OSHA, the Occupational Safety and Health Administration, the federal agency in charge of enforcing health and safety standards, classifies soils into four categories or groups, based on the fundamental soil characteristics, environmental conditions and soil stability. Soils also are classified according to their cohesiveness and the conditions they exist in. These categories are established after extensive manual and visual tests, conducted by knowledgeable and trained individuals. Soil classification systems are important for they determine the type of benching, sloping and protective systems required in a particular area.
Solid rock, also called stable rock, is naturally solid material that can be easily excavated but will remain intact upon exposure. This category of soil is deemed the most structurally stable and immovable and typically requires heavy equipment and/or machinery to mine and transport.
Type A soils are less stable than solid-rock soils but provide more stability than types B and C. Type A includes clay, sandy clay, silty clay, loamy clay and often sandy-clay loam and silty-clay loam soils. Most type A soils have a compressive, unconfined strength of 1.5 tons per square foot or higher. Soils that are fissured, cracked, subject to vibration (due to heavy traffic) or were previously disturbed are not classified as type A soils and are grouped as type B or C, depending on the extent of soil disturbance. Type A soils clump easily and are often difficult to remove due to their compactness.
Type B soils are moderately stable soils and include non-cohesive and cohesive soils, such as sandy loams, silts, unstable rock and medium clay soils. Type A soils with fissures, cracks and crevices may be classified as type B soils. Type B soils have unconfined compressive strengths that range between 0.5 tons per square foot (tsf) and 1.5 tons per square foot. These soils clump easily and may prove deadly upon collapse, easily burying people and equipment.
Type C soils are considered the most unsafe and unstable. These include cohesive, granular (including loamy sand and sand and gravel), submerged soil or rock and wet soils. According to Dave Heberle in the “Construction Safety Manual,” type C soils often have unconfined compressive strengths of less than 0.5 tons per square foot (tsf). They are typically easy to break apart and remove but increase the chance of injuries due to inhalation and/or collapse. Type C soils are also called free-seeping soils or submerged soils.