How to prevent a trench collapse


By Kelly Bubolz, Compliance Assistance Specialist, Appleton Area OSHA Office

The fatality rate for excavation work is 112 percent higher than the rate for general construction.  Excavations are inherently unstable, whether dug deep or shallow, which makes pre-planning, protective systems and evaluation by a competent person critical to keeping workers safe.  Between 50 and 100 workers die in trench collapses each year, according to NIOSH. Most injuries – 75-95% – occur in trenches lacking protective systems or found a protective system was used incorrectly in about one-quarter of trench deaths and, in 12%, a protective system was on site but not used in the trench.

Why Does a Trench Collapse Occur? Soil is normally kept in place by the pressure generated from the horizontal and vertical forces of the surrounding soil. During a conventional excavation operation, when the soil is being removed or dug-out in bulk form, the surrounding support is removed; the remaining soil becomes a vertical wall without lateral support. Because of that lack of surrounding pressure, most soil types will eventually collapse into the open excavation. This often happens suddenly, and usually, without warning.  Ground that previously has been disturbed may be more likely to cave in. But also, once a trench has been opened, the soil’s moisture conditions are constantly changing, and if there’s rain or seepage or anything like that it could encourage a cave-in of the walls of the trench.

OSHA regulations governing the protection of employees working in trenches state that workers must be protected from trench collapse/cave-ins by an adequately designed protective system; unless the proposed excavation depth is less than five (5) feet, and the examination of the soil by the competent person reveals no indication of a potential cave-in.


OSHA also requires that all workers be properly trained in understanding the importance and functionality of protective system(s) proposed for use. For excavations five (5) feet or more in depth, the following protective systems are commonly used:

  1. Sloping or Benching of the Soil – The simplest method of protecting workers is to slope or bench the walls of the excavation. The maximum angle of the soil slope will vary depending on the soil type. If the excavated walls are composed of stable rock, then the trench can be dug with a vertical slope. As the soil type or stability reduces, so too does the slope angle. OSHA requires that excavations over four (4) feet in depth have some form of access/exit, such as a ladder or ramp; and that access/exit points be located within 25 feet of employee(s). When the location and/or depth of the proposed excavation makes sloping or benching of the soil impractical, a protective system of either shoring or shielding must be used.
  2. Shoring – Shoring systems provide lateral support against the walls of a trench to prevent a collapse. Shoring systems can utilize metal or timber uprights, driven sheet piling, or other recognized methods. Shoring is used to protect large areas so that a crew can work inside, or adjacent to an excavation without danger of collapse.
  3. Shielding – Unlike shoring, shielding is not designed to prevent a collapse of the trench walls. Instead, shielding protects workers from cave-ins in a specific area of the trench where they are working. Shielding, also commonly referred to as a trench box, is usually designed to be portable and can be moved along a trench. If the excavation is using a shielding or shoring system, there must be a copy of the manufacturer’s information and technical data on site; as well as a copy of equipment inspection(s) performed by the competent person.

Regardless of the method used to guard against the collapse of excavated soils, workers must be protected from objects, debris, soil, etc. from falling into the trench and/or area of excavation. OSHA requires that all equipment, excavated spoil piles, etc. be positioned at least two (2) feet away from the edge of the trench.

Other trench hazards include accidentally striking an underground utility, exposure to toxic fumes, drowning in a flooded trench, and being hit by equipment or falling loads.  OSHA requires trenches to be inspected by a competent person daily and as conditions change. That person must be able to identify hazards and have the authority to take immediate corrective action. It is important to make sure he or she actually is onsite before and during the project.  Depending on the project, trench competence alone may not be enough.  Trench work also may involve fall protection or lockout/tagout, each of which requires different expertise.  There’s so many elements and so many different requirements that are standalone requirements that a competent person really has to be well-trained to pick up on.

Wisconsin OSHA Compliance Assistance Specialists:

Kelly Bubolz – 1648 Tri Park Way, Appleton, WI 54914 –  (920) 734-4521
Mary Bauer – 1310 W. Clairemont Ave, Eau Claire, WI 54701 – (715) 832-9019

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