Construction

Safety Assured

Safety practices for construction trenching and the equipment used to save lives

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Credit: National Trench Safety
Prior to his famous Antarctic expedition, Victorian explorer Sir Ernest Shackleton ran the following advertisement for men to join his team: “Men wanted for hazardous journey. Low wages, bitter cold, long hours of complete darkness. Safe return doubtful. Honor and recognition in event of success.” Needless to say, this was before modern worker safety and OSHA regulations. We no longer demand such a disregard for safety from our construction crews. This makes it imperative that a site operator and his contractors do all that they can to ensure the safety of their employees, especially in inherently unsafe operations such as trenching.

And make no mistake, trenching is one of the most dangerous construction activities. Trenching is rated as the most dangerous job site operation, ahead of falls from high ladders and scaffolding, crushing by falling loads from above, confined spaces and hazardous atmospheres, vehicle collisions and other mobile equipment accidents such as overturning, fires and explosions, or electrical shocks. Even partial burial by a collapsed trench sidewall can cause asphyxiation by the weight of the soil pressing against a worker’s diaphragm muscle, making it impossible to breathe even if his upper half remains unburied. Construction workers can lose their lives in a trench collapse, tragedies that are easily preventable if proper precautions are taken.

According to Blake Smith, sales and marketing director for United Rentals Trench Safety, “OSHA data clearly shows that trench collapses don’t discriminate by age, and trenches less than nine feet deep account for the vast majority of fatalities. A contractor who thinks a shallow trench poses less of a risk is making a serious mistake. Regardless of the project size or depth, if a Competent Person doesn’t do his or her job, injuries and fatalities will continue to occur. Responsibilities include conducting daily inspections before starting work and continuing to monitor the site throughout the day, after every rainstorm, and after every occurrence that poses a potential hazard.” Table 1 provides the 10-year statistics from OSHA.

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OSHA Trenching Standards—Slope and Benching Requirements
The Occupational Safety and Health Administration (OSHA) has established a set of definitions enshrined in Federal regulations (29 CFR 1926 Subpart P Appendix A) that describe and define soil conditions and their effects on trenching operations. These definitions are based on the soils’ physical characteristics, particle sizes, and mechanical strengths. Since a contractor has to accept the local soil conditions he will be working in, he must adjust his trenching and excavation operations to ensure job site safety. OSHA has accordingly grouped various soil types into four categories from strongest to weakest (Stable Rock, Type A, Type B, and Type C), while mandating different trench configurations and excavation procedures for each. These soil categories are summarized in Table 2.

Though there are many types of stable rock formations, they all meet one defining characteristic as defined by OSHA: “natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed.” Aside from large rocks and boulders encountered during excavation, rock formations are typically extensive and thick. Though by definition rock formations are safe to trench into, as a practical matter they are seldom found at construction depths at most job sites. Furthermore, rock is extremely expensive to excavate or trench into, so what it provides in terms of safety is lost by being prohibitively expensive to cut (often requiring explosives).

After stable rock, the next strongest soil is Type A (mostly stiff clays and hardpan), having unconfined compressive strengths in excess of 1.5 tons per square foot—material that is considered a true soil and not rock. No soil is Type A if it is fissured, receiving vibration from heavy vehicle traffic or equipment operations, disturbed by previous excavation or trenching operations (including blasting), or part of a sloped area steeper than four horizontal to one vertical (4H:1V) or part of multi-layered soil strata.

Soils of moderate strength, with unconfined cohesive strengths between 0.5 and 1.5 tons per square foot, are categorized as Type B. These tend to be less cohesive soils with significant amounts of gravel, sand, and silt. It also includes those soils that would initially be considered to be Type A but are subject to the environmental characteristics and/or operational damage described above.

Type C is the weakest and therefore the most dangerous soil to excavate a trench in. This is soil having an unconfined compressive strength of less than half a ton per square foot. This includes granular soil (gravel and sand) and soil subject to groundwater that is freely seeping from the sidewalls of the excavation. It also includes friable soil likely to crack and crumble. Type B soils that are subject to fissures, vibration from vehicle and equipment operations, and so on are also considered to be Type C. It also includes soil in a sloped area steeper than four horizontal to one vertical (4H:1V) or part of multi-layered soil strata.

