A Guide to the OSHA Excavations Standard

A Guide to the OSHA Excavations Standard Bobby R. Davis Series Editor Division of Occupational Safety and Health N.C. Department of Labor 0 Mail Service Center Raleigh, NC Cherie K. Berry Commissioner
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A Guide to the OSHA Excavations Standard Bobby R. Davis Series Editor Division of Occupational Safety and Health N.C. Department of Labor 0 Mail Service Center Raleigh, NC Cherie K. Berry Commissioner of Labor N.C. Department of Labor Occupational Safety and Health Program Cherie K. Berry Commissioner of Labor OSHA State Plan Designee Allen McNeely Deputy Commissioner for Safety and Health Kevin Beauregard Assistant Deputy Commissioner for Safety and Health Acknowledgments This edition of A Guide to the OSHA Excavation Standard has been updated to include material as prepared by the Construction Education and Research Institute, Department of Civil Engineering at N.C. State University (principal investigators were Paul P. McCain and David W. Johnston); U.S. Department of Labor, Occupational Safety and Health Administration; U.S. Department of Health and Human Services (NIOSH); and OSHNC personnel. This guide is intended to be consistent with existing state and federal OSHA standards. Therefore, if the reader considers a statement to be inconsistent with a standard, the OSHA standard should be followed. To obtain additional copies of this book, or if you have questions about N.C. occupational safety and health standards or rules, please contact: N.C. Department of Labor Bureau of Education, Training and Technical Assistance 0 Mail Service Center Raleigh, NC Phone: (99) or -800-NC-LABOR ( ) Additional sources of information are listed on the inside back cover of this book. The projected cost of the OSHNC program for federal fiscal year is $4,52,395. Federal funding provides approximately 30 percent ($4,528,766) of this total. Printed 02/02, M Part Page Contents Foreword iiv Introduction vii Common Soil Problems ii 2 Soil Types and Pressures ii8 3 Methods of Protection ii4 4 Installation and Removal of Protective Systems ii8 5 Residential Contractors and the Excavations Standard ii29 6 Worker Training and Jobsite Safety ii30 7 Safety Checklist ii34 Suggested Readings ii37 iii Foreword There is no reason why anyone ever has to die in a trenching accident. Modern technology has provided us with a variety of excellent shoring systems and trench shields. The OSHA excavations standard provides us with a set of clearly written and logical safety rules. Yet every year workers are killed and injured by cave-ins. Employers and employees can expect N.C. Department of Labor safety inspectors to make the most detailed, professional inspections possible whenever such accidents occur. A Guide to the OSHA Excavations Standard examines the standard s different sections, offering many illustrations and a safety checklist to help explain how to excavate and work safely in a trench. It also discusses the new rule concerning excavating and residential construction. In North Carolina, DOL inspectors enforce the federal Occupational Safety and Health Act through a state plan approved by the U.S. Department of Labor. The N.C. Department of Labor s Division of Occupational Safety and Health offers many educational programs to the public and produces publications, including this guide, to help inform people about their rights and responsibilities regarding occupational safety and health. When looking through this guide, please remember DOL s mission is greater than just to enforce regulations. An equally important goal is to help people find ways to create safe workplaces. This booklet, like the other educational materials produced by the N.C. Department of Labor, can help. Cherie K. Berry Commissioner of Labor v Introduction The Occupational Safety and Health Administration issued its first Excavation and Trenching Standard in 97 to protect workers from excavation hazards. Since then, OSHA has amended the standard several times to increase worker protection and to reduce the frequency and severity of excavation accidents and injuries. Despite these efforts, excavation-related accidents resulting in injuries and fatalities continue to occur. OSHNC has developed this guide in effort to address requirements of the standard, as well as provide information for equipment operators, workers and all others associated with trenching and excavating to help recognize hazardous conditions that could result in injury or a fatality. This guide discusses soil composition in moderate detail to provide a general overview of the various properties associated with different types of soil. A general understanding of the properties of soil is the first step in predicting the behavior of soils in varying condition. Some of the most common types of soil conditions that lead to trench and excavation failure are also discussed. Proper trenching operations are necessary to protect the workers from soil collapse. The basic trenching operations that help