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Excavation Safety Training

Excavation

Excavation work is hazardous. This guideline provides information on the potential hazards involved in excavation work so that workers and employers can work together to create a safe, injury and fatality free work site.

DEFINITIONS:

Excavation: means a dug out area of ground and includes a deep foundation excavation, trench, tunnel and shaft.
Open excavation means an excavation in which the width is greater than the depth, measured at the bottom.
Trench means an excavation that is deeper than its width measured at the bottom.
Trench Cage means a steel support structure designed to resist the pressure from the walls of a trench and capable of being moved as a unit.
Trench Jack means a screw or hydraulic jack used as a brace for a temporary support structure.
Shoring is an assembly of structural members designed to prevent earth or material from falling, sliding or rolling into an excavation.
Pile or Caisson means a slender,deep foundation unit made of materials or a combination of materials, such as wood, steel or concrete, which is either pre-manufactured and placed by driving, jacking, jetting or screwing, or cast in place in a hole formed by driving, excavation or boring.
Shaft means a vertical or inclined opening that leads to an underground working and is excavated below ground level.
Support structure means a temporary/permanent structure or device designed to provide protection to workers in an excavation, tunnel or shaft from cave-ins, collapse, sliding or rolling materials and includes shoring, bracing, piles, planks and trench cages.
Tunnel means a generally horizontal excavation that is more than a metre long and located underground.

HAZARDS TO WORKERS NEAR EXCAVATION SITES:

The most common hazards that exist in excavation work include:
  • cave-ins or excavation collapses
  • excavated material
  • falling objects or objects near an excavation
  • powered mobile equipment
  • slips, trips, and falls
  • hazardous atmospheres
  • flooding/water hazards
  • underground facilities

Cave-Ins or Excavation Collapses:

It is because both employers and workers often forget that when they remove earth from the ground it creates an opening, and the remaining earth surrounding the opening tends to relax. This increases the pressure towards the walls of the opening and makes the ground collapse. Water in the soil or ground also affects the stability of the walls by putting additional pressure on the walls & increasing the possibility of a cave in. Unless a horizontal distance equal to the vertical depth of the excavation walls is maintained, engineering controls must be used (ex: shoring, trench cages) to provide a safe and healthy workplace within the excavation area.

Excavated Material:

Injuries may also occur in excavation work when excavated material on the surface of the excavation is too close to the edge and falls into the excavation, or affects the structural stability of the walls of the excavation. Pile all excavated material so that the material cannot roll back into the excavation. The material must never be closer than one metre (three feet) from the edge of the excavation, and should be placed as far away as possible so it does not affect the structural stability of the walls. Ideally, the excavated material should be placed as far away from the edge of the vertical excavation as the excavation's height (d ³ h: see diagram below).

Falling Objects or Objects near an Excavation:

Place tools and equipment used at the excavation site so that they cannot fall into the excavation or affect the structural stability of the walls of the excavation. Use barriers to help keep tools and equipment at a safe distance from the edge of the excavation. Use ropes or other lowering devices to transport the tools or equipment into the excavation.

Powered Mobile Equipment

Workers may be exposed to hazards when powered mobile equipment is used near an excavation site. Powered mobile equipment includes backhoes, track hoes, concrete trucks, trucks removing excavated material and others. Common hazards related to powered mobile equipment include:
  • equipment placed close to the edge of an excavation may cause the excavation walls to become unstable. Powered mobile equipment can be placed near the edge of the excavation if a support structure, designed to consider the overload from the equipment, is installed in the excavation.
  • equipment vibration puts additional pressure on excavation walls, affecting the structural stability.
  • equipment operating on rough terrain, or too close to the edge of an excavation, may roll over and fall into the excavation. Ensure all equipment is equipped with roll over protective structures (ROPS).
  • workers riding on equipment without approved seats may be injured.
  • workers getting on and off equipment are at risk because balance can be affected by the vibration of the equipment. A worker may not be as sure footed getting off the equipment after operating it for a period of time.
  • workers may be injured by equipment. A safe distance must be maintained from moving equipment at all times. Operators must be aware of all workers near their work area.

Slips, Trips, and Falls

Slip, trip and fall hazards are common around excavations. Examples include:
excavation entrances and exits. A safe means of entering and exiting an excavation is required. Specifically, where an excavation is more than 1.5 metres (five feet) deep, a stairway, ramp or ladder is required. Workers must use both hands when climbing up or down ladders. Tools or equipment should not be carried up or down the ladder. In addition, the ladder must:
  • meet the standards outlined in  the Workplace Safety and Health Regulation, respecting Entrances, Exits, Stairways and Ladders.
  • extend one metre above the edge of the excavation
  • be located no more than three metres (10 feet) away from the workers inside the excavation.
uneven ground surfaces around or inside an excavation. It is important that a housekeeping program is in place and every effort is made to ensure walkways and pedestrian traffic areas are maintained.

