Trenches and excavations are very common in construction work. However, they can be extremely dangerous to workers; a partial collapse of an excavation can easily be a fatal accident. The Occupational Safety and Health Administration details the proper method for determining shoring depth for open-earth excavations in standard 1926.652. These standards should be used by professional engineers to ensure the safety of a trench or excavation against collapse. Any open-earth trench or excavation that will have workers inside must be properly shored.
The first step in determining shoring depth is to establish the type of soil through which the trench will be dug. OSHA classifies three types of soil based on industry and agricultural standards. Examples of Type A soil include "clay, silty clay, sandy clay, clay loam" and "cemented soils such as caliche and hardpan." Type B soils are "angular gravel (similar to crushed rock), silt, silt loam, sandy loam and, in some cases, silty clay loam and sandy clay loam." Type C soils are the most unstable types of earth and include wet soils and "granular soils including gravel, sand, and loamy sand." (See Reference 4)
After determining soil type, the depth of a trench is specified and then referenced to a chart keyed to soil type in order to determine the required shoring depth and configuration for proper excavation safety. Each chart specifies the size and spacing of wales (horizontal supports parallel to the face of a trench) and uprights (vertical beams that contact with the floor of the excavation). In addition, these charts provide the horizontal and vertical spacing of cross beams (horizontal supports anchored to the wales and uprights) based on trench depth and width.
OSHA excavation rules allow for many different types of shoring arrangements and the use of either timber support members or aluminum hydraulic shoring. Charts are provided for timber systems in Appendix C and aluminum hydraulic systems in Appendix D. Aluminum hydraulic shoring designs require fewer elements than timber systems but are more expensive than conventional lumber. Aluminum hydraulic installations also carry the advantage of being able to bear greater loads while taking up less space in an excavation, leaving more room for workers. In addition, aluminum systems can be used long term whereas timber systems are typically temporary structures.