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OSHA Excavation Standard Handbook
29 CFR Parts 1926.650 through 1926.652

TABLE OF CONTENTS

Note to user: It has been assumed that the user has a general knowledge of the OSHA requirements regarding excavations which are found in 1926.650 to 1926.652. This section is meant to serve as a handy reminder of the basic requirements of the excavation standard, but does not necessarily discuss every requirement of the standard. You should consult the standard directly when questions arise or additional clarification is necessary.

BEFORE YOU EXCAVATE

  1. Contact the utility companies or property owners and ask the companies or owners to find the exact location of the underground installations in the area. If the utility companies or owners do not respond within 24 hours or the period established by law or ordinance, or if they cannot establish the location of the utility lines, the excavation may proceed with caution. In this situation, the employer must provide employees with detection equipment or other safe and acceptable means to locate utility installations.

  2. Remove or adequately support objects in the excavation area that could create a hazard to employees. These may include trees, rocks, sidewalks, and other objects.

  3. Classify the type of soil and rock deposits at the site as either stable rock, Type A, Type B or Type C soil. The soil classification must be made based on the results of at least one visual and at least one manual analysis conducted by a competent person. (Note: Soil classification is not necessary if the excavation will be sloped to an angle of one and one-half horizontal to one vertical.)

SOIL CLASSIFICATION

Each soil and rock deposit at an excavation site must be classified by a competent person as stable rock, Type A, Type B, or Type C soil.

Stable rock refers to natural solid mineral matter which can be excavated with vertical sides and remain intact while exposed.

Type A soil is cohesive with an unconfined compressive strength of 1.5 tons per square foot (tsf). Type A soils include clay, silty clay, sandy clay, clay loam, earache, hardpan, and sometimes silty clay loam and sandy clay loam. No soil should be classified as Type A if it is fissured; subject to vibration from traffic, pile driving, or similar effects; previously disturbed; or part of a sloped, layered system where the slope is four horizontal to one vertical or greater.

Type B soil is cohesive soil with an unconfined compressive strength greater than .5 tsf but less than 1.5 tsf. Type B soils include granular cohesionless soils like angular gravel, silt, silt loam, sandy loam, and sometimes silty clay loam and sandy clay loam; previously disturbed soils that are not Type C; fissured soils and soils subject to vibration that would otherwise be classified as Type A; dry rock that is not stable; and material that is part of a sloped, layered system where the layers dip on a slope less steep than four horizontal to one vertical.

Type C soil is cohesive soil with an unconfined compressive strength of .5 tsf or less. Type C soils include granular soils such as gravel, sand, and loamy sand; submerged soil; soil from which water is freely seeping; submerged rock that is not stable; or material in a sloped, layered system where the layers dip into the excavation at a slope of four horizontal to one vertical or steeper.

Soil classification is to be based on at least one visual analysis and one manual analysis of the soil. Details of the acceptable visual and manual analyses may be found in Appendix A of the excavation standard.

PROTECTIVE SUPPORT SYSTEMS

OSHA requires that each employee in an excavation be protected from cave-ins during an excavation by an adequate protective system designed in accordance with OSHA standards. Protective system options include proper sloping or benching of the sides of the excavation; supporting the sides of the excavation with timber shoring or aluminum hydraulic shoring; or placing a shield between the side of the excavation and the work area. The employer is free to choose the most practical design approach for any particular circumstance. Once an approach has been selected, however, the required performance criteria must be met by that system.

No protective system is necessary if the excavation is made entirely in stable rock, or the excavation is less than 5 feet (1.52 m) in depth (provided there is no indication of a potential cave-in).

The following figure is a graphic summary of the preliminary decisions which need to be made in order to determine whether a protective system is needed for an excavation 20 feet or less in depth. Protective systems for use in excavations more than 20 feet in depth must be designed by a registered professional engineer.

SLOPING AND BENCHING

If sloping or benching is used to protect against cave-ins, there are four basic options that can be chosen for designing sloping or benching systems. First, if soil classification is not made, then the sides of the excavation can be sloped to an angle not steeper than one and one-half horizontal to one vertical (34&#f176;). A slope of this gradation or less is considered safe for any type of soil (see Figure 1).

Figure 1: Simple slope of 1-1 /2 H to 1 V; Suitable for all soil types in excavations less than 20 feet in depth.

The second option for designing a sloping or benching system is to use Appendices A and B of the excavation standard to determine the maximum allowable slope and allowable configurations for sloping and benching systems. These requirements are summarized in Table 1. The soil type must be determined in order to use this option.

