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OSHA Sloping Requirements

Trenches are commonly dug in construction to lay pipe or to pour foundations. If the trench walls have the incorrect slope, they can become deadly places to be in if the walls collapse or cave in. The Occupational Safety and Health Administration (OSHA) has therefore devoted an entire series of regulations to slopes. These regulations are designed with a run-over-rise method, or a degree system for different types of soils.

  1. Soil Types

    • OSHA classifies soils by their type, which are stable rock Type A, Type B and Type C, from the most stable to the least stable. Trench slopes are then designed according to the soil type. Note that all designs by OSHA are for trenches 20 ft. or less in depth. For trenches more than 20 ft. deep, OSHA stipulates that the trench wall slopes be designed by a registered professional engineer.

    Stable Rock

    • For stable rock, OSHA states that vertical wall slopes (90-degree) are acceptable. However, this criterion is very strict. If another soil type is found interspersed with the rock, the criterion for that soil type must also be adhered to.

    "Type A" Soil

    • For Type A soils, the maximum wall slope is a three-fourths-to-one ratio (three quarters of a unit of run to one unit of rise), or 53 degrees. For temporary trenches (open 24 hours or less) of 12 ft. or less, this ratio can be reduced to one-half to one. If a bench method (staircase method) is used to form walls, the staircase also must have a three-quarters-to-one ratio

      The staircase criteria are that the horizontal run can be no more than 4 ft. and the vertical rise can be no more than 5 ft. For trenches 8 ft. or less deep, the bottom can have a vertical wall of no more than 3 1/2 ft. deep. For trenches from 8 to 12 ft., a bottom vertical wall no more than 3 1/2 ft. is allowable, with a maximum side slope of a one-to one-ratio (45 degrees).

    Type B Soil

    • The slope for Type B soils is a one-to-one ratio, or 45 degrees. If staircased, then the maximum allowable rise and run of the staircase is 4 ft. each. The staircasing method of wall excavation is only allowed for cohesive Type B soils; all non--cohesive soils must have smooth slopes.

    Type C Soils

    • Type C soil is the loosest soil of all. For this soil, the maximum allowable slope is one-and-one-half-to-one, or a maximum angle of 34 degrees. Staircasing is not allowed.

    Layered Soils

    • Most subsurface topography is never composed of just one type of soil. Rather, the subsurface strata have different layers. For example, first topsoil is encountered; digging deeper reveals a vein of Type B soil, then a layer of stable rock. OSHA requires that for each layer encountered, the slope criteria for different soil types must be used.

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