make a trench safe for workers are described and illustrated. The methods of shoring installation are also discussed briefly. This document is not intended to be used as a stepby-step guideline in the excavation process. OSHA has completely updated the excavations standard, with focus on the existing standard to simplify many of the existing provisions, add and clarify definitions, eliminate duplicate provisions and ambiguous language, and give employers added flexibility in providing protection for employees. In addition, the standard provides several new appendices. Appendix A to provides a consistent method of soil classification. Appendix B to provides sloping and benching requirements. Other appendixes (appendixes C F) provide pictorial examples of shoring and shielding devices, timber tables, hydraulic shoring tables, and selection charts that provide a graphic summary of the requirements contained in the standard. For more information on the details of proper installation, please refer to the OSHA standard on excavation (29 CFR 926 Subpart P, which includes and appendixes A F) and to the Suggested Readings in this guide. Scope and Application OSHA s revised rule applies to all open excavations made in the earth s surface, which includes trenches. According to the OSHA construction safety and health standards, (b), a trench is referred to as a narrow excavation made below the surface of the ground in which the depth is greater than the width the width not exceeding 5 feet (4.5 meters). An excavation is any man-made cut, cavity, trench or depression in the earth s surface formed by earth removal. This can include excavations for anything, from cellars to highways. The standard does not apply to house foundation/basement excavations (including those that become trenches by definition when formwork, foundations or walls are constructed) when all of the following conditions are present: The house foundation/basement excavation is less than 7 / 2 feet (2.5 meters) deep or is benched for at least 2 feet (0.6 meters) horizontally for every 5 feet (.52 meters) or less of vertical height; The minimum horizontal width (excavation face to formwork/wall) at the bottom of the excavation is as wide as practicable but not less than 2 feet (0.6 meters); There is no water, surface tension cracks nor other environmental conditions present that reduce the stability of the excavation; There is no heavy equipment operating in the vicinity that causes vibration to the excavation while employees are in the excavation; All soil, equipment and material surcharge loads are no closer in distance to the top edge of the excavation than the excavation is deep; however, when front end loaders are used to dig the excavations, the soil surcharge load must be placed as far back from the edge of the excavation as possible, but never closer than 2 feet (0.6 meters). vii Work crews in the excavation are the minimum number needed to perform the work; and The work has been planned and is carried out in a manner to minimize the time employees are in the excavation. These conditions as applicable to residential construction are restated in another section of this guide, Residential Contractors and the Excavations Standard. The standard provides several options for designing trench protection measures. Under certain conditions, the design of the trench protection measures must be prepared by a registered professional engineer. Proper selection and installation of trench protection measures are very important. To comply with the standard, the employer must have a competent person: one who is capable of identifying existing and predictable hazards in the surroundings or working conditions which are unsanitary, hazardous, or dangerous to employees, and who has authorization to take prompt corrective measures to eliminate them. This competent person has critical inspection responsibilities regarding excavations. This person must inspect every excavation and protective system under his or her care daily, including areas adjacent to any excavation. Additional inspections must be conducted before starting work and as needed through a shift, for example, when any hazard-increasing occurrence (such as a rainstorm) takes place. When the competent person finds any evidence of a situation that could result in a cave-in, protective system failure or any other hazardous condition, employees are to be immediately removed from the danger area until the problem is fixed. Many companies have established a written policy that outlines specific safe trenching practices in detail. Such a policy should ensure adequate support for the trench and frequent inspections of the excavation site to detect any change in the soil conditions. When this type of policy is enforced, all employees understand their responsibilities, which helps to avoid unsafe practices. This document is not intended to be a guideline for compliance with all pertinent OSHA regulations but rather an overview of safe practices in trenching operations. Though the guide is not intended