Table 1
Maximum Allowable Slopes
SOIL OR ROCK TYPE MAXIMUM ALLOWABLE SLOPES (H:V) 1 FOR EXCAVATIONS LESS THAN 20 FEET DEEP 3
STABLE ROCK VERTICAL (90°)
TYPE A 2 3/4:1 (53°)
TYPE B 1:1 (45°)
TYPE C 1-1/2:1 (34°)

NOTES:

  1. Numbers shown in parentheses next to maximum allowable slopes are angles expressed in degrees from the horizontal. Angles have been rounded off.
  2. A short-term maximum allowable slope of 1/2H: 1V (63) is allowed in excavations in Type A soil that are 12 feet (3.67 m ) or less in depth. Short-term maximum allowable slopes for excavations greater than 12 feet (3.67 m) in depth shall be 3/4H: 1 V (53).
  3. Sloping or benching for excavations greater than 20 feet deep shall be designed by a registered professional engineer.

Figures 2 and 3 provide examples of simple slope excavations in Type A and Type B soils. Figures 4 and 5 illustrate the use of single and multiple benching in Type A and B soils.

Figure 2: Simple slope excavation in Type A soil with a 3/4H: 1 V slope.

Figure 3: Simple slope excavation in Type B soil with a 1 H:1 V slope.

Figure 4: Benched excavation in Type A soil with a maximum slope of 3/4H: 1 V.

Figure 5: Multiple benched excavation in Type B soil with a slope of 1H:1V.

Sloping and benching systems can also be designed using other tabulated data approved by a registered professional engineer or by having an engineer design and approve the system to be used.

There are a number of exceptions or special cases to these general sloping and benching guidelines. They are outlined below.

  1. In Type A soil, simple slope excavations which are open 24 hours or less (short term) and which are 12 feet high or less in depth may have a maximum allowable slope of 1/2 horizontal to 1 vertical.

  2. In Type A soil, all excavations 8 feet or less in depth which have unsupported vertically sided lower portions must have a maximum vertical side of 3.5 feet.

  3. In Type A soil, excavations over 8 feet but less than 12 feet in depth with unsupported vertically sided lower portions must have a maximum allowable slope of 1H:1V and a maximum vertical side of 3.5 feet.

  4. In Type A soil, excavations 20 feet or less with vertically sided lower portions that are supported or shielded shall have a maximum allowable slope of 3/4H:1V. The support or shield system must extend at least 18 inches above the top of the vertical side.

  5. In Type B soil, all excavations 20 feet or less which have vertically sided lower portions shall be shielded or supported to a height at least 18 inches above the top of the vertical side. The excavation shall have a maximum allowable slope of 1H:1V.

  6. In Type C soil, all excavations 20 feet or less which have vertically sided lower portions shall be shielded or supported to a height at least 18 inches above the top of the vertical side. The excavation shall have a maximum allowable slope of 1-1 /2H:1 V. See Figure 6 for an example of this type of excavation.

Figure 6: Excavation in Type C soil with shielded vertically sided lower portions. The excavation is sloped at 1-1/2H:l V.

LAYERED SOILS

When the excavation contains layers of different types of soils, the general sloping requirements do not apply. The excavation must be sloped according to Table 2. Figures 7 and 8 provide examples of excavations made in layered soils.

Table 2
Sloping Requirements for Layered Soils
Slope Required For Each Soil Layer
Layered Soil Type Type A Layer Type B Layer Type C Layer
B over A 3/4:1 1:1  
C over A 3/4:1   1-1/2:1
C over B   1:1 1-1/2:1
A over B 1:1 1:1  
A over C 1-1/2:1   1-1/2:1
B over C   1-1/2:1 1-1/2:1

Figure 7: Excavation in layered soil (Type C over Type A). The layer of Type C soil is sloped at 1-1/21, while the layer of Type A soil is sloped at 3/4:1.

Figure 8: Excavation in layered soil where Type A soil tops Type C soil. Both the Type A and Type C soils in the excavation must be sloped at 1-1/2: 1.

TIMBER SHORING

Designs for timber shoring in trenches can be determined using one of four methods: using the requirements set forth by OSHA in Appendices A and C of the excavation standard; using data provided by the manufacturer of the support system; using other tabulated data approved by an engineer; or having a registered professional engineer design the system.

The design specifications for timber shoring provided by OSHA may be found in Tables 3, 4, and 5 on the following pages.

These tables refer to the actual dimensions and not nominal dimensions of the timber. Those wishing to use nominal size shoring are directed to the additional tables found in Appendix C of the standard.

These OSHA design specifications apply only to trenches that do not exceed 20 feet. The soil type in which the excavation is made must be determined in order to use the OSHA data. The specifications do not apply in every situation experienced in the field; the data were developed to apply to most common trenching situations.

