There are 4 basic degrees of soil types that have lessening degrees of stability:
The definitions of these different soil types are as follows:
Stable Rock: Is the naturally solid mineral matter that occurs in nature and can be excavated with the vertical sides remaining intact while exposed.
Type A: Is the naturally occurring Cohesive Soils such as clay, silty clay, sandy clay, clay loam, also cemented soils such as caliche and hardpan. This classification of soil cannot have any type of fissures, been disturbed in anyway, or have been exposed to any type of vibration such as heavy traffic, or pile driving. It will also be carry a different classification type if there are any other factors that make it less stable.
Type B: Is the naturally occurring Granular Cohesionless Soils such as angular gravel, silt, silty loam, sandy clay loam, previously disturbed soils, soils that have been subject to vibrations, soils that have fissures. Dry rock that is not stable also falls under this classification.
Type C: Is the naturally occurring Granular Soils such as sand, loamy sand, non-angular gravel, submerged soils or soil where water freely seeps, and submerged rock that is not stable.
It is very important to survey your jobsite for all types of conditions to help you visually determine what soil types you may encounter. It is also extremely important to take several soil samples to help with the classification determination. Remember that as construction continues it is essential to continue to take soil samples. A soil classification can change with the depth of your trench, the width of your trench, and the length of your trench. Remember that properly classifying your soil type will help calculating your slope, your shoring and bracing. It will help keep your costs from running over and more importantly keep your job site safe and free of injuries.
For more detailed and in depth information you may visit the OSHA website.