By Expert No. 8901, P.E., C.I.H., C.S.P.

Excavation and trenching may present substantial risk of injury to person working in the excavation or adjacent to the edge. Safety guidelines have been developed through years of experience working with and in excavation. Many of these guidelines have been adopted by OSHA and have been made mandatory for all trenching and excavating activities.


Many jurisdictions require a permit before commencing large excavation activities to ensure the safety not only of the workers but also of the community. The permit notifies the regulatory agency of the excavation activity and requires the organization doing the digging to place a competent and knowledgeable person in charge of the dig. The permit may be specific for a single excavation or may be issued on an annual basis for a firm that does many excavations.

Competent Person

The competent person has the responsibility to examine the area of the excavation to determine that no recognizable conditions exist which would expose employees to injury from possible moving ground before work is permitted in or adjacent to the excavation. The excavation must not be opened until the competent person has determined that it is safe to do so. The competent person must inspect the excavation at the beginning of each day before workers are exposed to ensure that no changed condition, such as water entering the excavation, has increased the danger. Excavations must be inspected by the competent person after every rainstorm or other hazard-increasing occurrence and the protection against slides and cave-ins increased, if necessary, before excavation activities resume.

Below Grade Hazards

Before opening the excavation, the location of all underground installations such as sewer, water, fuel, electric lines, telecommunication lines, tanks, etc., must be located and marked so that they may be avoided or approached with caution. Many regions have a single telephone number that will alert the owners or managers of underground installations that an excavation is intended that may impact their facility or equipment. The operator of that underground installation then is required to mark on the ground their approximate location.

Surface Encumbrance

Trees, boulders, poles, structures, and other surface encumbrances that create a hazard or are endangered by the excavation must be avoided or relocated or made safe before opening the excavation. Additional shoring may be required to keep an object on the surface from being damaged or falling into the excavation. No excavation work may take place below the level of the base of an adjacent foundation, retaining wall, or other structure until it has been determined by the competent person that such excavation will in no way create a hazard to workers or until adequate safety measures have been taken for the protection of workers. Undermined sidewalks and pavements must be supported to safely carry all anticipated loads. If the stability of adjoining buildings or wall is endangered by excavations, either shoring, bracing, underpinning, or some other method affording equivalent protection for workers must be provided as necessary to ensure their safety. The competent person must inspect all shoring, bracing and underpinning systems daily or more often, as conditions warrant, to ensure that protection is effectively maintained.

Special safety provisions consisting of additional bracing or other effective means may be required at excavations adjacent to streets, railroads, or sources of external vibrations or superimposed loads. Similar provisions may be required in excavations made in disturbed soil and areas that have been previously filled.


Excavated material must be prevented from falling back into the area where employees are working. This is accomplished by locating the spoil at a distance from the edge of the excavation consistent with the character of the materials and the nature of the operations, but unless otherwise contained, experience has shown that the excavated material should be placed at least 2 feet from the edge of excavations 5 feet or more in depth. For lesser depths, a 1-foot clearance is generally sufficient. Not only does this prevent the spoils from falling back into the excavation, but reduces the load on the edge of the excavation. Loading the edge of the excavation may cause the edge to fail and fall into the excavation. Do not use a spoils containment method that disturbs the soil, such as driving stakes, as this weakens the edge and may lead to failure and cave in.


To avoid falling and injury, trenches should only be crossed where safe crossings have been provided. Walkways and bridges should be provided with standard guardrails and toeboards. Toeboards are particularly important when people are working in the excavation.

Excavating Equipment

Employees working in the vicinity of operating excavating equipment must be aware of the movement of the equipment and should only be in work areas that are safe from falling into or otherwise contacting the machine’s moving parts. Stop logs or barricades are needed when mobile equipment is used or allowed to operate adjacent to excavations to prevent the equipment from rolling over the edge into the excavation. The grade should always be away from the excavation.

Retaining Walls

No existing wall or other structure must be made by reason of an excavation or backfill, to function as a retaining wall until it has been determined that such wall will safely withstand all expected loads that otherwise might be a source of hazard to workers.


Diversion ditches, dikes, or other suitable means should be used to prevent water from entering an excavation and for drainage of the excavation. Accumulations of water in excavations that endanger the stability of those excavations or pose a hazard to workers must be controlled before further work proceeds. Dusty conditions during excavation should be kept to a minimum. Water used to control dust must not be allowed to accumulate in the excavation or be applied excessively to weaken the soil.

Trench Entry

Entering a trench or excavation presents many hazards. Appropriate precaution must be exercised to protect from being buried or inhaling a hazardous atmosphere whenever entering an excavation or trench. Work in an excavation should at all times be under the immediate supervision of the competent person who is authorized to modify the shoring or sloping to protect their safety.


Any trench more than 4 feet in depth should be provided with a safe means of access located no more than 25 feet of lateral travel from where the work area. The access may be a ramp at least 20 inches in width and sloped no steeper than 2 feet rise in 10 feet. If access is provided by a ladder, the ladder should be secured at the top to prevent displacement. Ladders with broken or missing rungs or steps, broken or split side rails, or other faulty or defective construction must be immediately removed from service and removed from the job site. Workers should not be carried in or on buckets, forklifts, or any other machinery not designed for transportation of personnel.

Cave In Protection

Any trench more than 4 feet in depth should be protected by a system of shoring, sloping of the ground, or benching, or other alternate means. Protection for employees who work in excavations less than 4 feet in depth is also needed when the worker’s head is below the edge of the excavation or when examination by the competent person indicates that hazardous ground movement may be expected. A registered civil engineer should design the protection system for any trench more than 20 feet in depth.

Atmospheric Hazards

The atmosphere in the excavation must be tested before entry to ensure the oxygen concentration is between 20 % and 25 %, the combustible gas concentration is less than 20 % of the lower explosive limit (LEL) and for any other hazardous contaminant that is likely to be present. Where airborne contaminants are likely, monitoring the atmosphere in the trench must be continuous. Contaminated soil and naturally occurring hazards can cause the concentration of hazardous gases such as hydrogen sulfide and carbon monoxide to change rapidly.

Expert No. 8901, PE, CIH, CSP is a Licensed Safety Engineer, Certified Industrial Hygienist and Certified Safety Professional with more than 30 years experience in accident investigation systems in industrial facilities, exposure assessment strategies, reconstructing exposures, Cal/OSHA, industrial and environmental hygiene. He has thorough knowledge of federal and state environmental and worker health and safety regulations and has managed numerous programs to evaluate and ensure compliance with these regulations. He has taught at UCLA and at Cal State – Dominguez Hills.

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