According to the Census of Fatal Occupational Injuries (CFOI) data, published by the Bureau of Labor Statistics (BLS), between 2011 and 2016 there were 130 fatalities recorded in trenching and excavation operations, with nearly half of all deaths occurring in 2015/2016. More than a third of all trench/excavation deaths occurred at industrial premises and another third occurred at private residences, while one-fifth occurred at streets or highways.
Trenching and excavation work presents serious hazards to all workers involved. Cave-ins pose the greatest risk and are more likely than some other excavation-related incidents to result in worker fatalities. One cubic yard of soil can weigh as much as a car—nearly 3,000 pounds.
Trench collapses (cave-ins) pose the greatest risk to workers’ lives. But there are numerous other hazards, such as falls, falling loads, hazardous atmospheres, and incidents involving mobile equipment.
“The biggest hazard in trenching is a trench collapse,” says Scott Smith, general manager, Groundworks Safety Systems. “Each year many people are killed while working in unprotected trenches. Ground pressure is so great; just four feet deep can be enough to kill a person, yet workers continually place themselves in harm’s way.”
“There are a lot of potential hazards in construction, including trenching and shoring,” says James McRay, director of marketing and media relations, Efficiency Production. “The difference is people are more likely to be killed in a trench than in many other construction applications. It is more likely to kill you than falling from a height or being struck. In trenching, the most common accident is also the mostly deadly. So, this makes trenching and shoring one of the most deadly among the construction trades.”
According to OSHA, it is “because of the continuing incidence of trench/excavation collapses and accompanying loss of life, [that] the agency has determined that these worksites continue to warrant an increased enforcement presence.” This National Emphasis Program (NEP) on Trenching and Excavation builds on OSHA’s longstanding belief that “employees exposed to potential cave-ins must be protected before the excavation face is in imminent danger of collapse” and that no trench is free from the risk of trench collapse.
“OSHA believes that the rate of deaths and serious injuries resulting from trenching and excavation incidents (mostly collapses) can be significantly reduced if OSHA concentrates resources to effectively engage trenching and excavation operations through both enforcement and compliance assistance activities,” says OSHA in the trenching and excavation regulation update.
Recent Legislative Changes
OSHA standards require that employers provide workplaces free of recognized hazards. In trenching applications, employers must comply with the trenching and excavation requirements of 29 CFR 1926.651 and 1926.652 or comparable OSHA-approved state plan requirements.
OSHA updated regulations surrounding trenching and excavation in October 2018. The instruction “provides updated guidance to Occupational Safety and Health Administration’s (OSHA) national, regional, area, and State Plan offices for continued implementation of an OSHA National Emphasis Program (NEP) to reduce or eliminate workplace hazards associated with trenching and excavation operations.” It has been more than 30 years (1985) since OSHA first implemented CPL 02-00-069 - Special Emphasis: Trenching and Excavation.
The updated instruction continues support for compliance assistance and inspection programs related to the implementation of a NEP for trenching and excavation operations. According to a statement put out by OSHA, the plan:
- replaces CPL 02-00-069 - Special Emphasis: Trenching and Excavation;
- provides a national reporting system for all OSHA trenching and excavation inspections by updating guidance for recording trenching and excavation inspections; and
- establishes the requirement for each region to create and deploy outreach programs in support of the new changes.
“Removing workers from and helping workers identify trenching hazards is critical,” says Deputy Assistant Secretary of Labor for Occupational Safety and Health Loren Sweatt. “OSHA will concentrate the full force of enforcement and compliance assistance resources to help ensure that employers are addressing these serious hazards.”
According to OSHA, “Any unprotected trench or excavation that is brought to the attention of the Area Office shall be evaluated, and, if appropriate, inspected. Inspections under this NEP shall normally be limited to evaluating worker exposure to safety and health hazards associated with the excavation. However, a Compliance Safety and Health Officer (CSHO) may expand the scope of an inspection if other safety and health hazards or violations are observed in plain view and/or brought to their attention.”
