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. This booklet highlights key elements of the standards and describes safe work practices that can protect workers from cave-ins and other hazards.
Soil Mechanics
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. The following diagrams show some of the more frequently identified causes of trench failure.
Tension Cracks
Tension cracks usually form at a horizontal distance of one-half to three-quarters times the depth of the trench, measured from the top of the vertical face of the trench.
Sliding or Sluffing
This may occur as a result of tension cracks.
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 below.
Heaving and squeezing can occur even when shoring or shielding has been properly installed.
Boiling
Boiling is evidenced by 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.
When must employers conduct site inspections?
Employers must ensure that a competent person inspects all excavations, adjacent areas, and protective systems daily for possible cave-ins, indications of failures in protective systems and equipment, hazardous atmospheres, and other hazardous Trenching and Excavation Safety 1 3 conditions. Inspections must be done prior to the start of work and as needed throughout the shift.
Inspections are also required after natural events, such as rainstorms, or other hazard-increasing occurrences, such as blasting work. If an inspector finds any unsafe conditions during an inspection, the employer must clear workers from the hazardous area until the necessary safety precautions have been taken.
