A number of stresses and deformations can occur in an open cut or trench. For example, increases or decreases in moisture content can adversely affect the stability of a trench or excavation. Below are some of the more frequently identified causes of trench failure (per OSHA’s soil mechanics).
Tension cracks usually form at a horizontal distance of 0.5 to 0.75 times the depth of the trench, measured from the top of the vertical face of the trench. See the accompanying drawing for additional details.
Sliding or sluffing may occur as a result of tension cracks, as illustrated below.
In addition to sliding, tension cracks can cause toppling. Toppling occurs when the trench’s vertical face shears along the tension crack line and topples into the excavation.
Subsidence and Bulging
An unsupported excavation can create an unbalanced stress in the soil, which, in turn, causes subsidence at the surface and bulging of the vertical face of the trench. If uncorrected, this condition can cause face failure and entrapment of workers in the trench.
Heaving or Squeezing
Bottom heaving or squeezing is caused by the downward pressure created by the weight of adjoining soil. This pressure causes a bulge in the bottom of the cut, as illustrated in the drawing above. Heaving and squeezing can occur even when shoring or shielding has been properly installed.
Boiling is evidenced by an upward water flow into the bottom of the cut. A high water table is one of the causes of boiling. Boiling produces a “quick” condition in the bottom of the cut, and can occur even when shoring or trench boxes are used.
For additional information, see OSHA Technical Manual (OTM) Section V: Chapter 2 (Overview: Soil Mechanics).
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