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OHS Guidelines Part 20 Construction, Excavation and Demolition - Guidelines Part 20 - Excavations - Fri May 27, 2016
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Guidelines Part 20
Guidelines Part 20 - Excavations
G20.78 Qualified registered professional and engineering documents
Issued August 1999; Editorial Revision February 7, 2006; Formerly Issued in G20.78(2) - Re-issued as G20.78 January 1, 2009
Section 20.78 of the OHS Regulation ("Regulation") states:
(1) Subject to this section, excavation work must be done in accordance with the written instructions of a qualified registered professional if (a) the excavation is more than 6 m (20 ft) deep,
(b) an improvement or structure is adjacent to the excavation,
(c) the excavation is subject to vibration or hydrostatic pressure likely to result in ground movement hazardous to workers, or
(d) the ground slopes away from the edge of the excavation at an angle steeper than a ratio of 3 horizontal to 1 vertical.
(2) Despite subsection (1), excavation work described in that subsection must be done in accordance with the written instructions of a professional engineer if the excavation requires or uses support structures.
(3) The written instructions required by this section must
(a) be certified by the qualified registered professional concerned,
(b) be available at the site, and
(c) specify the support and sloping requirements, and the subsurface conditions expected to be encountered.
The purpose of this guideline is to provide information about what is considered acceptable for written instructions required by section 20.78 of the Regulation. Written instructions
Verbal instructions from a qualified registered professional with no supporting documents are insufficient.
The following should be included as a minimum for a qualified registered professional's certificate on a site under this section:
Site address/location
Drawing/sketch, plan, and sections and/or clearly written instructions
Geotechnical description of the expected soil conditions, or confirmation upon site review
Limitations for machinery or equipment being adjacent to the excavation
Time period for which certification applies
Influence of changing weather conditions
Name of the certifying qualified registered professional, signature, and seal
Subsequent certifications may refer back to the initial certification documents, in which case such documents shall be available at the site. If conditions and/or instructions change with respect to the conduct of the excavation work, supplementary instructions and documentation are required.
If the certification is incomplete or deemed inadequate, work should stop in the hazard area until acceptable certification is available, or until remedial work is done so that the excavation complies with the Regulation.
G20.78(1)(c) Vibration, hydrostatic pressure or hazardous ground movement
Issued August 1999; Editorial Revision June 6, 2007; Formerly Issued as G20.78(1) - Re-issued as G20.78(1)(c) January 1, 2009
Section 20.78(1)(c) of the OHS Regulation ("Regulation") states:
(1) Subject to this section, excavation work must be done in accordance with the written instructions of a qualified registered professional if
(c) the excavation is subject to vibration or hydrostatic pressure likely to result in ground movement hazardous to workers,
The purpose of this guideline is to provide some explanation of the terms "subject to vibration" and "hydrostatic pressure likely to result in ground movement."
Explanation of "subject to vibration"
An excavation may be considered "subject to vibration" under section 20.78(1)(c) if there is activity such as heavy vehicle traffic, blasting, road compaction equipment, or compaction during backfill placement close to the excavation. The severity of the vibrations as well as the distance between the activity to the excavation shall be considered.
Hydrostatic pressure and hazardous ground movement
Hydrostatic pressure is a concern if water is coming out of the sides or base of an excavation. Engineering or other work done in accordance with the written instructions of a qualified registered professional is required unless an effective dewatering system can be implemented. If water can be prevented by the use of a dewatering system, hydrostatic pressure should not be a problem. Using a water pump to remove nominal surface runoff (such as from rainfall) should be acceptable without engineering or other work done in accordance with the written instructions of a qualified registered professional. If the soil adjacent to an excavation has undergone significant changes in moisture content, the stability of the excavation sides may be in question. Soil that is frozen, or may freeze due to the ambient air temperature during the excavation work, may cause development of hydrostatic pressure and thus such excavation work should only be undertaken following a qualified registered professional's instructions.