OSHA safety standards require (at minimum) the following safety measures during trenching operations to prevent sidewall collapse and cave-ins: sloping and benching the sides of the excavation; supporting the sides of the excavation; or placing a shield between the side of the excavation and the work area.

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Maximum allowable sloping (or battering) for each soil type in trenches no deeper than 20 feet are summarized in Table 3. This applies to both smooth slopes and equivalent slopes resulting from benching—5 feet maximum height for each bench—stepped into the side slopes of the trench.

Benching is allowed for only Type A and Type B soils. Type C soils cannot be benched and must be cut back with smooth side slopes. Trenches with vertically sided lower portion must have trench shield or shoring installed to protect workers at the bottom of the trench.

How exactly does a trench fail? Essentially, a mass of soil along the trench sidewall gives way and falls into the open trench, burying anything and anyone unfortunate enough to be in its path. What distinguishes trench failure into different modes is the shape of the failure plane passing through the soil behind the shifting soil. This plane will either take the form of a straight-line wedge or a rotational failure curve. The potential for any trench wall to collapse is a function of its inherent factor of safety against a particular failure mode.

Rotational failure occurs along an arc segment of a circle passing through the soil and centered on some axis of rotation located in space above the soil mass. The weight of the soil rotating about this axis is resisted by the soil’s strength (internal friction and cohesion) acting along the failure arc in the opposite direction. For this reason, cohesionless soils (especially Type C soils) are inherently less resistant to failure due to the lack of cohesive strength and having only internal friction to resist the force of its own weight. In both cases, the forces are resolved into vectors that act along the failure arc. The ratio between the two opposing forces gives the trench walls factor of safety against failure.

Failure along a wedged plane occurs in a manner similar to that of rotational failure, with the driving weight of the [sol]mass being resisted by the soil’s internal strength. Only in this case, failure can occur along a flat plane set at variable angles to the horizontal, creating a wedge of soil with the side wall of the trench. Instead of rotating around an axis, the driving and resisting forces act in opposite directions along the flat failure surface.

For long-term excavation and open trench conditions, higher factors of safety are required. Typically, a minimum factor of safety of at least 1.5 is required for permanent or long-term conditions. For short-term and temporary operations, a factor of safety of 1.2 is needed. For the rare possibility of seismic action and earthquake loads, a built-in factor of safety of at least 1.0 is required.

Other factors may reduce inherent strength. Excess water pressure in sandy soil can cause boils to appear at the base of the trench reducing the strength at the toe of the trench sidewall. Conversely, excessive drying can reduce the cohesive strength of soil with a large sand component. The combination of long-term open conditions and applied surcharge loads can deform soil, causing sidewall collapse. Leaving a trench inactive for long periods and exposing it to the effects of weather and climate can also create weaknesses. No trench should be used after prolonged inactivity without a thorough inspection being performed.

Current Methods and Equipment
When benching is not practical and shallow cut back slopes are not possible due to constrained site conditions, more active and direct methods are required. These include both shields and movable boxes to protect personnel in the trench and shoring structures to reinforce the trench sidewalls. Under OSHA rules, any trench deeper than 5 feet requires some sort of protective system.

Shoring is the more active measure with structures that actively brace themselves against the sidewalls of the trench. These consist of two parallel walls of panels (for trenching operations) or four-sided boxes (for pits and other deep digs). The panel walls are supported and braced by interior members placed horizontally to connect the two walls. There are three methods of providing this support: hydraulic shoring, driven steel and plate construction, and soil nailing. Hydraulic shoring utilizes hydraulic pistons manually or mechanically pumped with high pressure to force the two halves of the shoring member to push apart. This, in turn, applies pressure to the panel side walls and buttresses the soil’s internal resisting strength by creating active pressure to prevent soil failure. The panels themselves typically consist of reinforced plywood (also known as “finform”), aluminum, or steel sheets. Driven steel and plate utilizes steel I-beams driven vertically into the soil at the base of the trench at regular intervals along the length of the trench. Once the I-beams are in place, steel plates or plywood sheets (referred to as “soldier boarding”) are inserted between them and the trench sidewall. Lastly, soil nailing is a kind of reverse shoring. Instead of having internal structures to exert outward pressure on the trench sidewalls, soil nail inserts external reinforcing bars into the trench sidewalls themselves to anchor the soil in place. Inserted into holes drilled at designed intervals, spacing, and angles, the reinforcing bars are then grouted in place to secure the sidewall against failure or collapse.

