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Trench Excavation on Construction Site
16 April 2015

Buried Alive: Preventing Excavation and Trench Accidents

According to the National Institute for Occupational Safety and Health (NIOSH), on average around 35 workers die each year as a result of trench cave-ins or similar excavation-type accidents. Additionally, excavation activities cause many worker injuries on construction sites and other industrial workplaces throughout the US each year. Being aware of the inherent risks involved as well as preventative measures can be a matter of life and death. This guide will explore the causes, risk factors, and possible solutions to this problem to help keep workers safe.

Causes of Excavation / Trench Accidents

Trenching is sometimes necessary on a job site. A trench is a narrow excavation where the depth of the ditch is deeper than the width, usually not extending more than 15 feet wide at the bottom. Excavations can be any man-made cut, trench, cavity, or a depression in the Earth’s surface that is formed by the organized removal of earth.

Before digging can begin on a trench, a survey must be done to assess the location of any water and sewer lines, power lines, or Internet lines, or any other communications, piping, or other underground services.

If a proper survey is not done, this may contribute to a cave-in once digging begins, as service lines may be unintentionally disrupted during the digging process, loosening earth and causing it to cave-in.

By far, the most common reason a trench caves in is due to soil sliding. This is caused by pressure on the surrounding soil combined with excessive vibrations in the area, typically due to heavy materials or equipment being used in or around the trench.

Heavy machinery and equipment, like excavators, jackhammers, or even shoveling can contribute to, or directly cause, cave-ins.

Water is another potential contributing factor to erosion and soil slides, especially if trenches are excavated below the water table in sandy soils or soft clay or clay-like earth. Sudden changes in weather can also cause pressure on the trench, or change the pressure being exerted, potentially causing or contributing to a cave-in.

This is usually the case after a rainstorm or some other dramatic weather change, such as freezing and thawing or any hazard like tornadoes or other heavy storms.

Risk Factors

Soil Conditions of the Trench

Depending on its composition, and the current or recent weather, soil may contribute to a cave-in.

  • Clay: Clay is soil that is very finely grained. Rainfall will generally collect on the surface and evaporate off of it instead of absorbing underneath into the soil. However, water that has already penetrated the clay may contribute to a cave-in. Specifically, water that causes cracks, or freezes and causes separation, may cause or contribute to trench cave-ins or soil erosion.
  • Sand: Sand is granular soil, but not as fine as clay. Sandy soil usually has large pores, making it possible for groundwater absorption. When digging trenches in sand, soil erosion and sliding is a serious risk since sand does not typically have integrity under even moderate construction conditions.
  • Silt: Silt has soil properties which combine various aspects of both clay and sand. It’s the most sensitive to moisture, but crumbles when dry. For this reason, it presents one of the greatest challenges to digging and is a major risk factor for cave-ins.

Excavation and Digging Risks

When trenches are dug more than 5 feet deep, an increased risk of cave-in exists. Trenches that are more than 20 feet deep require professionally engineered solutions to prevent cave-ins.

Heavy equipment that is near a digging site should be kept clear of trenches. Workers, and engineers, should work to identify any sources that might negatively affect the stability or integrity of the trench before entering it or working on it.

Excavated soils must be kept at least 2 feet away from the trench edges, and a test for atmospheric hazards should be done to avoid toxic fumes and gasses when digging more than 4 feet into the ground.

Workers should never work under suspended or raised loads or materials, and never following a rainstorm until the trench has been verified safe. Trenches that have not been inspected should be considered inherently dangerous and unsuitable for working in.

Lack Of Personal Protection

When working in trenches personal protection usually means either safety harnesses or engineered safety and retaining walls designed to withstand any possible cave-in. A lack of these measures could result in serious injury or death.

When these are not used, or when the retaining walls are not sufficient to protect against a cave-in, workers may be seriously injured or die.

Prevention and Practical Solutions

Prevention is accomplished through a combination of safety and preparedness protocols. One of the ways used to prevent a cave-in is an excavation technique called “shoring” or “sloping.”

Shoring

Sloping methods require workers to dig a trench with an angled entry and angled sidewalls. This is not always possible on every job site as it may interfere with the project, damage adjacent buildings or make it impossible to use equipment.

When it is possible to use sloping, soil is placed 2 feet, or more, from the edge of the trench to prevent it from falling back into the trench while workers are working.

Hydraulic Shoring

Shoring is a process whereby posts, wales, or struts are used to hold back soil of a trench. Today, it’s common for hydraulic shoring techniques to be used on-site. These techniques utilize a prefabricated strut or wale made of aluminum or steel. They’re light, so they can be installed by one worker. They’re gauge regulated so that pressure from the soil is evenly distributed along the trench line and the side of the strut or wale facing the trench can be pre-loaded to take advantage of a soil’s natural cohesion – resisting soil movement or sliding. Finally, they can be changed and adapted to an almost unlimited number of depths and widths.