Key to the changes in this most recent NEP is the scheduling procedures. In order to address the sometimes short schedules of trenching and excavation operations, OSHA has made the following change: “Compliance Safety and Health Officers shall initiate inspections under this NEP whenever they observe an open trench or an open excavation, regardless of whether or not a violation is readily observed. These observations may occur during the course of their normal work-day travel or while engaged in programmed or un-programmed inspections.”
However, these new changes may not be uniform across the entire country. States are permitted to create their own plan, which may be different from the new regulation. According to OSHA, each region, whether they use an approved State Plan or the NEP proposed by OSHA, must market their new trench safety guidelines for 90 days before they can execute any trench or excavation investigations.
Identifying Hazards
Before trenching begins, there could be hazards in the work environment. Ensure that you have located all built structures, such as pipelines and cables, that are underground near the trench.
Then, a worker designated as a competent person, must classify the earth being trenched into one of four categories:
- Stable Rock – natural solid mineral matter that remains intact when excavated vertically
- Type A – cohesive soils, such as clay, silty clay, sandy clay, and clay loam, which have an unconfined compressive strength of 1.5 tons per square foot (tsf) or greater
- Type B – cohesive soils, such as angular gravel (similar to crushed rock), silt, silt loam, and sandy loam, which have an unconfined compressive strength greater than 0.5 tsf but less than 1.5 tsf
- Type C – cohesive soils, such as gravel and sand, which have an unconfined compressive strength of 0.5 tsf or less but also rocks submerged in water and material in sloped layers
From this classification, the competent person will determine what grade for the trench walls is safe. In stable rock, trench walls can be steep. In Type C soil conditions, trench walls need to be very sloped. A ratio of 18 inches in width for every 12 inches in depth is generally accepted as safe for all soil types.
The Competent Person
The competent person on a trench is the person trained to and tasked with inspecting trenches daily and as conditions change, and with identifying existing and predictable hazards or working conditions. They are also authorized to take action in order to eliminate any found hazards or conditions.
“The competent person must know OSHA standards, know how to identify soil, and know the correct use of protective systems. The competent person must be authorized to stop work when hazards arise and must be empowered to do so,” says Mitch Post, senior manager, operational & leadership training delivery, Sunbelt Rentals.
How deep is the trench? Trenches in Soil Types A, B, and C 5 feet deep or greater require a protective system, such as a trench box. At less than 5 feet deep, the competent person may determine that a protective system is not needed. Trenches 20 feet deep or greater require that a protective system that has been designed by a registered professional engineer be used.
“There’s a misconception that contractors don’t have to worry about any trench less than five feet deep,” says Post. “OSHA leaves it up to the judgment of the competent person, but if you get someone hurt or killed in a three-foot trench, you’re every bit as liable as if that trench was 30 feet deep.”
“Environmental conditions do change over the course of a project. Is there other construction happening in the area? Is there a change in weather? Rain can be damaging to trench integrity. Water changes the property of soil,” says McRay.
“Job site conditions can change from day-to-day and even throughout the day with weather and on-site activities,” says Post. “It is imperative that the competent person on site inspects the excavation each day prior to starting work and also throughout the day in case anything has changed that would compromise the system.”
Other Hazards
Besides cave-ins, there are many other worksite hazards, such as falling loads, hazardous atmospheres, hazards from mobile equipment, and electrocution or explosions, which can occur when workers contact underground utilities.
Heavy equipment is instrumental in trenching and excavation operations, but can also be damaging to trench integrity. Their weight can put a lot of pressure on the earth below, causing a trench to collapse or for an excavated vertical surface to fail.
Being struck by a machine or even struck by a protective device is possible. “The hazards of working with trench boxes are mainly during the assembly and disassembly processes, and especially when the trench box has wider spreads,” says Smith. “Much of this is never discussed within the trench safety industry. They call it trench safety, but the harsh reality is the handling of the product can be quite unsafe.”
