OSHA Facts and Myths:

Contemplating 'The 18" Rule'
The Myth:

Above: 18" must be measured from the top of the protective system, down to the begining of the slope. The slope must be made in accordance with the soil classification as specified by OSHA sloping charts.
One very common misconception among contractors is that any protective system is required to extend at least 18 inches above the top of the trench. Although many of them will tell you that they can't pinpoint or cite the exact wording of "the 18-inch rule" from the Federal regulation, they will still insist that the OSHA rules require the protective system to extend 18 inches above grade.
The Facts:

The Code of Federal Regulations actually cites the following situation:

When a protective system as been installed in an excavation but that system does not reach ground level, then if sloping is selected as the option to protect workers, the protective system must extend above the verticle side of the trench a minimum of 18 inches

The intent of this rule is to provide a barrier on slopes that will protect workers from any loose debris which might roll down the face of the slope and into the trench.

If the contractor determines that the roll-off protection is required under any other set of circumstances, OSHA simply states that protection must be "adequate and it must be spefically tailored to control the hazard"