Page 22 - Trench and Excavation

Maximum Allowable Slope

Cut-back distance (H) = .75 x Depth (V)

Steepest Actual Slope

Cut-back distance (H) = 1.25 x Depth (V)

Layered Systems

Digging in layered systems is essentially like digging two separate trenches one on top of the
other. In Appendix B, OSHA recognizes systems composed of only two layers. If more than two
layers are encountered, sloping should be designed to deal with the weakest soil type.

Two themes run through the layered soils sloping criteria:

1. If the stronger soil is below the weaker soil, the sloping for each layer is the same as the
simple slope for that soil type.

2. If the stronger soil is above the weaker soil, the entire system is sloped according to the
weaker soils simple slope.
The following diagrams will illustrate layered system sloping angles. When calculating cutback
distances, assume that the toe of the top layer is that point on the trench face where the top layer
begins.

Surcharge loads must be placed at least 2 feet back from the edge of the trench. When spoil or
surcharge loads are placed at the top of sloped or benched systems, they must stay at least 2
feet back from the edge. Further, the center of the spoil pile can be no closer than the imaginary
line formed by the continuation of the slope. This is true for positioning outriggers on heavy
equipment as well.

Shoring Systems

The safety concept behind timber and aluminum shoring is fundamentally different from shield
systems. Timber and aluminum shoring systems are designed to protect workers by pressing
against the face of the trench and preventing the cave-in from occurring in the first place.

Shoring System a structure such as a metal hydraulic, mechanical, or timber shoring system
that supports the sides of an excavation and which is designed to prevent
cave-ins.

The Competent Person has four shoring options from which to choose:
Option 1: Use the tables provided by OSHA. This requires soil testing.
Option 2: Use the manufacturer's tabulated data that is supplied with the shoring system.
Option 3: Use tables designed by a Registered Professional Engineer (RPE).
Option 4: Use a shoring system designed by a Registered Professional Engineer (RPE).

Trench & Excavation
Competent Person Training P3 Safety Solutions LLC
Page 22