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Excavating is recognized as one of the most hazardous construction operations. OSHA recently revised Subpart P, Excavations, of 29 CFR 1926.650, 29 CFR 1926.651, and 29 CFR 1926.652 to make the standard easier to understand, permit the use of performance criteria where possible, and provide construction employers with options when classifying soil and selecting employee protection methods.
This chapter is intended to assist OSHA Technical Manual users, safety and health consultants, OSHA field staff, and others in the recognition of trenching and shoring hazards and their prevention.
Accepted Engineering Practices are procedures compatible with the standards of practice required of a registered professional engineer.
Adjacent Structures Stability refers to the stability of the foundation(s) of adjacent structures whose location may create surcharges, changes in soil conditions, or other disruptions that have the potential to extend into the failure zone of the excavation or trench.
Competent Person is 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 authorization to take prompt corrective measures to eliminate or control these hazards and conditions.
Confined Space is a space that, by design and/or configuration, has limited openings for entry and exit, unfavorable natural ventilation, may contain or produce hazardous substances, and is not intended for continuous employee occupancy.
Excavation. An Excavation is any man-made cut, cavity, trench, or depression in an earth surface that is formed by earth removal. A Trench is a narrow excavation (in relation to its length) made below the surface of the ground. In general, the depth of a trench is greater than its width, and the width (measured at the bottom) is not greater than 15 ft (4.6 m). If a form or other structure installed or constructed in an excavation reduces the distance between the form and the side of the excavation to 15 ft (4.6 m) or less (measured at the bottom of the excavation), the excavation is also considered to be a trench.
Hazardous Atmosphere is an atmosphere that by reason of being explosive, flammable, poisonous, corrosive, oxidizing, irritating, oxygen-deficient, toxic, or otherwise harmful may cause death, illness, or injury to persons exposed to it.
Ingress and Egress mean "entry" and "exit," respectively. In trenching and excavation operations, they refer to the provision of safe means for employees to enter or exit an excavation or trench.
Protective System refers to a method of protecting employees from cave-ins, from material that could fall or roll from an excavation face or into an excavation, and from the collapse of adjacent structures. Protective systems include support systems, sloping and benching systems, shield systems, and other systems that provide the necessary protection.
Registered Professional Engineer is a person who is registered as a professional engineer in the state where the work is to be performed. However, a professional engineer who is registered in any state is deemed to be a "registered professional engineer" within the meaning of Subpart P when approving designs for "manufactured protective systems" or "tabulated data" to be used in interstate commerce.
Support System refers to structures such as underpinning, bracing, and shoring that provide support to an adjacent structure or underground installation or to the sides of an excavation or trench.
Subsurface Encumbrances include underground utilities, foundations, streams, water tables, transformer vaults, and geological anomalies.
Surcharge means an excessive vertical load or weight caused by spoil, overburden, vehicles, equipment, or activities that may affect trench stability.
Tabulated Data are tables and charts approved by a registered professional engineer and used to design and construct a protective system.
Underground Installations include, but are not limited to, utilities (sewer, telephone, fuel, electric, water, and other product lines), tunnels, shafts, vaults, foundations, and other underground fixtures or equipment that may be encountered during excavation or trenching work.
Unconfined Compressive Strength is the load per unit area at which soil will fail in compression. This measure can be determined by laboratory testing, or it can be estimated in the field using a pocket penetrometer, by thumb penetration tests, or by other methods.
Definitions That Are No Longer Applicable. For a variety of reasons, several terms commonly used in the past are no longer used in revised Subpart P. These include the following:
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 0.5 to 0.75 times the depth of the trench, measured from the top of the vertical face of the trench. See the accompanying drawing for additional details.
Text version of Figure V:2-1.
This figure illustrates how a tension cracks usually form at a horizontal distance of 0.5 to 0.75 times the depth of the trench, measured from the top of the vertical face of the trench.
Sliding or sluffing may occur as a result of tension cracks, as illustrated below.
Text version of Figure V:2-2.
This figure illustrates how sliding or sluffing may occur as a result of tension cracks.
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.
Text version of Figure V:2-3.
This figure illustrates how toppling occurs when the trench's vertical face shears along the tension crack line and topples into the excavation.
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.
Text version of Figure V:2-4.
This figure illustrates how 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.
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 the drawing above. Heaving and squeezing can occur even when shoring or shielding has been properly installed.
Text version of Figure V:2-5.
This figure illustrates how downward pressure created by the weight of adjoining soil causes a bulge in the bottom of the cut.
