Excavation and trenching - Washington

The state of Washington operates an OSHA-approved State Plan covering most private sector workers and all state and local government workers.

Washington’s Division of Occupational Safety and Health (DOSH) has adopted most Federal OSHA standards that relate to state and local government and private sector workplace operations by reference. However, there are unique state regulatory standards in the areas of excavation, trenching, and shoring. These unique standards include requirements that differ from the federal standards.

KEY DEFINITIONS

Excavation. Any person-made cut, cavity, trench, or depression in the earth's surface, formed by earth removal.

Hazardous atmosphere. An atmosphere which by reason of being explosive, flammable, poisonous, corrosive, oxidizing, irritating, oxygen deficient, toxic, or otherwise harmful, may cause death, illness, or injury.

Protective system. A method of protecting employees from cave-ins, from material that could fall or roll from an excavation face or into an excavation, or from the collapse of adjacent structures. Protective systems include support systems, sloping and benching systems, shield systems, and other systems that provide the necessary protection.

Shoring (shoring system). A structure such as a metal hydraulic, mechanical, or timber shoring system that supports the sides of an excavation and which is designed to prevent cave-ins.

Sloping (sloping system). A method of protecting employees from cave-ins by excavating to form sides of an excavation that are inclined away from the excavation to prevent cave-ins. The angle of incline required to prevent a cave-in varies with differences in such factors as the soil type, environmental conditions of exposure, and application of surcharge loads.

Support system. A structure such as underpinning, bracing or shoring, which provides support to an adjacent structure, underground installation, or the sides of an excavation.

Trench (trench excavation). 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. 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 or less (measured at the bottom of the excavation), the excavation is also considered to be a trench.

GENERAL PROTECTION REQUIREMENTS

Surface encumbrances. You must remove or support surface encumbrances that are located so as to create a hazard to employees, as necessary, to safeguard employees.

Underground installations:

• You must locate utility installations, such as sewer, telephone, fuel, electric, water lines, or any other underground installations that reasonably may be expected to be encountered during excavation work, prior to opening an excavation.
• You must contact utility companies or owners within established or customary local response times, advised of the proposed work, and asked to locate the underground utility installation prior to the start of actual excavation.
• When excavation operations approach the location of underground installations, you must determine the exact location of the installations by safe and acceptable means.
• While the excavation is open, you must protect underground installations, supported, or removed as necessary to safeguard employees.

Access and egress:

• Structural ramps that are used solely by employees as a means of access or egress from excavations must be designed by a competent person. Structural ramps used for access or egress of equipment must be designed by a competent person qualified in structural design and must be constructed in accordance with the design.
• Ramps and runways constructed of two or more structural members must have the structural members connected together to prevent displacement.
• Structural members used for ramps and runways must be of uniform thickness. Cleats or other appropriate means used to connect runway structural members must be attached to the bottom of the runway or must be attached in a manner to prevent tripping.
• Structural ramps used in lieu of steps must be provided with cleats or other surface treatments on the top surface to prevent slipping.
• A stairway, ladder, ramp or other safe means of egress must be located in trench excavations that are four feet or more in depth so as to require no more than 25 feet of lateral travel for employees.

Exposure to vehicular traffic. Employees exposed to vehicular traffic must be provided with high-visibility garments meeting the requirements of WAC 296-155-200, General requirements for personal protective equipment (PPE).

Exposure to falling loads. Employees may not be permitted to be underneath loads handled by lifting or digging equipment. Employees must 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 are equipped, in accordance with WAC 296-155-610 (2)(g), to provide adequate protection for the operator during loading and unloading operations.

Warning system for mobile equipment. When mobile equipment is operated adjacent to an excavation, or when such equipment is required to approach the edge of an excavation, and the operator does not have a clear and direct view of the edge of the excavation, you must utilize 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:

• Testing and controls. In addition to the requirements set forth in parts B-1 and C (296-155 WAC) and chapter 296-880 WAC to prevent exposure to harmful levels of atmospheric contaminants and to assure acceptable atmospheric conditions, the following requirements apply:
  • Where oxygen deficiency (atmospheres containing less than 19.5 percent oxygen) or a hazardous atmosphere exists or could reasonably be expected to exist, such as in excavations in landfill areas or excavations in areas where hazardous substances are stored nearby, you must test the atmospheres in the excavation before employees enter excavations greater than 4 feet in depth.
  • You must take adequate precautions to prevent employee exposure to atmospheres containing less than 19.5 percent oxygen and other hazardous atmospheres. These precautions include providing proper respiratory protection or ventilation in accordance with chapter 296-842 WAC.
  • You must take adequate precautions such as providing ventilation, to prevent employee exposure to an atmosphere containing a concentration of a flammable gas greater than 10 percent of the lower flammable limit of the gas.
  • When controls are used that are intended to reduce the level of atmospheric contaminants to acceptable levels, you must conduct testing as often as necessary to ensure that the atmosphere remains safe.