G20.78(1)(d) Ground slope adjacent to excavation work
Issued February 22, 2005; Formerly Issued as G20.78(1)(e) - Re-Issued as G20.78(1)(d) January 1, 2009
Section 20.78(1) of the OHS Regulation ("Regulation") states:
(1) Subject to this section, excavation work must be done in accordance with the written instructions of a qualified registered professional if (a) the excavation is more than 6 m (20 ft) deep, (b) an improvement or structure is adjacent to the excavation,
The requirements under section 20.78 address certain circumstances where the general sloping and shoring requirements under section 20.81 do not provide adequate protection for the safety of workers. In these special circumstances, a qualified registered professional is required to oversee assessment of the site and provide written instructions about performance of the work. This guideline discusses the circumstance set out in 20.78(1)(d)  as it applies to construction or maintenance work performed along an established road or other similar right of way.
Purpose of 20.78(1)(d)
The intent of section 20.78(1)(d) is to address the hazards arising from situations where the ground slopes up or down from the top edge of an excavation. Where the ground slopes up, the concern is for the increase in lateral ground pressure that arises due to the weight of soil positioned above a 3 horizontal to 1 vertical slope. This weight is an extra load that has not been allowed for in the sloping and shoring requirements specified in section 20.81 of the Regulation. Where the ground slopes down and away, the concern is the lack of adequate lateral support on the downhill side for any bracing specified in section 20.81 positioned against the downhill excavation face.
The control of surface runoff and soil erosion may also be a concern. These concerns arise particularly when the excavation work will be a side hill cut, bulk excavation, or a trench other than in a direction in line with the natural ground slope. Stated another way, if the excavation will generally be "across" the slope in a manner that will change the cross section profile of the slope, section 20.78(1)(d) will apply.
The slopes along a right of way will generally have been designed to provide stable faces to the extent necessary to balance safe and economical functioning of the right of way. Where the right of way was not so designed, but has been in place for at least a year (one full season cycle of rain and/or freezing weather as applicable for the geographical area), the slopes should have reached a natural, stable state. Any excavation activity that will alter the design profile, or naturally stable profile, of a cross section of a slope is within the scope of section 20.78(1)(d). Excavation work that does not change the cross section of the slope
Some maintenance or construction activities along a right of way may involve minor or localized excavation work that will not alter the "general" cross section profile of a slope in a manner that would affect the overall stability of the slope. Some examples of excavation activity of this type are Installing a utility pole, lamp standard, or signpost through use of an auger, drill, or similar device to dig a "post hole" or use of a back hoe or other equipment to dig a "bell hole"
Cleaning out a drainage ditch or clearing a buffer zone to restore the original design profile and grade
Installing or repairing a shallow culvert running generally in line with the natural ground slope (or perpendicular to the centre line of the right of way)
Shouldering work, profiling a road surface, or similar minor excavation work within a compacted roadway
Excavating a "bell hole" to repair a buried pipe or similar service
These excavations generally do not require any additional or special sloping or shoring considerations as the dimensions of the excavation are generally small enough that the natural "bridging action" of the soil provides enough support. Also, in most of these cases a worker does not enter into the actual excavation. These types of excavation activities generally do not require any written instructions from a qualified registered professional. It must be remembered however, that the hazard of loose material (such as rocks, logs, or trees) coming down from a slope above always needs to be evaluated and considered whenever work is being done near the base of the slope. Also, hazards may develop from weather conditions, such as extreme rainfall, or the potential for an avalanche. A "tailboard" or "tool box" meeting should be held with the crew to discuss the conditions in the planned work area prior to starting work. Loose material and weather condition hazards need to be assessed by a qualified person at least daily and preferably, before each work shift. The condition of the slope above must be examined and any loose material that could be a hazard should be removed or stabilized before work starts. If the daily assessment raises any concern regarding the overall stability of the slope, a qualified registered professional's assessment and advice should be obtained.
Excavation work that will change the cross section of the slope
There are some excavation activities related to servicing and maintaining a right of way that involve work at or near the base of an unstable or potentially unstable slope and will alter the overall cross section of the slope. Two examples of such activity are slope stabilization and the removal of rock or mud slide material. Prior to starting such work, a qualified registered professional should oversee the assessment of the conditions in the area to determine if there is a significant risk of substantial material flow (either immediately or as the work proceeds) that could endanger workers. If the qualified registered professional determines there is a significant risk, written instructions on how to proceed (covering aspects such as the sequence of work, selection of equipment, and work methods) need to be obtained from the qualified registered professional.