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.
Text version of Figure V:2-6.
This figure illustrates an upward water flow into the bottom of the cut. Boiling produces a "quick" condition in the bottom of the cut, and can occur even when shoring or trench boxes are used.
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 110 pounds to 140 pounds or more, and one cubic meter (35.3 cubic feet) of soil can weigh more than 3,000 pounds.
OSHA categorizes soil and rock deposits into four types, A through D, as follows:
Stable Rock is natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed. It is usually identified by a rock name such as granite or sandstone. Determining whether a deposit is of this type may be difficult unless it is known whether cracks exist and whether or not the cracks run into or away from the excavation.
Type A Soils are cohesive soils with an unconfined compressive strength of 1.5 tons per square foot (tsf) (144 kPa) or greater. Examples of Type A cohesive soils are often: clay, silty clay, sandy clay, clay loam and, in some cases, silty clay loam and sandy clay loam. (No soil is Type A if it is fissured, is subject to vibration of any type, has previously been disturbed, is part of a sloped, layered system where the layers dip into the excavation on a slope of 4 horizontal to 1 vertical (4H:1V) or greater, or has seeping water.
Type B Soils are cohesive soils with an unconfined compressive strength greater than 0.5 tsf (48 kPa) but less than 1.5 tsf (144 kPa). Examples of other Type B soils are: angular gravel; silt; silt loam; previously disturbed soils unless otherwise classified as Type C; soils that meet the unconfined compressive strength or cementation requirements of Type A soils but are fissured or subject to vibration; dry unstable rock; and layered systems sloping into the trench at a slope less than 4H:1V (only if the material would be classified as a Type B soil).
Type C Soils are cohesive soils with an unconfined compressive strength of 0.5 tsf (48 kPa) or less. Other Type C soils include granular soils such as gravel, sand and loamy sand, submerged soil, soil from which water is freely seeping, and submerged rock that is not stable. Also included in this classification is material in a sloped, layered system where the layers dip into the excavation or have a slope of four horizontal to one vertical (4H:1V) or greater.
Layered Geological Strata. Where soils are configured in layers, i.e., where a layered geologic structure exists, the soil must be classified on the basis of the soil classification of the weakest soil layer. Each layer may be classified individually if a more stable layer lies below a less stable layer, i.e., where a Type C soil rests on top of stable rock.
Many kinds of equipment and methods are used to determine the type of soil prevailing in an area, as described below.
Penetrometers are direct-reading, spring-operated instruments used to determine the unconfined compressive strength of saturated cohesive soils. Once pushed into the soil, an indicator sleeve displays the reading. The instrument is calibrated in either tons per square foot (tsf) or kilograms per square centimeter (kPa). However, Penetrometers have error rates in the range of ± 20-40%.
This test is conducted by molding a moist sample of the soil into a ball and attempting to roll it into a thin thread approximately 1/8 inch (3 mm) in diameter (thick) by 2 inches (50 mm) in length. The soil sample is held by one end. If the sample does not break or tear, the soil is considered cohesive.
A visual test is a qualitative evaluation of conditions around the site. In a visual test, the entire excavation site is observed, including the soil adjacent to the site and the soil being excavated. If the soil remains in clumps, it is cohesive; if it appears to be coarse-grained sand or gravel, it is considered granular. The evaluator also checks for any signs of vibration.
During a visual test, the evaluator should check for crack-line openings along the failure zone that would indicate tension cracks, look for existing utilities that indicate that the soil has previously been disturbed, and observe the open side of the excavation for indications of layered geologic structuring.
The evaluator should also look for signs of bulging, boiling, or sluffing, as well as for signs of surface water seeping from the sides of the excavation or from the water table. If there is standing water in the cut, the evaluator should check for "quick" conditions (see Paragraph III. F in this chapter). In addition, the area adjacent to the excavation should be checked for signs of foundations or other intrusions into the failure zone, and the evaluator should check for surcharging and the spoil distance from the edge of the excavation.
Shoring is the provision of a support system for trench faces used to prevent movement of soil, underground utilities, roadways, and foundations. Shoring or shielding is used when the location or depth of the cut makes sloping back to the maximum allowable slope impractical. Shoring systems consist of posts, wales, struts, and sheeting. There are two basic types of shoring, timber and aluminum hydraulic.
Text version of Figure V:2-7.
This figure illustrates an example of timber shoring, including various parts such as sheeting, crossbrace struts, uprights, and wales. The crossbrace struts connect to the wales, which provide support to the sheeting and uprights.
