Excavation and Ground Support

Risk Control

Tunneling industry is a special division of the construction industry; it requires skilled crews that are specialized in tunnel construction. Tunneling construction is a risk-prone industry; a small mistake can lead to complete failure or major damage to the project.

Hazard Identification for a Safe Work System

Effective hazard identification is necessary in order to prevent any risk arising from the hazard. good health and safe work performance must be the priority of the constriction team. The entire team is responsible for creating a safe workplace, identifying hazards, and helping to develop a safe construction process and environment. Risk identification and controlling is a continuous learning process that requires a team effort. Consistently, the risk identified should be avoided by taking preventive measures and should be recorded in a database for preventing future occurrences. The process should be carried out on a regular basis from the construction to the maintenance phase of the tunneling works.

Hazards in the tunneling industry may arise from the unstable ground, noise, dust, water, moving machinery, contaminated soil, the presence of gases, and many more. The hazards have the potential to harm a person working or living in the proximity of the construction area. The risk is directly proportional to the likelihood of occurrence of harm and the consequences if it does occur. The tunneling industry is a low frequency, but the high consequences industry, and the occurrence of any incident may be deleterious.

A structured approach should be undertaken to reduce risks and should include the following stages:

• 1. Hazard identification, which should take place during construction and maintenance stages, must be done carefully to control risk. Commonly, hazard identification is done using generic approaches, due to the fact that each project is different in nature and in its associated risk. Therefore, project-specific hazard identification is crucial.

  2. Hazards should be eliminated at the design stage where possible.

  3. Risk evaluation, for risk arising from residual hazards, should be undertaken.

Basic Principles

The basic principle of safe excavation should be to ensure ground stability, and to minimize surface settlement and ground movement at all times. This can be achieved by either:

• • Providing continuous ground support as part of the excavation sequence, or

  • Opening up only as much ground as is safely self-supported until permanent or temporary support can be provided.

As soon as the volume of the ground is excavated, there is a redistribution of stress in the remaining ground which can initiate ground movement. Therefore, time window response between ground excavation and temporary or permanent support for the ground depends upon the soil type. Soft clay or loose sand requires immediate support while excavation in hard rock can remain stable for weeks without any support.

Factors to take into consideration while determining the load of the ground-support system include:

1. The size and depth of the tunnel;

2. The shape of the tunnel;

3. The method and speed of excavation and lining;

4. The stiffness and water tightness of the lining system;

5. The underground regime (the geological structure);

6. The proximity of other surface and underground structures;

7. The construction of adjacent tunnels and other underground structures;

8. Vibration

Experience and sound judgment are essential in assessing how much ground can safely open up and for how long it can remain unsupported. In case of mechanized tunneling such as TBM, it is not always possible to see the excavation face. In this case, the alternative method should be used to assess the ground.

Note 1: Where sudden ground conditions are encountered in soil prone to sudden changes during ground excavation such as silt and sand in the loose ground, the face of the excavation should be stabilized as soon as possible. Mechanical closing down of the face or alternatively stuffing quickly with bags of straw or similar material can be effective (BS 6164: 7.2).

Ground Movement Control

Ground movement control should be considered while construction of the tunnel is in urban areas, as it can help to choose the best tunneling technique to be adopted.

Amount of ground movement around a tunnel is a function of the ground loss, stress relief and change to hydrology. The excavation process should be managed to ensure minimum ground disturbance. Monitoring of ground movement can give real-time results and where ground movement is critical; it should be included in the overall management of the ground construction process.

Made Ground and Contaminated Ground

Where the made ground is encountered, it should be investigated thoroughly, and sufficient information about the nature of the ground must be apprehended.

Examples of contaminated ground include the presence of hydrocarbon, solvents, bacteria or gases. In case of contaminated ground use of slurry, TBM should be considered as it can mitigate problems of exposure of contaminants in TBM (BS 6164: 7.4.4).

