Compressed Air Working Environment

The use of compressed air to manage underground water presents a hazard to both individuals working inside a tunnel and structures.

Structural Considerations

General

When the use of compressed air is foreseen, consideration should be given during the design and tender stages to quantify the risks involved in its use and prepare any required safety measures (BS 6164: 11.1.1).

MUST KNOW

As According to Alberta OHS Code Part 10, Section 171(8d)

A worker must ensure that compressed air is not used to blow dust or other substances from clothing.

Maximum Working Pressure

The pressure used in the design should be based on the maximum head of water likely to be encountered.

Initial estimates should be made to find the lowest point in the excavation, and an additional 10% should be added as a safety factor on top of the initial estimate (BS 6164: 11.1.2).

Ground Strength

The ability of the ground to sustain air pressure from within a tunnel or shaft can limit the pressure used. This should be determined by an assessment of the overburden at all parts of a tunnel or shaft system at all stages of construction. The assessment should include testing.

When a lack of ground strength limits the pressure that can be used, other methods of groundwater management should be considered to reduce the air pressure needed (BS 6164: 11.1.3).

Physical Effects of Compressed Air

Hydrostatic Balance

A sudden blow or blow-out situation arises when very high pressure is applied to the ground. Blow out may result in an enlarged passage through the overlying ground, which may result in a sudden and critical loss of air pressure and inflow of water and soil into the tunnel. To reduce certain events, the pressure should be assessed when the depth of the driving tunnel is shallow and the place towards the shaft meets the tunnel and creates low air pressure.

The hydrostatic balance in a totally fluid medium is impossible and can be practically achievable where the strata have some cohesion and some resistance to the flow of water and air.

To minimize the air pressure used, the balance level should be fixed at the minimum depth practicable.

In few cases, it can be appropriate to set the balance level above the crown of the tunnel, accepting the consequences of wet conditions in the tunnel and providing the face supports. It can be helpful in a situation where a large diameter tunnel is used and cover to the crown is shallow (BS 6164: 11.2.1).

Supporting Pressure & Ground Loading

When compressed air is used in impervious ground, it exerts a supporting ground pressure. Providing pressure can act as immediate ground support, but precautions should be taken to prevent ground instability in the event of air pressure loss. It may also be used in a tunnel with a very shallow clay cover to support the ground (BS 6164: 11.2.2).

Where the tunnel is being driven under the shallow cover, ground loading should be considered to reduce the danger of blow-out (BS 6164: 11.2.4).

Behavior in Permeable Non-Cohesive Ground

In case of tunneling through permeable water bearing gravel or sand, compressed air can be used to reduce the inflow of water and to stabilize the exposed material. Air can escape at the upper part of the face and can help in the tunnel driving process. However, more volume of air has to be supplied due to air escape. At the same time, water can inflow at the lower part of the face and can open up channels, thereby increasing the water inflow. To mitigate, it is advisable to provide timbering and clay, bentonite, grout, or other material for sealing off escaper air (BS 6164: 11.2.5).

Air Losses

Air loss must be minimized, as it might impact the adjacent structures, and traps beneath the impervious layer may uplift the ground. Also, air loss may affect the sewer and drain structures overlying the tunnel. To mitigate such risks, heavy gauge plastic sheeting can be highly effective as a temporary measure. Sprayed concrete or bentonite can also be used to control air leakage.

Compressed air usage is generally confined to tunnels lined with segmental cast iron linings or precast concrete. Air losses through segmental linings should be reduced by a sealing system. Elastomeric gasket should be used to eliminate air losses, but care should be taken for the segments to be installed, gasket should not be displaced and adequate grout should be applied (BS 6164: 11.2.6).

Depressurizing Working Chamber

Depressurizing should be performed gradually to allow the air to disperse outside the work area. Rapid depressurization is not recommended, as it can result in residual external pressure on the linings of shafts and tunnels (BS 6164: 11.2.7).

Shaft Sinking

General

When shafts are sunk using compressed air, the following two positions of the air deck are possible:

• • at or close to the surface, or

  • some way down the shaft

The shaft lining should be designed to compensate for additional load and stresses due to compressed air. The applied loading on the deck should be sufficient to prevent uplift loading at maximum air pressure. Flanged segmental lining is assumed to be unable to sustain longitudinal tensile forces and should be in compression at all times unless a special design is considered to withstand such forces (BS 6164: 11.3.1).

Underpinning

When a shaft is sunk by underpinning, particularly under soft soil or loose ground, care should be taken to ensure that the lost ground behind the lining is replaced with grouting promptly.

Caisson Construction

During caisson sinking of a shaft, the lining cannot be grouted in place until sinking is complete. To prevent air loss around the cutting edge, the cutting edge should be kept buried or at least below the point of hydrostatic balance and an airtight lining should be built for the shaft. The decent of caisson should be controlled.

Risk assessment should be carried out to detect when a person should be drawn from the working chamber in case of an emergency. Depressurization of the working chamber should not be used to aid sinking unless personnel are withdrawn beforehand (BS 6164: 11.3.2.2).

