Environmental Health & Safety Manual
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17 - EXCAVATION AND TRENCHING
"Competent person" means one who is capable of identifying existing and predictable hazards in the surroundings, or working conditions which are unsanitary, hazardous, or dangerous to employees, and who has authorization to take prompt corrective measures to eliminate them.
"Excavation" means any man-made cut, cavity, trench, or depression in an earth surface, formed by earth removal.
"Trench (Trench excavation)" means a narrow excavation (in relation to its length) made below the surface of the ground. In general, the depth is greater than the width, but the width of a trench (measured at the bottom) is not greater than 15 feet (4.6 m). If forms or other structures are installed or constructed in an excavation so as to reduce the dimension measured from the forms or structure to the side of the excavation to 15 feet (4.6 m) or less (measured at the bottom of the excavation), the excavation is also considered to be a trench.
- GENERAL REQUIREMENTS
Many on-the-job-accidents are a direct result of inadequate up-front planning. Correcting mistakes in shoring and/or sloping after work has begun slows down the operation, adds to the cost, and increases the possibility of an excavation failure.
Before starting a job, it is a good idea to develop a safety checklist and have all items that are needed on hand. The following site conditions should be taken into account:
- Nearness of structures and their conditions
- Surface and ground water
- The water table
- Overhead and underground utilities
- SPECIFIC EXCAVATION REQUIREMENTS
- Surface Obstacles
Remove obstacles on the surface that may create a hazard to people.
- Underground Installations
Before starting an excavation, estimate the location of utility installations such as sewer, telephone, fuel, electric, water lines, or any other underground installations. Contact utility companies within established response times. (Note that on campus "Miss Utility" will only locate telephone lines.) If the exact location of the utility installations cannot be found, proceed with caution, using detection equipment or other acceptable means to locate the utility installations.
When excavation operations approach the estimated location of underground installations, determine the exact location of the installations by safe and acceptable means. While the excavation is open, protect, support or remove underground installations as necessary to safeguard employees.
- Access and Egress
Construct ramps for access to or exit from excavations in accordance with designs developed by a person competent to do so.
When constructing ramps and runways, use structural members of the same thickness and connect the structural members together to prevent movement. Attach members used to hold runway parts together on the bottom of the ramp or runway. To prevent slipping, attach cleats to structural ramps used in place of steps.
Provide a stairway, ladder, ramp or other safe means of exiting trench excavations that are 4 feet (1.22 m) or more in depth, so as to require no more than 25 feet (7.62 m) of lateral travel for employees.
- Vehicular Traffic
Provide employees exposed to public vehicular traffic with warning vests or other suitable garments marked with or made of reflectorized or high-visibility material.
- Falling Loads
Do not allow employees to be underneath loads handled by lifting or digging equipment. Stand away from any vehicle being loaded or unloaded to avoid being struck by any spillage or falling materials. Operators may remain in the cabs of vehicles being loaded or unloaded when the vehicles have adequate protection for the operator during loading and unloading operations.
- Warning System for Mobile Equipment
When operating mobile equipment next to an excavation or when approaching the edge of an excavation, and the operator does not have a clear and direct view of the edge of the excavation, use a warning system such as barricades, hand or mechanical signals, or stop logs. If possible, the grade should be away from the excavation.
- Hazardous Atmospheres
It is necessary to consider that there may be harmful levels of atmospheric contaminants in an excavation, and an acceptable breathing atmosphere must be present prior to entry.
To assure acceptable atmospheric conditions, do the following:
- Before entering an excavation greater than 4 feet (1.22 m) deep, test the atmosphere where oxygen deficiency (less than 19.5 percent oxygen) or a hazardous atmosphere exists or could reasonably be expected to exist. An example is an excavation in an area where hazardous substances are stored nearby. Enter an oxygen deficient or other hazardous atmosphere only if wearing proper respiratory protective equipment or only after adequately ventilating the space.
- In an atmosphere that contains flammable gas concentration in excess of 20 percent of the lower flammable limit, take adequate precautions such as providing ventilation.
- When using controls to reduce the level of atmospheric contaminants to acceptable levels, conduct testing as often as necessary to ensure that the atmosphere remains safe.
- Emergency Rescue Equipment
Have emergency rescue equipment readily available where hazardous atmospheric conditions exist or may reasonably be expected to develop during work in an excavation. Emergency rescue equipment includes items such as a breathing apparatus, a safety harness and line, or a basket stretcher. When entering bell-bottom pier holes, or other similar deep and confined footing excavations, wear a harness with a lifeline securely attached to it. Lifelines must be separate from any line used to handle materials, and must be individually attended at all times while the person wearing the lifeline is in the excavation.
- Protecting from Water Accumulation Hazards
Do not work in an excavation in which there is accumulated water or in which water is accumulating, unless adequate precautions are taken. The precautions would vary with each situation, but could include special support or shield systems to protect from cave-ins, water removal to control the level of accumulating water, or use of a safety harness and lifeline. If water is controlled or prevented from accumulating by the use of water removal equipment, monitor the equipment and operations to ensure proper operation.
If excavation work interrupts the natural drainage of surface water (such as streams), use diversion ditches, dikes, or other suitable means to prevent surface water from entering the excavation and to adequately drain the adjacent area. Excavations subject to runoff from heavy rains require an inspection by a competent person to evaluate water accumulation hazards.
- Stability of Adjacent Structures
Where the stability of adjoining buildings, walls, or other structures is endangered by excavation operations, provide support systems such as shoring, bracing, or underpinning to ensure the stability of such structures.