Method of Excavation and Spoil Removal

Today, tunnels are frequently built using mechanical excavation and lining methods. However, hand excavation remains a viable technique due to its adaptability in confined spaces. It is commonly used for short tunnel drives in unsuitable ground conditions, breakout excavations from shafts, establishing short connections and back drives, accessing cutter maintenance chambers, and enlarging tunnels. Hand excavation should be employed only when mechanical excavation is not feasible (BS 6164: 7.6.1).

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 448(1)

An employer must ensure that work involving mechanical excavation equipment is prohibited within the hand exposure zone of a buried facility until the buried facility has been visually exposed.

(a) by hand digging,

(b) by a non-destructive technique acceptable to the owner of the buried facility, or

(c) by a method equivalent to clause (a) or (b).

According to Part 32, Section 448(2)

Despite AOHS Part 32: 448(1), an employer may use mechanical excavation if doing so does not present a hazard and

(a) if the buried facility is an electrical cable or conduit, the employer must ensure that

     (i) it is grounded and isolated so that its disconnection is visible, and

     (ii) the owner of the electrical cable or conduit is notified of the operation before it begins, or

(b) if the buried facility is not an electrical cable or conduit, the employer ensures that

     (i) it is no longer in use, and

     (ii) the owner of the buried facility gives the employer written consent to excavate or remove the facility.

According to Part 32, Section 448(5)

An employer must not allow the use of mechanical excavation equipment within 600 millimeters of a buried pipeline unless the use of the equipment is under the direct supervision of a representative of the owner of the buried pipeline.

Mechanized Tunneling

The safety of the machine should be considered an essential aspect of the overall tunneling safety. The machine must conform to relevant tunneling machine standards, such as BS EN 815, BS EN 12110, BC EN 12111 and BS EN 12336.

All machinery should be strictly operated in accordance with the manufacturer’s instructions (BS 6164: 7.6.2).

Conventional Tunneling

The conventional tunnel is excavated, and the excavated material is transported using various types of equipment. When tunneling through soil or rock, separate equipment may be necessary for excavation and removal of the excavated material. Workers should consistently maintain a safe distance from moving machinery and equipment, typically self-propelled.

Distinct risk assessments should be conducted specifically for moving machinery, and appropriate risk mitigation measures must be implemented. Whenever feasible, conveyors should be installed to minimize extensive vehicular movement within the area.

Hand Excavation

Hand excavation should be minimized by using mechanized excavation where practical. However, where hand excavation is exercised, risk mitigation strategies should be devised. Hand excavation is carried out by conventional picks and shovels, air-powered picks, breakers, and clay spades.

Hazards include confined space, noise, hand-arm vibration, manual handling, working at height and heat. To mitigate the risks, shorter shifts with frequent rotation of crews around different tasks should be considered. A proper platform should be provided where necessary (BS 6164: 7.6.4)

Ground Support for Conventional Tunneling in Soft Ground

Construction Risk

Some of the major construction risks for larger tunnels in the soft ground are the instability of the newly excavated ground, damage to the new lining and excessive ground movement. These can damage the underground utilities, harm the man working inside the tunnel, and destroy the structure itself. When necessary, the stability of the excavated surface should be enhanced by doming and applying a coat of sprayed concrete.

A complete system should be developed and followed to ensure that the ground condition, excavation and lining procedure, materials quality control, and post-lining ground movement meet the designer’s assumptions and requirements (BS 6164: 7.7.1)

Initial Support and Profile of SCL Tunnels

The excavated ground should be stabilized immediately after excavation by applying the first coat of sprayed concrete. For initial support, fiber-reinforced spray concrete should preferably be applied by robotic sprayers. Spraying should be followed by installation of lattice girders and mesh in the crown of the tunnel (BS 6164: 7.7.2).

A waterproof membrane is also required within the permanent lining (BS 6164: 7.7.3).

Sprayed Concrete

Sprayed concrete covers all pneumatically applied mixes of cement, water, additives, and aggregates, including shotcrete and gunite, and using a wet or dry process. The process is quick and efficient and provides ground stability momentarily after concrete placement.

