Welcome to this training session on Trenching safety. This session is intended for what OSHA calls a “competent person”—that is, a person who is knowledgeable about trenching operations. It provides important background information about trenching safety to support the competent person’s experience.

What Is a Competent Person?

What is a “competent person”? When it comes to trenching, a competent person is someone who:

• Identifies existing and predictable trench hazards, including working conditions that are hazardous or unsanitary for employees;
• Is authorized to take prompt corrective measures to eliminate hazards;
• Is responsible for daily trenching inspections;
• Is knowledgeable about soils, protective systems, and OSHA rules regarding trenching; and
• Determines which protective systems work best and works with the professional engineer, or PE, who designs protective systems for trenches that are greater than 20 feet deep.

Session Objectives

Here are the main objectives of this session. As a “competent person,” you will be able to:

• Recognize trenching hazards;
• Choose protective systems for trenches; and
• Identify the hazards and preventive measures for activities associated with excavation.

Trenching Fatalities and Injuries

Clearly, the overriding purpose of the session is to help prevent trenching fatalities and injuries. Here are some statistics about trenching accidents:

• Over 30 workers die and close to 1,000 workers have lost-time injuries per year. Note that 75 percent of fatalities at excavation sites are the result of cave-ins.
• The most common causes of trenching and excavation accidents include:

– No protective systems in place;

– Failure to inspect the trench and protective systems;

– Unsafe placement of the spoil pile; and

– Unsafe entry and exit to and from the trench.

What Is an Excavation? What Is a Trench?

What are the definitions of an excavation and a trench?

• An “excavation” is any man-made cut, depression, or trench that is made by removing earth.
• A “trench” is a narrow excavation made below the surface of the ground that normally is deeper than it is wide. The Occupational Safety and Health Administration, or OSHA, further defines a trench as an excavation that is not more than 15 feet wide at the bottom.
• Trench walls will eventually fail. This is an important rule to remember, and it shows why trenching should always be considered hazardous.

What Is a Cave-In?

What is a cave-in? The definition includes the following:

• The separation of soil or rock material from the side of a trench.

– Then the material falls or slides into the trench.

• The material moves suddenly.
• Many soil cave-ins occur when the soil in the trench wall gravitates downward because of its weight. The soil below is forced to move sideways because of the pressure of the soil above it, so it will move into the opening created by the trench.

Consequences of a Cave-In

What are the consequences of a cave-in? Here are important points to remember:

• Soil is dense and heavy—it weighs about 125 pounds per cubic foot and is heavier when wet.
• Cave-ins of soil can entrap, bury, injure, or otherwise immobilize a person.
• Suffocation can occur even if your nose and mouth are not buried. That’s because in a cave-in, heavy soil can put 800 pounds of pressure on your chest and make it impossible to breathe.
• Finally, a worker can become immobilized by a cave-in because soil exerts so much pressure that it makes rescue very difficult. For example, it takes about 750 pounds of force to remove a person’s leg that is buried by soil.

Soil Classification

To understand more about cave-ins and trenching safety, you need to understand about soil classification.

• Soil is classified into different types, depending on its characteristics.
• Each type requires a certain protective system—not all protective systems are appropriate for all soils. • Only a competent person can determine the soil type by using these classifications.
• Under OSHA rules, there are four primary categories or classes of soils:

– Stable rock, which is natural solid minerals such as granite or sandstone, that remains intact after it is excavated; and

– Soil types A, B, and C, which we’ll discuss in the next few slides.

Soil Stability

Soil stability is a key factor in classifying soils. Soil stability is affected by:

• Grain size. There are four basic types of grain size: gravel, sand, silt, and clay. Smaller grain sizes are more stable, so clay is the most stable, and gravel is the least stable.
• Water saturation, which is the amount of water the soil is holding. Soil that is completely saturated is much less stable than soil that is only slightly damp. However, soil with no water content at all is also unstable.
• Cohesiveness, which is how well the soil sticks together. The more it sticks together, the more stable it is. A field test for cohesiveness can be to roll some soil together in your hands and observe how and when it separates.
• Unconfined compressive strength, or UCS, which determines how easily the soil will shear and cave in, based on how much weight per square foot the soil can withstand.

