Trenching and Excavation Safety
The construction industry relies on trenches to provide a solid base for infrastructure development and maintenance. Professionals may dig a trench to install or repair utility lines. Trenches also support building foundations.
A trench is a type of excavation where the depth exceeds the width. An excavation is a hole or cavity in the ground created by removing soil or material. When soil gets removed from the ground, gravity pulls down the surrounding walls and causes them to collapse or cave-in, potentially trapping and suffocating employees.
SUFFOCATION BY SOIL
Soil weighs 2,750 pounds per cubic meter. A person typically stops breathing with just 40 pounds of pressure placed on their chest. When a trench collapses, soil can crush a person’s body and block their breathing even without covering the nose. People trapped in trenches rarely survive, succumbing solely to the soil’s weight.
Unprotected trenches can be an early grave for those who dare to enter them. Many employees climb into trenches to perform a quick fix. They may assure us by saying, “it will only take a minute!” or “I’ve done this a hundred times!” Lack of training leads to employees underestimating the risk of trench collapses and soil suffocation. Unplanned rescue missions may also prove fatal for the heroic if the same trench collapses again.
Trenches can collapse without warning, depending on the type of soil and level of protection. Parts of trenches can collapse one minute, while another part collapses ten minutes later.
The presence of water or vibration also increases the risk of cave-ins.
TYPES OF SOIL
The United States classifies soil types under three categories based on their “unconfined compressive strength”. Unconfined compressive strength refers to the amount of pressure required to cause soil to collapse. In the U.S., unconfined compressive strength is measured in tons per square foot.
- Type A Soil — Rigid, cohesive, and never been disturbed, this type of soil strongly binds together and does not break easily. Type A soil provides the most stability during an excavation with its high unconfined compressive strength (> 1.5 tons per square foot). Examples can include clay and other clay aggregates.
- Type B Soil — This type of soil has been disturbed by equipment, previous excavations, or sources of vibration. Soil particles do not stick together as well and provide less stability during a new excavation. The unconfined compressive strength lies between 0.5 and 1.5 tons per square foot. Angular gravel and silt are some examples.
- Type C Soil — The most dangerous of all soil types, type C soil crumbles and collapses very easily. Type C soil has the lowest unconfined compressive strength (< 0.5 tons per square foot). Examples include sand and gravel. Watery soil automatically classifies as type C due to its weak nature.
Soil types can vary within the same trench. Soil types often change along the depth and length of a trench. It is important to test several samples of soil prior to excavation.
SOIL ASSESSMENTS
By law, companies must conduct a soil assessment before excavation. A soil assessment involves a visual inspection and physical examination using a penetrometer.
A visual inspection can differentiate between Type A and Type C soil as they can look quite distinct. Poking the soil with a finger may also reveal soil compaction and hardness.
Companies can use a pocket penetrometer to obtain numerical measurements of compressive strength. The pocket penetrometer includes a gauge or scale along with a stick that penetrates the soil. A person pushes the device into the soil and reads the measurement off the gauge or scale. This measurement can then be used to determine the soil type.
UTILITY LINE COORDINATES
Utility lines carry services such as electricity, water, and gas from one place to another. Some utility lines may exist underground and disrupt excavation during construction projects. For example, if a backhoe punctures through a gas line, it can endanger employee and public safety.
Prior to excavation, companies must also pinpoint and protect utility lines from punctures and damage. In the U.S., companies can call 811, the federal utility location hotline, to request approximate coordinates of underground utility lines. Coordinates are provided using different colors, such as green for sewer, blue for water, or yellow for gas lines.
Excavators must stay at least 18–40 inches away from a utility line. Different states have different rules for maintaining a safe distance. To expose the exact location and depth of a utility line, excavators may safely drill small test holes by hand in a process called potholing. Once exposed, these utility lines must be protected, supported or removed as needed.
CONTROLS — SHORE IT. SLOPE IT. SHEILD IT.
Any trench that exceeds 5 feet must have a proper protection system if employees are required to enter. Control mechanisms include shoring, sloping, and shielding. To be on the safer side, install these protection systems at the 4-foot mark. A 4-foot deep trench can still collapse and crush vital organs.
Shoring
Shoring consists of a temporary hydraulic system with aluminum struts, pistons, cylinders, walers, and sheathing to stop trench walls from moving. Hydraulic shores apply pressure against the walls to prevent trench collapses. Shoring design depends on the soil type.
Sloping
Sloping involves angling the trench walls away from the trench floor. A sloped trench may resemble an upside-down trapezoidal shape instead of the normal rectangular trench shape. Angling trench walls weakens the gravitational pull and helps reduce the risk of cave-ins.