Precaution

Wet-process spraying is preferable with the help of remotely operated spraying equipment, to reduce exposure to dust and hazardous material.

Where remote techniques cannot be used, operators should wear full protective clothing, and use a respirator and eye protection. Exposed skin should be covered with a barrier cream. Flexible supply pipes should be wired or chained at joints to prevent them from flailing about if a joint burst while in operation (BS 6164: 7.7.4.2).

When installing sprayed concrete linings, particular care should be taken, and contingency and emergency plans should be in place so that they can be implemented when necessary along with the sufficient material supplies as planned (BS 6164: 7.7.5).

Tunneling Machines

Open-faced Shields

An open-faced shield provides initial support and protection during excavation and lining erection or during pipe installation. Precautions should be taken during tunnel excavation and should not be carried out beyond the area protected by the shield. In mixed soil conditions (e.g., rock, soft ground), the rock should be excavated clear of the cutting edge.

Face support must be provided for shields, which might be required throughout the construction process, and should be properly maintained.

On a large diameter shield, while excavating, the soil should be supported by the working platform, not by scaffolding itself. Responsibility for the design, safe installation, and operation of the working platform should be given to a designated person (BS 6164: 7.8.1.1.

Tunnel Boring Machines (TBMs)

A TBM has a rotary wheel excavator capable of cutting the whole face of the tunnel at each revolution of the wheel. The excavation can be carried out by a shielded or an unshielded operation. Excavated material is scooped out from the face and normally lifted onto a central conveyor. These machines are usually employed in the ground where face support is normally not required, although some machines are fitted with hydraulically operated mechanical doors to support the face when required.

A safe system of work at the tunnel face, for provision of rescue, inspection, maintenance, and changing of cutters, should be in place (BS 6164: 7.8.2.1).

Slurry Machines

Slurry machines are shielded TBMs with a fixed bulkhead, separating the excavation chamber from the rest of the machine. Pressurized slurry is pumped into the excavation chamber to support the face of the excavation. Excavated material mixed with the slurry is pumped out of the excavation chamber and piped to the surface for separation. Subsequently, the clean slurry is then pumped back to the excavation chamber.

There should be appropriate means of clearing the slurry and excavated material from the cutter head chamber before man-entry (BS 6164: 7.8.2.1).

Earth-Pressure Balance Machines

Earth-pressure balance machines are shielded TBMs with a closing bulkhead separating the excavation chamber from the rest of the machine. They have a balanced screw conveyor to remove the excavated spoil while maintaining a pressure in the remolded soil within the excavation chamber. Conditioning agents, such as bentonite, polymers and foam can be injected at the cutterhead to improve the efficiency of the machines and effectiveness of the groundwater, and for material control (BS 6164: 7.8.2.3).

Hard-Rock TBMs

Hard-rock TBMs are used for excavation of rock with a range of strength, abrasiveness, jointing, and water. They can be fully or partially shielded. The risk assessment should cover sudden changes in the rock conditions, and control measures should be devised (BS 6164: 7.8.2.4).

Segmental-Erection Equipment

The segment and other lining erection equipment should be sound enough to carry out safe operations and should be checked and maintained at regular intervals.

Site personnel, either on or behind a machine, should be given clear instructions and training to perform safe work. The tunnel lining must be erected using a purpose-designed full-circle erector where practicable. Where not, the lining should be erected by other appropriate means, including:

• • a roller path erector,

  • a bird’s wing erector, or

  • by hand, with mechanical assistance, such as winches with snatch blocks and roller bolts.

Wire ropes, connection pins and other lifting devices used in lining erection should be inspected frequently, as they are subjected to heavy wear and tear (BS 6164: 7.8.2.5.