• Emergency rescue equipment. Emergency rescue equipment, such as breathing apparatus, a safety harness and line, or a basket stretcher, must be readily available where hazardous atmospheric conditions exist or may reasonably be expected to develop during work in an excavation. This equipment must be attended to when in use.
  • Employees entering bell-bottom pier holes, or other similar deep and confined footing excavations, must wear a harness with a lifeline securely attached to it. The lifeline must be separate from any line used to handle materials and must be individually attended at all times while the employee wearing the lifeline is in the excavation.
  • Note: See chapter 296-62 WAC, Part M, for additional requirements applicable to confined space operations.

Protection from hazards associated with water accumulation. Employees must not work in excavations in which there is accumulated water, or in excavations in which water is accumulating, unless adequate precautions have been taken to protect employees against the hazards posed by water accumulation.

• The precautions necessary to protect employees adequately 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, the water removal equipment and operations must be monitored by a competent person to ensure proper operation.
• If excavation work interrupts the natural drainage of surface water (such as streams), you must use diversion ditches, dikes, or other suitable means to prevent surface water from entering the excavation and to provide adequate drainage of the area adjacent to the excavation.
• Excavations subject to runoff from heavy rains will require an inspection by a competent person.

Stability of adjacent structures:

• Where the stability of adjoining buildings, walls, or other structures is endangered by excavation operations, you must provide support systems such as shoring, bracing, or underpinning to ensure the stability of such structures for the protection of employees.
• You must not permit excavation below the level of the base or footing of any foundation or retaining wall that could be reasonably expected to pose a hazard to employees except when:
  • A support system, such as underpinning, is provided to ensure the safety of employees and the stability of the structure; or
  • The excavation is in stable rock; or
  • A registered professional engineer has approved the determination that the structure is sufficiently removed from the excavation to be unaffected by the excavation activity; or
  • A registered professional engineer has approved the determination that such excavation work will not pose a hazard to employees.
• Sidewalks, pavements, and appurtenant structures must not be undermined unless a support system or another method of protection is provided to protect employees from the possible collapse of such structures.

Protection of employees from loose rock or soil. You must provide adequate protection to protect employees from loose rock or soil that could pose a hazard by falling or rolling from an excavation face. Such 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.

You must protect employees from excavated or other materials or equipment that could pose a hazard by falling or rolling into excavations. Protection must be provided by placing and keeping such materials or equipment at least two feet from the edge of excavations, or by using retaining devices that are sufficient to prevent materials or equipment from falling or rolling into excavations, or by a combination of both if necessary.

Inspections. Daily inspections of excavations, the adjacent areas, and protective systems must be made by a competent person for evidence of a situation that could result in possible cave-ins, indications of failure of protective systems, hazardous atmospheres, or other hazardous conditions.

An inspection must be conducted by the competent person prior to the start of work and as needed throughout the shift. Inspections must also be made after every rainstorm or other hazard increasing occurrence. These inspections are only required when employee exposure can be reasonably anticipated.

Where 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, you must remove exposed employees from the hazardous area until the necessary precautions have been taken to ensure their safety.

Fall protection. You must provide walkways where employees or equipment are required or permitted to cross over excavations. You must provide guardrails which comply with chapter 296-880 WAC, Unified safety standards for fall protection where walkways are 4 feet or more above lower levels.

You must provide adequate barrier physical protection at all remotely located excavations. You must barricade or cover all wells, pits, shafts, etc. Upon completion of exploration and similar operations, you must backfill temporary wells, pits, shafts, etc.

REQUIREMENTS FOR PROTECTIVE SYSTEMS

Protection of employees in excavations. Each employee in an excavation must be protected from cave-ins by an adequate protective system except when:

• Excavations are made entirely in stable rock; or
• Excavations are less than four feet in depth, and examination of the ground by a competent person provides no indication of a potential cave-in.

Protective systems must have the capacity to resist without failure all loads that are intended or could reasonably be expected to be applied or transmitted to the system.

Design of sloping and benching systems. The slopes and configurations of sloping and benching systems shall be selected and constructed by the employer or employer's designee and shall be in accordance with the following options:

• Option 1--Allowable configurations and slopes. Excavations shall be sloped at an angle not steeper than one and one-half horizontal to one vertical (34 degrees measured from the horizontal), unless the employer uses one of the other options listed below. Slopes under Option 1 must be excavated to form configurations that are in accordance with the slopes shown for Type C soil in Appendix B.
• Option 2--Determination of slopes and configurations using Appendices A and B. Maximum allowable slopes, and allowable configurations for sloping and benching systems, must be determined in accordance with the conditions and requirements set forth in appendices A and B.
• Option 3--Designs using other tabulated data. Designs of sloping or benching systems must be selected from and be in accordance with tabulated data, such as tables and charts. At least one copy of the tabulated data which identifies the registered professional engineer who approved the data, must be maintained at the jobsite during construction of the protective system. After that time, the data may be stored off the jobsite, but a copy of the data shall be made available to the director upon request. The tabulated data must be in written form and include the following:
  • Identification of the parameters that affect the selection of a sloping or benching system drawn from such data;
  • Identification of the limits of use of the data, to include the magnitude and configuration of slopes determined to be safe;
  • Explanatory information necessary to aid the user in making a correct selection of a protective system from the data.
• Option 4--Design by a registered professional engineer. Sloping and benching systems not utilizing Option 1 or Option 2 or Option 3 must be approved by a registered professional engineer. At least one copy of the design shall be maintained at the jobsite while the slope is being constructed. After that time, the design need not be at the jobsite, but a copy must be made available to the director upon request. Designs must be in written form and include at least the following:
  • The magnitude of the slopes that were determined to be safe for the project;
  • The configurations that were determined to be safe for the project; and
  • The identity of the registered professional engineer approving the design.