If the qualified registered professional determines there is only a minimal risk of a substantial material flow, the work plan can be developed without further input from the qualified registered professional. Work of this type generally involves powered excavating equipment that is substantial in size and affords protection for the operator in the event of any minor material flow. If there is a danger of logs, trees, or other debris coming down the slope, operators of mobile equipment must be protected by suitable cabs, screens, grills, shields, guards or structures (see section 16.21 of the Regulation). It is necessary that such operations be carried out, as far as is practicable, so that the height of any unstable face being worked does not exceed the maximum safe reach of the excavating equipment being used (see section 20.93 of the Regulation). Pedestrian workers should stay clear of the working face, any other unstable faces, and operating equipment.
Rock scaling is a form of excavation, but is not generally an activity that will change the cross section profile of a slope. Scaling of slopes is not an activity normally considered within the scope of section 20.78, and is specifically addressed in sections 20.96 to 20.101 of the Regulation. If there is a concern regarding the overall stability of a slope that is to be scaled, a qualified registered professional's assessment and instructions should be obtained and followed.
G20.79 Underground utilities
Issued August 1999; Editorial Revision August 2004; Revised February 13, 2006; Revised February 24, 2006; Revised August 11, 2010; Editorial Revision to include February 1, 2011 regulatory amendment
Section 20.79 of the OHS Regulation ("Regulation") states:
(1) Before excavating or drilling with powered tools and equipment, the location of all underground utility services in the area must be accurately determined, and any danger to workers from those utility services must be controlled.
(2) Excavation or drilling work in proximity to an underground utility service must be undertaken in conformity with the requirements of the owner of that utility service.
(3) Pointed tools must not be used to probe for underground petroleum and electrical utility services.
(4) Powered equipment used for excavating must be operated so as to avoid damage to underground utility services, or danger to workers.
Section 4.18 of the Regulation states:
The purpose of this guideline is to highlight the following:
Reference the practice document related to the requirements under section 20.79 of the Regulation
Describe the use of hydrovacing to expose underground utilities
Highlight some jurisdictional considerations Explain the occupational health and safety obligations of employers when their work activities result in a hit or damage to a pipeline, buried electrical cable, or other such utility WorkSafeBC's practice document
Prevention of Damage to Buried Facilities in British Columbia provides practices and procedures for excavating near buried facilities. The information in the document should be considered in conjunction with applicable legislation and regulations. Use of hydrovacing equipment for locating buried utilities
Hydrovacing is the use of pressurized water to liquefy and loosen soil, which is then removed by the use of on-truck vacuum systems and hoses. It can be an effective, efficient, and safe means of accurately locating and exposing ("daylighting") underground utility lines. When performed in conformity with the requirements, restrictions, and prohibitions of the utility owner, and following safe work procedures that adequately address hazards that workers may be exposed to, hydrovacing can be as safe as hand digging. It is important to have the 'locate information' and any necessary permits prior to hydrovacing. Note that the various utility owners and regulators (e.g., water, petroleum product, electrical, sanitary sewer, and steam) may have different requirements or prohibitions and employers performing hydrovacing need to be aware of and adhere to these rules.
Other legislation governing excavations near underground utilities
Jurisdictions other than WorkSafeBC also have statutes and regulations that apply to excavation or drilling in proximity to an underground utility service. For example, the Gas Safety Regulation specifies the means by which excavators who work near a gas installation must determine the existence of underground gas services and the methods that excavators must use to find the exact location of services. See Prevention Manual Policy Item D1-108-1 Application of Part 3 - Where Jurisdictional Limits Exist for guidance on how WorkSafeBC prevention officers will exercise their powers in situations where there are jurisdictional limits on those powers.
Regulation requirements if contact with any underground facility occurs
Notify the owner of the utility without delay
Section 4.18 of the Regulation requires that if work activities conducted by or on behalf of an employer cause a utility service to be hit or damaged, the employer must notify the owner of the utility service without delay.