Conventional Tunneling in Rock

Excavation Techniques

Drill and Blast

Blasting pattern should be designed to reduce the risk of damage to the excavated surface, which can loosen the rock. Smooth blasting or pre-splitting techniques should be used where possible

The hazards associated with explosives include storage, handling, and use, which should be approached with proper caution. For the drill and blast techniques, hazards include noise, vibration, dust from the use of drilling equipment, fumes from explosives, and falls of ground.

Following blasting, and before re-entry, a check should be performed to ensure that the tunnel atmosphere is fit for respiration. The risk of rock fall is most acute in the period after the blast and before temporary support. A thorough inspection should be made in order to resume work after blasting.

Scaling should be done by mechanical means where practicable. Inspection should take place on both newly fired ground and previously scaled surface. Supervision should be provided during the scaling process. If support is necessary, it should be provided as soon as practically possible after scaling so as to maintain the overall integrity of the rock mass and to prevent minor falls. The use of air drills and pusher legs should be minimized to avoid any health risks. Flying rocks can easily damage machinery or service line. Therefore, vulnerable parts should be checked before operation (BS 6164: 7.9.1.1).

Where rock strengths or joint patterns permit, road headers or other part-face machines can be used for excavation (BS 6164: 7.9.1.2).

Steel Arches and Packing

Where steel arch ribs are used in conjunction with timber or steel poling to provide immediate support, they should be fixed, wedged and packed up as soon as it is practicable after excavation. Arch support types and spacings should be designed for the actual ground condition encountered.

A supervisor who has the experience to assess the ground condition should be available at the site. Sufficient predetermined supports regime should be available on site. Where necessary, scaling should be carried out on the site before frames and arches’ erection (BS 6164: 7.9.2.1).

Two-Part Arches

For tunnels up to 3.5 meter high, U-shaped arches with one bolted connection at the crown are commonly used. The foot of the arch should be capable enough to sustain and transfer the load above it. The foot should be rested on the rock or foot plates to transfer the load. The arches should be securely fixed to each other with ties and struts.

The packing between the arch and rock should be wedged tight, and the arches should be securely packed, particularly at the springing level, to stop side sways movement at that point (BS 6164: 7.9.2.2).

Large or Multi-Part Arches and Frames

In addition to the recommendations provided in 4.10.2.1 (BS 6164: 7.9.2.2), the following recommendations should be followed (BS 6164: 7.9.2.3):

• Bolted joints and packing should be accessed from a properly designed platform, staging, or mechanical/hydraulic access equipment.

• The Erection equipment should be properly designed for particular situations.

Steel Lattice Ribs and Support Between Arches

Steel lattice provides lateral support to the frame while applying sprayed concrete. These ribs should be installed in sections corresponding to the excavation stages, with bolted joints at the bench surfaces (BS 6164: 7.9.2.4).

When necessary, support should be provided between arches to prevent falls of loose rocks (BS 6164: 7.9.2.5).

Rock Bolting

Rock bolting, using either mechanically or chemically anchored bolts, is a common method used to provide temporary support or to be an element of the permanent support system. It is used fundamentally to prevent any gaps across discontinuities, such as joints, fissuresand bedding planes to maintain the integrity of the exposed rock structure.

The bolt pattern, types, length and diameter should be determined after studying the particular circumstances that change rapidly as the tunnel work progresses.

The anchorage should be of sufficient depth and should be designed to resist the full pull-out value of the bolt without slipping in the bore or crushing the rock locally. In situ load tests should be carried out on representative bolts (BS 6164: 7.9.3).

Compressed Air

Compressed air should be applied to control groundwater to improve the stability of the tunnel face, though additional face support might be necessary (BS 6164: 7.10).

MUST KNOW

Alberta Occupational Health and Safety Code Relevant to the Topic

According to Alberta OHS Code Part 32, Section 464(2)

An employer must ensure that a tunnel is provided with suitable and efficient machinery or another device for keeping the tunnel free from accumulations of water.

Geotechnical Processes for Ground Improvement and Water Management.