to be inconsistent with OSHA standards, if an area is considered by the reader to be inconsistent, the OSHA standard should be followed. viii Common Soil Problems This guide highlights the requirements in the updated standard for excavation and trenching operations, provides methods for protecting employees against cave-ins, and describes safe work practices for employees. A necessary first step in planning the approach to any trenching or other excavation project is to understand what could go wrong. This understanding can help avoid many of the problems associated with excavation. The terms soil and earth are commonly referred to in the excavation process to describe the naturally occurring materials uncovered on a project. Soil conditions vary from one site to the next. Soil may be loose or partially cemented, organic or inorganic. However, most soils can be referred to as a mixture or an accumulation of mineral grains that are not cemented together. An exception is hard rock, which remains firm after exposure to the elements. Soil failure is defined as the collapse of part or all of an excavation wall. The most common soil failure is typically described as an unexpected settlement, or cave-in, of an excavation. Soil sliding is the most common factor leading to soil failure. Proper planning and supervision can avoid the unsafe working conditions caused by soil sliding. Unless such safety precautions have been implemented, sliding soil failure can occur in all types of excavations (including sloped trenches and excavations with braced trench boxes). See figure. Overview: Soil Mechanics A number of stresses and deformations can occur in an open cut or trench. For example, increases or decreases in moisture content can adversely affect the stability of a trench or excavation. The following diagrams show some of the more frequently identified causes of trench failure. Tension cracks. Tension cracks usually form at a horizontal distance of one-half to three-quarters times the depth of the trench, measured from the top of the vertical face of the trench. See figure 2 for additional details. Figure Sliding Failure Tension Crack Failed Soil Mass Figure 2 Tension Crack Tension Crack H.5 to.75 H Sliding or sluffing may occur as a result of tension cracks, as illustrated in figure 3. Figure 3 Sliding Sliding Toppling. In addition to sliding, tension cracks can cause toppling. Toppling occurs when the trench s vertical face shears along the tension crack line and topples into the excavation. See figure 4. Figure 4 Toppling Toppling 2 Subsidence and Bulging. An unsupported excavation can create an unbalanced stress in the soil, which, in turn, causes subsidence at the surface and bulging of the vertical face of the trench. If uncorrected, this condition can cause face failure and entrapment of workers in the trench. See figure 5. Figure 5 Subsidence and Bulging Bulge Subsidence Heaving or Squeezing. Bottom heaving or squeezing is caused by the downward pressure created by the weight of adjoining soil. This pressure causes a bulge in the bottom of the cut, as illustrated in figure 6. Heaving and squeezing can occur even when shoring or shielding has been properly installed. Figure 6 Heaving or Squeezing Heave Soil Weight 3 Boiling is evidenced by an upward water flow into the bottom of the cut. A high water table is one of the causes of boiling. Boiling produces a quick condition in the bottom of the cut and can occur even when shoring or trench boxes are used. See figure 7. Figure 7 Boiling Water Table Boiling Unit Weight of Soils refers to the weight of one unit of a particular soil. The weight of soil varies with type and moisture content. One cubic foot of soil can weigh from 0 pounds to 40 pounds or more, and one cubic meter (35.3 cubic feet) of soil can weigh more than 3,000 pounds. A safe slope can be defined as the maximum angle of the edge wall or bank of an excavation at which sliding will not occur. The unique mixtures of the different types of soil (sand, clay, silt and rock) necessitate different safe slopes from one excavation site to the next. There are other complicating factors that can result in sliding soil failures. During an excavation, visibly different layers of soil maybe uncovered. Each of those layers may call for different safe slopes. It is essential to plan your excavation around the most gradual (rather than steepest) safe slope for all of the different soil types and layers encountered during the excavation. Another complicating factor is that soil composition mixtures may vary significantly from one area of the project to another. During an excavation, as the soil composition changes, the safe slope for trench wall excavation also changes. Thus, across an excavation site, the slope of the bank may need to be different to provide a safe working environment. Sliding and other modes of failure can also occur in soils that are not densely compacted. For example, a trench that is made close to a previously dug trench is very unstable. If uncompacted soil is discovered, the normal