Trench Safety in Retrospect
Steam Ditcher Company manufactured the first trencher—the Buckeye No. 88. The trencher became popular and soon other companies introduced their own trenchers.
As trenching became a greater part of construction, the disproportionate number of deaths became apparent.
“By 1925, most states had general construction safety orders in place,” says Smith. “There was already danger associated with excavation walls and rules for sheeting, and angled timber strut bracing the walls were set in place.”
Trench construction technology changed little until the 1950s. In that decade, manufacturers introduced drive trains and hydraulic motors, as well as several other features that made the machines more user-friendly.
After 1950, the regulations were upgraded with a statement pertaining to all excavations that “‘No employer shall cause or permit his employees to work in or adjacent to any excavation until a reasonable examination of the same has been made to determine that no conditions exist exposing them to injury from unstable ground,’” says Smith. “This is akin to the modern requirement that all excavations are inspected by a competent person.”
During the next few decades, trenchers, like many construction equipment categories, increased in size.
“In 1971, Efficiency designed our first trencher in response to a contractor’s desire to work more efficiently and to make more money by installing a shield to protect workers so he could move excavate less dirt and ,” says McRay. Efficiency Production is the first company to commercially manufacture steel trench shields, sometimes called trench boxes.
The desire to protect workers in trenches had both a commercial response and an institutional one. “In the ’70s, OSHA was formed to enforce the rules of workplace safety, and in 1979, it began to focus on existing excavation safety regulations with the intent of improvement,” says Smith. “Some of the most notable outcomes from this were standardization of sloping and soil classification that is still in place now.”
Trench safety products evolved from timber sheeting and bracing to steel and aluminum trench boxes and shores. “Most of the innovations for trench safety took place shortly after the Second World War when machines and steel manufacturing technology advanced and began to resemble modern trench safety,” says Smith.
“Until 2015, the common trench box of steel walls with pipe spreaders had changed little since its inception and at some point, someone needs to place themselves into a vulnerable position or someone needs to be under a heavy, suspended load,” says Smith. This is called “the line of fire,” meaning if something goes wrong, the person(s) in the line of fire is/are at very serious risk. Injuries ranging from missing fingers to crushed body parts and even deaths have taken place while workers have been assembling or disassembling trench shields.
Modern Protective Systems
There are several ways employees can be protected while working in a trench; OSHA recognizes slopes, benching, shoring, and shielding as legitimate ways of protecting workers in a trench.
Sloping involves removing more soil, so that the top of the trench is wider than the bottom of the trench. For trenches in Type C soil, OSHA requires 18 inches of width for every 12 inches of depth (53 degrees). For Type B soils, its 12 inches of width for every 12 inches of depth (45 degrees), and for Type A soils, its 9 inches of width for every 12 inches in depth (34 degrees).
Benching is similar to sloping, but instead of the trench wall being a direct line from the bottom of the trench to the top, it is a series of “steps” (perhaps 1 foot wide for every foot deep). The trench wall looks like a flight of stairs, which is great for ingress and egress, but it can’t be applied in Type C soils.
Shoring uses various support systems—the most popular of which are timber and aluminum hydraulics—to prevent soil and the trench walls from moving. When using this method, vertical shores are installed periodically throughout the trench and each one stretches the width of the trench.
“Shoring is a term that is used for any type of device or system or equipment that prevents walls from moving,” says McRay. “The whole goal of shoring is to prevent cave-ins by making sure trench walls are supported at all times and will not collapse. That was the original concept of trench safety.”
When an aluminum hydraulics support system is used, workers use a hand pump to pressurize the hydraulic cylinders until the vertical shores fit tightly in place.
“What has really become popular in the last 30 to 40 years is hydraulic shoring,” says McRay. “It is a device that prevents trench cave ins by applying hydraulic pressure into the banks of a trench. You activate it by using a hand pump that pumps water into the hydraulic cylinders and the pressure arcs into the walls and prevents them from collapsing.”
“Our extensive trench shoring equipment includes both shielding and shoring solutions. Excavation shoring support equipment includes sheet and frame systems, slide rail, monitoring equipment, and excavator attachments,” says Post.
