Page 6 - Trench and Excavation

Hazardous Atmosphere Parameters

Less than 19.5% oxygen concentration
Greater than 20% LEL for flammable gases
Greater than Permissible Exposure Limit (PEL) for toxic substances

Inspection- To formally examine the trench area to ensure that all aspects of worker safety are
acceptable and in conformance with the Standard.

Internal Friction- Inter-locking of soil particles which allows for a greater angle of repose.
Kick-Out- Accidental release or failure of a cross-brace.
Layered System- soil systems composed of two or more distinctly different soil or rock types
arranged in layers. Micaceous seams or weakened planes in rock or shale are considered layers.
Lateral Soil Pressure- The pressure exerted by the walls of the trench. This pressure is usually
a function of the soil type and moisture condition.
Loam- A soil mixture of clay, sand, and silt.
Mixed Soil- A soil composed of a mixture of gravel, sand, silt and/or clay.

Maximum Allowable Slope- the steepest incline that a trench face can be excavated. Trenching
conditions must be ideal and no signs of distress may be present.

Moist Soil- Condition where the soil feels damp. Moist, cohesive soils exhibit plasticity and moist,
granular soils exhibit apparent cohesion.
Micaceous Seam- A thin layer of mica, or other weak material, between two layers of rock.

Mudsill- a wale installed at the toe of the trench wall.
Penetrometer- A spring-loaded cylinder device used to determine a soil's unconfined
compressive strength.
Plasticity- A characteristic of soil which allows it to be molded and deformed without cracking, or
appreciable volume change.
Protective System- A method of protecting workers from material falling, or rolling, into an
excavation.
Raveling- Small particles ("crumbs") of soil falling from the trench face.
Registered Professional Engineer - A professional engineer who is competent in soil mechanics
and who can approve tabulated data or design protective systems for worker protection in
excavations.

Sand- Granular rock or mineral fragments which range in size from 0.05mm to 2.0mm.
Saturated Soil- Soil in which the void spaces are filled with water.
Scale- Loose clumps of unstable material on the trench face.

Section Modules- A measure of the bending strength of aluminum walers
Shale- A fine-grained sedimentary rock most likely formed by the compression of clay.
Shear Force- forces, which act perpendicular to the plane of movement.

Shear Vane Device- A rotary device used to determine a soil's unconfined compressive strength.

Trench & Excavation
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