TBM and Compressed Air

When compressed air is used in the TBM, the design should be performed and verified as suitable for the conditions by a TBM designer or another qualified person (BS 6164: 7.6.4).

TBM with Pressurized Plenum

This method enables TBM operation without persons, continuously under pressure during the construction cycle. Spoil is conveyed through the bulkheads by means of slurry pipework or an auger conveyor.

When maintenance is carried out, good communication should be kept at all times and the crew inside should be provided with the means to prevent rotation of the head. In the event of a fire, consideration should be given to its protection by the provision of insulation and or a water-cooling system (BS 6164: 11.4.2).

Entire TBM in Pressurized Atmosphere

TBM and compressed air are the best combinations to drive a tunnel. When carrying out front-end repairs to a TBM, the ground should be supported ahead of the cutter head and inflow of water should be controlled using compressed air over a short length of the tunnel. Undertaking such an operation, a comprehensive assessment of the hazard should be carried out with plans of emergency rescue for the most inaccessible areas of the operation (BS 6164: 11.4.3).

Air Supply

Quality

Air quality should be monitored regularly. The frequency of the monitoring should be determined depending upon the project specific duties. Electronic gas monitors should be installed in the working areas and recalibrated as per the manufacturer’s recommendations (BS 6164: 11.5.1).

Quantity and Compresses Air Plant

In order to maintain the quality of compressed air in the working chamber, the guidance recommended that fresh air should be supplied at a rate of 300 l/min per person at the working pressure. In practice, a higher rate of fresh air intake is required. 

In some compressed-air tunnels where the quantity of air needed for hydrostatic balance is not sufficient for ventilation, further quantity of fresh air should be supplied directly to the work areas (BS 6164: 11.5.2).

Compressed air plant’s reliability is an essential safety feature (BS 6164: 11.5.3).

Deoxygenated Air

Deoxygenated air is a hazard indirectly associated with compressed air working, and it can be drawn back from the ground. Hence, proper ventilation should be provided, and the oxygen content of air should be constantly monitored (BS 6164: 11.5.4).

Bulkhead, Airlocks, and Associated Compressed-Air Equipment

Design and Construction

Bulkheads in tunnel lining and airlocks should be designed as per BS EN 12110 and should have at least the following features (BS 6164: 11.6):

• • Airlocks and bulkheads should be strong enough to withstand any air pressures to which the structures could be subjected in use and in an emergency.

  • Dimensions should be adequate for the maximum number of persons likely to use the airlock at any time.

  • Anchorage of airlock should resist the thrust imposed by air pressure on the end of the airlock and should be designed to carry all loads safely.

  • The airlock itself should be airtight and satisfactory devices should be provided for sealing the doors, even at low pressure.

  • All materials used in the construction of airlocks should be non-combustible or fire-resistant when tested.

  • Pressure-relief valves

Testing in Installation

Testing for the compressed air installation should be performed as follows:

• 1. All mechanical equipment used in the system should be verified as adequate before installation.

  2. After installation and prior to the commencement of tunneling operations, the whole system should be verified and tested to the capacity of the lowest-rated component.

  3. After the installation of bulkhead and airlocks and the testing of the mechanical equipment (as detailed in b), and prior to the commencement of the pressurized tunnel works, the whole of the installation should be tested (BS 6164: 11.6.2).

The workings and individual airlocks should be protected from overpressure by means of one or more safety valves that are able to maintain pressure in the event of a control problem. A test should be performed to verify the effectiveness of these valves. Hearing protection should be worn during the test.

Test should be carried out under the supervision and at no time should the working pressure exceed the pressure to which an installation has been tested.

Fire and Rescues in the Compressed Air

Special Hazard

compressed air provides a greater mass of oxygen. Therefore, it escalates fire hazards. Materials that are flammable in free air burn more vigorously in compressed air, hence extra care should be taken in order to mitigate the risk of fire.

A hydraulic plant containing oil at pressure can be dangerous. A pinhole leak can produce a fine spray of oil, or an oil hose exposed to fire can burst, releasing a large volume of flammable oil. To mitigate such risk, low-flammability hydraulic oils shall be used.

Batteries should not be charged within the compressed air environment; if this is necessary in case of an exceptional condition, local ventilation should be provided.

The use of burning welding or grinding gear in compressed-air working is hazardous and can only be used when cold work is impracticable. To use the hot work, special permits should be drawn up with detailed precautions. Using fire blankets and extinguishers, and removing combustible debris and exposed oil alone, working should not be permitted and a fire-watcher should be appointed during and an hour after finishing of hot work (BS 6164: 11.7.1).

Firefighting Equipment and Special Training

Water is the principal resource controlling a fire in compressed-air workings. Fire can be reignited in compressed air, hence the extinguisher should be kept under continuous observation and the area should be wetted with water.

