Excavation, Trenching and Shoring Operations - SAIF Corporation

SC-116
June 2007
Risk Management – Safety Challenges
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Excavation, Trenching and Shoring
Operations
The Dynamics and Dangers of
Working with Dirt
© SAIF Corporation, Risk Management — Safety Challenges SC-116, June 2007
RISK MANAGEMENT
Safety Challenges
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
• heavy loads in the area.

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

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

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

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

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

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

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

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

Pre-Excavation Requirements Checklist
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• 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.
© SAIF Corporation, Risk Management — Safety Challenges SC-116, June 2007

Excavation Inspection Checklist
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• 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.
© SAIF Corporation, Risk Management — Safety Challenges SC-116, June 2007

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