Hazardous Atmospheres:

Hazardous atmospheres at excavation site may come from soils that are moved or from pipes and conduits disturbed during excavation. Where there is a potential for a hazardous atmosphere, a plan must be developed to ensure the workers in or near the excavation are not at risk. The plan must include the following steps:
Pre-Work Testing – The atmosphere must be tested before anyone enters the excavation to ensure they won't be exposed to hazards. Common atmospheric hazards include gasoline vapours, methane or other explosive gases and a lack of oxygen.
Periodic Testing – Periodic tests must be conducted to ensure the hazardous atmosphere is controlled and that workers are protected. All testing must be done by qualified personnel who have the knowledge and expertise required to keep workers safe. All tests conducted must be:
  • recorded
  • kept at the excavation site
  • made available to the workers upon request

Flooding/Water Hazards:

Because of the condition or location of excavation sites, water hazards may be present, including:
Flooding – An excavation may flood if the work is below the water table, near a watercourse bank or exposed to adverse weather conditions. When there is a risk of flooding, an emergency evacuation plan must be developed. The plan will include a full body harness with a lifeline (that meets the
requirements of Part 14 of the Workplace Safety and Health Regulation, relating to fall protection) to be worn by workers in the excavation. Workers must also have direct communication with the person located at the surface of the excavation.
Water Accumulation – This may be caused by an excavation near a ground water source, in wet conditions or because of equipment that uses water for operation near the excavation site. Water accumulation must be kept to a minimum to reduce risks such as slipping or tripping hazards, electrical hazards, equipment malfunctions or others.

Underground Facilities

Common underground facilities include:
  • electrical lines
  • oil and gas lines
  • telecommunication lines (phone, cable, computer)
  • water and sewer lines
  • traffic signal lines
  • steam lines
Special attention must be given to the hazards associated with underground facilities. Before beginning excavation work, proper planning must identify the location of underground facilities and any precautions needed to avoid contact with these facilities.

SOIL PROPERTIES:

Category 1 – Cohesive soils of firm to stiff consistency that are fissured (Category 1b) or unfissured (Category 1a). These soils are generally of medium to high plasticity but may also include glacial clay tills of low to medium plasticity. These soils usually have low moisture content and most often occur above the water table.

Category 2 – Cohesive soils of soft consistency and non cohesive silt soils. The cohesive soils can be of medium to high plasticity while the silt soils are of non to low plasticity. These soils typically have high moisture contents and will tend to fill voids left between the excavation walls and shoring.
Category 3 – Cohesionless soils of loose to medium dense (Category 3a) and dense to very dense (Category 3b) consistencies. The Category 3a soils are generally easy to excavate by hand and are easily disturbed by construction equipment, particularly when they are at or near the water table or become saturated.Category 3b soils are generally not easy to excavate by hand and are not easily disturbed by construction equipment, except if they are at or near the water table or become saturated.

Soil Types:

Cohesive Soils:
Silty clay – soil of medium to high plasticity of primarily lacustrine origin. The silty clay can range from soft to hard depending on the moisture content and is usually brown in the upper six to 10 metrs and grey below indicating the extent of previous oxidation and weathering. Typically, the upper three meters of lacustrine clay is weathered, fissured and nuggety.
Alluvial Clay – soil of medium plasticity, although plasticity can range from low to high. Alluvial clay can vary greatly in grain size distribution and consistency, but generally, the major constituent of this soil type is silt, followed by clay and then sand. Alluvial clay can range from very soft to stiff,
depending on moisture content. In a dry state, the soil may often appear to be cohesion less, while in a wet state, alluvial clay is often very soft and subject to sloughing.
Glacial clay till – heterogeneous mixture of boulders, cobbles, gravel, sand, silt and clay, generally of low to medium plasticity.Glacial clay till can vary from soft to hard,primarily dependent on moisture content and deposition characteristics.
Cohesionless Soils:
Silt – soil that is non-plastic to low plastic. Silt ranges from loose to extremely dense depending on moisture content and deposition characteristics. Silt is seldom encountered in a pure state, but normally has a significant fine sand component and occasionally a trace of some clay
Sand – sand can range greatly in grain size and density, and is often poorly graded (sorted). Typically, saturated sand exhibits a dilate behavior (fine grained sand), unstable with respect to excavations, and is subject to sloughing.
Gravel – like sand, gravel can have a wide range of grain size distribution and density. In a dry state, gravel is generally more stablethan sand (although still somewhat unstable) in vertical cuts, but still requires sloped excavation walls. Typically, on saturation, gravel becomes unstable (although less so than other cohesionless soils) with respect to excavation, and is subject to sloughing,
Glacial silt till – soil that is non-plastic to low plastic. Glacial silt till is a heterogeneous mixture of boulders, cobbles, gravel, sand, silt and clay.
Fill – fill can be a single soil type or a mixture of various soil types such as clay, sand, gravel, organic soils, etc. and may even contain non-soil materials such as demolition rubble or wood. It can vary widely in consistency, but is often softer or looser than the surrounding native soil, and has a greater likelihood of sloughing when encountered in excavations. In particular, it cannot be relied upon to be uniform, even over short vertical and horizontal distances, and may collapse in any one of several different modes, depending on its makeup.