Normal fire extinguisher does not perform effectively in elevated ambient air pressure. Hence, the effectiveness of fire extinguishers should be checked with the supplier.

Breathing apparatus for use in smoke and fumes should be used, and those who are trained to use it are the only ones to be employed on the job site (BS 6164: 11.7.3).

Firefighters should be provided with theoretical and practical training. The fire service, and local accident and emergency hospitals should be informed of the use of compressed air and should be invited to participate in contingency planning. The fire service personnel should have training in the operation of locking-in and locking-out in the compressed air construction. Training should also be provided to the designated site fire crew (BS 6164: 11.7.4).

Methane

The presence of methane should be assessed by detection and measurement, and if detected, extra precaution should be implemented. In areas such as coal seams, there is a danger of presence of methane (BS 6164: 11.7.5).

Rescue when Shaft Sinking with Vertical Airlocks

A plan should be laid out to remove an injured person from the working chamber of the shaft being sunk with the vertical airlocks.

The normal way to enter in such airlock from the chamber is by climbing a ladder through a hatch and into a small chamber in which it is only convenient to stand. Alternative means of egress for an injured person should be provided.

Rescue in Tunnels and from Machines

In addition to the fire hazard, a rescue plan should be worked out for the person injured by falls and other accidents. Lockkeepers and workers should be trained in the routine for rescuing a person injured in the working chamber. They should be familiar with the manufacturer’s instructions, specifications, and operation of machines (BS 6164: 11.7.7).

Inundation

Precaution and Escape

In compressed air tunneling, an increased risk of inundation arises if air escaping through the ground erodes a channel of increasing area through which there could ultimately be an uncontrolled loss of air pressure. Precautions should be including:

• • use of the minimum practicable air pressure;

  • constant inspection for air leaks at the face and surface;

  • sealing off any leaks, using bentonite, cement grout, plastic sheeting, bags, or other means of choking the airflow.

Special boxing-up and inspection procedures, preferably by remote means, should be implemented at weekends and stoppages (BS 6164: 11.8.1).

Escape routes should be carefully planned to get access to airlocks. In small-diameter tunnels, airlocks at higher levels, accessed by ladders in trucking, and affording a better place of safety than airlocks at the tunnel level should be used (BS 6164: 11.8.2).

MUST KNOW

Alberta Occupational Health and Safety Code Relevant to the Topic

As according to the Alberta Occupational Health and Safety Code (current as of January 1, 2019) an employer must follow the following:

According to Part 10, Section 171(1)

An employer must ensure that

(a) compressed or liquefied gas containers are used, handled, stored and transported in

accordance with the manufacturer’s specifications,

(b) a cylinder of compressed flammable gas is not stored in the same room as a cylinder

of compressed oxygen, unless the storage arrangements are in accordance with Part 3

of the Alberta Fire Code (1997),

(c) compressed or liquefied gas cylinders, piping and fittings are protected from damage during handling, filling, transportation and storage,

(d) compressed or liquefied gas cylinders are equipped with a valve protection cap if manufactured with a means of attachment, and

(e) oxygen cylinders or valves, regulators or other fittings of the oxygen-using apparatus or oxygen distributing system are kept free of oil and grease.

According to Part 4, Section 244(1)

An employer must determine the degree of danger to a worker at a work site and whether the worker needs to wear respiratory protective equipment if

(a) a worker is or may be exposed to an airborne contaminant or a mixture of airborne contaminants in a concentration exceeding their occupational exposure limits,

(b) the atmosphere has or may have an oxygen concentration of less than 19.5 percent by volume, or

(c) a worker is or may be exposed to an airborne biohazardous material.

According to Part 4, Section 244(2)

In making a determination under AOHS Part 4: 244(1), the employer must consider

(a) the nature and exposure circumstances of any contaminants or biohazardous material,

(b) the concentration or likely concentration of any airborne contaminants,

(c) the duration or likely duration of the worker’s exposure,

(d) the toxicity of the contaminants,

(e) the concentration of oxygen,

(f) the warning properties of the contaminants, and

(g) the need for emergency escape.

According to Part 4, Section 244(3)

Based on a determination under AOHS Part 4: 244(1), the employer must

(a) subject to AOHS Part 4: 244(3)(b), provide and ensure the availability of the appropriate respiratory protective equipment to the worker at the work site, and

(b) despite AOHS Part 4: 247, when the effects of airborne biohazardous materials are unknown, provide and ensure the availability of respiratory protective equipment appropriate to the worker’s known exposure circumstances.

According to Part 4, Section 244(3.1)

AOHS Part 4: 244(3) does not apply when an employer has developed and implemented procedures that effectively limit exposure to airborne biohazardous material.

According to Part 4, Section 244(4)

A worker must use the appropriate respiratory equipment provided by the employer under AOHS Part 4: 244(3).

According to Part 4, Section 247

An employer must ensure that respiratory protective equipment used at a worksite is selected in accordance with CSA Standard Z94.4-02, Selection, Use, and Care of Respirators