Trench Safety 101
In 2022, OSHA reported that 39 people in the United States died in trenching-related accidents. These deaths were caused by several violations of OSHA's trench safety guidelines. The four most common reasons included failing to inspect excavation sites daily, not providing a safe entrance/ exit, placing material too close to the trench's edge, and failing to provide cave-in protection to workers.
These violations indicate that unsafe trenching practices are a prevalent issue on many job sites. A lack of safety knowledge, little to no leadership buy-in, and cost reduction efforts are the main cause of trench safety violations and the deaths they're directly responsible for.
The 39 deaths in 2022 were twice the amount of cave-in deaths in 2021. Educating people on hazards and safety practices is the first step toward significantly reducing the death toll in 2023.
There's more to trenching than digging a hole in the ground. Multiple hazards can occur throughout the process which often aren't revealed until people are already inside. These hazards are important to prepare for prior to excavation.
Cave-ins: Cave-ins are the most common hazard. This occurs when the soil forming the trench walls fall into the trench, destroying it entirely. Improper reinforcement is the leading cause of cave-ins.
Exposure to Buried Lines: Power, gas, water, and telecommunications lines can all be disturbed during excavation. Disturbances can cause electrocutions, gas leaks, and other injuries.
Falling Objects: Soil, materials, and equipment can fall into trenches and onto the people working below. This often occurs when materials are kept too close to the trench's edge.
Flooding: Water within the soil or rainwater can accumulate at the bottom of the trench. Excess water causes flooding and reduces soil stability. This water should be removed and soil density should be retested before re-entrance.
Exposure to Airborne Toxins: When soil is disturbed during excavation, hazardous airborne materials can be released. This includes gas, biohazards, vapors, dust, and inadequate oxygen levels.
Entrance and Exit Injuries: Poorly installed entry and exit equipment can cause injuries as workers use them. Exits contained outside of reinforced trench space pose the greatest risk.
Employers and/or site supervisors are responsible for accident prevention on site. Before digging and sending people inside trenches, supervisors must complete the following requirements to create a safe environment.
Identify Buried Lines
As previously mentioned, coming into contact with buried lines can cause serious injury to workers and damage equipment. Before excavation, site supervisors should survey the area to locate buried lines. Utilities providers and municipalities should have this information. Certain projects may require power to be shut off, or lines may be avoided entirely.
Classify Soil Type
The soil type your excavation site is located in indicates its stability. Some soils are dense enough to support significant loads, while others are weaker and require reinforcement.
Soil types are categorized differently in various jurisdictions. Generally, soils are sorted into Types A, B, and C. Type A soils are clay-based, cohesive soils. Cohesive soils are non-crumbly soils that are held closely together by cohesive strength. Type A soils are solid and can withstand significant compressive stress.
Type B soils are cohesive, silty soils that withstand less compressive stress. Whereas Type C soils are the loosest, granular soils that have lower load-bearing capacity. In summary, Type A soils are the strongest, and all subsequent soils are weaker.
Contractors must classify the soil type before excavation and identify its angle of repose using a soil density test and inspection. It's important to note that multiple soil types may exist within a trench. Topsoil and the base of a six-foot trench often have different degrees of stability.
Identify Load-Exerting Structures
Structures and foundations in close proximity to the excavation area are exerting significant pressure on the earth. Depending on the soil's bearing capacity and the trench's proximity to these structures, the stability of its walls may be affected. It's important to inspect the environment before excavation, as additional reinforcement may be required to prevent cave-ins for soils in weakened states.
Everyone on-site should receive trench safety training. Many people, supervisors included, aren't aware of the risks deep excavations pose. Cave-ins, severe injuries, and deaths are direct consequences of poor safety training. Without this knowledge, your team won't understand how important it is to take precautions on excavation sites.
Supervisors are responsible for completing the following tasks during excavation.
Test Oxygen Availability and Air Quality
There needs to be adequate oxygen levels at all depths within the trench. Oxygen availability must be tested before workers enter the trench, throughout construction, and before re-entrance. Air quality tests should also check for the presence of hazardous gases and other toxins. Having the site inspected by a professional will prevent further injury down the road.
Install Structural Supports
Installing structural supports inside trenches is the best way to protect workers. Structural supports reinforce trench walls, which add stability to soil and prevents collapse. Common structural supports used are shoring, sloping, and shields. OSHA mandates shoring installation in trenches at depths equal to or greater than 5 feet. However, shoring may be required in shallower trenches when soil-bearing capacity is poor or the excavation site is deemed more hazardous than usual.
Install Proper Exits
Exits must be installed from the inside of the trench. The Canadian Centre of Occupational Health and Safety requires that exits are no more than 25' away from any person inside the trench. OSHA requires that exits for trenches 4' or deeper must surpass the trench's edge by at least 3 feet. So, exit ladders must be securely installed to prevent injury and extend 3' above the edge.
Equipment and Materials Placement
Another common injury common among workers is caused by materials falling into trenches and then onto people. These materials are tripping hazards, causing people themselves to fall into excavated ground.
The root issue behind these injuries is equipment and materials placement on site. OSHA requires all materials and equipment to be at least 2 feet away from the trench's edge. This prevents people from tripping and falling and objects from being knocked over.
As for equipment, running engines can cause vibrations within the surrounding soil. These vibrations can trigger a cave-in if they occur along the trench's edge.
Contractors should familiarize themselves with additional excavation safety requirements in their jurisdiction. Enforcing PPE use and safety compliance, establishing emergency procedures, and daily excavation inspections should all be regular practices on site.
There are several structural supports that protect people who work inside trenches, including sloping, shoring, and shields. Failing to reinforce deep trenches causes excavation- related deaths. Using these techniques is essential to creating a safe excavation site.
Sloping is a common technique used during construction. Contractors will cut the top portion of the trench walls at a sloped angle. As opposed to having the walls perpendicular to the ground's surface, slopped walls form a funnel leading into the hole.
The soil is sloped at the angle of repose, which is the maximum angle soil can be heaped to without sliding down the sides of the pile. Soil sloped at any angle less than or equal to the angle of repose won't slide into the trench. However, trench walls that aren't sloped are greater than the angle of repose and naturally slide. Soil classification and density tests will reveal the angle of repose.
For trenches deeper than 4 feet, shoring is the most effective reinforcement technique. Constructed with walls or beams, shores hold the sides of the trench in place by bearing the soil's weight. There are multiple shoring techniques that work best in different environments including hydraulic shoring, sheet pile shoring, and dead shoring, among other methods. Even though shoring isn't required in shallower trenches, it's a good idea to install it for additional reinforcement.
Trench boxes or shields are the last protective measure in place when cave-ins occur. They reduce the risk of injury by taking the brunt of the force imposed by sliding soil. The boxes should be used in addition to shoring, as their main purpose is to protect workers if shores fail to do so. Shields are formed with two metal panels that are attached to rods. Each panel lies parallel to a trench wall and people work inside the gap found between each plate.
The only way to reduce trench-related injuries and deaths is to actively incorporate safety practices into your operations. Inspecting your excavation site for hazards, providing safety training, and reinforcing the walls will keep more people safe on-site. Doing so will create a standard, which must be consistently enforced and modeled from the top down.
Frequently Asked Questions About Trench and Excavation Safety
What are some of the hazards associated with trenches?
There are multiple hazards involved, including cave-ins, inadequate oxygen supply, flooding, and exposure to airborne toxins, among others. However, taking the proper precautions and safety training will mitigate these hazards.
What are the three main protection methods against cave-ins?
The three main protective measures against cave-ins are sloping, shoring, and shielding.