This module describes the classification of soil and rock deposits based on site and environmental conditions, and on the structure and composition of the earth deposits. This appendix contains definitions, sets forth requirements, and describes acceptable visual and manual tests for use in classifying soils.
Key Definitions:
Cohesive Soil means clay, or soil with a high clay content, which has cohesive strength. Cohesive soil does not crumble, can be excavated with vertical side slopes, and is plastic when moist. The cohesive soil is hard to break up when dry and exhibits significant cohesion when submerged.
Fissured means a soil material that has a tendency to break along definite planes of fracture with little resistance, or a material that exhibits open cracks, such as tension cracks, in an exposed surface.
Granular means gravel, sand, or silt, (coarse-grained soil) with little or no clay content. The granular soil has no cohesive strength, cannot be molded when moist, and crumbles easily when dry. Some moist granular soils exhibit apparent cohesion.
Type A means cohesive soils with an unconfined compressive strength of 1.5 tons per square foot (tsf) or greater. Examples of cohesive soils are: clay, silty clay, sandy clay, clay loam, and, in some cases, silty clay loam and sandy clay loam. Cemented soils such as hardpan are also considered Type A. No soil can be Type A if fissured, subjected to significant vibration, or has been previously disturbed.
Type B means cohesive soils with an unconfined compressive strength greater than 0.5tsf but less than 1.5 tsf. Some examples are granular cohesionless soils including angular gravel, silt, silt loam, sandy loam, and , in some cases, silty clay loam and sandy clay loam. Type B also includes previously disturbed soils except those which would otherwise be classed as Type C.
Type C means cohesive soils with an unconfined compressive strength of 0.5 tsf or less. Some examples include gravel, sand, and loamy sand. Also included may be submerged soil or soil from which water is freely seeping, and submerged rock that is not stable.
Classification of Soil and Rock Deposits
_ Classification must be based on the results of at least one visual and one manual analysis conducted by a Competent Person.
_ In a layered soil system, the system must be classified in accordance with its weakest layer. However, each layer may be classified individually where a more stable soil lies under a less
stable soil.
Visual Tests
Observe samples of soil that are excavated and soil in the sides of the excavation. Estimate the range of particle sizes and the relative amounts of the particle sizes. Soil that is primarily composed of fine-grained material is cohesive material. Soil composed primarily of coarse-grained sand or gravel is granular material. Observe soil as it is excavated. Soil that remains in clumps when excavated is
cohesive.
Soil that breaks up easily and does not stay in clumps is granular. Observe the side of the opened excavation and the surface area adjacent to the excavation.
Crack-like openings such as tension cracks could indicate fissured material. If chunks of soil spill off a vertical side, the soil could be fissured. Small spills indicate moving ground and can pose potentially hazardous situations.
Observe the area adjacent to the excavation to identify previously disturbed soil (i.e. evidence of existing utility and other underground structures).
Observe the opened side of the excavation to identify layered systems. Examine layered systems to identify if the layers slope toward the excavation. Estimate the degree of slope of the layers.
Observe the area adjacent to the excavation and the sides of the opened excavation for evidence of surface water, water seeping from the sides, or the location of the water table level.
Observe the area adjacent to the excavation and the area within the excavation for sources of vibration that may affect the stability of the excavation face.
OR-OSHA 302 Excavation Safety
This material is for training use only Manual Tests
Plasticity and Pat Test - Mold a moist or wet sample of soil into a ball and attempt to roll it into threads as thin as 1/8 inch in diameter. Cohesive soil can be successfully rolled into threads without crumbling. If at least a 2 inch length of 1/8 inch thread can be held on one end without tearing, the soil is cohesive. Spread a 1/8 or 1/4 inch thick sample of wet soil on the palm of the hand. Wipe the
surface of the sample with a finger to remove visible water. With the palm facing up, slap the back of the hand moderately 5 to 10 times. If water rises to the surface of the sample (surface will appear shiny), then the soil is mostly cohesionless silt or sand. If no water appears, then the soil is mostly cohesive clay.
Dry Strength - Granular soil, when dry, crumbles on its own or with moderate pressure into individual grains or powder. Soils with clay content, when dry, crumbles into clumps which break up into smaller clumps but the smaller clumps can only be broken with significant force. If the dry soil breaks into clumps which do not break up into small clumps and which can only be broken with difficulty, and there is no visual indication the soil is fissured, the soil may be considered
unfissured.
Thumb Penetration - The thumb penetration test can be used to estimate the unconfined compressive strength of cohesive soils. This test should be conducted on an undisturbed soil sample as soon as practicable after excavating to reduce the chance of air drying the sample. If later the trench is exposed to moisture (rain, flooding, etc.), the soil classification must also be changed.
Type A soils can be readily indented by the thumb. However, they can be penetrated by the thumb only with very great effort.
Type C soils can be easily penetrated several inches by the thumb and can be molded by light finger pressure.
Other Strength Tests Estimates of unconfined compression strength of soils can also, be obtained by use of a pocket penetrometer or a hand-operated shearvane. Other tests can include drying and sedimentation.
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