- For Type A soil, the trench walls should be angled at 53 degrees with a height/depth ratio of 0.75:1.
- For Type B soil, the trench walls should have an angle of 45 degrees with a 1:1 height/depth ratio.
- For Type C soil, the trench walls should slope at a 34 degree angle with the greatest height/depth ratio of 1.5:1.
Shielding
Shielding includes the use of a steel or aluminum trench box. A trench box is a square structure with side walls and adjustable cross-beams. Employees conduct work from inside the box. Trench boxes do not prevent a cave-in but rather shield employees from the pressure and weight of the soil during a cave-in.
INSPECTIONS
Every trench requires a competent person to conduct inspections on a daily basis. A competent person refers to someone who can identify hazards and has the authority to control them. This person inspects soil conditions, reviews the integrity of protective measures, and examines the surrounding areas outside the trench. For the surrounding areas, keep all loose objects and equipment at least 2 feet away from the edge of the trench. These items can fall on workers and pose a struck-by or crush hazard.
In addition to the daily inspection, the competent person must conduct another inspection after every rainstorm. Water changes the soil type and largely increases the probability of a collapse. Workers cannot enter trenches with water accumulation unless the company takes proper precautions to remove the water.
Hazardous Atmospheres
Some trenches expose workers to hazardous atmospheres such as oxygen deficiencies, hazardous fumes, and toxic gases. For example, trenches near landfills and sewers may contain methane and carbon dioxide gases. Within the confined spaces of a trench, these gases can displace oxygen, leading to asphyxiation and death. Methane also classifies as a combustible gas, which can again endanger workers inside a trench. Welding operations within trenches can also emit hazardous fumes.
If a trench exceeds 4 feet and involves hazardous atmospheres, the company must conduct atmospheric testing and provide appropriate protection. Atmospheric testing can be conducted by collecting air samples with calibrated air monitoring equipment.
Appropriate protection can include respirators, self-contained breathing apparatus, harnesses, and emergency rescue equipment. Ventilation systems can also reduce concentrations of chemicals, toxic gases, asphyxiants, and hazardous fumes.
ACCESS AND EGRESS IN TRENCHING
Every trench exceeding 4 feet must also have a safe means of access and egress such as ladders, stairways, or ramps. Proper ladders, stairways, and ramps can prevent falls and ensure quick evacuation during emergencies. Access and egress-ways must be located within 25 feet of workers to enhance efficiency inside the trench.
For trenches with vertical walls, workers can use a ladder for entering and exiting. Always choose a non-conductive ladder in good condition with the ability to support the weight of the worker and their tools. Place these ladders on a stable surface.
Stairways work well for trench walls with an angle of 60 degrees or less. For ramps, trench walls cannot exceed a 20-degree angle. Stairways and ramps must remain free of obstructions and have a guardrail system to prevent falls.
CONCLUSION
The construction and maintenance industries present several hazards and result in many workplace fatalities. In the U.S., the construction industry sees more than 1000 fatalities every year. Trench collapses normally account for about 5% of these fatalities. Unfortunately, trenching fatalities doubled in 2022, when compared to the last three years.
Trenching fatalities are completely avoidable. Trenches entrap, bury, and kill. Proper planning, protection, and training can largely reduce the risk of injury and significantly curb the fatality rate. It may take one minute to quickly jump into a trench and get the job done, but it takes just a second for a trench to collapse and claim a life.
REFERENCES
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Hazardous atmospheres. (n.d.). Murraystate.edu. Retrieved March 28, 2023, from https://www.murraystate.edu/about/Offices/esh/OccupationalSafetyHealth/TrenchingShoring/HazardousAtmospheres.aspx
How to Provide Safe Access & Egress: Construction Trenching and Excavation| Sloping & Benching 2022. (2022). A&H Safety & Environmental. https://www.youtube.com/watch?v=HLwFU4yUX5Q&ab_channel=A%26HSafety%26Environmental
Pocket Penetrometer Test | What Engineers Must Know. (2020, July 14). Soil Mechanics and Engineering Geology. https://www.youtube.com/watch?v=kWaCaKE7jXc&t=112s&ab_channel=SoilMechanicsandEngineeringGeology
Soil classification. (n.d.). Osha.gov. Retrieved March 22, 2023, from https://www.osha.gov/vtools/construction/soil-testing-fnl-eng-web-transcript
Trenching and Excavation Safety. (2015). Osha.gov. https://www.osha.gov/sites/default/files/publications/osha2226.pdf
Trenchlesspedia, W. is P.-D. (2017, May 28). Potholing. Trenchlesspedia.com; What is Potholing? — Definition from Trenchlesspedia. https://www.trenchlesspedia.com/definition/2523/potholing
