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What to Know About Soil Before You Excavate

Construction workers are at risk for trenching cave-ins. On average, 40 workers die each year due to these types of accidents. Having an expert in soil classification on hand is important for everyone involved. Before an employee can perform an excavation, the Occupational Safety and Health Administration (OSHA) requires that they be properly trained g to identify potential hazards and ways of resolving them. A designated expert with specialized training in soil stability must be at the site to review the soil and make the necessary safety decisions.

Excavations four feet or deeper require an awareness of the type of soil surrounding the dig. If the excavation is more than 20 feet deep, a licensed engineer must first approve a protective system that will prevent cave-ins.

Classes of Soil

The stability of the soil determines into which class it falls. The cohesiveness of the soil also matters. Clay-rich and fine particle soil will stick together better than other types, reducing the likelihood of a cave-in. Granular soils typically include gravel or sand that have coarse particles. These are not likely to stick together, so excavators must take measures to ensure that a cave-in does not occur.

OSHA uses “unconfined compressive strength” to classify soils. This refers to the amount of pressure needed for the soil to collapse, measured in tons per square foot. Soil types are categorized into three types:

  • Type A – most stable: This soil has a compressive strength equal to or greater than 1.5 tons per square foot. This category includes soil that has clay, sandy clay, clay loam or silty clay.

  • Type B – cohesive but disturbed: This category has a compressive strength of 0.5 to 1.5 tons per square foot. While the soil can stick together, it won’t do it as well as Type A soils. These include silt loam, angular gravel, silt or soil that has been cracked.

  • Type C – least stable: These soils have an unconfined compressive strength of 0.5 tons per square foot or less. The particles don’t stick together well. This category includes soils that allow water to seep through them. Weak soils do not provide sufficient support for most buildings or walls.

Importance of Soil Tests

Before work can begin, a visual test of the site must be done. The designated expert on the site will determine the following:

  • Has the soil been disturbed?

  • Does it appear to be granular or cohesive?

  • Is water seeping through the soil?

  • Are there cracks in it?

  • Are sources of vibration near the site?

To further test the cohesiveness of the soil, a sample will be taken that is representative of the area. As the digging progresses, excavators must pull different samples since a trench can pass through distinct types of soil, especially in deeper trenches.

There are three main soil tests conducted on samples:

  • Plasticity/Pencil Test: A soil sample is rolled into a 1/8-inch by 2-inch piece. It is then held on one end to determine if it breaks. If it doesn’t, it is considered cohesive.

  • Thumb Penetration: A thumb is pushed into a sample of soil. If only a small indentation is made, then the soil is in the Type A class. If the thumb can go into the soil until the end of the thumbnail, it’s Type B. If it goes all the way in, it’s Type C.

  • Numeric: A pocket penetrometer is used to collect numerical data to determine if the soil is cohesive or non-cohesive.

Why Soil Tests Are Important

Soil tests are essential for safety. Failure to conduct proper soil testing puts workers at risk. Not performing tests, or being unable to prove that you completed them, can lead to OSHA sanctions.

Soil testing helps the designated on-site expert determine what type of benching, shoring, and sloping are needed to prevent cave-ins during the excavation. Construction work can't begin until testing is complete. Testing highlights hidden dangers that need addressing long before any jobs start. By understanding the physical and chemical compositions of the soil, you can determine whether the soil can bear the weight of the excavation and any construction work.

About CBS

Chamberlain Backhoe Service, established in 1957, has assisted thousands of contractors throughout Southern California with their heavy equipment rental and turnkey operator needs. We work with highly skilled operators who can assist you with all your excavation projects. Our team will listen to your requirements to determine what skills you need for your upcoming project to ensure that we send the right person for the job.

Learn more about how we can assist you today. Contact us now!


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