Design of support and shield systems, and other protective systems. Designs of support systems, shield systems, and other protective systems must be selected and constructed by the employer or employer's designee and must be in accordance with the requirements of one of the options below:

• Option 1--Designs using appendices A, C, and D. Designs for timber shoring in trenches must be determined in accordance with the conditions and requirements set forth in appendices A and C. Designs for aluminum hydraulic shoring must be in accordance designs using the manufacturer’s tabulated data, but if manufacturer's tabulated data cannot be utilized, designs must be in accordance with appendix D.
• Option 2--Designs using manufacturer's tabulated data. Design of support systems, shield systems, or other protective systems that are drawn from manufacturer's tabulated data must be in accordance with all specifications, recommendations, and limitations issued or made by the manufacturer.
  • Deviation from the specifications, recommendations, and limitations issued or made by the manufacturer may only be allowed after the manufacturer issues specific written approval.
  • Manufacturer's specifications, recommendations, and limitations, and manufacturer's approval to deviate from the specifications, recommendations, and limitations must be in written form at the jobsite during construction of the protective system. After that time, this data may be stored off the jobsite, but a copy must be made available to the director upon request.
• Option 3--Designs using other tabulated data. Designs of support systems, shield systems, or other protective systems must be selected from and be in accordance with tabulated data, such as tables and charts. At least one copy of the tabulated data, which identifies the registered professional engineer who approved the data, must be maintained at the jobsite during construction of the protective system. After that time, the data may be stored off the jobsite, but a copy of the data must be made available to the director upon request. The tabulated data must be in written form and include the following:
  • Identification of the parameters that affect the selection of a protective system drawn from such data;
  • Identification of the limits of use of the data;
  • Explanatory information needed to aid the user in making a correct selection of a protective system from the data.
• Option 4--Design by a registered professional engineer. Support systems, shield systems, and other protective systems not utilizing Option 1, Option 2 or Option 3, above, must be approved by a registered professional engineer. At least one copy of the design must be maintained at the jobsite during construction of the protective system. After that time, the design may be stored off the jobsite, but a copy of the design must be made available to the director upon request. Designs must be in written form and include the following:
  • A plan indicating the sizes, types, and configurations of the materials to be used in the protective system; and
  • The identity of the registered professional engineer approving the design.

Materials and equipment. Materials and equipment used for protective systems:

• Must be free from damage or defects that might impair their proper function.
• Must be used and maintained in a manner that is consistent with the recommendations of the manufacturer, and in a manner that will prevent employee exposure to hazards.

When damaged, a competent person must examine the material or equipment and evaluate its suitability for continued 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 such material or equipment must be removed from service and evaluated and approved by a registered professional engineer before being returned to service.

Installation and removal of support. Generally, the following requirements apply to installation and removal of support:

• Members of support systems must be securely connected together to prevent sliding, falling, kickouts, or other predictable failure.
• Support systems must be installed and removed in a manner that protects employees from cave-ins, structural collapses, or from being struck by members of the support system.
• Individual members of support systems must not be subjected to loads exceeding those which those members were designed to withstand.
• Before temporary removal of individual members begins, additional precautions must be taken to ensure the safety of employees, such as installing other structural members to carry the loads imposed on the support system.
• Removal must begin at, and progress from, the bottom of the excavation.
• Members must be released 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.
• Backfilling must progress together with the removal of support systems from excavations.

Additional requirements for support systems for trench excavations are as follows:

• Excavation of material to a level no greater than two feet below the bottom of the members of a support system may be permitted, but 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.
• Installation of a support system must be closely coordinated with the excavation of trenches.

Sloping and benching systems. Employees must not be permitted to 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. General rules regarding shield systems include the following:

• Shield systems must not be subjected to loads exceeding those which the system was designed to withstand.
• Shields must be installed in a manner to restrict lateral or other hazardous movement of the shield in the event of the application of sudden lateral loads.
• Employees must be protected from the hazard of cave-ins when entering or exiting the areas protected by shields.
• Employees must not be allowed in shields when shields are being installed, removed, or moved vertically.

Additional requirement for shield systems. Excavations of earth material to a level not greater than two feet below the bottom of a shield will be permitted, but 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.