Shielding is not the same thing as shoring. While shoring actively buttresses the trench sidewalls to prevent failure from happening in the first place, the point of shielding is to protect trench occupants should a failure occur. Though not as safe and secure as shoring, trench shields can be more than sufficient to provide needed safety while being significantly less costly. Used in either the form of prefabricated boxes or walls of sheeting, shields are typically mobile—they can be dragged down the trench as needed to provide protection where workers are located during the course of construction. For example, a trench cut to allow for the installation of buried pipelines can be used to protect pipe layers where they are working to join a pair of pipe segments together. Once that operation is done, the trench box can be dragged down the pipeline to the next pipe joint. Shields are easily installed, which makes them ideal for emergency trenching operations where time is of the essence.

The Human Element, Training, and the “Competent Individual”
With all this talk of soil analysis and protective structures, it is easy to forget the human element in trench safety. In all cases, OSHA requires that the safety conditions of any trench be judged by a “competent individual.” Furthermore, studies have shown that the single most important element in trench safety is the safety training given to trench workers. While a detailed analysis of trench conditions and the design of trench protective systems are best left to a registered professional engineer, it is in the field appraisal where competency is largely required. As such, the “competent individual” ensuring the safety of the trench operations is as important as any shoring or shielding.

Field appraisal involves both initial and daily site inspections of the trench and its construction activities. This is not just common sense; it is an OSHA regulatory requirement. The person performing the inspection does not need to be a registered professional engineer, but he or she must have the necessary experience and insight needed to identify potentially hazardous, unsafe, or unsanitary conditions. For trenching operations in areas where the accumulation of noxious or explosive gases are possible (such as the trenching to install leachate or landfill gas restriction pipelines in municipal solid waste landfills), competency also requires the ability to test and evaluate the atmosphere within the trench. Furthermore, the inspection will require the expertise necessary to take corrective measures to ensure trench safety. This includes the authority to stop all work if needed until these corrective measures can be put in place.

Safety includes actions as basic as how to properly enter and leave the trench (via ladders extending 3 feet above the top of the trench and installed every 245 feet along the length of the trench). Safety inspections are required not just at the start of the workday, but after events that could weaken the trench—such as after a major rainstorm. Surcharges from the weight of soil stockpiles, tool boxes, stacks of pipes, and heavy equipment should be avoided by keeping them at least 2 feet back from the edge of the trench.

And while it may seem counterintuitive, it is often the poorer soils that can be the safest overall. This is because the danger is apparent from the start and trenching operations can utilize proper safety structures and procedures from the outset. Contrarywise, soil that looks hard and competent may hide flaws below its surface. No “competent individual” would ever assume a soil to be safe merely from its appearance.

Trenching Safety Trends and New Trench Safety Technology
The first trench safety trend involves this very same human element. Training and outreach to workers are being expanded to include not just basic OSHA requirements but a more sophisticated knowledge of trench shoring methods, how they are installed, and how they should work. The workers in the trench may never be involved in shoring installations themselves, but a thorough understanding of how they work will provide technical/situational awareness and allow them to self-inspect while working and increase operational safety accordingly.

After making advanced safety training free and readily available to workers (preferably in a bilingual format), such training should include lessons learned from case studies and survivors of trench failures, video illustrations of the rapidity of trench collapse, classroom study to provide a theoretical knowledge of trench failure, interactive and web-based training modules and information libraries, how to monitor work practices, and the use of practical checklists for the inspection of trenches and protective equipment.

