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April 2013

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									                                                                                                April 2013
                                Soil Classification and Excavation Safety

An excavation—the act of creating a man-made cut, cavity, trench or depression in the earth’s surface—
is one of the most hazardous activities that we deal with in construction. This information will shed light
on proper soil classification; slope angle calculations and a simple rule that will help your employees make
safe excavation decisions.

Four Types of Soil

Employees who work on excavations must be trained in the four soil classifications: stable rock, Type A,
Type B and Type C. In general, stable rock is not common because we disturb it by excavating, leaving us
with the following three soil types to gauge our excavations.

Type A: This is the most stable of the soil classifications and implies that you have a slope angle of a
0.75:1 ratio, which means that for every foot of depth, the sides of the excavation will slope back three-
quarters of a foot or at a 53-degree angle. Type A soils are cohesive with an unconfined compressive
strength of 1.5 tons per square foot (tsf) or greater. Examples include clay, silty clay, sandy clay and clay
loam. Type A soil may also be “benched,” or set at specific angles for employee protection. Benching
creates a stair-step condition; the soil comes up 5 feet vertically from the bottom of the excavation, and
cuts back 4 feet horizontally at 90-degree angles on the sides. This is repeated to the top of the
excavation.

Type B: This soil is less stable than Type A type soil, but is very cohesive and still quite stable. The slope
angle for a Type B excavation is a 1:1 ratio or at a 45-degree angle. For every foot of depth, the sides of
the excavation must slope back 1 foot. Type B soil is cohesive with an unconfined compressive strength
greater than 0.5 tsf, but less than 1.5 tsf. Other examples include granular non-cohesive soils, such as
angular gravel, which is similar to crushed rock; silt; silt loam; sandy loam; previously disturbed soils
except those that would otherwise be classified as Type C soil; soil that meets the unconfined
compressive strength or cementation requirements for Type A but is fissured or subject to vibration; and
dry rock that is not stable. Type B soil may also be benched, coming up 4 feet vertically from the bottom
of the excavation and 4 feet horizontally at 90-degree angles on the sides, repeating to the top of the
excavation.

Type C: Of all the soil types, this is the least stable and most hazardous, and must be sloped at a 1.5:1
ratio or at a 34-degree angle. Depending on water saturation or seepage, the angles may need to be
greater than 34 degrees for employee safety. Type C soil is cohesive with an unconfined compressive
strength of 0.5 tsf or less. Examples include granular soils such as gravel, sand and loamy sand;
submerged soil or soil from which water is freely seeping; and submerged rock that is not stable.
Benching Type C soil is unacceptable and shall not be done.




The PEI Safety Program Audit is aimed at helping companies provide a safe place of work for all employees. The program is
completely confidential and available to distributors doing business in the United States. Each Safety Program Audit reviews
company health and safety philosophies, training programs and facilities, as well as written corporate safety policies, chemical
inventory sheets and various forms required by OSHA. For information, contact PEI at 918-494-9696 or cbouldin@pei.org.
Slope Angle Calculations

Determining your slope angle is not complicated; in fact, you don’t even need a protractor. This simple
equation will tell you the proper opening width: (depth x 2) x type slope ratio + width of original
excavation = top width. As an example, let’s calculate the slope angle of a simple trench that is 6 feet
deep by 2 feet wide, factoring in the type of soil.
• Type A: (6 feet x 2) x 0.75 + 2 feet = 11 feet wide at the top.
• Type B (6 feet x 2) x 1 + 2 feet = 14 feet wide at the top.
• Type C (6 feet x 2) x 1.5 + 2 feet = 20 feet wide at the top.

As you can see, there is a significant difference in the width at the top of the excavations, so proper soil
classification is a must for employee protection when using sloping or benching techniques. Remember,
when classifying soil, one visual and one physical test must be performed at minimum. Physical tests may
include a ribbon or thumb test or the use of a penetrometer to determine soil type.

Depending on the situation, shoring may become a viable option. It may be portable or permanent, but all
shoring must be designed by a professional engineer and accompanied by tabulated data that references
how many tons per square foot the shield will resist. This means you cannot go to the local hardware
store and buy plywood and timber; an engineer must run the numbers regarding the strength of the
installed shield. A shoring system can be installed by a qualified person who, by experience or degree,
recognizes the hazards of an excavation and is under the supervision of a competent person. When
installing shoring systems, keep in mind both the ends and the sides of the excavations.

Simple Rule

In addition to proper soil classification training, the “2 through 5 and 25” rule will help ensure your
employees make the proper and safest decisions in an excavation situation. Here are the basics of the
rule:

•   Keep tools, material, equipment and spoils 2 feet from the edge of an excavation.
•   Three feet of a ladder must extend above the edge of an excavation for proper ingress/egress.
•   At a depth of 4 feet, a ladder or other means of ingress/egress is required.
•   At a depth of 5 feet or more, proper shoring or sloping techniques shall be utilized.
•   An employee shall not laterally travel farther than 25 feet to reach a ladder.

Excavations are extremely hazardous and can even lead to fatalities. However, employees who receive
proper training before they are assigned to excavation work will have the tools they need to stay safe and
avoid an incident.

                                                                                              References
This SafePractices is based on the article Soil Classification and Excavation Safety by Lester Apley, CHST,
CUSP. Apley is a safety coordinator for Pike Electric Corporation. The article was originally published in
Incident Prevention Magazine on February 12, 2012.




The PEI Safety Program Audit is aimed at helping companies provide a safe place of work for all employees.
Each Safety Program Audit reviews company health and safety philosophies, training programs and facilities, as
well as written corporate safety policies, chemical inventory sheets and various forms required by OSHA. Want
to sign up? Visit www.pei.org and click on Safety Resources for more information.
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The PEI Safety Program Audit is aimed at helping companies provide a safe place of work for all employees.
Each Safety Program Audit reviews company health and safety philosophies, training programs and facilities, as
well as written corporate safety policies, chemical inventory sheets and various forms required by OSHA. Want
to sign up? Visit www.pei.org and click on Safety Resources for more information.

								
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