Prevent deaths and injuries from trench collapses with planning, education and execution

When it comes to trench safety, education is the first line of defense.

“Soil is heavy. A cubic foot can weigh as much as 114 lbs., and a cubic yard can weigh over 3,000 lbs. — as much as a pick-up truck. Most workers don’t realize the force that will hit them when a cave-in occurs. A person buried under only a few feet of soil can experience enough pressure on the chest to prevent lungs from expanding. Suffocation can take place in as little as three minutes,” says Philip L. Colleran, an occupational safety and health consultant who authored a paper for OSHA on the subject for the AFSCME Training and Education Institute.

The hazards

The most common excavation hazard is the threat of cave-in, which can occur when soil is disturbed and the force of gravity takes over. Cave-ins usually occur too quickly for a worker to react and are more likely to occur in unprotected excavations where digging takes place in unstable or previously disturbed soil; where there is excessive vibration from construction equipment or vehicle traffic; an excessive load near the sides of an excavation, often from equipment or the excavated material placed too near to the edge; an accumulation of water in the excavation; and/or changing weather conditions such as freezing, melting or sudden heavy rain.

These conditions can be controlled by routing traffic away from the excavation and keeping only necessary construction equipment near the excavation; keeping the spoil pile at least 2' back from the excavation edge; pumping water out of the excavation before workers enter it; and using the correct protective systems.

Other trench hazards include accidental contact with utility lines, crushing and striking hazards from construction equipment and hazardous atmospheres. These hazards, however, can be controlled with proper planning, starting with a pre-job survey of the planned trenching activities.

What OSHA requires

Title 29 Code of Federal Regulations, Part 1926.650-652, (Subpart P), OSHA’s Rules and Regulations for Construction Employment, covers trenching safety. OSHA defines an excavation as any man-made cut, cavity, trench or depression in the earth’s surface formed by earth removal. It also defines a trench as a narrow excavation where the depth is greater than the width and the width does not exceed 15'. If an excavation is more than 5' deep, a protective system must be in place while workers are in the excavation. Excavations more than 4' deep must have a way to get in and out, usually a ladder, for every 25' of horizontal travel.

OSHA requires a competent person to inspect conditions at the site daily and as frequently as necessary during work to assure excavation hazards are eliminated before workers enter.

A competent person must be versed in soil type classification, understand the different types and proper use of excavation safety equipment, and can recognize unsafe conditions. They must have the authority to stop the work when unsafe conditions exist, and must know how to correct unsafe conditions.

The competent person determines what soil types are present by using manual and visual methods. The ability to determine soil type correctly is critical because it helps determine the proper protective systems.

The three most commonly used protective systems are shoring, shielding, and sloping. Shoring uses timber or hydraulic shores that brace the walls of an excavation. Shielding devices, commonly called trench boxes, shields, or coffins, are not designed to prevent a trench wall from collapsing, but rather act as a shield for workers should a cave-in occur. Workers are only protected if they stay within the protective confines of the systems.

Sloping requires cutting back the trench walls at an angle that greatly reduces the chance for collapse.

Soil type affects safety

Soil is a mixture of sand, gravel, silt, clay, water and air. The combination of these materials affects the soil’s cohesiveness or how well a soil will hold together.

Cohesive soil does not crumble and can be molded easily when wet, and is hard to break up when dry. Clay is very cohesive; sand and gravel have little cohesiveness. Generally, the more clay that is in the soil being excavated, the better the trench walls will hold up.

Water – or lack of it – greatly affects soil cohesiveness. Saturated soil does not hold together well and is particularly dangerous in excavation work; but soil that is void of all water can crumble easily, and will not hold together when excavated.

OSHA classifies soils into four categories:

- Solid Rock,

- Type A,

- Type B, and

- Type C

Solid Rock is the most stable, and Type C soil is the least stable.

Type A soil can be clay, silty clay, or sandy clay, but it can not be fissured or compromised by heavy traffic vibration, pile driving or any activity that has disturbed the soil or soil that has been excavated and less stable soil is placed underneath more stable soils. Groundwater or freezing and thawing conditions can also require a Type A soil to be reclassified as a Type B or C soil.

Type B soils include silts, sandy loams, medium clays and unstable rock. Soils that might be classified as Type A but have fissures or are subject to vibration may be classified as Type B soils.

Type C soils are the most unstable and most dangerous. They will continually slough off the sidewalls of the excavation. If soil is submerged or water seeps from the sides of an excavation, it’s very likely a Type C soil. Soil may be classified as Type C if excavation reveals different soil types on top of each other. If an unstable soil type is underneath a stable soil type, consider it a Type C soil.

Once soil has been excavated, it will never return to its original natural composition and are usually identified as Type C soils. If you are uncertain of the soil type, assume it is a Type C soil.

A competent person must type soils by using at least one visual and one manual test. A visual test can include inspecting removed soil and the color and makeup of the soil in excavation walls. A manual test means working the soil with your hands or with an instrument designed to measure soil strength. If the soil can be worked into a long worm or ribbon, the soil is cohesive and may be classified as A or B, depending on other conditions.

A penetrometer, which measures soil strength, measures a soil’s unconfined compressive strength in tons per square foot (tsf) and can be used to help classify soils.

Protective systems

For trenches between 5' and 20' deep, shoring, sheeting, shielding, sloping and benching are acceptable protective measures. Construction project planners and the competent person on site must determine which system is most appropriate. If an excavation is greater than 20' deep, a registered professional engineer must design the protective system.

Shoring uses timber, mechanical or hydraulic systems to support the sides of an excavation. Sheeting can be driven into the ground or work in conjunction with a shoring system. Driven sheeting is often used if an excavation will be open for long periods of time; sheeting can be a part of an active strutted system that uses aluminum hydraulic shores.

A shield or trench box is a common protective system and excellent choice in continuous horizontal trenches. It’s placed in the trench and dragged along with the progress of the work. Workers should not be in the box when it is being moved. After a box is positioned, voids between the box and the trench wall should be filled, which prevents the box from moving in case of a cave-in.

Sloping and benching can also protect workers from cave-in hazards. They require excavating trench walls to an “angle of repose” which depends on the soil type. Sloping can be used as alone or with benching. However, few applications allow using sloping and/ or benching because the extra right-of-way needed for them is almost never available and the soil must likely be put back in place and compacted, adding time an expense to the project.

Material handling

Construction equipment poses crushing and striking hazards, particularly in excavation work. To protect workers, cordon off areas around the swing radius of excavators and move the barriers along with the progress of the work. Most excavators have blind spots, usually at the rear of the machine on a diagonal to the operator’s position.

Moving material suspended from buckets may catch a worker between the suspended load and the excavator.

When any load is brought under tension as it is hoisted, stay away from it. While aloft, the load should be controlled with a tag line and workers should only approach it after it is in position. No worker should ever be under any part of a suspended load.

Don’t rely on back-up alarms as the sole warning of machine movement.

Workers should make sure the equipment operator is aware of his or her position; reflective vests or clothing can be worn to make workers on the ground more noticeable.

Hazardous atmospheres

Hazardous atmospheres can go unrecognized by workers until it’s too late. Then, fellow workers attempt to rescue fallen workers and often become victims as well. In fact, 60 percent of all workers who die from such atmospheres are rescuers.

Planners who perform preconstruction site surveys must identify potential atmospheric hazards as well as cave-in hazards. The competent person must know how to use a detector that can identify oxygen deficient, flammable/combustible/ explosive and toxic environments.

Continuous monitoring is a good idea because changing conditions can create hazards where there was once none.

Copyright 2010 Urbain Communications. All rights reserved.