Excavation, Trenching and Shoring Operations Trenching and Shoring Operations RISK ... These are...

Risk Management – Safety Challenges www.saif.com

© SAIF Corporation, Risk Management — Safety Challenges SC-116, June 2007

Excavation, Trenching and Shoring Operations

RISK MANAGEMENT

Safety Challenges

The Dynamics and Dangers of Working with Dirt

Trench and excavation cave-ins account for a growing number of fatalities and serious injuries in construction. Too many contractors and their employees fail to realize the hazard of working in unprotected or poorly protected excavations. With little or no warning, an unsupported, improperly shored or sloped trench or excavation wall can collapse, trapping the workers in seconds.

Common causes of trench and excavation cave-ins include:

• inadequate shoring in an attempt to cut costs or save time

• misjudgment of soil conditions

• defective shoring materials

• failure to evaluate changing weather conditions

SC-116 June 2007

• heavy loads in the area.


Excavations Extreme caution must be used during any excavation. A trench is referred to as a narrow excavation in which the depth is greater than the width, and the width is not greater than 15 feet. An excavation is any man-made cavity or depression in the earth’s surface. This can include excavations for anything from basements to highways.

All excavations over four-feet deep (Washington), or five-feet deep (Oregon), must be sloped, shored, sheeted, braced or otherwise supported. When soil conditions are unstable, excavations shallower than four feet must also be sloped, supported or shored.

Sitework and Building Excavation

Site excavation involves factors beyond merely “digging a hole” for even relatively small buildings. Proper preparation for the site can be as involved as the erection and completion of the building itself.

Core borings taken at an appropriate number of locations will disclose the presence of one or more of the following:

1. Rock

2. Water

3. Sand - including quicksand



These are conditions for which allowance must be made in site work and excavation. Study all site plans and contact all known utility services to determine if there are any:

1. Telephone cables

2. Electric power cables

3. Water lines (in-service or abandoned)

4. Sewer lines (in-service or abandoned)

5. Tunnels (new, in-service or abandoned)

If any of these underground sources are identified, hold an immediate consultation with the utility companies involved. Any damage to existing utilities during sitework and excavation will add significant expenses to the project.

Safe Trenching The first step in safe trenching and excavation work begins in the pre-bid stage when you should make allowances in both time and cost for applying safe shoring and/or sloping systems. 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.

Use a checklist to establish pre-planning before the actual digging begins:

• Choose a support system sturdy enough to withstand the pressure of the soil.

• Choose a support system sturdy enough to withstand the stress which may be exerted by water, vibration, or heavy loads.

• Properly support any underground installations that are uncovered.



• Locate any underground installations - sewer, telephone, water, fuel and electric lines that may be encountered in the digging.

• Contact the utility companies involved and inform them of the proposed work before starting the trench or excavation.

Sloping for Safety

One method of ensuring the safety and health of workers in a trench or excavation is to slope the sides of the cut to the “angle of repose,” the angle closest to the perpendicular at which the soil will remain at rest. The angle of repose varies with different kinds of soil and must be determined on each individual project. When an excavation has water conditions, silty material or loose boulders, or when it is being dug in areas where erosion, deep frost, or slide planes are apparent, the angle of repose must be flattened.

A second method of support is shoring (sheeting, tightly placed timber shores, bracing, trench jacks, piles, or other materials) installed in a manner strong enough to resist the pressures surrounding the excavations.

You should also consider the use of a trench box. A trench box is a prefabricated movable trench shield composed of steel plates welded to a heavy steel frame. Use a trench box if the protection it provides is equal to or greater than the protection that would be provided by the appropriate shoring system.

Trenching Procedures

Trenching operations are hazardous even under the best of conditions and a thorough hazard analysis should be made prior to start up. The result of your hazard analysis should be previewed with all employees who may work near the trenching operation. If you are involved in trenching operations where the excavation is deeper than four feet, established procedures must be developed in accordance with all federal, state and company policies.

Your written procedures should include the checklists provided at the end of this section.

Support Systems

Designing a support system can be a complex operation because of the number of factors involved. Some of the consideration you must take into account:

• Soil structure

• Depth of cut

• Water content of soil

• Changes due to weather and climate

• Superimposed loads

• Vibrations

• Other operations in the vicinity



Shoring Whatever support system is used, workers should always apply shoring from the top of the trench or excavation and work down. In installing the shoring, care must be taken to place the crossbeams or trench jacks in true horizontal position and to space them vertically at appropriate intervals. The braces also must be secured to prevent sliding, falling, or kickouts.

