Shafts, Pits, and Piles

General

Among the safety-critical aspects of shaft construction, the following was found:

• • Great depth, which means that even small objects falling could cause injury.

  • Hoisting of heavy loads for which there are no protective structures (BS 6164: 20.1).

Design

Shafts can be permanent or temporary, and when they are less than 6m in diameter, present difficulties for protection against falling objects (BS6164: 20.2)

Shaft Sinking

Construction Techniques

General

A shaft sinking could potentially affect the general public, as well as, adjacent structures. The design should take into consideration ways to minimize disturbance on the site, including safety and environmental factors (BS 6164: 20.3.1.1).

Underpinning

Underpinning can be used in firm self- supported soils or rock. It is provided by pre-formed concrete elements or sprayed concrete, and minimum grouting should be done at the end of every shift, but it depends on the ground conditions (BS 6164: 20.3.1.2).

Caisson Construction

In case of a non-self-supporting ground, a caisson can be used. This method shall be limited to shafts with a diameter of 6m or less.

At the time the new units on top of the shaft are being installed, segments bolted or lining rings should be used.

As an alternative method to caisson construction, jack frames can be used. They should be designed for the anticipated loads, and the shoes or spacers (if used) should be secured from falling into the shaft (BS 6164: 20.3.1.3).

Shafts with Pre-Installed Linings

Pre-installed linings, such as driven piles, sheet piles, secant piles and diaphragm walls can be installed before shaft excavation takes place. If required by design, additional support should be placed as excavation advances (BS 6164: 20.3.1.4).

Drill and Blast

The drill and blast technique should be normally used in rocks only due to the risks that flying debris and vibration present in urban areas, including environmental problems (BS 6164: 20.3.1.5).

Shafts Under Construction

During shaft excavation, the number of persons allowed at the shaft bottom shall be kept to the minimum, especially if there are suspended loads.

Shafts that are less than 6m in diameter represent higher risks if they cannot be excavated mechanically and should be avoided if hand excavation is the only reasonable method. Appropriate measures to mitigate the risks of heat stress, noise and vibration should be put in place.

Ventilation is required in many shafts to maintain air quality or when air quality cannot be measured.

When using a crane or hoist to handle loads, special care should be placed to ensure that

• • the load does not swing or twist, striking the lining of the shaft,

  • the load does not catch a ledge, causing it to tip over or spill out its contents, and

  • the rope does not catch in some parts of the shaft structure.

All equipment transferred down the shaft should be designed and certified for hoisting.

As part of the standard procedures in lifting, the load should be lifted in short intervals, then stopped and inspected before hoisting resumes.

The shaft should be fitted with skip guides or partitioning for personal protection (BS 61614: 20.3.2).

MUST KNOW

Alberta Occupational Health and Safety Code Relevant to the Topic

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

According to Part 32, Section 461(1)

An employer must ensure that, during the excavation of an underground shaft that is between 1.5 metres and 6 metres deep, the walls of the shaft from the top down are retained by temporary protective structures strong enough to prevent the walls from collapsing or caving in.

According to Part 32, Section 461(2)

An employer must ensure that, during the excavation of an underground shaft 6 metres or more deep, the walls of the shaft from the top down are retained by temporary protective structures certified by a professional engineer as strong enough to prevent the walls from collapsing or caving in.

According to Part 32, Section 461(3)

An employer must ensure that

(a) a solid fence or equally effective means of

preventing workers, materials and equipment from falling into the shaft is provided around an underground shaft opening, and

(b) gates not less than 1 metre high are installed at each entrance of an underground shaft and are kept closed when they are not being used.

According to Part 32, Section 461(4)

Workers must keep a gate to the entrance of an underground shaft closed when it is not being used.

According to Part 32, Section 462(3)

Subject to Alberta OHS Code Part 32, Section 462(4), if a worker in a belled area of an underground shaft is exposed to falling materials and is unable to stand clear of the area, an employer must ensure that the worker precedes each load of excavated material to the surface.

According to Part 32, Section 462(4)

If a worker referred to in Alberta OHS Code Part 32, Section 462(3) cannot precede each load to the surface, an employer must ensure that

(a) the worker accompanies each load if the equipment is designed to safely transport both the worker and the excavated material simultaneously, and

(b) safe work procedures are prepared that include the procedures to be followed when the worker and the excavated material are moved simultaneously

Inclined and Escalator Shafts

Wherever possible, shafts should be driven downhill. If the shaft has to be driven uphill, it should be planned in a way to minimize the risk of the ground falling down.

The access of all personnel should be by stairs or walkways equipped with gripper battens (BS 6164: 20.3.3).

Transport Systems on Downhill Drives

Monorail

In this type of transport system, a roof-mounted beam is installed and the load is suspended from it. Limit runaway or another type of fail-safe system should be put in place (BS 6164: 20.3.4.1).

Invert Rail Mounted System

In this type of system, fail-safe winch units are placed at the head of the shaft and the wagons are winched from it.

Visual and/or audible warning systems should be installed to warn workers when the haulage system is operational. As additional safety measures, fail-safe breaking systems and safety gates should be installed on the incline, one of which should always be closed. The hoist operators should be trained and certified for the proper operation of the system. All safety plans and mechanisms should be inspected and serviced regularly, and all related records should be kept (BS 6164: 15.3.4.2).

