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 Tailgate 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 3/4:1 ratio, which means that for every foot of depth, the sides of the excavation will slope back three-quarters of a foot or 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 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 noncohesive 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 1-1/2:1 ratio or 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.

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 .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.

A 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.

About the Author: Lester Apley, CHST, CUSP, is a safety coordinator for Pike Electric, where he has been employed for nearly 30 years. While at Pike, he has held a variety of positions including groundman, operator, URD foreman, apprenticeship program instructor and lineman. Apley has been injury-free for 27 years.

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