Design Requirements For Benching In Excavation
Planning and designing a benching system in an excavation is critical to the safety of the excavation crew and equipment. Several factors must be considered when designing a benching system, such as the size and shape of the excavation, the type of soil, the weight of the material to be benched, and the equipment availability. In this blog post, we will discuss some of the critical design requirements for benching in excavation.
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 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 soil type classification. As per regulations, all trenches with a depth of five feet or greater must be protected against cave-in using shoring, shielding, sloping or benching.
The Design Requirements For Benching In Excavation
- Excavation Size and Shape: Before beginning the design of a benching system, it is essential to accurately measure the size and shape of the excavation. This will help determine how much material needs to be removed from the sides of the excavation to create safe and functional benches.
- Soil Type: The soil type present in the excavation will influence the design of the benching system. Soil types vary greatly, from loose and unstable soils to those with high compaction rates. It is important to understand how each soil type affects the stability of a benched excavation to create a safe and effective system.
- Weight of Materials: The weight of the material to be benched must also be taken into account when designing a benching system. This is especially important when dealing with large up-and-down benches, as too much weight can cause instability in the excavation.
- Availability of Equipment: Lastly, the availability and size of equipment play a major role in the design of a benching system. It is important to assess the type and size of the machinery that will be needed for completing the project, as well as any limitations that this machinery may impose.
By following these essential design requirements, you can ensure that your benched excavation is safe and effective. Bench planning and design are an essential factors in creating a successful excavation, so be sure to take the time to properly plan and design your benching system.
Also, consulting a professional engineer or architect can help you create a safe and effective benched excavation system. It is also essential to ensure that any excavations you undertake are properly inspected and adhere to all relevant workplace safety laws.
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 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, and 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.
There are two basic types of benching: simple and multiple.
The type of soil determines the horizontal to vertical ratio of the benched side.
- As a general rule, the bottom vertical height of the trench must not exceed 4 feet.
- Subsequent benches may be up to a maximum of 5 feet vertical in Type A soil and 4 feet in Type B soil.
- All subsequent benches must be below the maximum slope allowed for that soil type.
- Also, in Type B soil, the trench excavation is permitted only in cohesive soil.
- Type C soil is not stable enough for benching as a protective system: use sloping instead.