges, and sampling-tube delay. Record location and height, time, gas name, numerical result, and ventilation, equipment, and weather conditions.
In the following situations, do not reuse previous results; measure again.
At the start of a shift, when re-entering after a break, or when entering after the site has been left overnight
When excavation depth, face location, covers, partitions, or duct arrangements change
When a new stratum, abandoned pipe, manhole, void, contaminated soil, or groundwater is exposed
After a fan stoppage, reduced airflow, power failure, hose damage, alarm, or sensor fault
When the number or fuel type of equipment changes, or when work such as blasting, welding, painting, or cleaning changes
When odor, eye irritation, headache, dizziness, abnormal combustion, or worker symptoms occur
When atmospheric pressure, rainfall, wind, water level, or dewatering conditions change substantially
When an alarm sounds, prevent additional exposure before attempting rescue
If there is an alarm, abnormal oxygen reading, unexplained odor, or worker symptoms, stop work immediately and withdraw along the planned route. Account for everyone who entered from the surface, and establish the emergency-stop logic in advance so that cutting power does not also stop ventilation or drainage. If a leaking utility is suspected, do not manipulate a valve or damaged section arbitrarily; control the area and notify the utility owner and relevant authorities. An improvised rescue in which a coworker enters an unexplained hazardous atmosphere without protection can turn one casualty into many. Rescue must be carried out as planned by trained personnel equipped with external monitoring, communication, retrieval capability, and appropriate respiratory protection.
Re-entry is not the moment when the odor disappears or the fan starts again. It is the point at which the source has been isolated, every zone has been sufficiently ventilated, and measurements of anticipated gases and oxygen at multiple locations confirm that acceptable conditions are maintained. Inspect not only the original alarm point but also adjacent shafts, utilities, manholes, equipment rooms, and return paths to which gas may have migrated. Incident and near-miss records should include not only concentrations but also the excavation geometry, strata, water level, fan and duct configuration, equipment locations, and weather conditions at the time, so that the airflow map can be revised.
A good gas map changes as construction advances
A hazardous atmosphere in underground excavation is invisible, but it does not arise at random. Low points, shafts, and dead ends are important candidates; utility backfill and cracks are migration paths; and contaminated soil, groundwater, organic ground, internal-combustion engines, and blasting are sources. Connecting these factors with ventilation supply and exhaust and worker travel routes reveals where to measure first and after which changes to reassess.
The key is not to set sensor locations from one density table. Look together near sources, along migration paths, at upper, middle, and lower levels, in workers’ breathing zones, stagnant areas, and return air, and combine continuous monitoring with periodic rounds. Update the gas map whenever excavation advances or equipment arrangements change, and verify numerically that ventilation actually reaches the work face. This article presents general management principles. Actual work requires a site-specific risk assessment based on the relevant laws of the Republic of Korea, client and contractor procedures, geotechnical and contamination investigations, safety data sheets, utility-owner information, and the rescue plan.