Bedrock

The limestone and shale bedrocks are typically highly weathered and fractured when at or near the natural ground surface, but often become less fractured and more intact with depth.                     
The shale bedrocks are generally considered to be soft rock while limestone can vary from soft to hard. Generally, limestone bedrock is stable to very stable with respect to excavations or rock cuts. Similarly, shale bedrock can also be table to very stable but, in instances where the shale is fractured or contains existing failure planes, the shale can perform poorly.

Types of Soil Collapse:

General Zone of Exposure
(the area where workers are exposed to mass soil or rock movement)
Collapse Type 1
Spoil pile slide – improper excavating procedures occur when the excavated material is not placed far enough away from the edge of the excavation. The recommended minimum distance for location of excavated soil (spoil) from the edge of the excavation is equal to or greater than the excavation depth. However, the minimum permissible distance of spoil from the edge of the excavation is 0.6 metres for every one metre of excavation depth.
Collapse Type 2
Side wall shear – common to fissured or desiccated clay-type or alluvial soils that are exposed to drying.



Collapse Type 3
Slough-in (cave-in) – common to previously excavated material, fill, sand, silt and sand mix and gravel mix where the water table is above the base of excavation, or where soils are organic or peat.

Collapse Type 4
Rotation – common in clay-type soils when excavation walls are too steep, or when moisture content increases rapidly.

Soil is test:

A competent person must conduct visual and manual soil tests before anyone enters an excavation. Visual and manual tests are a critical part of determining the type of protective system that will be used.
Visual tests: Visual testing involves looking at the soil and the area around the excavation site for signs of instability. The competent person might do visual tests such as the following:
Observe the soil as it is excavated. Soil that remains in large clumps when excavated may be cohesive. Soil that breaks up easily is granular.
  • Examine the particle sizes of excavated soil to determine how they hold together.
  • Look for cracks or fissures in the faces of the excavation.
  • Look for layers of different soil types and the angle of the layers in the face of the excavation that  may indicate instability.
  • Look for water seeping from the sides of the excavation.
  • Look for signs of previously disturbed soil from other construction or excavation work.
  • Consider vibration from construction activity or highway traffic that may affect the stability of the excavation.
Manual tests:
Manual testing involves evaluating a sample of soil from the excavation to determine qualities such as cohesiveness, granularity, and unconfined compressive strength. Soil can be tested either on site or off site but should be tested as soon as possible to preserve its natural moisture.
Plasticity test: Shape a sample of moist soil into a ball and try to roll it into threads about 1/8-inch indiameter. Cohesive soil will roll into 1/8-inch threads without crumbling.
Dry strength test: Hold a dry soil sample in your hand. If the soil is dry and crumbles on its own or with moderate pressure into individual grains or fine powder, it’s granular. If the soil breaks into clumps that are hard to break into smaller clumps, it may be clay combined with gravel, sand, or silt.
Thumb penetration test: This test roughly estimates the unconfined compressive strength of a sample. Press your thumb into the soil sample. If the sample resists hard pressure it may be type A soil. If it’s easy to penetrate, the sample may be type C.
A support structure (shoring) is required, or the excavation walls must be sloped at an appropriate angle, before a worker enters an excavation considered to be:
A trench excavation exceeding 1.5 meters (five feet) in depth.
An open excavation is a man-made cut, cavity, or depression in the earth’s surface.
           Trench (d>w) Excavation                                                      Open (w>d)Excavation
An openexcavation is any excavation that does not meet the criteria of being a trench, shaft, tunnel or caisson.

Sloped Excavations:

Fully sloped (Vee’d) excavations besides use of a shoring support structure, a safe method to protect workers in an excavation is to slope the walls of the excavations at a grade of 1H:1V (45 degrees) or flatter. The 45-degree slope is required no matter what type of soil conditions exist.

Combination slope and vertical face – A combination 1H:1V (45-degree) slope and vertical face may be used in some soils, as long as the vertical face does not exceed one metre (three feet), the overall depth of the excavation is not greater than five metres (16 feet), and where the soil is not subject to sloughing when saturated (ex: silt, sand, alluvial clay).

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