Excavations in Construction/Soil Classification
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
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
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
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
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
Any soil that is not cohesive is automatically
classified as Type C, although some Type C soils
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
penetrometer test can be used.
A soil's compressive strength can be given a numeric
value by using the pocket penetrometer test.
There can be some variability in these results,
so it's a good idea to run this test on a few soil
samples from the same part of the excavation,
just to make sure your results are consistent.
A pocket penetrometer works much like a tire pressure gauge.
A thin, metal piston is pushed into a soil sample,
and the penetrometer records the compressive strength
of the soil.
Be sure that the scale indicator is inserted
into the penetrometer body until only the "zero"
mark is showing.
To conduct the test, push the piston into the soil
until it reaches the engraved line.
Then, simply take the reading from the scale indicator.
It's important to recognize that a penetrometer
may give false results if the soil contains
rocks or pebbles, which won't compress.
As you can see, Type A soil will measure at least
1.5 tons per square foot.
Be aware, however, that you can't classify a soil
as Type A if the excavation site didn't meet all
the conditions of the visual test:
if it is granular, near a source of vibration,
or there are signs of previously disturbed soil,
water seepage, or fissured soil.
For Type B Soil, the reading will be
between 0.5 and 1.5 tons per square foot.
Type C soils are equal to or less than
0.5 tons per square foot.
Let's review the main points from the video.
OSHA classifies soils into three main groups:
Type A, Type B, and Type C.
Type A is the most stable and Type C is the least stable soil.
To determine the soil type on a construction site,
there are several tests that a competent person
After performing a visual test, you can use the
plasticity test to determine if the soil is
cohesive or granular.
For cohesive soil, the thumb penetration and
pocket penetrometer tests help determine the
unconfined compressive strength.
Determining the type of soil on a site will help
a competent person decide which methods of sloping,
benching, or shoring are needed to prevent
cave-ins and keep workers safe.
For more information, visit the OSHA website at
www.osha.gov, where you can read the OSHA Excavation
Standard, or use OSHA's Construction e-tool for
Trenching and Excavation.
You can also contact OSHA at 1-800-321-OSHA.