Disused Shafts

Permanently Disused

After completion of its use, a shaft shall be decked over with a decking system designed specifically for that purpose. Even though the shaft’s use is permanently shut down, access should still be provided for maintenance purposes and records should be kept giving details of the shaft, tunnel and the method of capping (BS 6164: 20.4.1).

Temporarily Disused

Temporarily disused shafts should be securely covered to prevent unauthorized access. An opening should be maintained in the cover for escaping or to allow for inspection routines. The installed cover should be vented (BS 6164: 20.4.2).

Tunnel Eye

In case an opening is to be formed through a shaft, the shaft should be designed in a way that facilitates the safety of that opening. The groundwater level and pressure outside the shaft should be checked before a breakout takes place.

The shaft structure should be supported in the same way as for the tunnel opening.

Special care should be taken at the time of breaking out, this is due to the disturbance of the ground by the sinking of the shaft. Thus, immediate close support of all grounds should be essential.

If TBMs are to be entered through the tunnel eye, an appropriate seal to prevent the ground loss, lubricant, slurry or grout should be fitted.

In bad ground cases, the first ring to be built within the shaft should be fitted. Alternatively, a break-up for the full-size access tunnel can be built at a safe distance in the undisturbed ground (BS 6164: 20.5

Shaft Top Layout

The top of the shaft should be laid out in a way that prevents persons or material from falling in. The immediate area around the shaft should be leveled, clear of obstructions and adequately lit to provide a safe working area. Barriers and pumping for drainage should be provided if necessary.

Materials should be stored at a safe distance to avoid excessive ground pressure near the shaft, and all the services fed from the surface should be fixed to the shaft’s wall and protected against impact from swinging loads.

The shaft top should be guarded by the use of steel and mesh or segmental rings, which should be at least 1.2 m in height above ground level (BS 6164: 20.6.

MUST KNOW

Alberta Occupational Health and Safety Code Relevant to the Topic

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

According to Part 22, Section 318(6)

An employer must ensure that a safeguard around the surface opening of an underground shaft serving a tunnel

(a) is made of wire mesh or an enclosure material that is equally or more effective at containing equipment and materials, and

(b) is not less than 1 metre high from the surface

According to Part 22, Section 318(7)

An employer must ensure that a safeguard is installed on all sides of

(a) the cage of a building shaft hoist or a tower hoist, or

(b) a hoist cage in an underground shaft serving a tunnel.

According to Part 22, Section 318(8)

An employer must ensure that a safeguard used on a cage under AOHS Part 22: 318(7) is made of

(a) wire mesh, or

(b) an enclosure material that is equally or more effective at containing equipment and materials and at protecting workers from hazards associated with the movement of a cage in a shaft.

Personnel Access

Where it is reasonable, the access of personnel to the shaft can be made by providing fixed access equipment, such as a man-riding crane or a mast climbing hoist.

In case of plant breakdown, secondary ways of egress should be provided within a reasonable period of time. This allows for the use of a second crane or hoist.

Fixed access by the use of stairways or vertical ladders with protective hoops should be provided as early as possible, except when alternative safe pedestrian access is provided.

All the ladders should be fixed at the base and upper landing. Landings should be provided at intervals not exceeding 6 m and constructed with handrails, guardrails, toe boards and adequate illumination.

Every access provided should be inspected weekly and maintained when necessary (BS 6164: 20.7).

MUST KNOW

Alberta Occupational Health and Safety Code Relevant to the Topic

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 163(2)(b)

If the hazard assessment required by AOHS Part 2 determines that a work area is not a hazardous location, an employer must ensure that flammable substances stored or used at the work area are not stored within 30 metres of an underground shaft.

According to Part 32, Section 443(1)

Subject to AOHS Part 32: 443(2), an employer must stabilize the soil in

(a) an excavation by shoring or cutting back, or

(b) a tunnel, underground shaft or open pit mine by shoring.

According to Part 32, Section 443(2)

An employer may stabilize the soil in an excavation, tunnel, underground shaft or open pit mine using an artificial soil stabilization technique, including freezing soil by artificial means or grouting if the process used is

(a) designed by a professional engineer to control soil conditions, and

(b) performed in accordance with the professional engineer’s specifications.

According to Part 32, Section 446(1)

An employer must provide workers with a safe means of entering and leaving an excavation, tunnel or underground shaft.

According to Part 32, Section 446(2)

An employer must ensure that a worker does not enter an excavation, tunnel or underground shaft that does not comply with this Part.

According to Part 32, Section 446(3)

A worker must not enter an excavation, tunnel or underground shaft that does not comply with this Part.

According to Part 32, Section 463

A worker must not enter a belled area of a drilled or bored underground shaft if the worker is not protected by temporary protective structures.

According to Part 32, Section 461(5)

An employer must ensure that an underground shaft is provided with suitable and efficient machinery or another device for keeping the shaft free of accumulations of water.

According to Part 32, Section 462(1)

An employer must ensure that

(a) a worker who is required to enter a drilled or bored underground shaft is protected by a casing or temporary protective structure, and

(b) the casing or temporary protective structure extends and remains at least 300 millimetres above surface of the ground where the shaft is drilled or bored.

According to Part 32, Section 462(2)

An employer must ensure that a casing or temporary protective structure referred to in AOHS Part 32: 462(1) is certified by a professional engineer as having sufficient strength to resist the shifting of the surrounding materials