The Ventilation of deep wells and accessible tunnels During construction, the design is intended to maintain a breathable atmosphere, control contaminants, renew the air, dissipate heat, limit hazardous gases, and ensure safe working, access, and rescue conditions. In underground construction projects, tunnels and shafts may be considered hazardous areas due to their limited access and poor natural ventilation; the INSST includes shafts and tunnels among its examples of confined spaces, and Royal Decree 1627/1997 requires sufficient ventilation in workplaces to maintain an atmosphere suitable for breathing and not hazardous to health.
In projects of vertical pits for driving and microtunneling, pipe ramming, microtunneling in terrestrial and subway applications For tunnels open to the public, ventilation must be planned before interior work begins, especially if welding, electrical equipment, combustion, sludge, resins, ground gases, low natural air exchange, or complex access points are involved.
What Ventilation Should Achieve
The ventilation system must ensure:
- Supply of clean air to the work area.
- Sufficient renovation of the interior space.
- Dilution and removal of gases, dust, vapors, or fumes.
- Oxygen Control and Deficient Atmospheres.
- Control of toxic or flammable gases.
- Reduction of heat and humidity.
- Safe conditions for operators, inspection, maintenance, and rescue.
- Service continuity during critical phases of excavation, installation, or connection.
In confined spaces, the INSST recommends assessing the indoor atmosphere and monitoring it from outside while work is in progress, with continuous measurements as appropriate.
Information needed to design it
To size the ventilation system, the following are reviewed:
| Design Specification | Why It Matters |
|---|---|
| Well depth | It affects pressure drops, air inlets, air extraction, and air distribution |
| Length and Diameter of the Accessible Tunnel | Define the volume, required flow rate, and discharge/intake points |
| Number of employees | Identifies renovation and operational safety needs |
| Types of Jobs | Welding, cutting, cleaning, resins, or machinery can generate pollutants |
| Equipment Used | Engines, compressors, pumps, generators, or tools can generate heat or emissions |
| Geology and Ground Gases | There may be a risk from natural gases, radon, methane, CO₂, or other pollutants |
| Water Table and Humidity | Increased condensation, corrosion, thermal discomfort, and ventilation problems |
| Geometry of the Access | It affects ducts, fans, airflow, and rescue operations |
| Duration of the exhibition | Manages measurements, shifts, permits, and HSE compliance |
| Foreseeable Emergencies | Define redundancy, alarms, evacuation, and communication |
Common Ventilation Systems
| System | Operation | When to use |
|---|---|---|
| Clean Air Supply | Feed outside air into the bottom of the shaft or the working face | Deep wells, tunnels open to the public, or areas with low air exchange |
| Localized Extraction | Removes contaminated air from the emission area | Welding, fumes, dust, gases, or occasional tasks |
| Combined supply-and-exhaust system | It supplies clean air and removes pollutants | Long spaces, complex geometries, or multiple emission sources |
| Ventilation via flexible or rigid ducts | It carries air to areas far from the intake | Tunnels open to the public, galleries, or very deep shafts |
| Redundant ventilation | It has backup equipment or an alternative power source | Critical tasks, hazardous environments, or hard-to-reach areas |
| Emergency Ventilation | Triggers a reset in the event of an alarm, gas leak, smoke, or incident | Construction Projects with an Underground Rescue and Evacuation Plan |
How to Determine the Ventilation Flow Rate
The flow rate is determined based on an HSE and technical assessment of the space. It should not be determined solely by the volume of the shaft or tunnel, but rather by the actual risks of the operation.
The following are considered:
- Volume of the enclosure.
- Air changes are necessary.
- Number of people exposed.
- Anticipated pollutants.
- Heat sources.
- Combustion equipment, if any.
- Hot work or welding.
- Duct Length and Pressure Drops.
- Temperature and humidity.
- The need to maintain fresh air at the workstation.
- Evacuation and rescue capabilities.
In general, the design must ensure that clean air reaches the area where workers are operating, not just the wellhead. In deep wells, this requires ducts extending all the way to the bottom; in accessible tunnels, to the working face or active work area.
Which gases and pollutants are monitored?
During execution, the following can be monitored, as appropriate:
- Oxygen.
- Carbon monoxide.
- Carbon dioxide.
- Hydrogen sulfide.
- Methane or other flammable gases.
- Vapors from fuels, solvents, or resins.
- Respirable dust.
- Welding or cutting fumes.
- Aerosols or vapors generated by chemicals.
- Heat and humidity.
The Spanish legal definition of a confined space cited by the INSST includes areas with limited openings, inadequate natural ventilation, and the potential for the accumulation of toxic or flammable contaminants, or an oxygen-deficient atmosphere.
Operational Control During Execution
Ventilation should be integrated into the work procedure; it should not be limited to simply installing a fan. Before and during entry into the shaft or tunnel, it is recommended to check the following:
- Initial atmosphere measurement.
- Entry permit or access procedure.
- Ventilate the room before entering.
- Continuous or periodic measurement, depending on the risk.
- Alarms for oxygen, toxic gases, or flammable gases.
- Control from the outside.
- Constant communication with the team back home.
- Measurement Log.
- Rescue teams are on standby.
- Safe and secure power supply.
- Emergency and Evacuation Plan.
In tunnels open to the public associated with infrastructure crossings In urban environments, this monitoring must be coordinated with the security plan, the emergency plan, surveillance, access points, lighting, and communications.
Common Mistakes to Avoid
The most common failures in ventilation systems for accessible shafts and tunnels are:
- Ventilate only the wellhead; do not ventilate the bottom of the well.
- Do not account for pressure drops in long ducts.
- Do not check the atmosphere before entering.
- Use ventilation without gas monitoring.
- Do not provide for localized exhaust for welding or fumes.
- Failure to provide backup ventilation during critical operations.
- Do not record flow rates, alarms, or measurements.
- Failure to coordinate ventilation with rescue, communication, and lighting.
- Underestimating heat, humidity, and heat fatigue.
- Do not update the system when changing the construction phase.
Minimum Ventilation Design Checklist
Before performing work in deep shafts or accessible tunnels, it is advisable to confirm the following: depth, diameter, length, internal volume, number of workers, type of tasks, sources of contaminants, natural gases, water table, humidity, temperature, ducts, fans, airflow, pressure drops, gas monitoring, alarms, communication, lighting, access, rescue, power supply, emergency ventilation, and HSE procedures.
Request a Technical inspection of ventilation and safety in deep shafts or accessible tunnels Before planning interior work, maintenance, inspections, or on-site connections WITHOUT trenching.
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