Figure 9 illustrates the use of timber shoring as a protective system.

TIMBER TRENCH SHORING - MINIMUM TIMBER REQUIREMENTS*

SOIL TYPE A Pa = 25 X H + 72 psf (2 ft Surcharge)

DEPTH OF TRENCH (FEET) SIZE (ACTUAL) AND SPACING OF MEMBERS**
CROSS BRACES WALES UPRIGHTS
HORIZ. SPACING FEET WIDTH OF TRENCH (FEET) VERT. SPACING (FEET) SIZE (IN.) VERT. SPACING (FEET) MAXIMUM ALLOWABLE HORIZONTAL SPACING (FEET)
UP TO 4 UP TO 6 UP TO 9 UP TO 12 UP TO 15 CLOSE 4 5 6 8
5 TO 10 UP TO 6 4X4 4X4 4X6 6X6 6X6 4 Not Req'd ---    2X6 
UP TO 8 4X4 4X4 4X6 6X6 6X6 4 Not Req'd ---     2X8
UP TO 10 4X6 4X6 4X6 6X6 6X6 4 8X8 4   2X6  
UP TO 12 4X6 4X6 6X6 6X6 6X6 4 8X8 4    2X6 
10 TO 15 UP TO 6 4X4 4X4 4X6 6X6 6X6 4 Not Req'd ---    3X8 
UP TO 8 4X6 4X6 6X6 6X6 6X6 4 8X8 4  2X6   
UP TO 10 6X6 6X6 6X6 6X8 6X8 4 8X10 4   2X6  
UP TO 12 6X6 6X6 6X6 6X8 6X8 4 10X10 4    3X8 
15 TO 20 UP TO 6 6X6 6X6 6X6 6X8 6X8 4 6X8 4 3X6    
UP TO 8 6X6 6X6 6X6 6X8 6X8 4 8X8 4 3X6     
UP TO 10 8X8 8X8 8X8 8X8 8X10 4 8X10 4 3X6     
UP TO 12 8X8 8X8 8X8 8X8 8X10 4 10X10 4 3X6     
OVER 20SEE NOTE 1

*Mixed oak or equivalent with a bending strength not less than 850 psi.
**Manufactured members of equivalent strength may be substituted for wood.

TIMBER TRENCH SHORING - MINIMUM TIMBER REQUIREMENTS*

SOIL TYPE B Pa = 45 X H + 72 psf (2 ft Surcharge)

DEPTH OF TRENCH (FEET) SIZE (ACTUAL) AND SPACING OF MEMBERS**
CROSS BRACES WALES UPRIGHTS
HORIZ. SPACING FEET WIDTH OF TRENCH (FEET) VERT. SPACING (FEET) SIZE (IN.) VERT. SPACING (FEET) MAXIMUM ALLOWABLE HORIZONTAL SPACING (FEET)
UP TO 4 UP TO 6 UP TO 9 UP TO 12 UP TO 15 CLOSE 2 3    
5 TO 10 UP TO 6 4X6 4X6 6X6 6X6 6X6 5 6X8 5     2X6    
UP TO 8 6X6 6X6 6X6 6X8 6X8 5 8X10 5     2X6    
UP TO 10 6X6 6X6 6X6 6X8 6X8 5 10X10 5     2X6    
See Note 1                          
10 TO 15 UP TO 6 6X6 6X6 6X6 6X8 6X8 5 8X8 5   2X6      
UP TO 8 6X8 6X8 6X8 6X8 6X8 5 10X10 5   2X6      
UP TO 10 8X8 8X8 8X8 8X8 8X10 5 10X12 5   2X6      
See Note 1                          
15 TO 20 UP TO 6 6X8 6X8 6X8 8X8 8X8 5 8X10 5 3X6        
UP TO 8 8X8 8X8 8X8 8X8 8X10 5 10X12 5 3X6        
UP TO 10 8X10 8X10 8X10 8X10 10X10 5 12X12 5 3X6        
                           
OVER 20 SEE NOTE 1

*Mixed oak or equivalent with a bending strength not less than 850 psi.
**Manufactured members of equivalent strength may be substituted for wood.

