In the second part of this three-piece article on trench collapses, the civil engineers at Robson Forensic discuss why trenches collapse and provide an introduction to some of the most common methods used to protect workers in, and around, trenches.
Protecting Against Trench Cave-Ins & Collapses
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.
Protective methods and systems for excavated soils
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:
- 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. The maximum allowable slope as determined by OSHA for the respective soil type is as follows:
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.
- 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.
- 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.
INVESTIGATING TRENCH INJURIES
There are a number of variables on any site that determine the selection of protective trench measures; these variables are dynamic and in many cases, will be influenced by weather and surrounding conditions and activities.
Among the civil engineers at Robson Forensic, there are many construction professionals with firsthand experience managing site safety and engineering protective systems for trenches and other hazards.
Submit an inquiry or contact the author of this article to discuss your case.
Civil Engineer & Construction Safety Expert
Robson Forensic offers construction safety experts from many of our regional offices. This article was assembled with the assistance of several members from our construction safety group, but spearheaded by David Gardner.
Dave Gardner is a civil engineer with more than twenty-five years of professional experience. He provides investigations, reports, and testimony in matters involving construction claims and injuries. Dave’s project experience crosses various disciplines within civil engineering, including heavy highway and bridge, municipal engineering, and compressor stations/site work for the natural gas industry.
Contact David or your local Robson Forensic office to discuss your case. Based on geographic location and technical specifics of your case, we can help determine which of our experts is most appropriate to aid in your investigation.