Innovative trench box design is another advancement. These designs utilize advanced composite materials and configurations to maximize strength while minimizing weight. Advanced trench box engineering allows an engineer to pick a protective system that best fits the trench site’s needs. At its most basic, a trench box consists of two parallel sides connected by fixed cross beam members of a required length to match the width of the trench. The aluminum panels can be layered with foam filling between the aluminum plates for reduced weight. Aluminum boxes are high strength and lightweight. Both characteristics translate into improved trench construction safety and productivity.

For deeper sewer and pipeline work, and in environments that are intently more dangerous (subject to applied loads or vibrations from equipment), steel trench boxes are preferred. These are strong enough to protect against failure while being rugged enough to be subject to continuous relocation in dig and push trenching operations­—or long-term open trench conditions. Heavy-duty steel boxes can come with hydraulically adjustable cross members, while beam and plate designs ensure cost-effective safety.

At all times, trench contractors need to realize that there is no “one size fits all” solution to trench safety. Again, from Blake Smith, sales and marketing director for United Rentals Trench Safety: “Another thing to bear in mind is that, given today’s complex excavations, a manufactured shoring system may not be adequate to ensure safety. In this event, the project would require a site-specific, engineered solution. United Rentals has an internal engineering department dedicated to custom solutions, often integrating multiple systems in combination to create a safe work zone. As cities continue to grow, and their infrastructures continue to age, job site safety has become more complex. A strong safety program—including a Competent Person—has become vital. Trenching deaths are preventable. No worker should ever be put at risk. Our industry can do better by leveraging the knowledge and training available.”

Credit: Efficiency Production

Major Suppliers
With over 45 years of experience, Efficiency Production has met the trench shielding needs of underground utility and construction excavation contractors. They provide a wide selection of trench boxes meeting the most stringent OSHA standards. Each of their protective steel boxes is certified by a professional engineer that it meets OSHA excavation safety standards.

Efficiency was one of the first shoring manufacturers to specifically address a common problem for underground utility contractors: existing utilities that cross through new trenches and excavations. Cross-trench utilities are even more prevalent when excavating in urban areas for upgrade work on America’s aging infrastructure. There are very few places a contractor can excavate and not encounter pipes and conduits of varying sizes and depths already in the ground. Early on, Efficiency Production designed and engineered their SHORE-TRAK Cross Trench Utility Shielding System to combat this problem.

Credit: United Rentals
United Rentals can offer Efficiency Production’s
Build-A-Box system.

SHORE-TRAK consists of a series of steel sheeting guide frames that sit level on the ground or in an excavated pilot hole and connect together in a four-sided configuration typically by sliding the ends of the guide frames into Slide Rail System corner posts; alternatively, the installer can connect the guide frames with specially designed corner brackets that pin over spreader collars welded onto both ends of the guide frame. In this configuration, Slide Rail posts are not used, which simplifies installation. “Compared to H-Beam and Lagging, the Efficiency Shore-Trak was faster to put in and there was also a cost saving,” says Pan-Oceanic Engineering’s Project Superintendent, Erik Kohman. Other applications of Shore-Trak utilize standard trench box spreader pipe, connecting cross-trench to the spreader collars in a more traditional trench shielding operation.

The Slide Rail described above is installed by sliding steel panels—similar to trench shield sidewalls—into integrated rails on the vertical posts, and then pushing the panels and posts incrementally down to grade as the pit is dug as part of a “dig and push” shoring system. Efficiency’s Panel Guides replace the Slide Rail panels that slide down the open-faced inside rails on the posts. This allows contractors to place short lengths of KD-6 sheeting through the 7-inch slot in the Panel Guides to tightly shore around existing utilities. “We considered tight-sheeting, but that would require us to take out both the 60- and 90-inch sewers and use bypass pumps the whole time, which would increase the cost of the project dramatically,” says Zac Birnbaum, American Excavating’s Project Manager. “Instead we went with an Efficiency Production Slide Rail System because we could use panel guides and short lengths of KD-6 sheeting to tightly shore around both of the big pipes, without having to remove them.”