TIMBER TRENCH SHORING - MINIMUM TIMBER REQUIREMENTS*

SOIL TYPE C Pa = 80 X H + 72 psf (2 ft Surcharge)

DEPTH OF TRENCH (FEET) SIZE (ACTUAL) AND SPACING OF MEMBERS**
CROSS BRACES WALES UPRIGHTS
HORIZ. SPACING FEET WIDTH OF TRENCH (FEET) VERT. SPACING (FEET) SIZE (IN.) VERT. SPACING (FEET) MAXIMUM ALLOWABLE HORIZONTAL SPACING (FEET)
UP TO 4 UP TO 6 UP TO 9 UP TO 12 UP TO 15 CLOSE 4 5 6 8
5 TO 10 UP TO 6 6X8 6X8 6X8 8X8 8X8 5 8X10 5
2X6
      
UP TO 8 8X8 8X8 8X8 8X8 8X10 5 10X12 5
2X6
     
UP TO 10 8X10 8X10 8X10 8X10 10X10 5 12X12 5
2X6
      
See Note 1                        
10 TO 15 UP TO 6 8X8 8X8 8X8 8X8 8X10 5 10X12 5
2X6
      
UP TO 8 8X10 8X10 8X10 8X10 10X10 5 10X12 5
2X6
     
See Note 1                        
See Note 1                        
15 TO 20 UP TO 6 8X10 8X10 8X10 8X10 10X10 5 12X12
5
3X6    
See Note 1                       
See Note 1                       
See Note 1                       
OVER 20SEE NOTE 1

*Mixed oak or equivalent with a bending strength not less than 850 psi.
**Manufactured members of equivalent strength may be substituted for wood.

Figure 9: This illustrates timber shoring in a trench approximately 13 feet deep and 5 feet wide in Type B soil. Using OSHA specifications described in Table 4, the 6 x 6 crossbraces have been placed at 6 feet horizontally and 5 feet vertically; the 8 x 8 wales are positioned at five feet vertically; and the 2 x 6 uprights are placed every two feet.

ALUMUNUM HYDRAULIC SHORING

Designs for aluminum hydraulic shoring should be based upon manufacturer's tabulated data and should be in accordance with the manufacturer's specifications, recommendations, and limitations. Deviations from the manufacturer's specifications, recommendations, or limitations are only allowed upon written approval of the manufacturer. The written approval must be kept at the jobsite during construction of the protective system.

If the manufacturer's tabulated data cannot be utilized, the aluminum hydraulic shoring can be designed using the OSHA specifications found in Appendix D of the excavation standard. Before using the OSHA data, the soil type must be determined. Other options for the design of aluminum hydraulic shoring systems include using other tabulated data approved by an engineer or having a registered professional engineer design the system.

Figures 10 and 11 provide examples of vertical aluminum hydraulic shoring and a horizontal waler system. These illustrations use OSHA specifications for the shoring design.

Figure 10: Vertical aluminum hydraulic shoring in a trench dug in soil Type B. The vertical shores and 2 inch diameter cylinders are placed 5.5 feet o.c. horizontally and 4 feet o.c. vertically. Plywood is used behind the shores to prevent local raveling between shores.

Figure 11: A horizontal waler system in a trench dug in Type C soil. Horizontal 2 inch cylinders are spaced at 6.5 feet o.c. horizontally. Wales are spaced 4 feet o.c. vertically and 3 x 12 solid timber sheeting is used.

GENERAL REQUIREMENTS FOR EXCAVATIONS

  1. Employees exposed to public vehicular traffic must wear warning vests or other suitable garments made of reflectorized or high-visibility material.
  2. A competent person must inspect the excavation and the adjacent areas on a daily basis for possible cave-ins, failure of protective systems and equipment, hazardous atmospheres, or other hazardous conditions. Inspections are also required after the occurrence of any natural (such as rain) or man-made events (such as blasting) that could increase the potential for hazards.
  3. A warning system should be used to alert operators of the edge of an excavation.
  4. Adequate protection must be provided to protect employees from failing rock, soil, or other materials and equipment.
  5. Employees should not be permitted under loads that are handled by lifting or digging equipment. Employees should not be allowed to work in the excavation above other employees unless the lower level employees are adequately protected.
  6. While the excavation is open, underground installations must be protected, supported, or removed as necessary to safeguard employees. Adjacent structures must be supported to prevent possible collapse.
  7. Employees should not be permitted to work in excavations where water has accumulated or is accumulating unless adequate precautions have been taken. Diversion ditches, dikes, or other means must be used to prevent surface water from entering an excavation and to provide drainage to the adjacent area.
  8. Before an employee enters an excavation greater than 4 feet in depth, a competent person must test the atmosphere where oxygen deficiency or a hazardous atmosphere exists or could reasonably exist. Emergency rescue equipment must be readily available and must be attended when hazardous atmospheric conditions exist or may develop.
  9. Sufficient means for exiting excavations 4 feet deep or more must be provided and must be within 25 feet of lateral travel for employees.
  10. Guardrails must be provided if there are walkways or bridges crossing over an excavation.


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