Figure 6: Excavation in Type C soil with shielded vertically sided lower portions. The excavation is sloped at 1-1/2H:l V.
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.
Figure 5: Multiple benched excavation in Type B soil with a slope of 1H:1V.
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.
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.
29 CFR Parts 1926.650 through 1926.652
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.
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.
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).
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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.
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.
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Figure 2: Simple slope excavation in Type A soil with a 3/4H: 1 V slope.
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.