Soil classification is an important part of preparing for excavation or trenching work.
According to the Occupational Safety and Health Administration (OSHA), trenching is one of the most dangerous types of work in the construction sector. In fact, 40 construction workers are killed annually from being caught in trench cave-ins. However, with proper precautions, these deaths are largely preventable.\
One method for detecting hazards in trenching operations is to conduct investigations in order to identify trenching conditions. Part of the identification process is having a competent person classify the soil that is being excavated to determine the hazards that could occur with that particular soil.
This guidance discusses the different types of soil classifications, how to identify them and how different soils impact excavations.
Soil Classifications
OSHA measures unconfined compressive strength to classify soil. This refers to the amount of pressure the soil can take before it collapses. Soils can be classified as Type A, Type B or Type C. The most stable soil is Type A, while the most unpredictable soil is Type C. Each type of soil has its own description:
- Stable rock is cohesive. This is natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed.
- Type A soil is cohesive. This type of soil is normally known as clay, silty clay, sandy clay and clay loam. This is the most stable soil to excavate, with a high unconfined compressive strength of 1.5 tons per square foot, or greater.
- Type B soil is also cohesive, but has been cracked or disturbed. This type of soil does not stick together as well as Type A soil. This soil is best known as angular gravel, silt and silt loam. It also includes soil that is fissured or located near sources of vibration. This soil has a medium unconfined compressive strength of between 0.5 and 1.5 tons per square foot.
- Type C soil is the least stable soil for excavating. This soil is best known as gravel or sand. In this soil, the particles do not stick together, and it has a low compressive strength of 0.5 tons per square foot, or less. If there is water seeping through soil, it is automatically classified as Type C soil, regardless of its other characteristics.
How to Determine Soil Classifications
Soil classification is based on determining the unconfined compressive strength of the soil and must be made based on the results of at least one visual and at least one manual analysis.
These tests must be completed by a competent person.
Any tests used must be recognized methods of soil classification by either the American Society for Testing and Materials or the U.S. Department of Agriculture textural classification system. Some common tests used include:
- Visual tests
- Using a pocket penetrometer
- Observations of the soil being excavated
- Observations of the excavation itself and the areas adjacent to the excavation
- Manual analyses of the soil
- Plasticity tests
- Dry strength tests
- Thumb penetration tests
- Drying tests
When determining soil types, it is important for the competent person to complete an inspection of the surrounding area in which the construction will take place. Questions a competent person should ask include:
- Are there sources of vibration near the excavation?
- Are there signs of disturbed soil?
- Is there water seeping into the soil?
When observing the site, the competent person should pay attention to whether the soil that is being excavated is clumpy or granular. If the soil is clumpy, this means the soil is cohesive and stronger.
The Impact of Soil Classifications
Once soil has been classified, this information is used to determine what trenching protective systems should be used to prevent cave-ins and whether any additional support systems are needed to ensure adjacent structures remain stable for the protection of the workers.
Protective systems that can be used in excavations to prevent cave-ins include:
- Sloping the sides
- Benching the sides
- Using shields between the side of the excavation and the work area (trench box
- Shoring
It is important to understand the soil classifications and the processes to identify hazards that could occur during excavations. Regardless of your job title, working safely is everyone’s responsibility at.
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