Freezing

Water-bearing ground can be strengthened by freezing pore water into ice, which increases the soil's capacity to bear loads. It can be achieved by circulation of a coolant at a significantly low temperature. Freezing might be impossible if there is an underground flow of water which brings in heat at a rate faster than the freezing process can extract.

It is very important to develop and maintain the frozen state of the soil, and it should be monitored by a thermocouple installed at the site. There is a residual risk that pockets of the unfrozen ground might remain, and contingency plans should be put in place to avoid such risk. In addition, the risk of ground heave must be considered as well, which can damage overlying and buried structures and services.

Water mains within the freezing zone should be insulated. Where ambient temperature is sufficiently low leading to discomfort, the crew must be protected from the cold. Self-rescuers appropriate for cold environment should be provided (BS 6164: 7.11.1.1).

Case report: Collapse of a TBM associated with soft ground occurred in Germany while working on a 17 km long tunnel project to increase the capacity of the Rhine Valley rail corridor from two track to four track as part of an underground rail network construction beneath the City of Rastatt. A 4.27 km long portion of the tunnel was passing through loose soil, gravel. To control loose sand and gravel deposits, ground freezing option was chosen. On August 12, the ground freezing substantially failed to support the ground and a subsidence depression of 500 mm occurred beneath the railway tracks. As part of the safety protocol, the railway operation was stopped and further investigation and corrective actions were performed (Wallis, 2017).

Brine

If brine is used as a coolant, tunnel excavation should not be performed within 0.5 m of live freeze pipes.

Liquid Nitrogen

Risk assessment should be performed on handling a cryogenic liquid and on leaks of liquid nitrogen evaporating in the tunnel.

Liquid nitrogen should be stored on the surface. All surface pipework should be protected from impact damage. Pipes should be installed vertically where possible. In case of horizontal pipe drilling, the pipe should be tested for leak tightness before use. Pipework should be sized to minimize the amount of liquid nitrogen in order to reduce the risk of discharge in the tunnel due to leaks and breaks.

Atmospheric monitoring equipment are required near the frozen ground to test any leaks. Any nitrogen leak has a tendency to congregate in the sump, and therefore proper monitoring system should be installed.

Additional emergency ventilation might be necessary while using the system; any exhaust nitrogen should be ventilated by a  chimney on the surface and directed away from site boundaries.

Risk assessment should be carried out if the excavation is conducted in proximity to the freeze pipes.

A bulk storage unit is required for the liquid nitrogen cooling operations, which should be secured in a protected area on the site. Assessment should be performed on both workforce and public for transportation and onsite storage (BS 6164: 7.11.1.3).

Ground Injection

Cementitious or chemical ground treatment in advance of tunneling can usefully enhance safety, particularly in open-face excavations. It improves the characteristics of the ground to be excavated, indirectly seals off water, and strengthens the overlying or surrounding ground. 

If sand or gravel is expected in the tunnel face, particularly if it is water-bearing, its permeability can be significantly controlled by the injection of a suitable grout mixture, and by the spacing and pattern of the injection holes. Progressively finer grouts of lower viscosity should be used in progressively finer soils (BS 6164: 7.11.2).

Note: Clays cannot be treated by permeation grouting. When compressed air is used in a tunnel, it can result in the grout being blown aside during injection. Where excessive fine content in the ground prevents permeation grouting, jet grouting can be considered.

Proper assessment and risk mitigation strategies include:

• Excessive pressure that can cause ground movement and structure damage should be avoided.

• The toxic and environmental risks of chemical ground should be assessed.

• Arrangement of collection and disposal of waste should be made in advance and should be approved by all relevant authorities.

• Precautions should be taken due to the toxic nature of some grouts, where protective clothing and appropriate respirators should be used. Grouting in confined spaces such as a pilot tunnel bears even more risk, and proper assessment should be made.

Dewatering

Dewatering can be an efficient solution in water-bearing sands and gravels. For shallow tunneling, it can be achieved by well-point dewatering. In case of deep tunneling, deep wells or submersible pumps could be effective.

