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  • In the U.S., more than 800 construction workers die

  • every year while on the job.

  • One of the most dangerous types of construction work

  • is trenching, which kills 40 construction workers

  • every year.

  • Workers can suffer death or serious injury within

  • minutes of being caught in a trench cave-in.

  • But these deaths can be prevented.

  • The video you are about to see shows one of the steps,

  • classifying soil, that employers must follow

  • so that trenching work can be done safely.

  • This video is not intended to be a complete educational

  • tool, instead it is meant as an introduction for

  • people who want to know more.

  • Employers have a responsibility to provide a safe workplace

  • and required protective equipment.

  • You'll learn how having the right information

  • about a construction site can help save lives.

  • Each employee who enters a trench must be protected

  • from cave-ins by a protective system if the

  • excavation is 5 feet or greater in depth,

  • unless it is dug into stable rock.

  • A support system is not required if the trench is less

  • than 5 feet in depth and examination of the ground

  • by a competent person provides no indication of

  • a potential cave-in.

  • One cubic yard of soil can weigh as much as a car,

  • 3,000 pounds, and comes in many varieties.

  • Some types of soil are stable and some are not.

  • When digging a trench, it's important to know the type

  • of soil you're working with so you know

  • how to properly slope, bench, or shore the trench.

  • This can help prevent a cave-in.

  • OSHA requires that employers have a competent person to

  • determine the soil type.

  • A competent person is someone who can identify

  • conditions that are hazardous to employees and who also

  • has the authorization to correct these hazards.

  • All trenches that are five feet or deeper must follow

  • OSHA's rules.

  • The appendices of the OSHA Excavation Standard show

  • the various types of support systems that may be used,

  • up to a maximum depth of 20 feet.

  • Any excavation deeper than 20 feet must use a protective

  • system approved by a professional engineer.

  • For all excavations, a competent person must

  • conduct a full investigation every day, or when any

  • trench conditions change, to identify and remove

  • any potential hazards.

  • In this video, you will see how a visual inspection

  • of a construction site's soil is performed.

  • You will also see how to test the soil using three

  • of the most common methods: the plasticity test,

  • the thumb penetration test,

  • and the pocket penetrometer test.

  • For best results, OSHA recommends that the competent person

  • use more than one of these methods to test the soil.

  • Knowing the type of soil makes it possible to determine

  • the right protective system to keep workers safe

  • when they're working in an excavation.

  • Soil can either be cohesive or granular.

  • Cohesive soil contains fine particles and enough clay

  • so that the soil will stick to itself.

  • The more cohesive the soil, the more clay it has,

  • and the less likely a cave-in will happen.

  • Granular soils are made of coarse particles,

  • such as sand or gravel.

  • This type of soil will not stick to itself.

  • The less cohesive the soil, the greater the measures

  • needed to prevent a cave-in.

  • OSHA uses a measurement called "unconfined compressive strength"

  • to classify each type of soil.

  • This is the amount of pressure that will

  • cause the soil to collapse.

  • This value is usually reported in units of tons per

  • square foot.

  • Soils can be classified as Type A, Type B, or Type C.

  • Type A soil is the most stable soil in which to excavate.

  • Type C is the least stable soil.

  • It's important to remember that a trench can be cut

  • through more than one type of soil.

  • Let's look at each type of soil.

  • Type A soil is cohesive and has a high unconfined

  • compressive strength; 1.5 tons per square foot or greater.

  • Examples of type A soil include clay, silty clay,

  • sandy clay, and clay loam.

  • Soil can not be classified as type A if it is fissured,

  • if it has been previously disturbed,

  • if it has water seeping through it,

  • or if it is subject to vibration from sources such as

  • heavy traffic or pile drivers.

  • Type B soil is cohesive and has often been cracked or

  • disturbed, with pieces that don't stick together as well

  • as Type A soil.

  • Type B soil has medium unconfined compressive strength;

  • between 0.5 and 1.5 tons per square foot.

  • Examples of Type B soil include angular gravel, silt,

  • silt loam, and soils that are fissured or near

  • sources of vibration, but could otherwise be Type A.

  • Type C soil is the least stable type of soil.

  • Type C includes granular soils in which particles

  • don't stick together and cohesive soils with a

  • low unconfined compressive strength;

  • 0.5 tons per square foot or less.

  • Examples of Type C soil include gravel, and sand.

  • Because it is not stable, soil with water seeping

  • through it is also automatically classified as Type C soil,

  • regardless of its other characteristics.

  • Before testing the soil, it's useful to perform a

  • visual test of the construction site.

  • This will help determine if there are factors

  • on site that will lower the strength of the soil.

  • Here are some observations to make when performing a

  • preliminary visual test of the soil around an excavation site:

  • First, as the soil is being excavated, does it come out

  • in clumps, or is it granular?

  • Clumps mean that the soil is cohesive.

  • Are there sources of vibration near the excavation?

  • Are there signs of previously disturbed soil,

  • such as utility lines?

  • Are there signs of water seeping through the soil?

  • Is the soil fissured? Signs of fissuring include

  • crack-like openings, or chunks of soil that crumble

  • off the side of a vertical excavation wall.

  • If any of these conditions are met, the soil

  • cannot be classified as Type A.

  • When performing a soil test, it's important to

  • choose a good soil sample.

  • Soil samples should be typical of the surrounding

  • soil in the excavation, and additional samples should

  • be taken as the excavation gets deeper.

  • While the excavation wall is one place to take samples,

  • OSHA recommends taking a large clump from the excavated

  • pile, as long as the soil in the pile is fresh and

  • hasn't been compacted.

  • Test results can change as the soil dries up,

  • so for the best results, samples should be taken

  • and tested as soon as practical.

  • Now let's look at the three main types of soil tests.

  • The plasticity test, which is sometimes called

  • the pencil test, is used to determine if the soil

  • is cohesive.

  • This test is performed by rolling a moist soil sample

  • into a thread that's one-eighth of an inch thick

  • and two inches long, resembling a short, slim pencil.

  • If the sample can be held at one end without breaking,

  • it is cohesive.

  • Here's an example of what the results will look like

  • if the soil is cohesive.

  • Notice how the sample stays in one piece.

  • Now look at what happens when the soil

  • is not cohesive.

  • The soil doesn't have the strength to hold

  • itself together.

  • Any soil that is not cohesive is automatically

  • classified as Type C, although some Type C soils

  • are cohesive.

  • The thumb penetration test is used to quickly estimate

  • the compressive strength of a cohesive soil sample.

  • To perform the thumb penetration test, simply press

  • the end of your thumb into a fresh clump of soil.

  • If the soil sample is Type A, your thumb will only

  • make an indentation in the soil with great effort,

  • as you can see demonstrated here.

  • If the soil sample is Type B, your thumb will sink

  • into the soil up to the end of your thumbnail,

  • just like this.

  • If the soil sample is Type C, your thumb will sink

  • all the way into the soil clump,

  • as you can see here.

  • Your results for this test will probably be somewhere

  • in between these results.

  • For a more numeric measurement, the pocket