All materials used for shoring must be:

• in good condition

• free of defects

• of the right size

Timbers with large or loose knots should not be used.

Installation

Installing the shoring should immediately follow the excavation work. It is dangerous to allow trenches to remain unshored even if no work is being done in them: dirt walls will slough off, causing dangerous overhangs. The longer a trench is left unsupported, the greater the chance of a cave-in.

Removal

As soon as work is completed, the trench should be backfilled as the shoring is dismantled. After the trench has been cleared, workers should remove the shoring from the bottom up, taking care to release jacks or braces slowly. In unstable soil, ropes should be used to pull out the jacks or braces from above.

Inspections

Shoring systems must be inspected daily by a competent person. Inspections also are required after rainstorms or any changes in conditions that can increase the possibility of a cave-in or slide. If dangerous ground movements are apparent, such as subsidence or tension cracks, all work in the excavation must be stopped until the problem has been corrected.



In case of an emergency, workers must be able to leave the trench quickly. When employees are required to be in trenches four feet deep or more, adequate means of exit, such as a ladder or steps, shall be provided and located so as to require no more than 25 feet of lateral travel.

Trenching and excavation work leaves no room for risk-taking and requires that safe working conditions be provided for all employees working in excavations.

The supervisor who designs the protection and the employee(s) who install it can help create a greater awareness of the safety problems to help end cave-in hazards in construction.

Other Conditions to Consider

The soil structure must be carefully identified. Excavations in wet soil, sandy soil, or areas that have been backfilled are relatively unstable and must have strong support. Even hard rock sometimes can be hazardous; faults in the strata can make it unstable when cut.

Unstable Conditions

Sometimes you will have to guard against an unstable excavation bottom, such as below the water line. Sheeting may have to be driven below the bottom of such an excavation to add to the soil stability.

Diversion dikes and ditches or other suitable means must be used to prevent surface water from entering an excavation and to provide adequate drainage of the area adjacent to the excavation. Water causes soil erosion and softening and should not be allowed to accumulate in a trench or excavation.

Shoring for large excavations should provide long-range protections from a variety of weather conditions. In some cases, additional precautions are necessary to protect excavation walls, such as covering them with plastic sheeting or spraying the soil bank with a moisture-limiting chemical.

Superimposed loads in the vicinity of a trench or excavation increase the pressure on excavation walls. Heavy equipment and materials such as pipes or timbers should be kept as far back from the excavation as possible.

Buildings, curbs, trees, utility poles, and other structures adjoining the excavation area also can place more stress on a trench side than it can safely accommodate. In these instances, bracing, or underpinning must be provided as necessary not only to protect workers but also to prevent the dislocation of the soil beneath the structures in the vicinity.

Spoils or excavation material can exert great pressure on the excavation walls. Spoils must be stored at a distance equal to the depth of the excavation, and be barricaded or retained in an effective manner.

Vibrations or sudden shock from passing vehicles or railways, blasting, equipment such as trucks or pile drivers, and some tools can contribute to cave-ins by loosening the soil. Even machines operated in nearby buildings, such as punch



presses, can create enough vibrations to endanger a shoring system. If these conditions exist near an excavation site, stronger support is vital.

General Trenching Operations Checklist 1. Before trenching or excavation, check the following:

• Soil Conditions or other material to be due.

• Proximity to utilities, buildings and sources of vibrations.

• Owners of utilities, service, or transmission piping, etc., and arrange for shutdown or relocation of facilities, if necessary.

• For previously distributed ground.

• For trees, boulders, or other hazards.

• Adequacy and availability of all equipment, including personal protective gear, shoring materials, signs, barricades, and machinery.

Notes: __________________________

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2. During trenching or excavation, regularly check for:

• Changing ground conditions; particularly after rainfall.

• Possible oxygen deficiency or gaseous conditions.

• Adequacy of shoring and/or sloping as work progresses.

• Maintenance of entrance and exit facilities.

• All sheeting, bracing, shoring and underpinning.

• Changes in vehicular and machinery operational patterns.

