In this article Civil Engineer, Bill Brewer discusses construction trench safety and provides an overview of the various safety measures used to protect workers from deadly trench collapses.
The civil engineers at Robson Forensic are frequently retained to provide expert witness investigations in a variety of construction injury cases, including trench collapses.
Trench Safety – Selection of Protective Measures
Previous articles by Robson Forensic Civil Engineers have provided information about trench safety including identification of soil conditions and provided an introduction to methods used to protect workers against trench cave-ins and collapses. This article will briefly touch on those topics while providing a basic understanding of factors involved in selecting appropriate protective measures.
Definitions of a few terms may be helpful. An excavation is any manmade alteration of the earth’s surface by removal of soil (i.e. rock, gravel, sand, silt, clay, etc.). A trench is generally defined as an excavation that is narrower than it is deep.
Trench work is among the most hazardous construction activities. Regulations and industry standards have been developed which provide performance criteria and guidance for selection of appropriate protective measures. Conditions where trench excavation work must be performed vary from site to site, and from day to day. Simply stated, no two trench conditions are alike and they do not remain unchanged over time. As a result of varying conditions, the hazards and appropriate measures to eliminate or control these hazards vary.
Various factors/conditions effecting any trench include:
The site, adjacent structures, above ground and buried utilities, trees, soil type and structure, ground water levels, weather, external loading or surcharge, required width and depth of the trench, type of work/installation, duration of the work, site constraints/space limitations, access to the site, previous disturbance of the soil, etc.
OSHA regulations and industry standards require that all employees performing trench work be trained in hazard recognition. Further, a Competent Person must make the determination as to the trench conditions and the appropriate protective system in view of the conditions. A Competent Person is defined as an individual who is capable of identifying existing and predictable hazards or working conditions that are hazardous, unsanitary, or dangerous to employees, AND who has the authority to take prompt corrective measures to eliminate or control these hazards and conditions. For some conditions regulations and industry standards require that a professional engineer must design or approve of a protective system (e.g. trenches 20 feet deep or greater require that the protective system be designed by a professional engineer or be based on tabulated data prepared and/or approved by a professional engineer).
All trenches 5 feet deep or greater require a protective system unless the excavation is made in stable rock. For trenches shallower than 5 feet, a Competent Person may determine that a protective system is not required.
Soil conditions, including soil type and ground water levels, are two critical factors effecting the type of trench hazard(s) and choices of protective systems. These conditions should be considered during the design phase as well as the construction phase of a project. Soil types were discussed in a previous article by J. David Gardner, P.E. in greater detail. In brief summary, soils are classified as noncohesive (e.g. sand, gravel, etc.) or cohesive (e.g. silt, clay, etc.). The properties of soils are dependent upon soil material and particle size distribution, soil loading conditions, soil water/moisture content, and the effect of moisture on stability of particular soil types.
Protective systems fall into three general categories:
- Sloping or benching
Sloping is the most basic protective measure as it simply requires cutting the walls of the trench back sufficiently to leave the trench sides sloped at a stable angle. This is most effective and efficient for shallow trenches in noncohesive soils where trench width is unconstrained by above ground or subsurface obstructions, and where ground water is not a factor or can be controlled by dewatering methods.
Benching is similar to sloping in that the trench sides are cut back. However, benching leaves the trench sides with a stepped appearance. This tends to be effective for previously undisturbed, more cohesive soil types. As with sloping, this is most effective where trench width is not limited by above or below surface obstructions and where dewatering can control ground water conditions.
Shielding, which protects workers from cave-ins, is commonly used for deeper trenches where sloping or benching may be impractical and/or where there are reasons to limit trench width. Types of trench shields include trench boxes, slide rail systems, etc.
Trench boxes are particularly well suited to pipeline installation work. Trench boxes may be stacked to protect workers in deeper trenches and are readily moved as pipe laying operations progress. Sloping or benching are required in conjunction with shielding for trench sides that are higher than the top of the shield structure.
Shoring, which is designed to prevent soil movement, is required for conditions including: nearby structures, subsurface utility protection, uncontrollable ground water, longer duration projects, etc. Sheet piling is a common type of shoring where interlocking steel sections are driven along the sides of a proposed trench. Other shoring methods include timber, aluminum or steel members with hydraulic jacks. Shoring is generally the most costly alternative.
- Link to Part 1 - Soil Identification
- Link to Part 2 - Protecting Workers Against Cave-Ins & Collapses
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 & Land Development Expert
Bill has over 30 years of experience in large-scale engineering, construction, and real estate development projects. He provides expert analysis in land acquisition, strategic planning, entitlements, design program management, site development, pipeline construction, and heavy-civil construction issues. He has specialized expertise in the construction of large-scale, master-planned communities and environmental issues including wetland mitigation, mass earthwork, blasting, and erosion control.