The risk of loss of fine particles should be prevented. The main risk arising from dewatering is the loss of ground stability by settlement and/or failure of the dewatering system during tunnel driving.

Settlement can affect a third-party property or cause injury. Mitigation measures include:

• • The use of a piezometer to monitor the ground.

  • Installation of cut-off walls to protect the sensitive foundations.

Surface pipework should be protected, and a backup system should be employed where possible to mitigate the risk of failure (BS 6164: 7.11.3).

Depressurization

It should be noted that there is a risk of instability in a conventional tunnel being driven in clay but close to an interface with granular material in which there is artesian pressure. The granular material should be depressurized (BS 6164: 7.11.4).

Small Headings and Small Tunnels

Risks and hazards in small tunnels are intensified due to confined space, time and resources. Therefore, only a competent crew should work under such places to reduce risk (BS 6164: 7.12.1).

Hand-Driven Segmental Lined Tunnel Without a Shield

Occasionally, when small tunnels are excavated, precast segments are built without the use of shield. The excavation of a full tunnel face is carried out without the use of a shield by securing the top and face as it is exposed by excavation. The face is excavated in steps or benches. In firm ground conditions, it is safer to take the entire face vertically with the help of very little timbering. In some cases, depending upon the ground condition, there is no need for support. It is necessary to foresee health and safety risks for working under such confined space work environment (BS 6164: 7.12.2).

Timber headings

Timber headings can be used in case of temporary support. The highest standard of workmanship is required in initial timbering, and subsequent backfilling should take place. Timbers used as timber headings should be strong in nature, and not less than 38 mm thick. The selected timber should be checked for imperfections which could reduce the wood strength. The timber should be readily available near the working face (BS 6164: 7.12.3).

The gap between the timber and soil should be backfilled by concrete and back grouted.

Rescue and Escape

The small heading can be considered as a special case, due to very tight workspace. Only one person can work near the excavation face, but a second person should always be available at the heading. The public emergency services might not be able to enter small headings and tunnels. As a result, site-specific arrangements should be made to rescue the worker in time of injury, illness or any unexpected event (BS 6164: 7.12.5).

Ventilation

Ventilation can be a challenge in the small headings, particularly when such headings or tunnel passes are near ground with organic content or with another contamination.

A force ventilation system should be employed to supply fresh air to the face (BS 6164: 7.12.6).

Pilot Tunnels

Pilot tunnels are built to provide detailed information on ground conditions, including groundwater regimes. They should be used in soft ground where settlement control is important (BS 6164: 7.13).

Pipe and Box Jacking

Pipe jacking is a system in which pipes or similar structures are thrust through the ground, and material from the face is removed through the pipeline (BS 6164: 7.14.1).

A thrust wall or an abutment should be designed and constructed to resist high jacking load. Comprehensive risk assessment should be performed to reduce the risk of injury in high stress environment.

• • Workers should be protected and withdrawn from the vicinity of high stressed equipment.

  • The use of lubricants on a sliding surface during operation reduces the jacking loads.

  • Proper eye protection is required during high-pressure lubricants’ injection.

  • All workers in the jacking pit should be provided with shelter or protection within the part/completed pipeline or elsewhere.

Box jacking works on similar principles as pipe jacking. The major difference is the use of thick walled concrete rather than circular pipes. A typical application would be the formation of an opening through a rail embankment for the passage of a new road.

Box jacking is often performed in shallow depth. Therefore, precautions must be taken to control the horizontal and vertical movement of overlying or adjacent infrastructures. An anti-drag system, in combination with an appropriate lubricant such as bentonite should normally be used above and below the box to minimize the ground movements.

In poor ground conditions, ground improvement techniques, such as dewatering, grouting, or ground freezing should be considered (BS 6164: 7.14.2).

Soil Conditioners and lubricants

Compounds, such as bentonites, polymers, and foams are increasingly used to modify ground to support the ground face during tunneling works. It is used to improve material transportation from face to surface and to lower the pipe jacking force.