Pneumatic Shoring

An alternative to hydraulic shoring is pneumatic shoring. This process is similar to hydraulic shoring except that pneumatic shoring uses air pressure instead of hydraulic pressure. There is an increased risk with this type of structure, however, because the worker must be in the trench when adjusting the struts.

Screw Jacks and Trench Boxes

Screw jacks differ between hydraulic and pneumatic systems in that they must be adjusted manually. When these are used, there is significant risk to the worker since he must be in the trench to adjust them.

Single-cylinder hydraulic shores are usually used in a water system, where stability is paramount. Underpinning is a process of engineering a foundation and other structures for the trench, using it to stabilize retaining walls.

Trench boxes take an entirely different approach to prevention. Instead of shoring, they are intended to prevent cave-ins or soil slides. The space between the trench boxes (the side walls) is back-filled to prevent lateral movement. Struts are used to prevent the soil from falling in on the workers.

Trench boxes must be engineered by a professional engineer, as they must be able to withstand the external forces placed on them. If they fail, a cave-in will occur, which may result in serious injury or death.

Benching

Benching is a process of building what are essentially steps or a step leading down into a trench. A single bench is a single “step” that leads into a trench which can be no more than 4 foot deep. Multiple bench setups consist of a series of steps, the first of which must be no deeper than 4 feet, while the remaining steps may be up to 5 feet in depth.

Reducing The Risk of Excavation Accidents

Companies concerned with the risk of cave-ins can also reduce the risk of this occurring by removing surface debris near the trench that could be hazardous, checking weather conditions prior to the construction project, as well as during each day of the project, and ensuring that workers are not working on job sites where rain or ice has compromised the integrity of shoring attempts or trench boxes.

Companies should examine the particle sizes of excavated soil to make sure that it holds together prior to building trench boxes or digging, and they should also look for signs of disturbed soil from previous construction projects.

Consider normal vibration in the area, plus any that will be introduced on the job site by construction equipment. Make sure that adjacent structures are stable, that workers have safe passage for entry and exist of the trench, and that there is an emergency response plan in place should the an accident occur.

Education and Training

Employees should be trained and educated about the risks of the job site. A designated “competent person” should be appointed to oversee the safety measures at the job site. Utility lines should be marked prior to digging.

A competent person, acting as a safety officer, should evaluate the soil to determine its stability. If necessary, an engineer should be brought in for further analysis. Soil conditions can change dramatically within days, so this process should be repeated every day, especially if humidity or moisture levels change.

The job site should have designated “safe locations” which are away from the trench, and spoil piles for heavy equipment routes.

If trenches are more than 20 feet deep, education should be given to employees about the inherent risks of trench boxes, and designs that attempt to hold back earth in the trench. Additionally, engineered solutions should be employed to reduce the risk to all workers in the trench.

For example, workers should be made aware that engineered solutions may provide additional safety, however, trench boxes, struts, and other safety devices are not fail-safe.

A trench emergency plan should describe the steps necessary in the event of an emergency. The safety officer should inspect the excavation, adjacent areas, and any protective systems used on the site every day before work begins. Workers should not be asked to work prior to this safety inspection.

Sub-contractors, who do not work for the company, should be given training in education classes when they arrive on the site.

Any ladders used should never be more than 25 feet from any worker in the trench. This is primarily for safety. If a worker must leave the trench quickly, the ladder is usually the only means of escape. Having it near the worker minimizes the risk that he or she will be caught in a landslide / soil side or cave-in.

Protection and More Resources

Workers should always wear all protective gear assigned to them. This may include, but is not limited to, safety goggles, protective equipment and reflective gear, hard hats, gloves, respiratory gear, and durable boots with steel toe linings.

When possible, personal rigs should be worn to help assist with quick evacuation of a trench.

When A Cave-In Occurs

When a cave-in does occur, options may be limited, depending on where it happens and the nature of the soil. If a cave-in happens, there should be protocols in place to have emergency personnel called so that they can help rescue trapped individuals.

Rescue personnel include emergency services, fire and rescue, and possibly local police.

In some situations, it may be possible to excavate the individual to safety, either by digging or by removing soil from the side of the trench to allow for a secondary cave-in away from the individual.

These methods are particularly dangerous and may result in further injury or the death of the individuals trapped. Therefore, the safety protocol should be strictly followed in cases of emergency. If it does not allow for further excavation, do not excavate. In some situations, it may put more workers in danger.

There are many situations where a cave-in results in death, either immediate or within seconds or minutes, and there is no practical way to save the individuals who are trapped. This is why prevention is such an important aspect of trenching and excavating.

If you’ve been injured on a construction site, or if you feel a site you are currently working on is unsafe, don’t hesitate to contact a lawyer and discuss the situation. You have rights, and you shouldn’t be afraid to assert them.

 

Additional References

Information For The Landscaping Profession

OSHA: An Unprotected Trench Is An Early Grave

OSHA: Trenches and Excavations

ACR Publications: Causes Of Cave-Ins

Excavation, Trenching, and Shoring Safety