Soil Safety Tips

 Soil definitions:

• Apparent cohesion is when sand or granular soil is held together by water filling the voids to give the impression that the soil is cohesive. As the soil dries it becomes weaker and will eventually fracture. Example: sand castle at the beach.
• Cemented soil is soil with the particles held together by a chemical agent, such as calcium carbonate. It cannot be easily crushed. It sometimes resembles concrete. It is frequently referred to a caliche or hardpan.
• Plastic is the property of soil that allows it to be deformed or molded without cracking or appreciable volume change.
• Saturated soil is soil in which the voids are filled with water. Saturation does not require flow.
• Submerged soil is soil that is under water or that is freely seeping water. Soils below the water table are generally freely seeping water.
• Layered soil is made up of two or more distinctive layers of soil.

Unconfined Compressive Strength

The unconfined compressive strength of a soil is the load-per-unit area at which a soil will fail in compression. It can be estimated in the field by using the thumb penetration test, pocket penetrometer, a shearvane, and other methods.

Type A Soil (Lateral pressure per foot of depth = 25 lbs)

Type A soil is a cohesive soil with an unconfined compressive strength of 1.5 tons per square foot (tsf) or greater, e.g., clay, silty clay, cemented soils.

Example

Stable Rock

Natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed. Many engineers believe that if the rock is disturbed, the walls will be cracked and therefore the rock would not be stable.

If the cracks slope into the trench the rock is not stable. If the cracks slope away from the trench the rock is stable. It is impossible to determine which way the cracks are sloped, therefore how can a competent person reasonably be able to classify the rock stable?

No soil is Type A if it:

• Is fissured or cracked
• Is subject to vibration
• Has been previously disturbed
• Is subject to other factors making it less stable, such as water

Type B Soil (Lateral pressure per foot of depth = 45 lbs)

• Cohesive soil with unconfined compressive strength greater than 0.5 tsf but less than 1.5 tsf
• Granular cohesionless soils, e.g., gravel, silt, silt loam or sandy loam
• Previously disturbed soils, except those otherwise classified as Type C
• Type A fissured or subject to vibration
• Unstable dry rock

Type C Soil (Lateral pressure per foot of depth = 60 lbs)

• Cohesive soil with unconfined compressive strength of 0.5 tsf or less
• Granular soil including gravel, sand, and loamy sand
• Submerged soil or soil from which water is freely seeping
• Submerged rock that is not stable

Cohesive soil

• Clay or soil with a high clay content
• Does not crumble and can be excavated with vertical sides
• Plastic when moist, hard to break when dry

Granular soil

• Gravel, sand, or silt with little or no clay content
• No cohesive strength
• Cannot be molded when moist and crumbles easily when dry

Cohesive vs. Granular

Before determining a soil type, the competent person must determine if the soil is cohesive or granular. This may be accomplished in several different ways that will be explained in the next few slides.

Once it is determined if the soil is cohesive or granular, other tests may have to be performed. If the soil is granular it is Type C. If the soil is cohesive, an unconfined compressive strength test using a pocket penetrometer, shearvane, or your thumb must be performed to estimate the unconfined compressive strength of the soil. An explanation of the use of these methods will follow.

If the unconfined compressive strength of the soil is 1.5 tons per square foot (tsf) or greater the soil is Type A. If it is 0.5 to 1.5 tsf, it is Type B. If it is less than 0.5 tsf, it is Type C.

Calculating Depth Rating Example

If a shield has a 1500 psf rating then divide the psf rating by the lateral pressure related to the depth of the shield.

1500 / 25 (Type A Soil) = 60 ft.
1500 / 45 (Type B Soil) = 33 ft
1500 / 60 (Type C Soil) = 25 ft.