Soil Types – Type A

The next three slides will cover basic soil types, A, B, and C. The soil type often determines the kind of trenching protection that is required.

Soil Type A is:

• The most stable type of soil if we don’t characterize stable rock as a “soil”;
• The most cohesive soil, with a UCS of 1.5 tons per square foot, or tsf, or more;
• Predominantly composed of clay, meaning fine grained;
• Not fissured or subject to vibrations; and
• Not previously excavated or disturbed.

Soil Types – Type B

Soil Type B is:

• Cohesive soil with a UCS of more than half a ton per square foot but less than 1.5 tons per square foot, or
• Granular, cohesionless soil including angular gravel (similar to crushed rock), silt, silt loam, sandy loam and, in some cases, silty clay loam and sandy clay loam.
• Previously disturbed soils except those that would otherwise be classified as Type C soil.
• A Type A soil that has fissures or is subject to vibration.
• Dry rock that is not stable.

Soil Types – Type C

Soil Type C is soil or material with any of the following characteristics:

• Cohesive soil with a UCS of less than half a ton per square foot;
• Granular soil, including gravels and sand;
• Submerged soil or soil with seeping water;
• Submerged rock that is unstable;
• Material in a sloped, layered system where the layers dip into the excavation; or
• Material in a slope at an angle ratio of 4 horizontal to 1 vertical or steeper.

Soil Tests

There are various types of soil tests, both visual and manual. Manual tests include these four main types:

• The Plasticity Test. For example, a wet ball of soil that can be rolled into a thread and held on one end without tearing is considered to be cohesive soil.
• The Dry Strength Test. For example, dry soil might:

– Crumble on its own or with moderate pressure into powder; or

– Fall into clumps but break into smaller clumps only with difficulty; this could be true of clay combined with gravel, sand, or silt; or

– Be unfissured; it breaks into clumps that do not break into smaller clumps without difficulty, and it has no visible fissures.

• The Thumb Penetration Test. For example, a ball of Type A soil requires a great effort to penetrate with the thumb, although it can be indented. However, a ball of Type C soil can be easily penetrated and molded.
• The Pocket Penetrometer Test and Hand-Operated Shear Vane Test help determine the unconfined compressive strength of soils. They are small, specialized instruments for soil testing. Follow the manufacturer’s instructions for using these instruments.

Signs of Soil Distress

Signs of soil distress signal the likelihood of a cave-in. These signs include:

• Fissures or cracks in the wall or face of a trench or excavation;
• The wall of the trench slumps or falls into the excavation;
• Soil from the trench walls bulges or heaves, which could mean the wall is about to break down into the trench;
• The edge of the excavation begins to sink; or
• Material, such as pebbles or clumps of dirt, begins to roll or trickle down the excavation wall.

Soil Distress – Conditions

Soil distress can result from a number of conditions, such as:

• Nearby vibrating machinery that can vibrate the soil and cause it to settle downward and inward toward the trench.
• Nearby moving, heavy loads, such as street traffic. This can compress the soil downward, which will push soil inward toward the trench.
• Water, such as rain, seeping into a trench, indicating a changing soil condition. This could mean a change in soil type, which could require a different protective system.
• Hot, dry weather that can also affect the stability of the soil because it might remove moisture and reduce cohesiveness.

Trenching Hazards

Now is the time for you to ask yourself if you understand the material presented so far. It is important for your safety and the safety of others that you understand:

• Trenching hazards,
• Cave-ins, and
• Hazards related to soil types.

Trench Protective Systems

There are several types of trench protective systems, and we’ll discuss some of the more commonly used types in the next few slides. These include:

• Sloping and benching, which is pictured here;
• Shoring; and
• Shielding

Sloping and Benching

Sloping and benching are simple and effective forms of trench protection that can be used if the width of the upper opening does not have to be narrow.