Notes: ________________________

Notes: ________________________

Notes: ________________________

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Notes: ________________________



3. After trenching or excavation, check the following:

• Depth of trenching or excavation, its sloping and shoring.

• Sloping of banks, sides and walls in relation to depth of cut, water content of soil, vibrations.

• Entrance and exit facilities.

• Location of heavy equipment - power shovels, derrick, trucks.

• Excavation material is two feet or more from edge of opening.

• Portable trench boxes or trench shields are adequate, if used.

• Cross braces or trench jacks are correctly positioned to prevent sliding falling or kickouts.

Notes: ________________________

Notes: ________________________

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Notes: ________________________

Notes: ________________________




Underground Excavation and Trenching Procedures

• Submit a detailed drawing of the proposed excavation route to the superintendent, project foreman, construction manager or general contractor prior to the start of excavation. The drawing should include areas outside the site fence line and in the parking lot.

• The superintendent or designated authority will review and locate all underground utilities on your proposed drawing, make final approval and sign off.

• Any changes in excavation routing must be approved and signed off by the superintendent or designated authority prior to beginning work.

• A copy of the approved drawing must be reviewed by and held in the possession of the operator actually performing the work.

• Once the excavation and installation is complete, each contractor must submit to the general contractor an as-built drawing locating all newly installed piping and utilities.

• The general contractor will then update the original drawing locating all newly installed underground piping and utilities on jobsite.



Pre-Excavation Requirements Checklist • Prior to opening and excavation, effort shall be made to determine whether

underground installations such as sewer, telephone, water, fuel, electric lines, etc. are present, and where they are actually located.

• The walls and faces of all excavations in which employees are exposed to danger from moving ground shall be guarded by a shoring system, sloping of the ground or some other equivalent means.

• The determination of the angle of repose and design of the supporting system shall be based on careful evaluation of pertinent factors such as: depth of cut, possible variation in the water content of the materials from exposure to air, sun, water or freezing.

• All slopes shall be excavated to at least the angle of repose except for areas where solid rock allows for line drilling or presplitting.

• The angle of repose shall be flattened when an excavation has water conditions, silty materials, loose boulders, and areas where erosion, deep frost action and slide planes appear.

• In excavations which employees may be required to enter, excavated or other materials shall be effectively stored and retained at least 2 feet or more from the edge of the excavation.

• Banks more than 4 feet high (Washington), 5 feet high (Oregon), shall be shored, laid back to a stable slope or some other equivalent means of protections shall be provided.

• When employees are required to be in trenches 4 feet deep or more, an adequate means of exit, such as a ladder or steps, shall be provided and located so as to require no more than 25 feet of lateral travel.



Excavation Inspection Checklist • Daily inspections of excavations and trenches shall be made by the

Competent Person. If evidence of possible cave-ins or slides is apparent, all work in the excavation or trench shall cease until the necessary precautions have been taken to safeguard the employee. In addition, all open trenches require high visibility perimeter flagging.

• All excavation and trenches shall be inspected by the Competent Person after every rainstorm or other hazard-increasing occurrence and the protection against slides and cave-ins shall be increased, if necessary.

• Figures 1 and 2 shall be used as guides for reference to angle of repose and shoring techniques used in excavations and trenches. These tables show the minimum requirements; added measurements must be taken if the conditions warrant added protection to avoid slides and cave-ins.


A Competent Person Must:

1. Be able to recognize hazards.

2. Have authority to stop work and correct hazards.

3. Be knowledgeable of the standards that apply.

4. Be able to recognize or train qualified workers.

After trenching or excavation, be sure to check the following:

• Depth of trenching or excavation, its sloping and shoring.

• Sloping of banks, sides and walls in relation to depth of cut, water content of soil, vibrations.

• Entrance and exit facilities.

• Location of heavy equipment - power shovels, derricks, trucks.

• Excavated materials are two feet from the edge of opening.

• Adequacy of portable trench boxes or trench shields, if used.

• Correct positioning of cross braces or trench jacks to prevent sliding, falling, or kickouts.

Excavation and trenching requirements:

• All trenches 4 feet or more in depth (Washington), or 5 feet or more (Oregon), must be shored, sloped or protected by a trench box.

• All material must be at least two feet from the edge of excavation.

• Access ladders must be provided every 25 feet in all trenches 4 or more feet (Washington) or 5 or more feet (Oregon) in depth.


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