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.
There are other complicating factors that can result in soil failures.
Sliding and other modes of failure can occur in soils that are not densely compacted.
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:
A number of stresses and deformations can occur in an open cut or trench causing soil instability and failure.
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.
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.
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.
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.
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 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.
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.
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.
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Note: The case described above was selected as being representative of fatalities caused by improper work practices. No special emphasis or priority is implied nor is the case necessarily a recent occurrence. The legal aspects of the incident have been resolved, and the case is now closed.
Learn to properly use adequate protective systems while working in trenches to avoid possible hazards through this Northwest Lineman College video demonstration.