• Sloping angles the walls of the trench in the form of an incline and protects employees from cave-ins. The angle is determined by the soil type, environmental conditions, and surcharge loads.
• Benching means excavating the sides of the trench in a series of horizontal levels or steps, usually with vertical or near-vertical surfaces between the levels.
• Soil type determines the angle of the slope or benches:

– Type A soil requires a slope of 3 feet horizontal to 4 feet vertical, or a 0.75-to-1 ratio.

– Type B soil requires a 1-to-1 slope ratio.

– Type C soil requires a 1.5-to-1 slope ratio, or 6 feet horizontal to 4 feet vertical. Benching is not permitted for Type C soil because this soil is too unstable to permit near-vertical surfaces between benches.

Shoring

Shoring is another protective system for trenches, as well as other excavations.

• Shoring uses support walls in trenches or other excavations to prevent a cave-in from occurring.
• It is usually built in place and designed by an engineer. Shoring can be made from a variety of materials, including wood, but the trend is toward prefabricated systems made of steel or aluminum.
• Components of a shoring system typically include:

– Uprights, or sheeting, that are vertical members placed in contact with the soil;

– Wales, which are horizontal support members placed parallel to the face of the excavation and bear against the uprights, or sheets; and

– Cross braces, which are horizontal support members placed perpendicular to the sides of the excavation and bear against either the uprights or the wales.

Shielding

Shielding is another form of trench protection.

• Shielding withstands the forces of a cave-in. However, shielding is not designed to prevent a cave-in, but rather to protect workers from a cave-in.
• Shielding can be permanent structures, or they can be portable structures that are moved along as trenching work progresses.
• Trench boxes are a type of portable shielding that can either be purchased premanufactured or built on-site in accordance with OSHA standards.

Shielding – Trenching Boxes

Shielding with trench boxes requires you to observe certain safety rules.

• Trench boxes are often designed to be stacked, which allows for additional height protection. Always stack trench boxes in accordance with the manufacturer’s instructions.
• Sheeting should not be used for height extension on a trench box. If a cave-in were to occur, the additional weight against the extended sheeting could cause the trench box to collapse.
• Trench boxes can be used in conjunction with sloping and benching for deep trenches. In these cases, start the slope at least 18 inches below the top of the trench box, and create the slope in accordance with the soil type.
• Finally, no one should be allowed inside or underneath the trench box when it is being raised or lowered.

What’s Wrong Here?

What’s wrong with this picture? Look at the photograph on this slide and see if you can identify the hazards and the missing protective systems and protective measures.

• The main potential hazard, of course, is a cave-in from falling soil or rock that could cause entrapment, burial, or suffocation.
• The missing protective systems and measures include no sloping or benching, shoring, or shielding. In addition, there is no proper means of entry or exit to or from this trench.

Trench Protective Systems

Do you understand trench protective systems? Do you understand the hazards of trenching?

• Do you understand the various soil types that you need to recognize?
• Do you understand the types of protective systems needed for trenching?

It is time to ask yourself if you understand the material presented.

It is important for your safety and the safety of others that you understand trench protective systems.

Hazardous Atmospheres

Now we’ll discuss some other hazards at excavation sites. Hazardous atmospheres is one of these, when trenching near sewers, landfills, or piping systems used for gas or other fuel. In these situations:

• Test the atmosphere before any workers are allowed to enter the excavation. Continue to test throughout the job to make sure the atmosphere remains safe. In particular, test for reductions in oxygen levels, but also be on the lookout for explosive or flammable gases and toxic gases such as hydrogen sulfide.
• Ventilation is the primary mitigation for a hazardous atmosphere.
• Personal protective equipment, such as a respirator, may be needed.
• Keep rescue and emergency equipment on-site, such as a breathing apparatus, a safety harness and retrieval line, or a basket stretcher, in case workers are overexposed to a hazardous atmosphere.