Some materials are hazardous in their disposal. Risk assessment for the tunneling process should be carried out prior to the use of these compounds. The manufacturer’s safety data sheets (MSDS) give recommendations on handling, and personal protective equipment should be complied with (BS 6164: 7.15).

Settlement Control – Mechanized Tunneling

In TBM-driven tunnels, the volume of removed spoil should be reconciled against the volumetric advance rate on a routine basis. Contractors should establish a system to monitor the full range of parameters which could give early warning in case of any undesirable event such as over excavating (BS 6164: 7.16).

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 444

If there is a danger of a worker or equipment falling into an excavation, an employer must ensure that workers are made aware of the excavation through flagging, marking, safeguards or other appropriate and effective means.

According to Part 32, Section 464 (1)

An employer must ensure that, during the excavation of a tunnel, the walls of the tunnel 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 447(1)

For the purposes of AOHS Part 32: 447(1.1) and 448, an owner means an owner or the owner’s designate of a pipeline that is within 30 metres of the work site or any other buried or concrete-embedded facility that may be affected by the ground disturbance or removal of existing concrete.

According to Part 32, Section 447 (1.1)

Before the ground is disturbed or existing concrete is removed at a work site, an employer must

(a) contact the owner,

(b) advise the owner of the proposed activities,

(c) ask the owner to identify and mark the location of the buried or concrete-embedded facility, and

(d) not begin disturbing the ground or removing the existing concrete until buried or concrete-embedded facilities have been identified and their locations marked.

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 443 (3)

A person must not use natural freezing of the soil as an alternative or partial alternative to a temporary protective structure, or to stabilize the soil in an excavation, tunnel or underground shaft.

According to Part 32, Section 451

If the walls of an excavation are cut back, an employer must ensure that

(a) if the soil is classified as “hard and compact soil”, the walls are sloped to within 1.5 metres of the bottom of the excavation at an angle of not less than 30 degrees measured from the vertical,

(b) if the soil is classified as “likely to crack or crumble soil” the walls are sloped to within 1.5 metres of the bottom of the excavation at an angle of not less than 45 degrees measured from the vertical, and

(c) if the soil is classified as “soft, sandy or loose soil” the walls are sloped from the bottom of the excavation at an angle of not less than 45 degrees measured from the vertical.

According to Part 32, Section 456(1)

An employer must ensure that temporary protective structures in an excavation

(a) 3 metres deep or less are of sufficient strength to prevent the walls of the excavation from caving in or otherwise moving into the excavation, and

(b) more than 3 metres deep are designed, constructed and installed in accordance with the specifications of a professional engineer.

According to Part 32, Section 456(3)

An employer must ensure that, before beginning an excavation, a foundation that may be affected by the excavation is supported by a temporary protective structure designed, constructed and installed in accordance with the specifications of a professional engineer.

According to Part 32, Section 445

An employer must ensure that an excavation that a worker may be required or permitted to enter is kept free of an accumulation of water that may pose a hazard to the worker.

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(3)

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

According to Part 32, Section 450(1)

Before a worker begins working in an excavation that is more than 1.5 metres deep and closer to the wall or bank than the depth of the excavation, an employer must ensure that the worker is protected from cave-ins or sliding or rolling materials by

(a) cutting back the walls of the excavation to reduce the height of the remaining vertical walls, if any, to no more than 1.5 metres for “hard and compact soil” and “likely to

crack or crumble soil”,

(b) installing temporary protective structures, or

(c) using a combination of the methods in clauses (a) and (b).

According to Part 32, Section 450(2)

AOHS Part 32: 450(1) does not apply if a trench is constructed in solid rock throughout the entire trench.

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 442(1-3)

Soil is classified as “hard and compact”, “likely to crack or crumble” and “soft, sandy or loose”.

According to Part 32, Section 442(4)

If an excavation contains soil of more than one soil type, for the purposes of this Part an employer must operate as if all of it is the soil type with the least stability.