Of the myriad OSHA standards that govern construction work, those directed to excavation hazards are among the most obviously needful to prevent deaths and serious injuries.
While some few of OSHA’s standards may lie on the esoteric side, it doesn’t take a high level of technical sophistication to foresee the probable outcome, if an employee, too careless to fasten his lanyard to an anchor point, falls from an elevated work platform.
No less obvious, we propose, is the inadvisability of a worker, ever, going down into an unprotected trench. The incidence of such a thing happening in your company should be zero. Excavation safety presents some technical issues (for example, differentiating among soil Types “A,” “B,” and “C”) but nothing you can’t handle. An important calculation to keep in mind is that just one cubic yard of soil weighs about as much as a car. Even when workers survive cave-ins, it’s often with gruesome and permanent, disabling injuries. You don’t want an employee in an unprotected trench, even for just a couple of minutes. We have read of fatal cave-ins occurring almost instantaneously, after workers (or a single worker) had entered the trench, not to perform work, but only to shut off a valve, or retrieve a tool.
While there are additional hazards posed to employees working in or close to trenches (e.g., falls, falling loads, water accumulations, hazardous atmospheres, and incidents involving mobile equipment) it’s cave-ins that cause the greatest numbers of deaths and serious injuries. As of 2011, about 2 construction workers a month were dying in cave-ins. Alarmingly, in 2016, the number of fatalities in cave-ins had crept up to almost double the average figure for the preceding five years. That led OSHA to make excavation safety a priority for 2018. June 18-23, 2018 has been declared a “Trench Safety Stand Down” week. So, while this is not our first installment on this subject, we think reminding our readers of its importance is in order.
It’s sometimes said that trenches less than five feet deep require no protective measures. That’s not exactly correct. The rule is that, unless built in stable rock, a trench five feet or more in depth will always require a protective system. The condition for shallower trenches not to require special measures is that a “competent person” determine that none are needed. In this context, a“competent person” is one capable of identifying existing and predictable hazards or working conditions that are hazardous, unsanitary or dangerous, soil types and appropriate protective systems, and who is authorized to take prompt corrective measures.
While we use the terms “excavation” and “trench” interchangeably in this article, technically an “excavation” is any man-made cut, cavity, trench or depression in the Earth’s surface made by earth removal, and a “trench” is a narrow excavation (in relation to its length).
The most fundamental rules regarding trench safety are as follows:
If an excavation is to be 20’ deep or deeper, it must be designed by a licensed Professional Engineer, or be based on tabulated data prepared and/or approved by a licensed Professional Engineer. Whenever an employer designs and implements a sloping or benching system, the underlying data must be in writing, contain explanatory information justifying the choices made, and identify the engineer who approved it. At least one copy must be kept onsite during construction, and even after completion, the written information must be retained, and made available to OSHA on demand.
Soil classification is done by a competent person or a testing laboratory. An excavation may be made in stable rock, or any of soil types “A,” “B,” or “C.” As its name implies, “stable rock” is natural solid mineral matter, in which an excavation having vertical sides will remain intact while exposed. Type “A” soil, the next most stable material, is cohesive soil, not fissured or previously disturbed (typically, clay, silty clay, and clay loam) with an unconfined compressive strength greater than 1.5 tons per square foot.
Type “B” soils include cohesive soil with an unconfined compressive strength less than 1.5 tsf but greater than 0.5 tsf plus granular and cohesionless soils (such as angular gravel, similar to crushed rock, silt, silt loam, sandy loam, and, in some cases silty clay loam and sandy clay loam).
Type “C” soils include cohesive soils having an unconfined compressive strength of 0.5 tsf or less, granular soils (gravel, sand, and loamy sand) submerged soil, soil from which water is seeping, unstable submerged rock, or material in a sloped, layered system with a slope of 4:1 or steeper.
In addition to protecting against a possible collapse of the sides of the excavation, the employer must also provide shoring, bracing or underpinning when necessary, to ensure that adjoining buildings, walls, sidewalk, etc. remain stable. Excavation below the base or footing of any foundation or retaining wall is prohibited, unless the excavation is in stable rock, or a support system (such as underpinning) is provided, or a professional engineer has determined that the adjoining structure is sufficiently distant from the excavation that it will not be affected, and that the excavation will not endanger workers.
Whenever support systems are being installed or removed, such operations must be carried out in a manner that protects workers from cave-ins, structural collapses, and struck-by injuries. Members of support systems must be securely connected, and not overloaded. The removal of support systems must begin at, and progress from, the bottom of the excavation, and backfilling must progress concurrently with the removal of the support system.
Materials used in support systems must be examined by a competent person, to ensure that they are free from defects or damage. Materials must be used within their rated capacities, and consistent with the manufacturer’s recommendations. If materials are found to be damaged, or a competent person cannot assure that they are suitable for use (and able to support the intended loads) they should be removed from service, until evaluated and approved by a professional engineer.
The hazards related to excavations in addition to cave-ins include, among other things, hazardous atmospheres, water-related hazards, materials falling or rolling into (or within) the excavation, mobile equipment too close to the edge, employees working at levels lower than the site of the excavation (and thus being exposed to material or equipment falling, rolling, or sliding onto them) and excavated materials falling or spilling, while being loaded for removal.
Critical to excavation safety is careful planning, that should begin even before you submit your bid. It’s essential to familiarize yourself with the jobsite, and to consider not only soil classification, but (among other things) nearby structures, if any, surface and ground water, utilities, site traffic, fall protection needs, number of ladders needed, etc.
Virtually all cave-in injuries, the #1 hazard associated with excavations, are preventable. No matter how many jobs you have done in the past that included excavation support systems, you should treat each new job as one presenting its own unique characteristics, and requiring particularized evaluation and planning.