Do not excavate below the level of the base or footing of any foundation or retaining wall if it could pose a hazard unless:
- A support system, such as underpinning, is provided to ensure the safety of employees and the stability of the structure;
- The excavation is in stable rock;
- A registered professional engineer has determined that the structure is sufficiently removed from the excavation so as to be unaffected by the excavation activity, or that such excavation work will not pose a hazard to employees.
Do not undermine sidewalks, pavements, etc., unless a support system or other protection is provided to protect employees from the collapse of such structures.
- Protecting Employees from Loose Rock or Soil
Safeguard employees from loose rock or soil that could fall or roll from an excavation face. This protection must consist of scaling to remove loose material; installation of protective barricades at intervals as necessary on the face to stop and contain falling material; or other means that provide equivalent protection.
Protect employees from excavated or other materials or equipment that could fall or roll into excavations. Place and keep such materials or equipment at least 2 feet (.61 m) from the edge of excavations, or use retaining devices that will prevent materials or equipment from falling or rolling into excavations, or combine both if necessary.
On a daily basis, a competent person must inspect excavations, the adjacent areas, and protective systems for evidence of a situation that could result in possible cave-ins, indications of failure of protective systems, hazardous atmospheres, or other hazardous conditions. Inspections must be conducted prior to the start of work and as needed throughout the shift. Inspections must also be made after every rainstorm or after any other occurrence that may increase hazards. These inspections are only required when employee exposure is anticipated.
If the competent person finds evidence of a situation that could result in a possible cave-in, indications of failure of protective systems, hazardous atmospheres, or other hazardous conditions, remove exposed employees from the hazardous area until the necessary precautions are taken to ensure safety.
- Fall protection
Install walkways where employees or equipment will cross over excavations. Provide guardrails where walkways are 6 feet (1.8 m) or more above lower levels.
Provide barriers at all remotely located excavations. Barricade or cover all wells, pits, shafts, etc. Backfill temporary wells, pits, shafts, etc., upon completion of exploration and other similar operations.
- PROTECTIVE SYSTEMS
- Protecting Employees in Excavations
Protect each employee in an excavation from cave-ins with a well-designed protective system except when the excavations are:
- Made entirely in stable rock, or
- Less than 5 feet (1.52 m) in depth and examination of the ground by a competent person provides no indication of a potential cave-in.
- Design of Sloping/Benching, Support, Shield, and Other Protective Systems
Designing a protective system can be complex because of the number of factors involved- soil classification, depth of cut, water content of soil, changes due to weather and climate, or other operations in the vicinity.
When constructing protective systems for excavations, consult OSHA Standard 29 CFR 1926.652, Appendices A through F for guidance. These appendices include information on the following:
- Appendix A: Soil classification
- Appendix B: Sloping and benching
- Appendix C: Timber shoring for trenches
- Appendix D: Aluminum hydraulic shoring for trenches
- Appendix E: Alternatives to timber shoring
The following figures summarize the requirements contained in the OSHA standard for excavations 20 feet or less in depth. Protective systems for use in excavations more than 20 feet in depth must be designed by a registered professional engineer.
- Materials and Equipment
For protective systems, use materials and equipment that are free from damage or defects that might impair their proper function. When using manufactured materials and equipment, use and maintain them in a manner that is consistent with the recommendations of the manufacturer and in a manner that will prevent employee exposure to hazards.
When material or equipment that is used for protective systems is damaged, a competent person must examine the material or equipment and evaluate its suitability for use. If the competent person cannot assure the material or equipment is able to support the intended loads or is otherwise suitable for safe use, then the material or equipment must be removed from service. It must then be evaluated and approved by a registered professional engineer before being returned to service.
- Installation and Removal of Support
Securely connect members of support systems together to prevent sliding, falling, kickouts, or other failure. Install and remove the systems in a way that protects employees from cave-ins, structural collapses, or from being struck by members of the support system. Do not exceed design loads on individual members of support systems.
Before beginning temporary removal of individual members, take additional precautions to ensure the safety of employees, such as installing other structural members to carry the loads imposed on the support system. Begin removal and progress from the bottom of the excavation. Release members slowly so as to note any indication of possible failure of the remaining members of the structure or possible cave-in of the sides of the excavation. Backfill as support systems are removed from an excavation.
At times it is acceptable to excavate material to a level no greater than 2 feet (0.61 m) below the bottom of the members of a support system. This applies only if the system is designed to resist the forces calculated for the full depth of the trench, and there are no indications while the trench is open of a possible loss of soil from behind or below the bottom of the support system.
Closely coordinate installation of support systems with the excavation of trenches.
- Sloping and Benching Systems
Do not work on the faces of sloped or benched excavations at levels above other employees except when employees at the lower levels are adequately protected from the hazard of falling, rolling, or sliding material or equipment.
- Shield Systems
Do not place loads on shield systems that exceed design. Install shields so as to restrict lateral or other hazardous movement of the shield in the event of the application of sudden lateral loads.
Protect employees from the hazard of cave-ins when entering or exiting the areas protected by shields. Do not enter shields when they are being installed, removed, or moved vertically.
It is sometimes acceptable to excavate material to a level no greater than 2 feet (0.61 m) below the bottom of a shield system. This is true only if the shield is designed to resist the forces calculated for the full depth of the trench, and there are no indications while the trench is open of a possible loss of soil from behind or below the bottom of the shield.