Notify WorkSafeBC where required
Damage to underground facilities can cause death or serious injury to a worker. Section 172 of the Workers Compensation Act ("Act") specifies the accidents that an employer must immediately report to WorkSafeBC, including any accident that results in serious injury or death of a worker, or involves the major release of a hazardous substance. An important factor in determining whether there is a major release of a hazardous substance is the seriousness of risk to the health of workers that the release presents. Policy includes some factors that determine the seriousness of the risk. Situations involving natural gas that should be considered a "serious risk" include the following: It was necessary for people to be evacuated from buildings
Gas seeped into sewers or drains
Any person required medical treatment
The gas leak ignited
Repair workers entered the gas envelope when the atmosphere contained flammable gas or vapor concentrations in excess of 20% of the lower explosive limit (LEL)
A non-gas company worker entered an excavation, after a strike, to attempt to stop or slow the flow of gas
Policy Item D10-172-1 Accident Reporting and Investigation - Immediate Notice of Certain Accidents (Major Release of Hazardous Substance) discusses the meaning of the "major release of a hazardous substance" under section 172 of the Act.
Investigate the incident Under section 173 of the Act employers must immediately undertake an investigation into the cause of any accident or incident that had a potential for causing serious injury to a worker. Any gas line hit or high voltage contact is likely to fall into this category and an investigation is required.
Also, under section 173 of the Act the employer must investigate the occurrence of any accident that involved the major release of a hazardous substance. See section 173 of the Act for additional incidents that employers must investigate.
G20.81 Sloping and shoring requirements
Issued August 1999; Editorial Revision August 31, 2007; Editorial Revision January 1, 2009
Section 20.81(1) of the OHS Regulation ("Regulation") states:
Subject to section 20.78, before a worker enters any excavation over 1.2 m (4 ft) in depth or, while in the excavation, approaches closer to the side or bank than a distance equal to the depth of the excavation, the employer must ensure that the sides of the excavation are (a) sloped as specified in writing by a qualified registered professional, (b) sloped at angles, dependent on soil conditions, which will ensure stable faces, but in no case may the slope or combination of vertical cut and slope exceed that shown in Figure 20-1,
(c) benched as shown in Figure 20-2,
(d) supported as specified in writing by a professional engineer,
(e) supported in accordance with the minimum requirements of section 20.85, or
(f) supported by manufactured or prefabricated trench boxes or shoring cages, or other effective means.
The purpose of this guideline is to provide explanatory material for the Regulation requirements in sub-sections 20.81(a)–(f).
Explanatory information for subsections 20.81(a)–(f)
This section is subject to section 20.78, which requires written instructions from a qualified registered professional in certain situations.
"Excavation" is defined in section 20.1 as meaning any "cut, cavity, trench or depression in the earth's surface resulting from rock or soil removal." This includes, for example, a hole dug in the ground, as well as the cutting of material away from a slope, such as occurs on roadway construction.
Section 20.81 requires that one of clauses (a) to (f) be met in all cases where an excavation over 1.2 m (4 ft) deep has a width that is equal to or less than twice its depth. If the excavation is wider than twice its depth, this will only have to be done if the worker will approach "closer to the side or bank than a distance equal to the depth of the excavation." The latter includes situations where there is no measurable width because the excavation is, for example, a cut into a slope. In these situations, the worker must enter and exit the excavation by a safe route, for example, a properly sloped or shored portion of the excavation.
Section 20.81(1)(e) refers to section 20.85. Section 20.85, and by reference Table 20-1, contain detailed specifications for "trench support structures." A "trench" is defined in section 20.1 as "an excavation less than 3.7 m (12 ft) wide at the bottom, over 1.2 m (4 ft) deep, and of any length." Section 20.81(1)(c) and section 20.85 do not apply to excavations over 12 feet wide, or excavations that have no measurable width.
Section 20.81(1)(f) permits the use of manufactured or prefabricated trench boxes or shoring cages. These may be used in accordance with the manufacturer's instructions or engineering certification for the device, unless the instructions for use do not adequately include the circumstances of use or questions arising at the site, or where the situation is covered in the scope of section 20.78. The instructions for use and/or certification of a trench box or cage should specify the types of soil conditions for which it is intended and the instructions relating to each. A copy of this information should be on site.