Falling Soil or Equipment

Workers in a trench must be protected from falling soil and equipment.

• To protect against loose rock or soil that might fall from the face of an excavation:

– Use scaling to remove loose materials before it can fall down; or

– Use shoring or shielding devices to prevent loose soil from affecting workers.

• To protect against falling material or equipment:

– Keep excavated rocks and soil at least 2 feet from the edge of the excavation. Tools and equipment, such as pumps, must also be kept at least 2 feet from the edge; and

– Use retaining devices designed to keep materials from falling into the excavation.

Adjacent Structures

Trenching and other excavations must consider the possible effects on adjacent structures.

• Excavations could endanger the stability of nearby buildings, walls, and other structures. You may require a professional engineer to determine if a nearby structure is stable or if it needs to be shored up or otherwise protected.
• Sidewalks and pavements must not be undermined by excavating unless they are supported to prevent them from collapsing onto excavation workers.
• Use shoring, bracing, or underpinning to stabilize nearby structures or to prevent a structure that hangs out over the excavation, such as a sidewalk, from collapsing.

Water Accumulation

Water accumulation in a trench can be very dangerous. Water seepage into a trench means the soil is very unstable.

• The risk of drowning is serious under these conditions; workers have been known to drown in a trench.
• Take special precautions when working in an excavation with accumulating water. Such precautions include:

– Use a special shoring or shielding system.

– Use a water removal system such as a pump. With a pump, a competent person must monitor the conditions to make sure it is operating properly.

– Workers should use a safety harness and a lifeline if water continues to be a problem. The lifeline should be continuously monitored by someone outside the excavation.

Entry and Exit

Proper entry into and exit from trenches are important safety considerations for preventing accidents. Trenches that are 4 feet deep or more must have:

• A means of entry and exit within 25 feet of every worker; and
• Proper means of entry and exit, which include:

– Ladders;

– Stairways; and

– Sloped ramps.

Other Trenching Issues

There are a number of other trenching issues relating to safety. These include:

• Marking all underground utilities near the trench before beginning any excavations. Make sure workers use caution when digging near the mark, since it may not be precisely accurate.
• Standing away from lifting or digging equipment while this equipment is in operation, to make sure the equipment or the material it is moving does not strike them.
• Using warning systems, such as lights or alarms, or barricades if vehicles will be approaching or working near the excavation.
• Wearing hard hats at all times in excavations to protect against falling soil, rocks, tools, and debris.

Other Trenching Issues (cont.)

Other important trenching issues related to safety include:

• Using fall protection systems to prevent people from falling into excavations. These systems may include guardrails, barricades, or covers over the excavation.
• Not allowing work above other workers unless there is some form of protection from such hazards as falling rock or tools.
• Finally, monitoring of the excavation work, which should be performed by a competent and knowledgeable person who is familiar with trenching hazards. The monitor should be outside the trench, observing workers and conditions. The monitor should have the authority to stop work and tell workers to exit the trench immediately if he or she notices any signs of a possible cave-in.

Trench Inspections

Trench inspections are to be conducted by a competent person.

• Inspect the trench before work starts, throughout the shift, and after rainstorms or any other events that might have affected the stability of the excavation.
• Inspect the excavation for all hazardous conditions, including:

– Evidence of possible cave-ins;

– Indications of failure of protective systems

—shoring, sloping, or shielding; and

– Potentially hazardous atmospheres.

• Remove workers from the excavation if a hazardous condition exists, and keep them out until the condition is corrected or necessary precautions are taken.

Key Points To Remember

The key points to remember about this training session on trenching include the following:

• First, be aware of all the hazards associated with working around trenches.
• Next, recognize that cave-ins can occur suddenly and can entrap, bury, and injure workers; therefore, taking the right safety precautions is extremely important.
• Finally, make sure you always use the right protection systems for the conditions at the trench or other excavation.

This concludes the training session on Trenching—Competent Person.