Definition - What does Benched Excavation mean?
A benched excavation also called benching can be defined as a type of excavation that consists of a series of horizontal steps with near-vertical surfaces between them. This is done as a safety measure to prevent the excavated walls from collapsing inwards and trapping the construction workers.
The angle of the slope or bench varies depending on the classification of soil type. As per regulations, all trenches with depth five feet or greater must be protected against cave-in using shoring, shielding, sloping or benching.
Trenchlesspedia explains Benched Excavation
Classification based on soil type will determine the slope of the trench. Soil is classified into three types:
Type A soil: These are cohesive soils such as clay, silty clay, sandy clay and clay loam with an unconfined compressive strength of 1.5 tons per square feet (tsf).
Type B soil: These are cohesive soils such as angular gravel, silt, silt loam, disturbed soils (unless classified as Type C), with an unconfined compressive strength of 0.5 tsf.
Type C soil: These are cohesive soil such as gravel, sand, loamy sand, submerged soil, unstable submerged rock, soil from which water is seeping, with an unconfined compressive strength of 0.5 tsf.
The rise/run ratio is the same for both sloping and benching. It is recommended that all soil be considered as class C and sloped or benched accordingly. Occupational Safety and Health Administration (OSHA) recommends that all benched excavations 20 feet or less in depth should have a maximum allowable slope of 3/4 to 1.