This GBCA Safety Toolbox Talk discusses sloping and benching, two protective systems to prevent cave-ins in excavation and trenching activities. Click below to download the Toolbox Talk as a multi-page handout.
Trenching and Excavation: Shielding and Shoring
Excavation and trenching are among the most hazardous construction operations. The Occupational Safety and Health Administration’s (OSHA) Excavation standards, 29 Code of Federal Regulations (CFR) Part 1926, Subpart P, contain requirements for excavation and trenching operations.
PROTECTIVE SYSTEMS: HOW TO PREVENT CAVE-INS
OSHA generally requires that employers protect workers from cave-ins. Two methods covered in this toolbox talk at sloping and benching the sides of the excavation.
Maximum allowable slopes for excavations less than 20 ft (6.09 m) based on soil type and angle to the horizontal are as follows:
Table V:2-1 Allowable Slopes
|Soil Type||Height:Depth Ratio||Slope Angle|
|Type A (short term)||0.5:1||63°|
(For a maximum excavation depth of 12ft)
The above figures illustrate the different types of slope excavations for Type A, B, and C soils in single slope excavations.
Sloping with Layered Soil
For areas with layered soil, the slope can follow the soil types only if stronger soil is below weaker soil. If the soil below is weaker, you must follow the weaker soil slope angle for the entire slope of excavation. Total depth of excavation of 20’-0” maximum.
The following figures illustrate the different types of slope excavations in layered soils.
Type A and Type B
Type A and Type C
Type B and Type C
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.
The figures below illustrate two types of excavations made in Type A soil: Single Bench Excavation and Multiple Bench Excavation.
The figures below illustrate two types of excavations made in Type B soil that are permitted in cohesive soil only: Single Bench Excavation, and Multiple Bench 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.
This figure (above) illustrates temporary spoil being placed at a minimum of two feet from the surface edge of the excavation.
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