Why Soil Fails

Example of sliding failure from tension crack.

Soil failure. Soil failure is defined as the collapse of part or all of an excavation wall.

Safe slope. A safe slope can be defined as the maximum angle of the edge wall or bank of an excavation at which sliding will not occur. The unique mixtures of the different types of soil (sand, clay, silt and rock) necessitate different safe slopes from one excavation site to the next.

Soil Sliding. Soil sliding is the most common factor leading to soil failure. The most common soil failure is typically described as an unexpected settlement, or cave-in, of an excavation.

Complicating Factors

There are other complicating factors that can result in soil failures.

• Layering. During an excavation, visibly different layers of soil may be uncovered. Each of those layers may call for different safe slopes. It is essential to plan your excavation around the most gradual safe slope for the different soil types and layers encountered during the excavation.
• Soil composition. Another complicating factor is that soil composition mixtures may vary significantly from one area of the project to another. During an excavation, as the soil composition changes, the safe slope for trench wall excavation also changes. Thus, across an excavation site, the slope of the bank may need to be different to provide a safe working environment.

Sliding Problems

Sliding is the most common cause of soil failure.

Sliding and other modes of failure can occur in soils that are not densely compacted.

• If a trench that is made close to a previously dug trench is very unstable.
• If uncompacted soil is discovered, the normal safe slope for dense soil will not be enough to prevent sliding. Bracing or more gradual sloping may be necessary.
• If tension cracks are observed in rocky types of soil, sliding has already occurred. These cracks should signal that a more gradual slope for excavation is needed because the rocky soil is very susceptible to slides and other types of failure.
• If, after prolonged exposure to the elements, the moisture content in stable soil may increase due to rainfall, a broken water line, or other reasons. The extra soil moisture tends to speed up sliding soil failures.

Vibration, encumbrances, and weather conditions cause soil failure.

The Three Common Factors

Soil failures that cause sliding can occur for any number of reasons. However, three of the most common factors that increase the chances of soil failure from sliding are:

1. excessive vibration,
2. surface encumbrances, and
3. wet conditions.

Wet conditions speed up sliding soil failures.

Soil Instability

A number of stresses and deformations can occur in an open cut or trench causing soil instability and failure.

Moisture

Increases or decreases in soil moisture due to rain or underground seepage can decrease soil cohesion while at the same time increase the weight of the soil. These two factors can adversely affect the stability of a trench or excavation.

Example of a tension crack.

Tension Cracks

Tension cracks usually form at a horizontal distance of 0.5 to 0.75 times the depth of the trench, measured from the top of the vertical face of the trench. They may result in sliding or sluffing.

During a visual test, the evaluator should check for crack-line openings along the failure zone that would indicate tension cracks, look for existing utilities that indicate that the soil has previously been disturbed, and observe the open side of the excavation for indications of layered geologic structuring.

Soil Instability (Continued)

Toppling

In addition to sliding, tension cracks can cause toppling. Toppling occurs when the trench's vertical face shears along the tension crack line and topples into the excavation.

Subsidence and Bulging

An unsupported excavation can create an unbalanced stress in the soil, which, in turn, causes subsidence at the surface and bulging of the vertical face of the trench. If uncorrected, this condition can cause face failure that may trap or bury workers in the trench.

Heaving or Squeezing

Bottom heaving or squeezing is caused by the downward pressure created by the weight of adjoining soil. This pressure causes a bulge in the bottom of the cut, as illustrated below. Heaving and squeezing can occur even when shoring or shielding has been properly installed.

Boiling

Boiling is evidenced by an upward water flow into the bottom of the cut. A high water table is one of the causes of boiling. Boiling produces a “quick” condition in the bottom of the cut and can occur even when shoring or trench boxes are used.

Real-World Accident

Click to enlarge.

Brief Description of Accident

Employees were laying sewer pipe in a trench 15 feet deep. The sides of the trench, 4 feet wide at the bottom and 15 feet wide at the top, were not shored or protected to prevent a cave-in. Soil in the lower portion of the trench was mostly sand and gravel and the upper portion was clay and loam. The trench was not protected from vibration caused by heavy vehicle traffic on the road nearby. To leave the trench, employees had to exit by climbing over the backfill.

As they attempted to leave the trench, there was a small cave-in covering one employee to his ankles. When the other employee went to his co-worker's aid another cave-in occurred covering him to his waist. The first employee died of a rupture of the right ventricle of his heart at the scene of the cave-in. The other employee suffered a hip injury.

Inspection Results

Following an investigation, citations were issued alleging three willful, four serious and two non-serious violations of construction standards. If the trench was shored to prevent slides or cave-ins and had employees been trained to recognize and avoid unsafe conditions, the accident could have been prevented.

Click on the button below to see OSHA's recommendations make sure this kind of accident doesn't happen again.

VIDEO

Learn to properly use adequate protective systems while working in trenches to avoid possible hazards through this Northwest Lineman College video demonstration.

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