The trend today is toward the use of hydraulic shoring, a prefabricated strut and/or wale system manufactured of aluminum or steel. Hydraulic shoring provides a critical safety advantage over timber shoring because workers do not have to enter the trench to install or remove hydraulic shoring. Other advantages of most hydraulic systems are that they:
All shoring should be installed from the top down and removed from the bottom up. Hydraulic shoring should be checked at least once per shift for leaking hoses and/or cylinders, broken connections, cracked nipples, bent bases, and any other damaged or defective parts.
Text version of Figure V:2-8.
This figure illustrates four vertical aluminum hydraulic shoring variations: Vertical Aluminum Hydraulic Shoring with Spot Bracing, Vertical Aluminum Hydraulic Shoring with Plywood, Vertical Aluminum Hydraulic Shoring (Stacked), and Aluminum Hydraulic Shoring Water System (Typical). The first variation uses a hydraulic cylinder to exert force against a vertical roll, which transfers the force the the wale. The second variety adds plywood between the vertical roll and wale. The third variety stacks multiple arrangements of the first type vertically. The fourth variety uses a hydraulic cylinder exerting force on a horizontal walethat presses against upright sheeting.
Pneumatic Shoringworks in a manner similar to hydraulic shoring. The primary difference is that pneumatic shoring uses air pressure in place of hydraulic pressure. A disadvantage to the use of pneumatic shoring is that an air compressor must be on site.
Text version of Figure V:2-9.
This figure has a diagram showing the various components of upright sheeting, along with pneumatic/hydraulic jacks and a screw jack. The various jacks press up against the wale, which in turn keep the shoring in position.
Trench Boxes are different from shoring because, instead of shoring up or otherwise supporting the trench face, they are intended primarily to protect workers from cave-ins and similar incidents. The excavated area between the outside of the trench box and the face of the trench should be as small as possible. The space between the trench boxes and the excavation side are backfilled to prevent lateral movement of the box. Shields may not be subjected to loads exceeding those which the system was designed to withstand.
Text version of Figure V:2-10.
This figure illustrates a trench shield, consisting of a knife edge, sidewall, and struts. The knife edge is at the bottom edge of the sides of the trench shield, and struts are going between both sides of the shield, providing support.
Text version of Figure V:2-11.
This figure illustrates two trench shields, stacked upon each other.
Combined Use. Trench boxes are generally used in open areas, but they also may be used in combination with sloping and benching. The box should extend at least 18 in (0.45 m) above the surrounding area if there is sloping toward excavation. This can be accomplished by providing a benched area adjacent to the box.
Earth excavation to a depth of 2 ft (0.61 m) below the shield is permitted, but only if the shield is designed to resist the forces calculated for the full depth of the trench and there are no indications while the trench is open of possible loss of soil from behind or below the bottom of the support system. Conditions of this type require observation on the effects of bulging, heaving, and boiling as well as surcharging, vibration, adjacent structures, etc., on excavating below the bottom of a shield. Careful visual inspection of the conditions mentioned above is the primary and most prudent approach to hazard identification and control.
Text version of Figure V:2-12a.
This figure illustrates how a support or shield system can be used in Type A soil. The slope alongside it is 18" minimum, 20' maximum, with a 1/0.75 slope.
Text version of Figure V:2-12.
This figure illustrates how a support or shield system can be used in Type B soil. The slope alongside it is 18" minimum, 20' maximum, with a 1/1 slope.
Text version of Figure V:2-12c.
This figure illustrates how a support or shield system can be used in Type C soil. The slope alongside it is 18" minimum, 20' maximum, with a 1/1.5 slope.
Maximum allowable slopes for excavations less than 20 ft (6.09 m) based on soil type and angle to the horizontal are as follows:
|Soil type||Height/Depth ratio||Slope angle|
|(For a maximum excavation depth of 12 ft)|
Text version of Figure V:2-13.
This figure illustrates the different types of slope excavations in layered soils. This includes Type A, B, and C soils in single slope excavations, and different permutations such as: Type A soil over both Types B and C individually, Type B soil over Types A and C individually, and Type C soil over Types A and B individually.
Text version of Figure V:2-14.
This figure illustrates the types of excavations made in Type A soil: Two types use an unsupported vertically sided lower portion (with maximum depths of 8 Feet and 12 Feet), and the three remaining types include single bench excavation, simple slope - short term, and multiple bench excavation.