safe slope for dense soil will not be enough to prevent sliding. Bracing or further sloping may be necessary. If cracks are observed in rocky types of soil, sliding has already occurred. These cracks should signal that a more gradual slope for excavation is needed because the rocky soil is very susceptible to slides and other types of failure. Excavations that have been stable for long periods are also subject to sliding types of failure. After prolonged exposure to the elements, the moisture content in the soil may increase. This increase in moisture may be due to various causes, such as rainfall or a broken water line. The extra soil moisture tends to speed up sliding soil failures. Determining the correct safe slope can be quite difficult for certain types of soil. The OSHA standard has developed a simple method of determining safe excavation bank slopes for different soil types. This new method will be discussed in more detail in a later section of this document. 4 Soil failure can occur for any number of reasons. Factors that increase the chances of soil failure are:. excessive vibration 2. surface encumbrances 3. weather conditions Cave-ins and Protective Support Systems Excavation workers are exposed to many hazards, but the chief hazard is danger of cave-ins. OSHA requires that in all excavations employees exposed to potential cave-ins must be protected by sloping or benching the sides of the excavation, by supporting the sides of the excavation, or by placing a shield between the side of the excavation and the work area. Designing a protective system can be complex because of the number of factors involved-soil classification, depth of cut, water content of soil, changes due to weather and climate, or other operations in the vicinity. The standard, however, provides several different methods and approaches (four for sloping and four for shoring, including the use of shields)* for designing protective systems that can be used to provide the required level of protection against cave-ins. One method of ensuring the safety and health of workers in an excavation is to slope the sides to an angle not steeper than one and one-half horizontal to one vertical (34 degrees measured from the horizontal). These slopes must be excavated to form configurations that are in accordance with those for Type C soil found in appendix B of the standard. A slope of this gradation or less is considered safe for any type of soil. (See figure 8). Figure 8 Type C Soil Simple Slope Excavation 20' Maximum /2 All simple slope excavations 20 feet (6. meters) or less in depth must have a maximum allowable slope of.5:. A second design method, which can be applied for both sloping and shoring, involves using tabulated data, such as tables and charts, approved by a registered professional engineer. These data must be in writing and must include sufficient explanatory information to enable the user to make a selection, including the criteria for determining the selection and the limits on the use of the data. At least one copy of the information, including the identity of the registered professional engineer who approved the data, must be kept at the worksite during construction of the protective system. Upon completion of the system, the data may be stored away from the jobsite, but a copy must be made available upon request to OSHNC. Contractors may also use a trench box or shield that is either designed or approved by a registered professional engineer or is based on tabulated data prepared or approved by a registered professional engineer. Timber, aluminum or other suitable materials may also be used. OSHA standards permit the use of a trench shield (also known as a welder s hut) as long as the protection it provides is equal to or greater than the protection that would be provided by the appropriate shoring system. (See figure 9.) *See appendix F to the standard for a complete overview of all options. 5 Figure 9 Trench Shield Strut Sidewall The standard does not require the installation and use of a protective system when an excavation is made entirely in stable rock or is less than 5 feet deep (.52 meters) and a competent person has examined the ground and found no indication of a potential cave-in. Vibrations Knife Edge Any large, heavy movement near an excavation results in vibration of the surrounding soils. This movement can result in soil failure. Moving machinery, nearby traffic, pile driving and blasting all cause vibration in surrounding soils. Vibration-related soil failures can occur in all types of soil. However, certain types of soils are more susceptible to vibration failures than others. For example, sandy soils tolerate less vibration than clay soils. Since actual soil conditions may be a mixture of more than one soil type, it is better to play it safe when planning the slope of an excavation. Figure 0 shows typical situations where vibrations can result in soil failure. Figure 0 Two Examples of Vibration Failures Soil affected by the movement
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