Quicksheet Guideframes have “mitered” corners with overlapping pockets that pin together like a door hinge, creating a 4-sided “picture frame” system that can be set up on the ground or in a shallow pilot hole. The 4-foot-tall, 24-inch-wide sheeting guide frame has a 7-inch slot where sheeting can be stood up and overlapped, then pushed down with an excavator bucket. The Build-A-Box Sheeting Guide Frame also utilizes lightweight corrugated sheets of aluminum to shore closely around existing cross-trench utilities. The sheets can be installed by hand, and the guide frame panel integrates seamlessly into any Build-A-Box configured system.

National Trench Safety (NTS) is a national provider of trench and traffic safety equipment and site-specific engineering for the underground construction industry. NTS carries one of the most robust and diverse rental fleets of trench and traffic equipment in the industry and also has a nationwide, strategically-located branch network to support the needs of its customers/contractors. NTS carries a complete range of trench boxes, slide rail, large hydraulic braces, vertical shores, steel plate, and other items that most contractors use routinely. What has separated NTS within the industry over the last several years is its in-house engineering group and its focus on large, complex, and highly technical projects that require unique and site-specific solutions. In 2014, NTS introduced the patented Work Zone Safety System to the industry, which provides integrated fall protection, ladder access systems, and worker retrieval systems specifically designed for the trench safety industry. The NTS Work Zone Safety System has won several industry awards and has received universal praise, as well as leading the introduction of other competitively produced systems. In 2015 NTS also introduced the Lite Guard Aluminum Shield to the North American market. Designed in Australia, the system makes use of a proprietary aluminum extrusion design to deliver a rugged, lightweight aluminum shield that’s about half the weight of a steel trench box while retaining the depth ratings that steel shields can provide. Excavation work has evolved dramatically over the past decade with projects increasing in scope, complexity, and depth. In 2018 NTS expanded its operations into the UK with its first international location in Leeds, England. The company is now providing its high level of expertise of highly engineered product solutions for some of the larger projects in the UK. Today’s contractor is faced with a much more complex job site and the increased use of site-specific engineering has helped to bridge the complexity while also providing the contractor with reasonable, cost-efficient shoring and shielding solutions. As the nation’s only true singular focused national trench and traffic safety specialist, NTS remains committed to the industry, its trends, and being on the leading edge of introducing new technologies, products, and services to the underground construction industry.

Pacific Shoring LLC is a leading manufacturer of both trench shoring assemblies and related safety equipment. They utilize vertical aluminum panel hydraulic shoring for linear trenching operations. These are assembled and installed from outside the actual excavation. Their shoring assemblies are hinged to allow for easy folding and insertion into a trench while they are flattened out. Once in position, the panels can be opened up into their proper alignment and spacing. Once in proper position, they are pressurized and reinforced by a manually operated hand pump. In addition to parallel two-sided trench assemblies, they provide four-sided hydraulic bracing systems for pits and other vertical excavations. Like the trench panels, these are also hinged to allow for ease of placement.

Always an innovator in the field of trench safety, Speed Shore Corporation was the originator of aluminum hydraulic trench shoring. For over 50 years, Speed Shore has led the industry with its technologically advanced line of trench shoring equipment. Their Modular Aluminum Panel System (MAPS) is engineered to be lightweight and strong. The light weight allows for easy transportation even in vehicles as small as pickup trucks. The design allows a two-man crew to quickly assemble the panels into two-sided, three-sided, and four-sided configurations. A mini excavator suffices for placement of the shoring assembly into the trench or excavation pit. The system’s adjustable spreaders, main panels, and end members are constructed from foam filled double-walled aluminum. The extruded end members vary in length from 2 feet to 10 feet and are designed with manually fitting pin-and-keeper connections. Several types of spreaders are available including telescoping steel spreaders, screw jacks, and hydraulic cylinders.

United Rentals is the trench safety leader in North America, both in terms of dedicated locations and fleet. The company believes that technological breakthroughs are an important driver of job site safety and will continue to lead to new solutions that solve specific operational and budgetary needs. United Rentals’ trench safety rental range includes engineered systems such as beam and plate shoring, steel sheeting and bracing, and mega-brace; aluminum hydraulic shores and boxes; aluminum shields and modular systems; steel shields, arch spreaders, and man-guards; bedding boxes; safety equipment for confined spaces; lasers and optical instruments; road plates and traffic control equipment; and testing devices. GX_bug_web

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