There are two basic types of benching, simple and multiple. The type of soil determines the horizontal to vertical ratio of the benched side.
As a general rule, the bottom vertical height of the trench must not exceed 4 ft (1.2 m) for the first bench. Subsequent benches may be up to a maximum of 5 ft (1.5 m) vertical in Type A soil and 4 ft (1.2 m) in Type B soil to a total trench depth of 20 ft (6.0 m). All subsequent benches must be below the maximum allowable slope for that soil type. For Type B soil the trench excavation is permitted in cohesive soil only.
Text version of Figure V:2-15.
This figure illustrates two types of excavations made in Type B soil that are permitted in cohesive soil only: Single Bench Excavation, and Multiple Bench Excavation.
Text version of Figure V:2-16.
This figure illustrates temporary spoil being placed at a minimum of two feet from the surface edge of the excavation.
Temporary spoil must be placed no closer than 2 ft (0.61 m) from the surface edge of the excavation, measured from the nearest base of the spoil to the cut. This distance should not be measured from the crown of the spoil deposit. This distance requirement ensures that loose rock or soil from the temporary spoil will not fall on employees in the trench.
Spoil should be placed so that it channels rainwater and other run-off water away from the excavation. Spoil should be placed so that it cannot accidentally run, slide, or fall back into the excavation.
Permanent spoil should be placed at some distance from the excavation. Permanent spoil is often created where underpasses are built or utilities are buried. The improper placement of permanent spoil, i.e. insufficient distance from the working excavation, can cause an excavation to be out of compliance with the horizontal-to-vertical ratio requirement for a particular excavation. This can usually be determined through visual observation. Permanent spoil can change undisturbed soil to disturbed soil and dramatically alter slope requirements.
The designated competent person should have and be able to demonstrate the following:
Surface crossing of trenches should be discouraged; however, if trenches must be crossed, such crossings are permitted only under the following conditions:
Access to and exit from the trench require the following conditions:
Procedures to protect employees from being injured or killed by vehicle traffic include:
Employees must be protected from loads or objects falling from lifting or digging equipment. Procedures designed to ensure their protection include:
The following steps should be taken to prevent vehicles from accidentally falling into the trench:
Employees shall not be permitted to work in hazardous and/or toxic atmospheres. Such atmospheres include those with:
All operations involving such atmospheres must be conducted in accordance with OSHA requirements for occupational health and environmental controls (see 29 CFR Part 1926 Subpart D) for personal protective equipment and for lifesaving equipment (see 29 CFR Part 1926 Subpart E). Engineering controls (e.g., ventilation) and respiratory protection may be required.
When testing for atmospheric contaminants, the following should be considered:
Employees required to wear respiratory protection must be trained, fit-tested, and enrolled in a respiratory protection program. Some trenches qualify as confined spaces. When this occurs, compliance with the Confined Space Standard is also required.
Emergency rescue equipment is required when a hazardous atmosphere exists or can reasonably be expected to exist. Requirements are as follows:
Methods for controlling standing water and water accumulation must be provided and should consist of the following if employees are permitted to work in the excavation:
Inspections shall be made by a competent person and should be documented. The following guide specifies the frequency and conditions requiring inspections:
29 CFR 1926, Subpart P. Excavations.
Construction Safety Association of Ontario. Trenching Safety. 74 Victoria St., Toronto, Ontario, Canada M5C2A5.
International Labour Office (ILO). Building Work: A Compendium of Occupational Safety and Health Practice. International Occupational Safety and Health Information Centre (CIS): ILO, Geneva, Switzerland.
National Safety Council. Accident Prevention Manual for Industrial Operations, Engineering and Technology, 9th ed., Chicago, IL: National Safety Council.
National Safety Council. Protecting Worker's Lives: A Safety and Health Guide for Unions. Chicago, IL: National Safety Council.
National Safety Council. Industrial Data Sheets: I-482, General Excavation, and I-254, Trench Excavation, Chicago, IL: National Safety Council.
National Utility Contractors Association, Competent Person Manual-1991.
NBS/NIOSH, Development of Draft Construction Safety Standards for Excavations. Volume I, April 1983. NIOSH 83-103, Pub. No. 84-100-569. Volume II, April 1983. NIOSH 83-2693, Pub. No. 83-233-353.
Scardino, A.J., Jr. 1993. Hazard Identification and Control--Trench Excavation. Lagrange, TX: Carlton Press.
During first and subsequent visits to a construction or facility maintenance location, the compliance officer (or the site's safety officer or other competent person) may find the following questions useful.
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