Section 20.81(1)(f) also permits the use of "other effective means." Means of shoring other than "manufactured or prefabricated trench boxes or shoring cages" referred to under section 20.81(f) should be done in accordance with the written instructions of a professional engineer under section 20.81(d). G20.85 Trench support structures
Issued August 1999; Editorial Revision January 1, 2009
Section 20.85 of the OHS Regulation ("Regulation") states: (1) Trench support structures, other than those designed by a professional engineer, must comply with Table 20-1 for the following relevant soil conditions:
likely to crack or crumble
soft, sandy, filled or loose
(2) If Table 20-1 is to be used for a combination of supporting and sloping, the selection of shoring elements must be based on the overall depth of the excavation, and the arrangement must conform to Figure 20-3.
(3) Cross braces and trench jacks must be installed in a horizontal position and must be secured against dislodgement.
(4) The minimum number of cross braces at each cross bracing location is determined by the trench depth as follows:
Depth at location
Number of braces
up to 2.4 m (8 ft)
2.4 m to 3.7 m (8 ft to 12 ft)
3.7 m to 4.6 m (12 ft to 15 ft)
4.6 m to 6 m (15 ft to 20 ft)
(5) At each cross bracing location the cross braces must be less than 1.2 m (4 ft) apart, and the uppermost cross brace must be within 60 cm (2 ft) of ground level.
(7) Hydraulic or pneumatic trench jacks must have a means of ensuring that they will not collapse in the event of loss of internal pressure.
(8) Uprights must not spread outwards more than 15 degrees from the vertical when viewed along the trench.
(9) Plywood may be substituted for two inch thick shoring elements provided that
(a) the plywood is not less than 19 mm (3/4 in) thick,
(b) the trench is not over 2.7 m (9 ft) in depth,
(c) uprights are installed at not over 60 cm (2 ft) centres,
(d) cross braces do not bear directly on plywood, and
(e) cross braces bearing on uprights or walers are located at all joints in plywood sheathing.
The purpose of this guideline is to provide information about requirements for trench support structures and types of soil. The guideline also provides information about combining sloping and shoring. Trench support structures and soil type
Section 20.85 of the Regulation and its accompanying tables and figures set out detailed requirements for trench support structures required under section 20.81(1)(e). Any trench support structures not covered by these requirements, other than "manufactured or prefabricated trench boxes or shoring cages" allowed by section 20.81(f), must be covered by a professional engineer's certificate under section 20.81(d).
The requirements of section 20.85 depend on the type of soil. Type A soil is described in the section as "hard and solid." No soil is type A if it:
Is fissured
Is subject to vibration from heavy traffic, pile driving, or similar effects Has been previously disturbed
Is part of a sloped or layered system where the layers dip towards the excavation on a slope of 4H:1V or greater
The material is subject to factors that would require it to be classified as a less stable material
Type C soil is described in section 20.85 as "soft, sandy, filled or loose." It typically has a low, unconfined compressive strength. It can be
A granular soil, including gravel, sand, and loamy sand
Submerged rock that is not stable
Material in a sloped, layered system where the layers dip towards the excavation on a slope of 4H:1V or greater
A more detailed, technical classification of soil types is available in Occupational Safety and Health Administration (OSHA) Part 1926, Subpart P, Appendix A. Visual and testing parameters are provided for analysis.
Combination of sloping and shoring
Section 20.85(2) and Figure 20-3 specify an option if a combination of sloping and shoring is to be used. The following additional information may help clarify the relationship between H, the 1:1 slope, and the 1.5H horizontal dimension in Figure 20-3. If the broken reference line extending on a 1:1 slope from the toe or base of the excavation meets the surface of undisturbed ground within a distance of 1.5 times the depth of the trench, the trench support structures specified by section 20.85 can be used. If not, the original ground slopes upward steeper than a 1 vertical in 3 horizontal slope, and a professional engineer's certificate must be obtained under section 20.81(d) and followed.