Shielding uses trench shields (also called trench boxes) to prevent collapsing soil. “You have to pick one that is right for the soil type,” says McRay. “Trench boxes are composed of two large plates held across by four cross members, and the plates are positioned to extend 18 inches higher than anything that can cave in.”
“For shielding solutions, Sunbelt Rentals offers steel and aluminum trench boxes, steel manhole and water-tap boxes, height adaptors, bedding boxes, hydraulic shoring shields, hydraulic vertical shores, aluminum lite-shields, and steel road plates,” says Post.
“If you have a trench box that’s not large enough for your excavation, or strong enough, or installed improperly, then you don’t have a trench box,” says Post.
When inspecting trench boxes, ensure the boxes are free from damage and defect, the plates have no deformities or holes, the welds are not cracked or bent, that all struts are accounted for, and that the position of the trench box hasn’t shifted.
Choosing a protective system for a trenching job can be complex because there are so many factors that must be considered, including soil classification, depth of cut, water, content of soil, changes caused by weather or climate, surcharge loads (e.g., spoil, other materials to be used in the trench), and other operations in the vicinity.
“Our trained shoring professionals are experienced in specialty applications and can assist you with pit-box configurations, slide rail systems, sheet and frame systems, and site-specific engineered systems. We can offer PE-stamped plans from our in-house engineers for all of our shoring projects,” says Post.
Trenching and Shoring Tips
There are hazards throughout the entire trenching process, and hazards are easier to avoid if you avoid the circumstances that lead to potential hazards, such as properly using protective systems and proper planning. Workers and employers both have responsibilities to mitigate hazards.
Here is a list of tips for trenching and shoring:
Before Digging
- Survey the entire work area for potential hazards. Log them. Communicate them to staff. This includes all structures above and below the surface, as well as environmental conditions.
- Call 811. Have utility companies locate their utilities. Electrocution and explosion are other ways workers die in trenches.
- Identify the soil type(s) located at or near where the trench will be dug.
- Consider all local legislation regarding trenching and or/shoring.
- Plan your work and work your plan. Consider protective structure needs, equipment needs, labor needs, expected weather, etc. Have back up plans.
- Check areas next to the site for potential hazards and for anything that could impact the stability of soil, such as nearby vehicles and equipment that can cause the soil to vibrate and collapse. Determine if any of them could affect trench integrity.
- Properly train workers in their respective roles, trench safety, and general safety awareness.
When Digging
- Plan appropriate organization of the work site. Keep excavated soil (spoils) and other materials, including equipment, at least 2 feet from trench edges.
- Inspect the trench at the start of each shift. Daily inspect the ground around trenches for tension cracks, which sometimes develop parallel to the trench. Examine the soil inside the trench.
- Inspect trenches after any event that could have changed conditions in or around the trench.
- Ensure the trench remains free of water; the presence of water in soil changes the soil and is a common cause of trench wall failure.
- Protect workers from falling into the excavation by clearly marking the trench edges.
- Have a means of exiting the trench or multiple exit points if the trench is long.
- If necessary, protect, support, or remove any underground installations.
- Communicate all potential hazards to workers and train them on how to deal with the hazard, as well as to avoid it. Communicate to them of any new hazards or other changes in work site conditions.
After Digging
- Inspect the trench at the start of each shift. When more than 4 feet deep, test for oxygen levels, hazardous gas, vapours, and dust. Never enter a trench that hasn’t been properly inspected.
- Use the protective system that is best suited for the specific trench that is being dug.
- Daily inspect protective systems. Look for cracks, bends, or holes; these could compromise the ability of the protective system. Check if equipment has shifted; this could be a sign that the soil is moving.
- Do not work under suspended or raised loads and materials. “Know the situation and stay out of the line of fire,” says Smith. “There is only one trench box that can be assembled and disassembled without placing workers in the line of fire and that is GroundWorks Safety Systems.”
